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Old 9th Jan 2020, 12:14 am   #41
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Default Re: Westminster S470M power supply problem?

Happy New Year everyone.... tonight I have "returned" to the radiogram after a few weeks away... still very little progress but noticed something which may be significant: When I power the amp through a 100-watt bulb in series, I get around 215V at the amp's mains in - the current drawn stabilises at around 50mA, the dropper resistor stays lukewarm and the set functions normally; I had it playing for about 20 mins with no problems. As soon as full mains power is applied (247V) the problems start again - current drawn 125mA and rising, HT voltage dropping until the audio distorts and fails completely, dropper resistor too hot to touch. In a previous post I commented that the resistors were all 20% tolerance items and they are all high in value, so I think I'm going to replace them all as a matter of course; I wonder if the combined effect of all of them being out of value could tip the balance and de-stabilise the circuit? Much as I hate the idea of wholesale replacing of random components, without an official schematic with listed voltages, I'm still stabbing in the dark. The grid voltages on the triode and pentode sections of the UCL82 are both between 1 to 4 volts negative with respect to cathode, which I think is a good sign... as usual any further info or advice appreciated....
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Old 9th Jan 2020, 8:34 am   #42
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Default Re: Westminster S470M power supply problem?

Hi!

Returning the output valve's cathode resistor to chassis/HT– via the LS/output transformer secondary is a fairly common and economical method of applying modest amounts of negative feedback to the stage used by a number of manufacturers!

The output from the UY85 rectifier cathode in the circuit given should be in the region of 210–220V when the set is working correctly – if you're only getting 150V either the UY85 is very low emission, you've used too high a surge–limiter resistor between mains and the UY85 anode, or you've got a leaky grid coupling cap or faulty UCL82 causing excess h.t. current to flow!

Short the G1 pin of the UCL82 pentode to chassis and measure the h.t. again – if it is appreciably higher than your initial reading of 150V, both the grid coupling cap.and the UCL82 are likely to need replacement!

If shorting G1 of the UCL82 pentode to chassis makes no difference to the reading, you'll need to check the UY85 by substitution or temporarily try a 1N4007 silicon diode across it between pins 3 and 9, cathode (marked) end to pin 3 – this will probably provide much higher h.t. than the original valve but it's safe to do it for a very quick test!

A low–emission rectifier valve should of course, be replaced.

Excessively leaky h.t. smoothing caps wouldn't drop the h.t. by such a large amount as you found without also getting very warm unless the rectifier was also very low emission, but fitting a silicon diode rectifier would almost certainly cause their final demise if they're that bad anyway!

Normal voltages on UCL82 pentode are typically:–

Anode:– 180–200V, Screen Grid G2:– 150–175V, Cathode 10–12V.

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Old 9th Jan 2020, 5:30 pm   #43
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Default Re: Westminster S470M power supply problem?

Hello Chris, thanks for your posting. Unfortunately a lot of what you suggest I've already done. I have tried substitutes of both the rectifier and the output valve, and the output transformer, these 3 seeming the most likely culprits. The dropper resistor, which was the reason for the set being dead in the first place, has been replaced with a 10-watt one of the same value as the bunt-out one (200 ohms). The smoothing and coupling caps are new items. As the RF section functions perfectly with the amp disconnected, I'm still inclined to think the problem is somewhere around the UCL82. The HT starts at around 300V, falls to about 220 while the amp warms up (so far so good..) - but then continues to fall....I've never had such a simple circuit that has caused me so much head-scratching! I've even considered building a replica amp circuit from new parts on a separate board and comparing readings between the two! Anyway... back to it tomorrow... thanks again
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Old 12th Jan 2020, 10:08 pm   #44
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Default Re: Westminster S470M power supply problem?

Another session tonight and a lot of progress. I replaced all of the resistors around the output valve (all of which were high and outside of their tolerance), and for good measure put the "new" UCL82 valve in also - (previously substituting this had made no difference), and now I seem to have stable HT rails and the set plays normally. However, there is still something puzzling me which I'd like advice on: The mains dropper resistor is 200 ohms. The current flowing through this when the set is warmed up is around 90mA, let's say 0.1A. Is that normal? The voltage drop across the dropper is about 20V (about 249 to about 228. And if I understand Ohm's Law correctly, that just about calculates correctly. But that also means the set is consuming about 25W and therefore the dropper resistor should be rated at this.. but most designs using a similar seem to have this resistor as a 5-watt or 10-watt component. Despite the seemingly normal operation, the dropper is still very hot (much too hot to touch). Am I still missing something?
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Old 12th Jan 2020, 10:26 pm   #45
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Default Re: Westminster S470M power supply problem?

The current through the dropper resistor is 0.1A -- this is what the U series valve heaters are rated for. 0.1A flowing through 200Ω gives a voltage drop of 0.1 * 200 = 20V (remember V = I * R). 20V at 0.1A gives a power dissipation of 2W (P = V * I).

The 25W is a total figure which will be distributed among the valves, the dropper resistor and any other components in the heater chain, in proportion to the resistance of each element; since the voltage across an element whose resistance is R = I * R, and so the power dissipated is I * I * R (or I ** 2 * R), but I is the same for each element.
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Old 12th Jan 2020, 10:37 pm   #46
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Default Re: Westminster S470M power supply problem?

According to the OP's schematic of the arrangement the 200 ohm resistor is the surge limiter for the rectifier, if that's what's being talked about the current and voltage pertaining to it should be measured with a True RMS meter.

If no True RMS meter, a rough rule of thumb for surge limiter dissipation is to use the DC load current for the calcs with the value of the surge limiter then whatever that comes to multiply it by four for a dissipation figure then double that for the resistors power rating.

Having said that, if I'm off beam with all this then my apologies.

Lawrence

Last edited by ms660; 12th Jan 2020 at 10:44 pm. Reason: alteration
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Old 13th Jan 2020, 1:23 am   #47
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Default Re: Westminster S470M power supply problem?

Just adding to Lawrence post, When you first turn the set on, the first AC cycle will start charging the reservoir capacitor through the rectifier, and each further cycle will add further charge. Meanwhile the set will be drawing a near constant current from the reservoir capacitor causing the voltage to decay during the part of the AC cycle when the rectifier is not forward biased but it will still retain most of the voltage. Eventually an equilibrium is reached when only a small part of the peak of the positive cycle of the input waveform is forward biasing the rectifier resulting in a current waveform that is far from sinusoidal. A non-RMS meter will measure the average current but the power dissipated in a resistor is I^2 * R so the current seen on the meter cannot be used to calculate the dissipated power using ohms law.
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Old 13th Jan 2020, 10:36 am   #48
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Default Re: Westminster S470M power supply problem?

Thanks for the recent postings. Yes, the resistor I was referring to is the surge limiter, the 200R, not the 800R heater dropper, sorry for the wrong terminology. Maybe I'm making too much of this but, as the original reason for failure was an open-circuit surge resistor, the fact that the new one, a 10-watt, runs very hot is giving me cause for concern, despite seemingly normal readings in the output stage. Having said that, the heater dropper resistor is also very hot and there seems to be no fault in this part of the circuit, so perhaps things are back to normal. I don't have a True-RMS meter, but I have two digital multimeters, so I can monitor current and voltage at the same time. I think having got this far I may run the set for a while tonight, keep an eye on it and take some readings around the UCL82 when it's fully warmed-up. Since posting last night I have read Paul Stenning's excellent article about audio output stages on another part of this site so lots of new stuff to take on board
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Old 13th Jan 2020, 6:38 pm   #49
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Default Re: Westminster S470M power supply problem?

Quote:
Originally Posted by julie_m View Post
The current through the dropper resistor is 0.1A -- this is what the U series valve heaters are rated for. 0.1A flowing through 200Ω gives a voltage drop of 0.1 * 200 = 20V (remember V = I * R). 20V at 0.1A gives a power dissipation of 2W (P = V * I).
Thanks for that info - I spotted my error after writing the post... so... even allowing for the fact that the non-RMS meter wouldn't give an accurate current rating, surely a 10-watt resistor running at 2 watts wouldn't get too hot to touch? This has been at the heart of the problem all along and I'm still not convinced I've nailed it..!
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Old 13th Jan 2020, 6:47 pm   #50
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Default Re: Westminster S470M power supply problem?

Quote:
Originally Posted by lesparapluies View Post
I spotted my error after writing the post... so... even allowing for the fact that the non-RMS meter wouldn't give an accurate current rating, surely a 10-watt resistor running at 2 watts wouldn't get too hot to touch? This has been at the heart of the problem all along and I'm still not convinced I've nailed it..!
How do you know it's dissipating 2 watts if you don't know what the RMS current is flowing through the surge limiter?

I would check the DC load current first and see what that is.

Lawrence.
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Old 13th Jan 2020, 7:26 pm   #51
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Default Re: Westminster S470M power supply problem?

Quote:
Originally Posted by ms660 View Post
Quote:
Originally Posted by lesparapluies View Post
I spotted my error after writing the post... so... even allowing for the fact that the non-RMS meter wouldn't give an accurate current rating, surely a 10-watt resistor running at 2 watts wouldn't get too hot to touch? This has been at the heart of the problem all along and I'm still not convinced I've nailed it..!
How do you know it's dissipating 2 watts if you don't know what the RMS current is flowing through the surge limiter?

I would check the DC load current first and see what that is.

Lawrence.
Missed the EDIT time, my apologies....read above as.....How do you know it should be dissipating 2 watts.....

Lawrence.
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Old 13th Jan 2020, 11:43 pm   #52
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Default Re: Westminster S470M power supply problem?

I see exactly what you mean, and I understand what RMS is; I just kind of assumed that, even if a normal meter didn't give an accurate current measurement at this point of the circuit, it wouldn't be out by a factor of 5 (2 watts against 10 watts)... but I bow to superior knowledge! I have run the complete set tonight for about an hour (both radio and amplifier) and all seems fine, voltage readings on the UCL82 are fairly typical, just the hot resistor which concerns me but... heigh ho...I'm repeating myself!
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Old 14th Jan 2020, 11:34 am   #53
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Default Re: Westminster S470M power supply problem?

It probably isn't a factor of 5 out, but "hot" in human terms and "hot" for a power wire wound resistor are rather different, maybe 50C vs 200C.
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Old 14th Jan 2020, 12:54 pm   #54
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Default Re: Westminster S470M power supply problem?

Re: Surge limiter power dissipation....

2nd page, 2nd para. here:

http://www.r-type.org/pdfs/uy41.pdf

13. Protective Resistors For Mains Rectifying Tubes here:

http://www.nj7p.info/Manuals/PDFs/Tu...art-4-1972.pdf

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Old 14th Jan 2020, 5:53 pm   #55
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Default Re: Westminster S470M power supply problem?

Thanks for both these replies - interesting reading. After posting my last one I realised that the amount of heat dissipated by the voltage drop on this resistor is probably exactly the same no matter what the resistor's power rating, it's just that the 10-watt component is better designed to withstand it than the 5-watt one, would that be a correct assumption? The links make interesting reading and once again fill in little gaps of knowledge. Now... before I put this set to bed.. just one more slight anomaly.... there have been acres of print on this site alone devoted to the subject of the earthing of live chassis sets and the general opinion is that it's not a good idea... but this model has a 3-core mains lead (definitely original); the earth goes to the amp chassis via a ceramic disc cap and the turntable chassis is connected directly to earth, any final thoughts on this one?
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Old 24th Jan 2020, 10:06 am   #56
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Default Re: Westminster S470M power supply problem?

Just about to put everything back into the cabinet and draw a line under it. Just one small point - there are 4 large ceramic disc capacitors on the chassis - one across the mains, one from earth to chassis, and one on each lead to the cartridge - would it be advisable to change these as a matter of course, maybe for Y2 types, or let sleeping caps lie? It's just I've heard that the one across the mains has a habit of going up in smoke and it would be a shame if that happened in a few weeks time after spending so much time on it so far.. thanks again for all the previous advice
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Old 24th Jan 2020, 11:30 am   #57
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Default Re: Westminster S470M power supply problem?

My inclination would be to replace those caps with modern Y2 types for the sake of safety and reliability.
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Old 24th Jan 2020, 3:47 pm   #58
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Capacitors sitting between line and neutral need to be X rated. Y rating is for capacitors sitting between line or neutral and earth or exposed metalwork.
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