VCM 163 setting up procedure.

[HBWOODY]

Yes, taken on the AC range.

[Dekatron]

Did you observe that they mention RV1 on the oscillator board and also RV1 on the potentiometer board? They are not the same RV1, the one on the oscillator board is a trim potentiometer whereas the one on the potentiometer board is the one on the separate board at the side, where you can access it through the hole in the side panel.

You should anyway have some 81mV on the output at the transformer on the oscillator board if the ma/V switch and resistors are correct. If there is anything wrong with those components the output could be loaded too much so that the voltage drops too low. What open circuit voltage do you get on the transformer if you de-solder one of the leads?

Look at page 47 & 48 in my circuit comparison document where you see a description of the circuit and you'll also see the expected voltages there, also for the calibration setting - there the voltage should be close to 15mV in the Cal. setting.

[HBWOODY]

I am aware of the two RV1 potentiometers.
I will check out the components on the ma/V switch and study pages 47/48 of the circuit comparison document.
When I turned on the VCM163 this morning the needle move very close to the CAL point.

[Dekatron]

Could it be that you have a sticky meter, either due to poor needle bearings or due to static electricity?

If you have another 50uA movement that you can adjust to get 1500 Ohm internal resistance, for instance a Taut Band movement from Simpson, you could test with that to see if it behaves better. Since you are looking to get to the 75% point on the scale it might be easiest to use another meter for comparison.

[Bazz4CQJ]

The Gm meter on my 163 is "temperamental" and always has been. Never had any trouble at all with Ia meter, but it's sibling makes up for it .

B

[HBWOODY]

I have have tried it with another meter and the results were the same.
Yesterday when I turned on the VCM163 and turned the mA/V to the CAL position the needle did rise to the CAL position on the scale and has done so again today. Could some component be failing?

[Dekatron]

When you say "I have have tried it with another meter and the results were the same" - do you mean that the other meter behaves as erratically as the original VCM163 meter, i.e. not reaching the CAL. position on some occasions? If so it must be a problem somewhere in the VCM163, unless the other meter isn't working properly too.

Since you already had one broken diode on the amplifier it could be more poor components there. The quickest thing would be to replace all semiconductors and then also check the electrolytic capacitors and replacing those that need replacing.

[Bazz4CQJ]

While it does not help HBW's immediate problem, this is the kind of thing that is driving me (very, very slowly) to produce one new board with a newly designed Wien oscillator, IC amplifier and DC PSU on it. It will be the size of one of the existing boards so that it will fit directly in place and it will use identical transformers to the original boards in order to interface with the rest of the VCM without problem (kind thanks to Ed Dinning for the transformers).

B

[HBWOODY]

The meters are fine they both work as they should and produce identical results. I suspect the problem is a failing component on the amplifier board.

[Dekatron]

It could also be low output signal from the oscillator board.

[HBWOODY]

Could the wires S3A/5 and S3A from the mA switch to the meter M2 have any negative effects on the meter readings?

[Dekatron]

They are used to short the gm-meter in the "Transit position" of the Ia-switch if it is set to the black dot (the IA-meter is shorted via S3C-5 & S3D-5). So if anything is wrong with those contacts and that circuit, which I think is highly unlikely, they might form a path with some resistance partially diverting current from the meter.

[HBWOODY]

Checked the amplifier board and found that the meter needle fluctuated when I was checking around VT2. VT2 tested as good (quick diode test) but decided to replace it with the satisfying result that calibrating to the CAL position is now as one would expect it to be.
So now both my VCM163,s fully operational, calibrated and giving results that would be expected.
Once again thank you all for your input especially Martin who has played a great part in getting me to this point.

[Bazz4CQJ]

Well done, both you and Martin. It would be interesting if you could find a little time to explore the problem with VT2 a little more. Is there any chance it was just the soldering on the PCB or is it definitely the component itself?

Are we in the position where, just as we accept replacing the electrolytics is necessary, so too, the transistors?

B

[Dekatron]

Thank you HBWOODY!

I am glad that I could help you and that you got it working finally!

In answer to Bazz4CQJ's question on replacing semiconductors - I regularly do it for those who want a fully working tester and who aren't interested in having it historically correct. I have seen bad semiconductors in all positions in the VCM163 as well as in older AVO VCM's. I have spoken to people who were designing with these semiconductors when they were manufactured and even then they were poor they told me, especially the diodes manufactured by Lucas who are used in the VCM163 in several positions (black outer casing filled with red glue). The transistors are prone to develop tin whiskers both outside of the casing but also inside due to poor molding I was told. A gentleman who worked at Welwyn told me that all resistors except those green ceramic ones with a shiny surface went bad earlier than expected and that they were moisture sensitive and that their resistance changed faster than expected, this included the black, red & silvery types usually found in AVO VCM's. When it comes to capacitors the electrolytic ones are worth replacing but the other types usually don't need replacing.

[Bazz4CQJ]

Thanks for those comments, Martin, which I think you have consistently promoted for sometime.

As you know, I have the osc board out of my 163 at the moment, having installed the new transformer made by Ed. I will now replace all the transistors on both that board and the amp.

A question about the amp, as recently pointed out by Wrangler, the 1.99uF and 1990pF polystyrene caps will be hard to find replacement for if so desired. For a negative tempco, I wonder if polypropylene will do and will two 1uF caps in parallel replace the 1.99uF?

My recollection is that LP filter at the front of the amp has such a low cutoff frequency that I guess the value of the 1.99uF is not critical, though I do note the unusual value and tight tolerance of the cap they used, so maybe my guess is wrong?

B

[Ed_Dinning]

Hi Folks, the transistors used had a wide gain range and may have been selected for a specific gain band in use.
I believe one of the contributors analysed this and thought it would make little difference.
Device Ft should have little effect at the frequency of interest; but leakage may be of importance (Developing tin whiskers?)

The circuit forms a type of bridge and an analysis of the numbers shows why 1.99uF was used. It would be interesting to know how vital this is. BUT the oscillator and bridge should have closely matching frequencies for optimum performance. The Vincor transformer has a core that could be adjusted to get both osc and amp on the same frequency

Ed

[Bazz4CQJ]

In my post#36, I said the filter on the amp is LP, when I should have said HP.

My understanding is that one of its primary functions is to take 50Hz out. Now if was a passband filter, clearly component values would be critical, but for a HP filter I would have thought less so

For my "feasibility study" of a "one-piece osc/amp/psu" replacement, I looked at these polypropylenes as possible replacements for the 1.99uF polystyrene https://www.ebay.co.uk/itm/323731614...r=512722824294.

Any comments welcome.

B

[Dekatron]

You could ask Solen (www.solen.ca) if they can provide a 1.99uF Polypropylene capacitor, they have offered other special values in the past to a friend of mine.

[Bazz4CQJ]

Thanks for that info Martin, but is it necessary? I guess they would have a minimum quantity level, so that would be an issue.

Is there a real problem with two off-the-shelf polyprop 1uF caps?

B