Upgrades to VCM3

AVO Ltd London made VALVE TESTERS.

Upgrades to VCM3

Postby danyavo3 » Sat Oct 20, 2018 2:31 am

Hi to all ! This's about upgrades to my VCM3 w/ a bit of history.
I got this off eBay from a close seller to Toronto where i'm located. The seller said that the tester was operative & c/w copied info & a CD ROM.I paid US$900, because he reduced it by $100 if I picked it up - very nice!
It proved to be OK, but I proceeded to calibrate. 1st I tested the mtr which required 33.5 uAmp for full scale(30 is the correct value). No matter what I did w/ the internal resistor didn't help. However, I moved the 0 slightly - the mtr was extremely linear between 10-100. In analog mtrs we hardly use the lower end anyway. Calibration went OK. I obtained a Yokogawa 30 uAmp mtr w/ the same arc for US$15 off eBay & the seller sent to me free postage from the US. I was going to install its guts into the AVO's case - in the future. You can also get a Simpson mtr. But I've a CT-160 w/ a dud mtr - I'll use the Yokogawa's internals for it.
There's a big problem w/ adjusting the -Vg pot exactly, even w/ using a DMM. It affects a lot the Ia & the Gm.
- Upgrade #1. Adding a 0.5 M pot in parallel to the voltage pot will make for very precise adj - in conjunction w/ the DMM.
But it so happened that a prowler broke in our house & jolted the AVO = the mtr died! Our insurance paid me Cad$4K for repairing. I sent it to Jac where he installed a new mtr - Taut w/ LEDs instead of the rly bulb function - works excellent. Of course, he also cal it.
A month ago the -Vg pot cooked up - must have been due tp a grid short. Jac could not help me w/ another one, but sent links to 3 pro pots()1oK). He claims that the original pot is very precise, but I maintain that no matter what, the pot will never be 100%. The same w/ the geared VCM4. I ordered 2 pots from China & I've yet to see how linear a & repetitive they're.
- Upgrade #2. In the mean time I installed a 10K 10 turns Bourns pot(very cheap from china) - it's a real pleasure to adj - don't forget the DMM.
Also the fil affects a lot the Ia & the Gm. Jac suggested a 15V x'former in reverse polarity w/ a variac to exactly adj the fil. But I don't want to complicate matters.
\- Pgrade #3. The ~ is set for the middle on the center knob(per cal, while our AC is 125.5V. By changing between -3 & +1 = left & right, I can obtain close to the proper fil for many tubes. There remains one problem - Av & Sv which are affected by shifting the ~ knob. However, AVO says in the cal info that -3% - +5% of these voltages(unloaded) are of no concern. A sample of 6BL7 6J5 6SN7 50L6 - are within the +/- voltages %. Only 12A6 & 6080 are a bit off. Don't forget to monitor w/ a DMM.
All the above DMM values can be accessed on the sockets panel of the AVO.
- Upgrade #4. I purchased off eBay from China few small multi digits panel mtrs: -Vg, Vf, Ia. for this last one install a 2 ohm resistor in series w/ AVOs analog mtr(the change means nothing to its 3250 ohm resistance). Then use a 200mV DM to pick up on the 2 ohm to relate to the different ranges of the Ia.
There's still one more problem: The -Vg changes every time the dryer, stove, AC, furnace, ECT, come on/off. I may be able to alleviate the problem by stabilizing the -Vg w/ Zeners, or by using a Sola i'm working on - stay tuned!
BTW, I'll be incorporating the same upgrades of -Vg & Ia on my Triplett 3444(actually to show 0-100mA Ia instead of only 0-50mA. Also Ia on my different Hickoks for Ia.
Dany, Toronto
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Joined: Mon Jul 25, 2016 2:43 am

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