Hewlett Packard 130C Oscilloscope

I use no digital oscilloscope, because I prefer the live picture of an analog oscilloscope. Also, I do not like menus and submenus with tiny little buttons, and I need a lens to read what they are for. My requirements are for low noise and ease of use. That is all, but there I want the very best there is. And no, that can NEVER be achieved with digital scopes. Perhaps I can get the low noise of HP130C with a 3k$ new scope (I doubt it...), but I will never get close to the ease of operation. When I only think of the dual banana inputs of the 130C. Which scope has that today? Yes, banana plugs. And no, that is not silly or old fashioned at all. The HP130C was MADE for this intentionally. When you connect a BANANA to BNC connector to the HP130C, like I did at first, and plug a normal probe on it, of course it works then like any scope. At it's most sensitive range of 200uV/div , of course it shows also a hum signal on the screen, which is not even bad, because at 5mV/Div it is gone, or almost gone, same as with any new made scope. No difference here.

But then, I found out, the real way, to use those two banana inputs. The negative input has to be connected to the ground of the device under test. So just with an alligator clip and a banana cable. This (negative) input has a complete vertical amplifier behind it, and it's only function is to substract the ground loop (error signal) from the measurement. Now, suddenly the hum signal is GONE. Also at 200uV/div. So I can measure fully hum free, at 200uV/div using not a probe, but a banana cable with a probe tip,same as a multi meter. This works so incredible well, I can only bang my head against the wall, because I worked so many years without this.

So the 130C is only 250kHz? Well, yes and no. Because the horizontal amplifier has not just this "pull to magnify" button. It can go as far a 1uSec/cm, which can be magnified in steps from 1...50x. Then we have 20nSec/Div.

The weak spot is however 250kHz bandwidth on the vertical amplifier. However that is fine for my tube audio applications, which is the intended use. .

The Hewlett Packard 130C Oscilloscope, some people (like me) call this the last good, and also the best Audio oscilloscope Hewlett Packard ever build.

There is also the 132A, A dual channel scope with dual beam tube, but the one you see here, the 130C is already much more bulky in real life, as on this picture, and the 132 is 1.5x the height. That is too much for me. I am afraid the 132A is too hard to service. Besides, I need the 130C only to do very sensitive measurements, and I never needed two channels for that at the same time.

The 130 is fitted with tubes for the signal amplifiers. The deflection and some other high voltage things, is done with transistors. And not just any tubes are used, but Telefunken E88CC, made in Germany, marked with a green dot on top of it. Lovers of old oscilloscopes call them "warm" oscilloscopes, and though you may be afraid of tubes and tube problems, my experience is another. When something is defective, it is never a tube. Also, there is nothing wrong with it, when moderate heat from inside prevents moisture from penetrating capacitors and resistors. So any defects? Sure it can happen, but overall, old tube instruments are more reliable than old transistor instruments, not to speak about older instruments with integrated circuits.

In case you would be interested in such a scope for low signal measurements, I can really recommend 130C very much. I bought mine as "defective" but it was not. At first it appeared more old fashioned as I thought. The knobs are much heavier as I expected, but for that, they don't crack as with old Tektronix scopes, and all switches and potentiometers need nothing but some exercise. Which I can not say about the Tektronix, you're always servicing switches and potentiometers with Tektronix, and a bad capacitor here and there. The HP front plate is engraved, so no wiped off texts. And after I had it working, it made me happy, every time I use it for very low signal measurements. I have similar experience with other old HP instruments, they're a bit bulky, but I never need to repair anything at all.

Ebay, sometimes you don't get fooled

I bought in on Ebay, from somebody who said he switched it on, and something gets visible, but for the rest he knew nothing about it. Normally this means he tried even the crudest repair, but no result. Well in this case, it was just working normally. You just need to know how to use it. You need to select one of the horizontal magnifications (normally 1x) or otherwise it goes into X-Y mode, and it's hard to understand what you see then, and it will not trigger also. So you get a total mess on the screen and you don't know what to do.

So yes, this time I had an honest seller, he indeed knew nothing about it. The scope worked just fine. The E88CC tubes (OEM Telefunken, W-Germany, with a green dot on it) were all worn out, but the circuits work at very low plate current, and worked fully normal, even worn out tubes.THAT is good circuit design. In an article in HP Journal, I read a text in HP Journal, by one of the main designers, Bob de Vries. (who died in 2012). He writes there, this low current was chosen to reduce drift of the DC coupled tube amplifier over time. A used but still good Telefunken E88CC is worth 100 Euro. So when you are lucky, the tubes are worth more than the whole scope. Well I replaced them all with burned in, NOS 6922, Philips ECG, and it worked same as before. The E88CC tube sockets are in a special construction, suspended in foam, and wired to the rest of the circuit with ultra flexible wire, to reduce chassis microphonics. The foam which keeps the tube sockets in place, was already a bit soft, so I replaced it with the same foam as we use in the Emissionlabs tube boxes. That is temperature proof foam, so people can put back a hot tube in the boxes, and the foam doesn't melt. After exercising the switches, the scope worked fine, and keeps on doing so since many years now. It did not even need a calibration, and all capacitors are original still. It's my #1 tool for low voltage signals. I have never found something which can replace this performance.

Why HP130C was made

You have to understand the purpose of this oscilloscope. It was made to analyse tube audio circuits. Generally, there is no such thing as high bandwidth and high sensitivity in the same product. So when we talk about audio test instrument, it is good enough when it is 500kHz bandwidth, but for that it has NO visible amplifier noise on the screen, even when going to 200uV/Div sensitivity in differential mode. Which is really amazing. I have a few Tektronix scopes, and some come close in terms of (specified) sensitivity, but the amplifier noise at such settings becomes too nasty, and the use of this is only theoretical. The HP130C much to the contrary can really do this 20x better, which is no surprise because it was MADE for this purpose.

Differential Banana Plug inputs

When I saw those, my first thought was: Oh my God.... such old fashioned banana plugs. But it is not like that! On the left you see two red and one black banana plug. The red ones are two identical amplifiers apart that one has an inverted input. The outputs are added to each other. In short, this is a differential input. On top of that, each of the two amplifiers is balanced from the beginning to the end, to increase DC stability. So for the vertical amplifier, we have a total of four amplifiers used with E88CC inputs, of which two are connected to the outside world, and the other two are grounded, to give DC drift compensation. Another four of those amplifiers is use for the horizontal pre amplifier, which is only needed for the X-Y mode. (Lissajous figures, and phase measurement) Since the whole X-Amplifier is identical to the Y-Amplifier, It is tempting to modify this into a two channel scope, and I whish they had done so at HP already.

So to measure versus ground you MAY connect one of the two red plugs to the scope ground, and measure with the remaining input. Initially I did it myself like that, with an Banana to BNC adapter plugged into that, so I could connect a normal coaxial probe. Which was kind of clumsy, and I was unhappy about it. That was so terribly WRONG to do! The good thing of these differential inputs is, you don't a need a coaxial probe! In case you measure vs ground, you just connect one input to the ground of the instrument you are working on, (not to the scope ground) and use the other for testing. Unlike with a coaxial probe, this creates no ground loop via the probe ground. This ground loop inevitably carries a small AC current, which flows capacitive via the two mains transformers, and it ends up as a hum signal which lays over the measured signal. With a differential input however, BOTH lines are high impedance, and there will be no ground loop current.

Like this, you can use a normal, single wire probe lead, like from a multi meter, and measure fully free of hum, and any "approach" effects of your hands getting near to the signal cable. In case you do not measure versus ground, just connect the differential cables where you like.

And yes, I am sure you can buy today new made differential FET probes for a few hundred Euro, doing the same thing. But try to set those to 200uV sensitivity, and then connect them to 500V by mistake. Good luck with that.

A differential measurement with a normal 5mV/Div normal dual channel scope is troublesome. First, you can't do better than 5m/Div, but also such scopes are noisier at 5m/Div as a scope which can do 200uV/Div. Also, when you change the gain, you need to balance by hand the two channels for every new measurement. This stays a tedious way to work.

Horizontal gain magnification

Many oscilloscopes have this 10x button to magnify the vertical sweep. The HP130 has a whole series of ranges for this, from 1x to 50x. I never saw this before. This is so comfortable, and it gives even 20nS/Div horizontal speed. Which is fast enough to look at any glitch or spike in detail. The horizontal position potentiometer which you need to use for this, must be very stabile, and the one used is incredible good quality. Unlike all those scratchy potentiometers in my old Tektronix equipment, they all work like new, without overhauling them.

Using the 130C in X-Y mode.

Myself I have no use for this, it is is a bit of an overkill today, but the 130C has a differential input horizontal amplifier too. The electrical circuits of the horizontal and vertical amplifiers are identical, allowing very precise Lissajous diagrams with differential inputs even to 200uV. So the 130C has four amplifier channels, each by itself constructed with differential circuits. Several such measurements can are done much better with differential inputs, like making the curve of a diode visible, using an external oscillator.

CRT tube

The picture quality is amazingly good. Ok, it is round, but the brightness and sharpness is a dream. The CRT tube was made by HP itself, in their own factory.

Hybrid construction

Tubes were only used where low noise, and input safety was needed.

The old style, HP drop test

Specified by the requirements of the "Environmental Manual", an HP wide used book, this oscilloscope had to survive a drop test in it's factory packaging. It was dropped from a height of 130cm on a steel floor. First, on each of the eight corners, and then flat on each of the six sides. After that it must come out of the box undamaged and within calibration. I worked at Hewlett Packard Boeblingen, and we used the same test still, from the same company internal instructions, for new designed medical equipment. That is why I know for sure, this requirement was not just theory.

Original Manual of HP130C