AVO VCMs a comparison of their designs - huge document split into chapters.

[Dekatron]

Hi all!

I have now finally come as far with this document that I am satisfied that I can publish it. It would have been published a long time ago if I had had the time and energy to focus on it wholeheartedly but private faily matters has pulled me down mentally.

I would like to thank everyone who has contributed by sending me copies of manuals and other material and also all whom I have had discussions with over the years which has led me onwards in my struggles to understand the AVO valve testers. I won't mention anyone special, so no one is left out, but more so since my memory of names is absolutely terrible!

In this document I have tried to explain how each circuit works in the AVO VCMs and why they have been constructed as they have and I have also described why all components need to have the values (and as a side effect also tolerances) that they have.

The document is in no way 100% complete as there are a few areas left on especially the VCM163 amplifier and oscillator but I'll leave that for now, I have tried to make it as complete as possible without delving into those two circuits. I have however included a few pages from the manual of the AVO Transistor Tester TT537 which uses an almost identical oscillator and amplifier for the measurements. There you can read how and why they were designed like they are. I might write a document on those circuits and some repairs that I've done in my VCM163 in the future if I ever get the time to do so.

I hope that you find it good reading!

If you have anything to contribute, discuss or ask please send me a PM or an email! I'll try to answer as fast as I can but unfortunately it might take a while sometimes.

[Dekatron]

The last chapters!

[Dekatron]

If you find anything that is wrong please don't hesitate to comment but please also write me a PBM or email!

[Dekatron]

I have accidentally named the last file incorrectly, the one containing chapter 6 with all schematics, it contains all of the last pages up to page 77 but has been named 61-72.

[davidgem1406]

Hello Martin,

An excellent piece of work and quite some undertaking as well. A most useful addition to the library for anybody who is interested in or has an AVO VCM.

Our most grateful thanks.

Dave.

[Dekatron]

Thanks Dave!

I really hope that it will show that these instruments are not that complicated and that it will also show the importance of the correct values of all components. Plus it will also give a better understanding of how each circuit works.

I have had a lot of questions from people who have had problems with their AVO VCMs and in many cases the errors were due to wrong component values, mostly due to ageing of the components but in some cases bad or poor repairs. There have also been a few where the AVO had not been repaired/restored in the correct way and that has led to incorrect measurements. The hardest ones to find are of course the dry joints or partially working components, resistors that change value when heated or when current flows through them, and in some cases also with solder joints that had not been soldered at all!!

So you will need to check every component in your AVO VCM including the meter if you want to be sure that you get the correct measurements.

OT:

I've just finished building my RoeTest and I am going to compare the measurements from a bunch of valves with the measurements on my AVOs to see how well they compare. I'll also use the measurement capability of the RoeTest to make reference/calibration valves to test on my AVOs.

[einovaan]

Originally Posted by Dekatron

I have accidentally named the last file incorrectly, the one containing chapter 6 with all schematics, it contains all of the last pages up to page 77 but has been named 61-72.

Fine job Martin!

I don't find pages 73 ... 77 from file named to 61-72. They are missing.

[Dekatron]

Dear Eino,

Thanks for pointing that out to me, you are correct.

I have included a new document here, which I will check to see that it is complete, which contains pages 73-77.

Sorry for this mistake, I must have had a blackout while checking it earlier!

(Moderator's note, file now added to post #2 at Martin's request).

[Radio Wrangler]

Wow!

Going straight into my archive. I don't want to lose this sort of information, and who knows when I might need it?

David