Fault Finding; VCM163 Osc Board.

[Bazz4CQJ]

The E113A is called up in the Service Manual for Avo's VCM163 to set the correct output level from the AF oscillator to 15mV (at ~14kHz). I don't have a 113 and have always had to go a scope to set that level.

I cannot find a user manual on the web for the 113; can anyone confirm what the inputs specs are for the 30mV AC range?

That voltage level is actually measured across a resistor of just 10R, so I guess it does not have to be set with a high impedance voltmeter, and as it it is a good sinewave signal, I guess that it also doesn't matter whether the meter is measuring average or true RMS, but please feel free to tell me if these conclusions are wrong?

With the my (old) scope, pk-pk V needs converting to RMS with pen and pencil. Of course, the measured Gm values on the 163 are directly proportional to the 15mV setting, so getting it right is good!

My oscillator board is currently not working at any level, so I will have extract it and see what's wrong with it .

Thanks

B

[Radio Wrangler]

The EA113 is a rectifier meter, so it really responds to peak voltage, and the scale is calibrated for RMS volts assuming the waveform is sinusoidal. It uses an AC amplifier and an active rectifier.

I checked mine against a Datron calibrator and found it -3dB a bit above 200kHz.

David

[Bazz4CQJ]

Thanks for that; I will proceed on the basis that a service engineer working on a 163 simple expects to read 15mV on the meter, and that is RMS.

I have the board out now, having checked that it was being fed with 24VAC and was converting that in to 12VDC for itself and the amplifier board. The only visual notable on removal were a few white deposits (looked like it was possibly flux) but only seen in one spot between a connection on one of the trimmer pots but spreading on to an adjacent pcb track. Came off easily with a Q tip soaked in 50/50 IPA water.

As life is short , and replacing 5 electrolytics is cheap (and quick and easy on such a sparsely populated board), I think I'll do that and see where that takes me.

B

[pmmunro]

Bazz,

Please send me a private message if you would like a copy of the user and service manuals.

PMM

[Bazz4CQJ]

Well, there are 3 off 10uF (Wima) caps on that board (and now removed) and one is measuring 700pF, ESR 7k and the other two are measuring between 2 and 3uF, with ESR's of over 10.

The three larger electrolytics (47, 100 and 220uF) tested on the board, look OKish.

B

[Station X]

Thread title changed at OP's request.

[Bazz4CQJ]

I recently found that this oscillator board was showing zero output at the two test points, but I replaced the electrolytics and immediately got the required value of 15mV RMS (42mV pk-pk on my scope) on the RED/BROWN test points.

Before fitting the board back in the VCM, I thought I would just check the output level from the transformer. This is running at about 1V...but, the level is not being controlled by RV2, which has no visible effect on the level shown on the scope.

I pulled out all the fixed resistors associated with the output transformer and checked that the two thermistors seem to be ok (both up around 10k resistance, one very sensitive to temperature change, one less so). The resistance of the transformer windings were around 90R on the primary and 27R on the secondary.

My recollection is that RV2 should have a big effect on the output level (it actually sets the Gm CAL level for the 163). Can anyone suggest where the problem may lie? Circuit attached.

Thanks

B

[Radio Wrangler]

If you're measuring the board alone, RV2 shouldn't affect the output into a high-Z meter much at all.

It's a variable series resistance and it will vary the output voltage, but only when it's working into a load. A high Z load on the secondary of T1 means that T1 primary looks to RV2 as almost an open circuit at the operating frequency.

You've just proven that T1 is well designed.

David

[Bazz4CQJ]

Originally Posted by Radio Wrangler

If you're measuring the board alone, RV2 shouldn't affect the output into a high-Z meter much at all.

It's a variable series resistance and it will vary the output voltage, but only when it's working into a load. A high Z load on the secondary of T1 means that T1 primary looks to RV2 as almost an open circuit at the operating frequency.

You've just proven that T1 is well designed. David

Thanks David. That did actually cross my mind...and then I rejected it as the explanation .

For anyone who is interested, I measured ~3.5V pk-pk of drive at the R12 end of C6. Anyway, as Martin (Dekatron) has advised elsewhere on the forum, I've replaced the resistors with new metal-film ones, but the ones taken out all tested very well, so I don't think I'll replace them all en-mass.

I've also got the amplifier board out to replace the electrolytics on that. The boys at Avo had used some kind of thread-locking substance on the 4BA nuts which hold the board support on to the chassis. I managed to soften that up nicely using my beloved Wickes paint and varnish remover, and that meant that I was able to get the board out without taking the bottom off the chassis, which saved some time .

Now that the osc and amp pcbs are out of the box, access to the various components mounted around the transformer is good, so it is tempting to change those. They include two high-voltage electrolytics and a bunch of 1uF Hunts 400V caps. Hard to know where to draw the line!

B

[Dekatron]

I am usually short on time when repairing valve testers like the VCM163 so I just go for replacing all of the old components. I've never been asked to do a repair which should keep the historical value, all I've repaired have been repaired to work with new components.

I know from experience that a lot of the old resistors change their values over time but also from heat and moisture when in use (also from talking to one of the designers over at Welvyn who told me that the resistors used in the VCM163 were never intended to live this long and that they are susceptible to moisture) and that most of the capacitors also don't fare so well when they age, a lot of the non-electrolytic capacitors have had cracks in their plastic molding which have led to moisture damaging them.

I usually replace all capacitors except for the two 1990pF and the 1.99uF capacitor on the amplifier/oscillator boards (resistors, transistors, trimmers and diodes are also replaced). I also replace the trim potentiometers on the "potentiometer board" next to the oscillator/amplifier boards as I've come across ones that were broken (they usually have a broken track). The large 1uF capacitors on the transformer are also replaced as some I have seen have worked better as a resistor than a capacitor.

/Martin