VCM 163 setting up procedure.

[Radio Wrangler]

Yes, TH2 will be a very low thermal mass thermistor designed for low dissipation giving large self-heating.

Its time constant will be rather short in order for the oscillator amplitude bounce to allow reasonably quick stabilisation. It will be an earlier version of the STC R53 type, made specifically for this purpose.

They are unobtainable except from scrap wien bridge oscillator instruments. Some designers used FETs as variable resistance elements, controlled by signal level rectifiers and loop amps. But that again is a big board change.

The first thermistor stabilises the oscillator amplitude, but the equilibrium level is a function of ambient temperature. So, the other thermistor is provided to partially compensate for this.

Things can be done to find substitutes for the second thermistor, If you don't mind doing temperature runs and fiddling with network values to get it reasonably good. But the first one is a major problem. I think it would just be easier to start again.

David

[Dekatron]

Originally Posted by Radio Wrangler

Is the thermal mass/timeconstant suitable?

The STC ones were tiny beads in an evacuated glass bulb. The thermistor isn't a temperature compensator, it's an amplitude to gain feedback element.

David

Thank you for this comment, that led me into looking more into how the original thermistors were designed by looking at a few modern ones from the few manufacturers out there that still make this type with an evacuated glass envelope. I then spotted that these types have very different time constant and dissipation factors compared to modern glass bead ones, mainly due to the casing since they use the same internal structures from modern manufacturers, self heating and external heating are of course different too. Time constants and dissipation factors are 50-100 times lower in the evacuated types compared to the glass bead types. I wasn't aware of this fact earlier, but after reading some more documents I see the reason for this, especially in the way that they are used (which you also mentioned in your post: https://www.vintage-radio.net/forum/...4&postcount=61).

[Bazz4CQJ]

One of the factors which influences my thinking on the 163 boards is that mine has a damaged Gm meter, I guess shorted turns, so I have had to install an op-amp driver (and the psu to power it) to recover the use of the meter. All of this makes my 163 somewhat "non-original".

This is a major factor why I have been dabbling with ideas to replace the two existing boards with just one board of the same size, with modern circuits for;

• an oscillator,
• an AF amplifier,
• a "meter driver" to allow for non-original or damaged meters,
• and a psu for the whole lot.

Where we may want to draw a line is in retaining the transformer that connects the oscillator to the test valve, and the transformer which connects the valve to the AF amp. Ed has very kindly supplied me with both those transformers and I am now moving (at a snail's pace) towards investigating the amp transformer and it's filter components to try and assess how critical their values are.

All that said, I do understand why some owners are committed to retaining originality.

B

[Dekatron]

Originally Posted by Bazz4CQJ

If taking the option of staying with the existing boards, can you not gain any advantage Martin, if you relax the original range of acceptable ambient temperature (as required by the military) to a narrower range?

I doubt if my 163 often get's used outside of the range of 19 to 23'C (which is the range of temperature where the operator is most reliable!). Would doing that make the task of finding a usable modern thermistor easier?

I suspect that even without any temperature compensation, over a narrow range of temperatures, the change in measured Gm would be very small indeed!

B

I already did so in my simulations with LTSpice, limiting the temperature range to 10-40°C range and focusing on getting the resistance right at the R20 & R25 points and as close a B-value as possible. Since the tolerances are +/-20% for the original thermistors the circuit will be possible to adjust with modern types.

The other thing which David (Radio Wrangler) pointed out (which I just commented on after having read more on thermistors) is the time constants and dissipation factors, mainly from self-heating as well as from external heating.

I'll have to read more on this but I guess that if we limit the temperature range the error won't be that big. However it would be interesting to try to see how large effect the external temperature has on the output transformer.

[Dekatron]

Originally Posted by Bazz4CQJ

All that said, I do understand why some owners are committed to retaining originality.

B

I am mainly thinking about doing the least amount of work when it comes to actually repairing a broken 163 by replacing components with new ones if possible. But there comes a time when that isn't possible and the right thing is to replace the boards with something new.

[Bazz4CQJ]

Martin, is that range of 10-40'C what you expect to see inside the case of the 163, so it's switched on a cold Monday morning and left on all day?

If the effect of the transformer temp is not something that be be calculated, I do have my osc board free at the moment and I have small laboratory oven that could be pressed in to service.

B

[Dekatron]

It's just two temperatures that show me a more or less linear portion of the NTC thermistor when simulating in LTSpice and they will probably fit everyone's needs for normal working conditions for their 163's

I'll ask Ed if he can give me the figures for the transformer resistance in the 10-40°C range.

[Bazz4CQJ]

The 400V CBB Metallized Polypropylene 1uF caps (candidates to replace the 1.99uF polystyrene on the amp board) arrived from China today. They are nominally 5% components, but in the pack of 10, some are very close to 1uF (within 1%), as measured on my Wayne Kerr bridge and they are neg temp co. (£3.30 for 10 inc P&P off ebay).

B