New L3-3 not working properly

Brand name believed to be KALIBR.

New L3-3 not working properly

Postby bartola » Sat Apr 08, 2017 6:20 pm

Hi all,
I'm new to the forum, so apologies if I'm posting in the wrong place.
I purchased a claimed "NOS" L3-3 from "any-devices" eBay seller. It seems like I'm not the first one struggling with it. Anyway, the tester is complete with all the cards and connectors.

The tester arrived safely yesterday. I follower the instructions to warm it up (I live in the UK), I connected the ground pin to the mains ground on the plug and left it for 40min running. I can adjust 250V, Us and grid (Ug) but can't measure Ua. At some point I had an intermittent measure when pressing the measure button. I suspect it may be the button and the contacts?
Also I can't calibrate the GM either. When calibrating it, the range is between 30 and 60. I can't take it up to 120.
Any suggestions on what should I start looking into?
I haven't opened the tester to inspect inside yet.

Image
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Thank you all for your help

Some pictures attached.
Ale
bartola
 
Posts: 5
Joined: Sat Apr 08, 2017 6:05 pm

Re: New L3-3 not working properly

Postby bartola » Mon Apr 10, 2017 8:04 pm

I opened the tester and cleaned up the 8 contacts from the measurement (left) button. I didn't want to reverse the button as suggested by Jacmusic as I want to isolate first where the issue is.
As a basic setup I connected pins as follows:
20/I (US range at 300V)
26/I (UA range at 300V)
1/I (grid at -65V range)

However when I measure with the voltmeter the anode voltage on 12/II (shunt pin of the Ua stabiliser circuit) I can measure voltage range from couple of volts to 500V. I'd expect the regulation circuit to limit the output voltage.
Still, the meter doesn't work on "Ua" position.
I suspect is either the Ua stabilise circuit (VL1, VL2 and VL4) or the rotary switch. The latter scares me a bit to dismantle it given the amount of connections!

I will test all the tester valves independently first and measure voltages around the stabiliser circuit as first step.
Any suggestion on how to trouble shoot this are welcomed!
Cheers
Ale
bartola
 
Posts: 5
Joined: Sat Apr 08, 2017 6:05 pm

Re: New L3-3 not working properly

Postby bartola » Fri Apr 14, 2017 8:29 pm

I changed the pots for Ua, Ug2 and Ug1 (x2). The latter 2 pots being 10T. I can get Ug1 to work fine.
The reference 250V is working well. I measured all the valves and found a couple of 6J3P tetrodes to measure low. I changed them for spare ones which measured about 65%.
The anode voltage spans from 120V to 500V. Definitely there is an issue with the rotary switch as I can measure Ua and Ug2 from the pins, but Ua isn't displaying at all.

I'm not very keen to dismantle the rotary switch to see where the Ua isn't connecting properly, too many cables!!

I will measure the Ua regulation circuit next....

Any advice?
thanks
Ale
bartola
 
Posts: 5
Joined: Sat Apr 08, 2017 6:05 pm

Re: New L3-3 not working properly

Postby bartola » Sat Apr 15, 2017 12:26 pm

I checked all the voltages across the regulator valves as per the manual instructions. Everything ok within the tolerance levels.
I can get to read Ug2 and the Ug2 is present on the 4mm connector for valves with top screen pins. However, the Ua can't be read or isn't present on the anode 4mm connector.

I suspect I will have to remove the 5 wafer rotary switch and dismantle it for cleaning. Anyone has done this before and can suggest best way for doing this task?
Thanks
Ale
bartola
 
Posts: 5
Joined: Sat Apr 08, 2017 6:05 pm

Re: New L3-3 not working properly

Postby bartola » Sun Apr 16, 2017 2:20 pm

I made the following changes:

1. Re-tested all valves and changed the ones measuring low. I ordered new ones as have some 6N3P unmatched sections (V18). This could the reason why I can't calibrate the uA voltmeter given the mismatch between triodes.

2. The Ug1 pots changed to 10T wirewound
3. The Ua and Ug2 pots changed to new 1Meg ones.

I tested all valves per manual and the voltages are ok. I re-calibrated the transconductance meter and struggled to get the 450mV reference. The voltage drops quickly from 500mV down to 20-50mV when adjusting R157. The frequency was set to 1400Hz and voltage left to 500mV. It seems to work fine.
The filament supplies are ok. I can set grid voltages accurately, however there is a load impact when switching from "Ug1" to "Ia". For example, I was testing a CX301a and set the voltage to -9V in "Ug1" that changed (measured at the grid with external voltmeter) to -8.8V.


So what is not working:

1. I can't measure the Ua on the meter. It sounds like a bad contact on the rotary switch. Any tips for dismantling it and cleaning it? It seems like a complex stuff. Im scared with the amount of connections and whether I could bring all the wafers together again!

2. The anode shunt resistors are out of tolerance (>0.2%). I found 2 measuring out of 0.2% - R50 (4R889) and R46 (97R78). I contacted the seller to see if he can send me them. When measuring a CX301a traced with the Utracer V3, transconductance was very close, however the Ia measurement was way off. This led to investigate and found the resistor drift issue.

3. The screen current resistors are within tolerance (<0.2%). However, when following the screen current calibration/test from the manual (e.g. I used a variable CCS as an external load with a series precision current meter) the meter goes into full scale regardless on the range. Something is wrong here


4. The meter seems to be working fine. Although is not the originally specified meter it has 745R impedance but the R100 measures differently from what it should be. However, the meter seems accurate when measuring Mains, 250V and Us. Also the transconductance values are ok when tested some sample valves. This is leading me to suspect that R100 was chosen to match the actual impedance of the meter in place.

Frustrated, seems to be a long process to hopefully restore this beauty of tester.

Hope any folks out there can give me some advice
cheers
Ale
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bartola
 
Posts: 5
Joined: Sat Apr 08, 2017 6:05 pm


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