Tektronix 576 Curve Tracer System
M HAMEG 203-4 oscilloscope has a simple, but very good working curve tracer build in. I found interest ins looking at curves of passive components. It is like these things we think we know it already, but do we really? So I opened my old parts boxes, and tested used coils, old transistors, lamps and diodes, just anything. And the things I have learned sort of pop up again. So I got the taste of this, and I wanted to have a curve tracer which can do more.
In case you are interested, I can recommend a Hameg Scope for this with a nice little component tester build in. Several Hameg types have that. Just look for this button below the picture tube where it says COMPONENT TESTER. It works really accurate, and you need to know nothing about it's operation. Just connect a diode under test with two banana plugs between ground and where it says "Component Tester". So the red and the black banana plugs you see in the picture. Then press the button and there you are: A two quadrant diode curve. (So forward and reverse in one picture). If you consider a scope for Audio, consider the 203-4. It is a very basic, but usefully model, and the component tester inside comes for free. After playing with this option, I became interested in it, and I thought I wanted to have the Tektronix 570 curve tracer, the first and last one by Tektronix, made for tubes. Still I decided for the Tektronix 576 transistor curve tracer. In the next article, I will explain why. Also show the repair details if the one I bought, and end with some testing on tubes and parts which I have here.
Also I wanted to study a just for my interest, what information can be extracted from components of any kind. Like see the effect of the heater voltage with tubes, and compare two curves charts of a double triode instantaneously. Now of course this can be done with digital curve tracers as well, but that is only half what I intended. A digital curve tracer is very boring to use, because you have to wait all for the curves to be finished point by point.
The best digital curve tracer I have, is the Sofia by Audiomatica Italy. The hardware is software protected against overload, and it switches off reliable with tubes. It should be able to test Field Effect Transistors as well, but I am not going to try it. Resistance of FETs is much lower, and I don't know if that was anticipated in the design.
The AT1000 curve tracer has very robust hardware, more forgiving with mistakes, but the software is has many shortcomings, as it can not work lower than 75 Volts. Resolution below 1mA is in steps of 0,1mA. It is too slow, and curves have no smoothening function. So curves sometimes look like made with a Commodore 64.
There are now some low cost digital tube tracers around, but these are limited in speed, voltage and current. The next lower level would be oscilloscope based testers. Even Tektronix had a module for that for the 7-series scopes, but that is a low power device, hardly suited for tubes.
There are a few others, but 570 and 576 are the top models.
These differ at following points:
Initially I was interested in a Tektronix 570. However this page is not going to be a tribute to this model. Though they are really nice and very useful, my main objection is the plate voltage can go only up to 500 Volts, and the price is crazy for what you get. When you use a tube at (say) 380 Volts, the curve chart should be up to 600Volt or more. Otherwise it becomes a bad compromise. Better is the Sofia, as it can trace up to 750V. Still a tube like 845 can not be tested under real use conditions at only 750V. You need 1200V or more.
However, objection with the 570 are many. First, the relation between what you pay and what you get is fully wrong. This is caused by people that pay 4000 Euro for them. Which they may be worth or not, decide that for yourself. In Jan 2018 I saw one go on Ebay (Auction nr 232611931723) for 1032 Euro + expensive shipment. It was said to be stored for 20 years, and that seemed like stored in a chicken shack. There was a layer of rust on the whole inside, and from underneath the scope tube socket was white oxide power coming out, and quite a lot actually. Ideally, it was put in the chicken shack in working order by mistake, and all it needs is you to find it in Ebay, and brush away the rust. Not so ideally, it was put there 20 years ago, with such bad defects, the chicken shack was the right place to put it.
You can even conveniently buy such an item, check if the scope tube is at least ok, and if the transformers are still good. If not, sell it on Ebay, and the next one will buy it for a higher price when it is cleaned nicely.
With 570, we have the same situation as with all tube testers, people think they can fix all problems with a soldering iron and a multi meter. Then after they bought it, they find out several owners before them already failed. So you're the next one in line, and good luck with it, because here comes the problem: Either you buy it from somebody who knows MORE about it than you, and he gave up on it. Or, you buy it from somebody who knows LESS about it than you, and you have to clean his mess. I see no other options.
Here is an observation I just made, looking for a Tektronix 5030 scope. I found information and pictures here. Then, I looked on the German Ebay site, and hey.... I see there EXACTLY this oscilloscope for sale, described on W140.com. Same series number! Incredible, but true! Really cheap, the auction number from Jan 2018 is: 182648785851. There are ways to look at old auctions, also with Ebay. Now, on w140.com this scope, with that SAME series number, is described here with the SAME pictures as on Ebay, by somebody who supposedly is an expert with that product. . On Ebay is written he "knows not much about the accuracy". Do you think what I think? And even so a 5030 is what nobody wants to have, but for a 570 they are all haunting.
When the tester was stored dirty, and somebody try to fix the whole thing with contact spray, you will never get happy with it any more. Do not confuse this with the later 576! Though so close related part numbers, the 576 is stuffed with the best and finest parts that were available at the time. There is gold plating simply for everything where they had the choice. Even so, most of the switches in the 576 are just, to switch RELAYS. Some of which have gold plated lead wires. That is a total different way of doing things. The many relays were for the card interface which existed, so most functions of the 576 are basically relay operated and not switch operated. Pull out the interface unit, and you will see there is a whole series of connectors behind it. These allow access to all relays, and other functions. Via those, you can for instance disable the vertical sensitivity as chosen by the knobs, and choose another sensitivity via the relays.
Though errors with tube equipment are usually not so tricky, the 570 has just very many of them. The air ventilator has the habit to slow down when it gets older and there is some risk your 570 was over heated at some point in time. So the 570 is something you should not buy unseen. Bringing it back to instrument condition ( in case you dream about that) means a full restoration. So take it completely apart, replacing every bad item you find inside, and that will be many. Just imagine what to do, after you have de soldered a selector switch, and you find it leaky between all contacts. I mean like some Mega ohms between the contacts. Well you could leave it as is, but no... this is nothing for me. Fixing only what seems broken is no option for me. I tried that out with other tube equipment, but when you repair one thing, soon after the next thing gets broken, and this just doesn't stop. Yet a full restoration of a Tektronix 570 will consume half a year of my time, not to speak of the costs, and many of the objections I wrote down here, do not exist with the 576. ( I fully restored mine in just several days work)
So I had to let go of the 570 idea, I just see too much of the usual risk, somebody sell it for reasons he knows damn well, and I don't want to be his fool. Besides the 576 costs only 1/4 of a 570, it can do 3x higher voltage and current. In fact it's quite amazing to see you can have 1uA per division vertical scale, and even zoom in on that a factor 10, with a delay function as with any oscilloscope, thus getting 100nA per division. Whereas you can also set it to 2 Ampere per division, giving 20 Ampere over the full picture tube. Oh man, the range of this curve tracer is incredible.
Generally the picture tube is a bit flickering, but that is something that belongs to the concept, and they used normal oscilloscope tubes in the 570 and 576 as well. A bit of a pity, because special tubes with very long light up time would have been better. Right now, it triggers to the mains frequency, so the horizontal (voltage) is coming from the mains directly, by using a 1600 V AC transformer, and connect that to the mains via a variac. Even so, the relatively low frequency of the mains is more or less the right one, because a higher frequency would give difficulties with the capacitance of devices. (So called looping problems). So the whole concept makes sense, just not so much the scope tube, but that is how it is.
Quality of the picture tube:
The graticule of the 576 is inside the tube, at the phosphorous layer, so it is parallax free. The tube is flat, square, large sized, and brilliantly sharp. The tube of the 570 is more old fashioned, round and not flat, and I believe the graticule is not parallax free. For the rest, I trust it is as good as any fine Tektronix tube, though myself I find a green scope tube a bit difficult to look at. One way or another, blue is more pleasant.
570 tube. Here you see a picture which is not looping the curves very nice. If it was mine. I would not rest until I have found the cause. Or perhaps never find the reason at all, meaning you have to accept this as is. It's just these kind of issues I try to avoid.
So after all those considerations I choose for the 576. At Ebay, every day you can buy a few, but these are always untested of "expected to be probably good". Many people have parts for sale. Prices of parts are high, but product offerings are many. The 576 has specific issues too, but as far as I know it is only minor things. So even though complexity of the 570 is lower, you can restore a 576 far more easier to instrument condition, because most things inside are still good. They don't make them like that any more. Printed circuit boards are glass fiber, gold plated, with thick solder islands that don't come off when you use a desolder iron. Several boards can be removed easily, because you can pull off all wires with small connectors. Exceptions are the readout card, which can be pulled out from the front after you take off the scope cover. (See next pictures below). Also some boards are underneath a layer of other boards, which is not really easy to service, but I went through that, and it was not so terrible to do.
Yes I know, there are people who bought an all working perfect 576 for a low price. My 576 was as expected. I bought from the University of Boston. So probably used a lot when it was new, and then less and less, until it got stored for some years. I received it with shipment damage, due to bad packaging, and they gave it to me for a low price. If somebody has two lower side panels for me... please let mw know. So the ones it stands on.
It was still "working" indeed, but the rest, the more I looked into it, the more needed to be done nevertheless. Luckily there was never worked on the inside guts, except for the readout card, which could pulled out from the front, and some unsuccessful repair attempts were done seemingly by professor Duck and his assistant Goofy. After it didn't work they just plugged it back in, and that was it. Though I am happy, they never tried to "repair" other things. So chances are very high, it was never damaged during it's professional career at the university.
Also, this gives an idea about what is the difference between a "working" one for 800$, and one which is in instrument condition.
What is very helpful is the documentation. There is a first class pdf document. I printed this double sided on 100 gram paper, and used a 26 holes binder. The result is better than a book. There is really everything described in there. All internal part numbers for the semiconductors are listed there nicely with the standard industry replacements.
REPAIRS: Two defective switches
There is the switch to find the zero of the screen. So when you press it, the beam should point to the center of the tube. I needed to wiggle it occasionally and then it worked again. Then I needed to wiggle it more often, and it became a problem. Something similar was with the "invert" switch, which is needed to display curves with negative voltage upside down, so they look normal again. Such curves for instance for NPN transistors are displayed normal, and curves for PNP transistors are displayed inverted. As these are hidden beneath a layer of other boards, and you need to take all knobs off the whole front, and remove all those connector pins from the boards, I had the idea trying to spray some C60 contact oil in the switches. You can access them a little bit by pulling of the knobs. That turned out to be a very bad mistake, and it was the last time I use that contact oil probably. Initially this solved the contact problem, but it made the cause of the problem get worse. So after 3 years, both switches gave up function. The invert switch didn't toggle any more, and the Zero switch sticked out 2mm too far and it had no function any more. So what I tried to avoid, remove all boards, had to be done now anyway, and as I will show later, the contact oil damaged one of the switches.
This is the board before repair-
What you can not see, the contact oil has also loosened the bottom of the middle switch, and that made the arm come out 2mm too far. I could clean it, the green stuff resolved in alcohol, the brown stuff would come off with fine abrasive paper. The switch bottom had no great force on it, it could be repaired as you can see on the next picture. I have been looking a little bit for such a replacement board, but it was kind of rare, or too expensive. However the switches as such can be found still from other Tektronix oscilloscope boards, and perhaps from the manufacturer too, but I don't know who that is. They look kind of standard switches to me. Note, to have more room for the repair, those white cubes are relays and you can pull then out of the board just like that.
Look at the spray container, the C60 contact oil has created a fat, sticky brown layer with rust. Similar
REPAIRS: Defective lamps in the readout card.
The 576 has a read-out card which is made by lighting into fiver optics with a series of light bulbs. This is one of the best features I have ever seen, and it shows what the Tektronix designers wanted to have, but digital electronics was just not ready for it yet. Though developments were very close actually, and right after these readout cards, first ICs came on the market, so called Character Generators, which could display texts on an oscilloscope tube. However this light bulb solution is amazingly well done, and the card itself is very compact also. Also the cards shows things as they really are. So when you magnify with some factor, the card corrects it's reading. Or the step function (3rd number) can be in mA or in Volts, depending if you test a transistor or a FET. This is done all correct by some arrays to diodes which in the end operate the driver IC's for the light bulbs. You really can't see this is an analog + light bulb solution.
Nice, big indicators, that change their readings with relay clicks, and normal white light. They have build this from fiber optic, and normal light bulbs. There is not one light bulb per dot, but whole groups of fibers are lead to one lamp. That reduces the number of bulbs needed, but there are still some 50 lamps used. So to change from the letter "V" to "A", there are thee light bulbs needed. One will light up all the dots (with separate fiber optic cables) that belong to both letters. The second one lights up the complimentary dots to make a "V", the third one lights up the complimentary dots to make it an "A". The driver IC has a diode matrix inside, and the input is just c "1", or logic"0" and "V" or "A" will burn. All of this becomes very handy when the 576 is programmed EXTERNALLY. Many are not aware of this option! So when you remove the hand-operated unit from the bottom right, you can replace it by a card unit. The card unit will automatically do the complete settings, and you will see that on the dot matrix display. With some technical skills, I think it is not even difficult to operate a TEK576 from a computer that way. All you need is the programmable module, it had all connectors.
The bottom unit is for the steps. For transistors you set to uA base current, and it gives the Beta. For FETS and Triodes, the steps are in Volt, and you get the Gm. Note, a FET and a Triode are essentially the the same for a curve tracer. So even on the very old 570 that was made before FETs existed, you can test FETs.
The Quality of the components inside.
At this point, the 576 differs massively from the 570. The 576 is a new generation of products. The change was made from hand soldered technique, to printed circuit boards, and not just some boards. The PCB's are glass fiber and gold plated in my 576. Augat Brand IC sockets everywhere, and many gold plated IC's even. Though I have seen some from Italy without IC sockets, and without the gold plating. As these sell probably for the same price, make sure you
REPAIRS: Cardanic Axis
REPAIRS: The 5V power supply.
With respect to the Tektronics designers, but the rectifier circuit is strange. The output signal is 5V DC, 100mA to feed 6 TTL IC's. Only 100mA, and they are using a big chunky capacitor of 11.000uF, followed by a series regulator, with a heavy power transistor. Very bulky, and I am sure this could have been done with less parts. Or perhaps it was right after all, read below under ESR meter. Such things do contribute to the massive weight, and I see this filosophy, that "cost, weight and size play no role", throughout the whole machine. But let's not criticize Tektronix for that, because any 50 years old equipment with savings on material cost, too simple circuits, and too small build, are not fun to work on.
In my 576 was a defective rectifier capacitor in the 5V electronics, and the regulator circuit worked very strange. I had a hard time finding out what the heck caused all of this. Whereas in the end this is only a 5V 100mA power supply, feeding 6 TTL IC's with 5V. Well I was lucky, the output voltage was too low. It was 4.2 Volts. When it gets too high it damages the ICs, and they are hard to substitute. Later I learned, other people have the same problem. So when you buy a 576, check the 5V circuit not just to be nicely 5V, and also check for no ripple on the input. As a good 11.000uF capacitor will give no ripple at 100mA. I would say, it is better to check the all rectifier capacitors with an ESR meter (of the kind you buy for 20 $ on Ebay, they work very good). ESR = Equivalent Series Resistance of the capacitor. You can call it Ri if you like. I found it to be in the range of 0,8...1 Ohms Ohms for all power supply capacitors. (apart for the broken one).
The use of an ESR meter
ESR stands for Equivalent Series Resistance. Such a meter cost only 20$ on Ebay, and these get more refined every year. Originally a Danish designer made an Arduino design with this, which worked excellent. He published the code for public use. I made a copy of this long ago. Later, some Chinese said thank you, and took it into production. They improve the graphic interface, but I think not it's functioning. So at the moment you can buy those with a color screen and much better graphic resolution that the first ones. The test routine seems unchanged ever since. All of that for just 20$. Such a meter first ramps up a very small voltage to see what happens. When current begins to flow, it does some other tests, to find out what might be connected, and it continues until any three lead device is identified, and tested. So much better than any 800$ Fluke, which pumps 1mA (at 9Volt open circuit) into any unknown device, that can really damage anything. The ESR tester also uses a low voltage, low current oscillator signal, and it measures current + voltage. When all tests are done, the software can say if it found a diode, a transistor, a FET, and keep PNP and NPS apart. Measure a .resistor, a capacitor, a short, a battery, a zener, or a normal diode, and tell the value. If a capacitor, it will give the value, and the equivalent series resistance. The nice part is, you can leave the capacitor in the circuit, and due to the low signal it doesn't damage any semiconductors. Don't worry about how this is done. The point is: It works. So you just connect it to a capacitor in a circuit. Suppose it is printed on the capacitor: 8000 uF. The ESR meter will detect at the connections for instance 2000uF and 100 Ohms ESR. So definitely this capacitor is bad, though it will have some function still. Or, when it says: 6000uF and 5 Ohms, it is still good. Of course this is not a 100% sure test, but to my experience you are close to 100% in all practical circuits. When ESR is too high, the capacitance itself is too low, that comes together in case of detoriation. When ESR is too high, but capacitance still good, the capacitor has a contact problem inside. For all easy to remove leads, I did solder them off still, and then you can test any capacitor precisely. However, as I said, most of the time you can find the bad capacitors in the circuit, because normally ESR is too high then. ESR should be a few ohms only. There are tables for that, but this approach is relatively new, and I rather compare it with good brand capacitors of the same kind. So you can not compare an bulky 5000uF cap from Sprague, with a Chinese one, and you wonder how they can make them so small and so cheap. . The answer is found in ESR changing already after a few years of use.
A very useful application is test capacitors you would not necessarily test, because they are somewhere on a PCB which works good, and you don't want to damage something. Even new capacitors don't get any better from soldering them in, but un solder 50 years old capacitors, and solder them back in, is not what you should do without need. Then they have been soldered three times instead of only one. WIth an ESR meter, you can quickly test every capacitor and not desolder it. If you never worked with such a meter, spend 20$ on it, and it will change the way you work on broken equipment for ever.
Still, because one appeared bad, I decided to I un solder all capacitors of the power supplies. There are two PCBs, mounted over them, and you need to remove those, by unplugging the connectors. Also take your chance to correct the connectors a little bit, as some of them got a bit loose ever since 1968 when it was made. My ESR meter indicated all capacitors "like new" except for C759 of the 5V supply. It had so high ESR, it was regarded broken. Other 576 users report C759 broken as well, so it is a common fault error. I opened it up so see what's inside, and it was very wet still. So sure not dried up. Still ESR was "infinite" on the tester. So this is really interesting to see, so many of those old electrolytic capacitors were still perfect ever since 1968. Here comes a point, all good electronic designers know: Lifetime of electrolytic capacitors increases drastically when you have a low AC ripple current through them. So with this power supply, perhaps also 1/3 of the capacitance value might have done the job as well, but then ratio of ripple current and capacitance would have been "3 squared" higher, so 9x higher, and lifetime would have been even more drastically reduced than a factor 9. There are tables for that, in any capacitor meant for power supplies, and you can bet those guys at Tektronix knew how to test capacitors with their own curve tracers. So on the one hand, the capacitors seem extremely over dimensioned, but on the other hand, what is sure, almost every one is still good now ever since.
To remove the capacitors, first you need to pull of the plastic cap, by hand. To get the capacitor out, you can do this from the visible side, but you can't get the screws back in for the replacement. To do this, you can carefully take out the small High Voltage box. No need to remove the oscilloscope tube. Hint: There is one nasty screw that you can't get out easily. Use a normal, flat screw driver, and then it works. You can use that under an angle. Like this I could remove the screw, slide out the box, and reach the capacitors from the other end, with my hand inside. I was able to fit new capacitors very easily inside the cap of the old one. Just saw off the cap. It was almost no work, and result was perfect. You can hardly see the repair afterwards! After I did this, the 5V supply was good again, the digital step generator worked, and the 12V circuit worked better also. Don't know exactly how, but the different voltages relate to each other. Like the 12V circuit produces the reference voltage for the 5V circuit.
All in all this whole power supply is a strange thing, and I would not have made it like this. I would have created the +12V with a 5 Volt 2Watt Zener diode as a reference. Then, take 100mA from the Zener diode to feed the 6 TTL ICs with. As simple as that :) But ok, 1968 is a long time ago. Things were different by then, designers liked to present fancy circuits. Those guys were the kings in any electronics company. I am sure, older analog designers know what times I talk about.
This is the power unit, with it's cover off. On the left are the series resistors, with a thermal protection mounted on it. Note, the inside chassis is nicely sprayed in Tektronix blue. This is waist of money and resources if you ask me. But as I wrote before, 1968 was another time.
REPAIRS: Design Error in the Step Generator
Yes you read it correct, a design error. In a Tektronix product. That's definitely what it is. The step generator had the nasty habit to skip a step sometimes, and it began to get worse, until in the end only every seconds step was displayed. I found a very nice article in the internet about this, in German language, which made it a lot easier for me.
It begins already here, at IC U3D. This IC generates a needle impulse every time the mains goes through zero. Now U3A, U3B, U3C, U3D is just an quad NAND gate, SN7400. They must have found it pretty interesting at Tektronix to alter the symbols for those, perhaps showing how they used it. However this altering is full of bad errors. That doesn't matter much for the real function of course. Just I want to point out here, what they printed in the circuit for U3D is a NOR gate, but what is used there is an SN7400 NAND Gate. And that is not the same. This is a totally wrong symbol in the drawing for U3D. Also for U3C, now it is suddenly is an AND gate with inverted inputs. So at a "0" on both inputs, you would get a "1" at the output. That is of course what a NAND gate does also, but NOT for all of the other conditions. So frankly, these symbols in the drawing are dead-wrong. But it's only the symbols. The function is good, when you want to call it like that. Look at what they connect to the output of U3C. A capacitor with a discharge e-function on it. This means the input of pin 12, U3D is an analog signal, and that is not allowed with IC's like SN7400. If you ignore that, the output becomes undefined as long as you are in the "forbidden" zone of the input voltage. Undefined means, the logic state is unknown, or the output may oscillate. Whatever the IC does, you can't tell. And that's what it does indeed. However due to this extremely curious circuit, they cut off the "undefined" phase, and what comes out is indeed the required a needle impulse needed for the step counter. However the needle impulse is analog. So the edges are no steep enough for TTL. And now the problems begin.
So look at the inputs of U22. There are two analog needle impulses there, with some very small phase shift, and the result is a small (now digital...) needle on the output of U22A pin 3. The analog signals on pin 1 and 2 however may sometimes cause a second impulse on Pin3, and that causes a failure with the step counter. After replacing U22 (SN7400) with an 74LS132 the problem was gone. This is a quad NAND with Schmitt Trigger inputs. I am not so extremely happy with this, because I do not know the reason why they made this illegal use of the SN7400. Was it ignorance or did they do something extremely special? We won't know. The machine is from 1968 and the designers probably don't work there any more. In case somebody has more information, please let me know.
When I have more time, I want to see how R8 needs to be adjusted in order to get the best needle impulse at U22A pin 3, and check what is the change when going from SN7400 to 74LS132m, as logic levels of 74LS132 are not the same. Because it's LS, and because it's a Schmitte Trigger.
WARNING: Later I learned how to burn those resistors. There are two in there, one for each position (left-right) of the fixture switch. I received my 576 with one resistor burned, I soon burned the other one myself. This is how it happened: On the text fixture is CBE for transistors, and two additional holes for whatever it is. When attaching a device to those two holes, you can nicely do AC measurements on it. However when you try to test a high current device there, those 22 Ohms resistors go up in smoke. High current testing I do via the Collector-Emitter terminals, and then it works fine.
Of course you need to use the original calibration procedure, and ideally you own the calibration fixture. Here is just a short cut I took, and it seemed to me it was within calibration anyway, apart from very minor adjustments in the 1% range. Also if you find, calibration is totally bad, you need to look for other things first before you mess up the rest. Like one of the internal voltages is wrong, or something with CRT tube. So never try to "calibrate away" a problem, without fixing it at the root. I know people LOVE this method, but I hate it, as it never made me happy. So any problem, find the root cause first, and fix it before calibration.
Here is how I went, but again this made only sense as the machine was pretty good condition anyway. If it is full of problems, better fix first these issues one by one, and do not waste time on calibrating an instrument with problems inside.
The 576 was never intended for tube testing. As you can see clearly, at Tektronix they must have thought the days of tubes were over at that time. Well we all thought that, though it was wrong. It is a pity, because for testing triodes all it lacks is a heater voltage. This can all be simply added to the fixture with two banana plugs and an external power supply. Probably the superior versatility of the 576 will allow some testing options, the 570 can not do. I expect to find that out later, and will update this page when I know more.
On the left here you see the Quadrants. These are numbered in electronics counterclockwise, for whatever the reason. Here you see zener diode tested. So the forward voltage is in the first quadrant, with positive voltage and positive current, whereas the Zener part is in the third quadrant, with negative voltage and negative current. These kind of curves are displayed in the "AC mode", so for testing two lead devices.
Three lead devices have a control input, which the 576 can supply either with a voltage or with a current. So you see, this is already something the 570 can not do, but a control current may be interesting when checking positive grid voltage driving a tube. Just to name something. Also the 576 can produce a sweep with a negative voltage. This is interesting perhaps sweeping the grid voltage, and stepping the Anode voltage. Or work with a higher external anode voltage and show al least one curve. So this would show a 2nd. Quadrant curve like this. Though I really have no idea right now if this will be possible, and I want to look at such options all on the end of this project, and not at the beginning.
Things to take care of, and what we need add to the set up when testing a tube with the 576.
THAT WAS ALL, and my TEK576 is in perfect shape now.
Conclusion: The TEK576 is a highly recommended, extremely useful machine.
Well it took me three weeks, every day 1 hour, but it was fun doing. What I can tell you, having a curve tracer really makes you understand two-lead devices better. As far as transistors are concerned, it amazed me that Germanium transistors have nice curves too. I always thought they were "primitive" or so, with bad curves and lots of distortion on it. However, curves of OC71, AD150, AC126, they were all so nice looking, and hey.... a LOT better than any pentodes I have seen.
The Tektronix 576 is so nice build, it is unbelievable. I would not know what to improve, apart from the weight which is really crazy.
Interesting idea: but I am not going to follow up on this: If you buy the programmable fixture on Ebay, you can access all connectors and wiring to control the TEK576 by computer, using some USB relay cards. May be you don't know, but most settings are relay controlled anyway. So when you choose 20V per division, it's just some relays you operate, also when you use the rotate knob. The knob itself doesn't do the switching, but it switches the relays. WIth the programmable fixture you can access all those relays yourself.
Prices of the Tek 576. From what I found, it was it least build for 21 years from 1969 to 1990, and last units were sold in 2000. Perhaps somebody has more information for me. In 1969 it was more expensive than a Chrysler Rambler 6-Cylinder, two door Sedan Coupe. In 1990 it was sold for the price of a Mercedes Benz. It seems as if they tried to stop sales of it, by price increases, but that didn't work.
Some specific parts of the circuitry works relatively difficult. It seems to me, the designers had great fun, trying to invent smartest possible circuits. Which is good and bad. What is good about it, such circuits WORK, as they are made by people that know the classical circuits, and then try to outdo them. What is not so good, when there is a defective part, you will spend a lot of time finding out how the "clever" circuit works, which is very difficult when the circuit is malfunctioning. So you have funny signals on a funny circuit, and that can give you quite an head ache. Well, the good thing as I said before, the used parts are generally very good, and nothing much seems broken anyway. The machine is well build from very high quality material, well designed, and made with good consideration somebody needs to be able to service it, and take it apart.
Mechanical parts contains plastic parts that detoriate, or becomes cracked. However real problems from cracking plastic part are small, and it seems replacement parts can be found still. Even the plastic cardanic connections for the pot meters, that can only break off, I found some NOS replacements on Ebay within minutes, and they were even cheap.
Weight is crazy and excessive. They really took no effort to save on weight. So you will see two pot meters in the back, connected with 30cm long metal rods to the front, using cardanic coupling pieces, and the pot meters mounted on thick aluminum panels. Protection caps on the internal high voltage units are from (too) thick metal, etc. Everywhere you look, it's twice the weight needed. So in the end the whole item gets twice as heavy. You can hardly ship it. So beware, you can NOT put this in a card box. This needs a wooden palette to ship. If the post man refuses to hurt his back, and simply drops such a box, I can't blame him.
Lamps are expensive. The "type 349" lamps that illuminate the scope are expensive, they cost 2...8$ one on Ebay.
The red hazard lamp on the right burns the voltages exceed the maximum of 15 Volts. So far so good, but if this lamps breaks, there is a safety precaution switching down the high voltage. They used the safety switch of that, so the one that is underneath the Test fixture. There is a relay which only is "on" when the switch of the safety cap is closed. However when the hazard lamp itself is broken, the tester behaves as if the safety cap is still open, which by itself makes the yellow warning light burn. You can try that out, just set up a test, and while it's running, pull out the hazard lamp. The same moment the tester shuts downs. So yes, really safe, and at 1500 Volts, that is a good idea. On the other hand, I wonder how many 576 were send in for repair, because of this. This behaves like some defect with the high voltage, it refuses to switch on. So you think your 576 is defective, but it's only the hazard light bulb that is broken.
The fuses are at the back. There is also a thermal fuse inside on the high voltage series resistors, which will reset after some time. You can choose from a low, medium, or high mains voltage setting. So save the instrument, best is to take the lowest one.
The high voltage for the horizontal sweep, up to >1500 Volts peak, is generated with a variac, and actually they use the sine wave of the mains voltage to do the horizontal sweep. The nice part is, you can not damage this circuit so easily, it has a thermal break out, which you can re set. By using the variac you can conveniently ramp up from 0 to 1500 Volt, or whatever maximum you take. Also there is current limiting on this, by an selectable series resistor bank. This is strong, safe, effective and will take some amount of abuse and errors.
This is it's primary function, and it can do so many things, it surprises me every time how nice and accurate it does so. I feel a little bit like somebody using an oscilloscope for the first time. So you make a sweep with from 0V to a maximum positive voltage for NPN transistors (or whatever other device), or switch it for a negative voltage sweep. In accordance with the four mathematical Quadrants , the negative sweep is done from the right to the left, and the (negative if course....) current from top to bottom, but in case that is too confusing for the user, the picture can be rotated 180° optically. Or, you can do an AC sweep, displaying Quadrants 1+3 at the same time. (A quadrant is this here)