Role of 1uF Caps on HT secondary taps

[Bazz4CQJ]

I've been doing some capacitor changing on the oscillator and amplifier boards of my VCM163, and a little bit of "mission creep" is threatening to take hold.

Attached is the circuit diagram of the multi-tapped HT winding which supplies the anode voltage in the 163. Capacitors C1-4 are wired across the taps at ~100V intervals, have values of 1uF and working voltages of 400VDC.

Can someone enlighten me as what those caps do? I don't recall seeing this arrangement previously and Mr Google has not yet offered an answer.

Although both the circuit diagram and the parts list specify those caps at 400VDC, the ones actually in place are rated at 250VDC, being Hunts Type M312 (quite small and flat with a green plastic outer wrapper).

Thanks in advance.

B

[G8HQP Dave]

Switch arc suppression?

[Radio Wrangler]

Rather big capacitance for suppression.

Looks almost like someone's resonating the transformer to fix manky mains waveforms.

Power factor correction

David

[Bazz4CQJ]

Arc suppression seems very plausible Dave, but if they are there for that alone would you expect them to be as big as 1uF?

The 163 depends on having a nice sinewave (or half a sinewave to be precise) going in to the valves. Could those caps have anything to do with that, helping avoid flat tops for example?

B

Wrangler and my last post overlap in time.

[Bazz4CQJ]

So, if there is a role in power factor correction, three thoughts occur to me;

1. failure of any of those caps by shorting could be seriously bad news for the transformer
2. Deterioration of those caps in any other way could adversely affect the performance of the tester.
3. The manual specs them at 400VDC, but they are actually 250, and made by Hunts, whose reputation was not the best.

Case for changing them for something of good quality?

B

[m0cemdave]

Originally Posted by Bazz4CQJ

Although both the circuit diagram and the parts list specify those caps at 400VDC, the ones actually in place are rated at 250VDC, being Hunts Type M312 (quite small and flat with a green plastic outer wrapper)

The 250V M312 (polyester) capacitors were rated at 100V AC, so just about OK between 100V taps.

[Bazz4CQJ]

Mmm...I note the word "just" and that was as rated 50 years ago. All the covers are already off the 163 and the amp PCB is out so there's access, and its winter so I have the time and 400VDC Y Class caps are not expensive, but a replacement transformer would cost....

Maybe I should start a Poll

B

[Ed_Dinning]

Hi Bazz, Y class caps at 1uF will be very expensive. They are difficult to find above 4n7.
X calss caps should do as rated for at least 250 or 440 (depending on type) working.
Mode of failure is to gradually reduce in capacitance over time. They are easily available to 2u2.
They could be being used to improve PF by resonating with the primary magnetising current. There is an article in the RSGB handbook power supply section that explains how values are calculated.

Ed

[saddlestone-man]

This looks similar to the constant voltage transformer arrangement used in my Advance CM1 Q meter. If you google 'constant voltage transformer' you'll see how they work. The theory seems to suggest that the resonant winding is separate from the ones used to produce HT, etc but maybe you can put a load on it.

When Advance stopped making test gear they concentrated on the cvt business and presumably made a good living out of it.

Best regards. Stef.

[boxdoctor]

For what little it costs, Bazz, I would replace them with 400 or 630Volt polyester types, (not class X) and then you can relax, knowing that the mains transformer is not going to be frazzled by one of them going S/C.
It's certainly an unusual circuit arrangement, - almost worthy of Philips ! Tony.

[Bazz4CQJ]

Thank you for those inputs. I’ll have a look at my old RSGB Handbook and see what that says. In terms of stabilising the HT, of course, the 163 does not have any stabilising circuit like a gas-filled tube, so again, anything those caps can do to keep the HT flowing consistently is a plus.

The reference to Y caps was a senior moment; I’ve actually now ordered some polypropylene X2 caps rated at 310VAC.

While I’ve left my days as a “mass-recapper” behind, this is the first time I’ve ever had both the oscillator pcb and the amplifier pcb out of the unit (a number of the small electrolytics on those boards had gone virtually open circuit). While I’ve got access to C1-4, I think I should replace them now and so the 163 should be good for another 50+ years… and could well be someone else’s problem by then .

B

[Electronpusher0]

Originally Posted by saddlestone-man

This looks similar to the constant voltage transformer arrangement used in my Advance CM1 Q meter. If you google 'constant voltage transformer' you'll see how they work. The theory seems to suggest that the resonant winding is separate from the ones used to produce HT, etc but maybe you can put a load on it.

Constant Voltage Transformers have a separate resonant winding and a magnetic shunt, usually just some "I" laminations shoved between the bobbin and the outer laminations. See here
http://www.aelgroup.co.uk/faq/faq001.php

Peter

[Ed_Dinning]

Hi Bazz, yes X2's should be fine, with the advantage that a failure of the cap is self isolating and only reduces it's value.
Polyester can fail S/c, admittedly unlikely but easily avoided

Ed

[Maarten]

Unlike Y-class capacitors, X-class capacitors (polyester, polypropylene or paper) may also fail short circuit, though especially the cheaper ones are indeed prone to opening up before they would ever get a chance of shorting out. In places like this, it's probably best to use a somewhat overdimensioned polypropylene type, X2 or otherwise. 310VAC caps will indeed be fine.

[John_BS]

I suspect the role of those 1uF caps might be to reduce the HF source impedance of the transformer: AVO may have found the odd valve which would take off due to an inductive anode load.

Alternatively, they act as a LPF, untilising the leakage L of the transformer. This would remove any large HF mains spikes which might upset things.



John

[Bazz4CQJ]

Thanks for the suggestions John. Those caps may well have fulfilled a number of roles, but no doubt they eliminated at least one problem they were seeing in development.

B