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Old 6th Oct 2019, 10:25 am   #21
Herald1360
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Default Re: Westminster S470M power supply problem?

Two cathodes swapped was confusing. Looks like the NFB is just round the output stage. The resistor (+dc resistance of TX secondary // speaker) will be providing the bias voltage.


Not seen that arrangement before- maybe the designer liked Quad IIs?
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Old 6th Oct 2019, 12:19 pm   #22
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Default Re: Westminster S470M power supply problem?

If the voltage drops then either the rectifier is poor or a capacitor is leaky.
Measure the voltage across the rectifier source resistance, the cathode voltage, the voltage across the primary of the transformer and the voltage between the two smoothers.
Calculate the current distribution.
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Old 15th Oct 2019, 11:48 pm   #23
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Default Re: Westminster S470M power supply problem?

Thanks for recent suggestions. Spent an hour or so on it tonight and very frustrating, virtually impossible to obtain any accurate readings as the numbers are constantly changing. Basically, the two HT rails coming off the smoothing caps (which are both brand new) start at around 340V then gradually drop to about 160V over about a minute. The 220R dropper gets very hot (this is brand new, old one was o/c, understandably) The current through the output transformer primary is at an average of about 40mA, rising further as the voltage drops. The current to pin 7 of the UCL82 is around 10mA. Both the 25uF cap and the 0.047 caps are likewise new items. I have tested the output transformer; it seems to be OK and there are no signs of burning etc. All the resistors are fairly near their marked values. The voltages on pins 2 and 3 of the UCL82 are anything from 12 to 20V... it goes on... I think I may just take a punt and order new valves as I seem to have eliminated virtually every other component... unless anyone has any further input?.. (btw I am enjoying the challenge despite all...!)
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Old 16th Oct 2019, 3:09 pm   #24
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Default Re: Westminster S470M power supply problem?

The only valve that could pass enough current to pull down the supply voltage is the UCL82. If the 47nF capacitor to pin 3 has been replaced with a new, modern one, then the valve could be faulty. I would have thought a valve fault unlikely, but once you have eliminated the impossible .....

Try another UCL82; but before you go ordering one, try a new grid capacitor, just in case the new one you've put in is faulty. It's also not altogether unheard-of for valve holders to go faulty and become conductive, especially if they have been overheated in the past.
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Old 17th Oct 2019, 9:47 am   #25
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Default Re: Westminster S470M power supply problem?

Just one other check before we can conclude it is most likely the UCL82, with the UCL82 removed, measure the resistance from the pentode grid (pin 3) to chassis, it should be about 480K.

What you are seeing is typical thermal runaway, the valve runs at too high current, it overheats, extra heat makes it conduct more.... This is often caused by impurities that have condensed on the inside of the valve causing leakage to the grid. Try monitoring the voltage from chassis to the grid (pin 3), it should be <0.5V but I suspect it will slowly rise as the valve gets warm.
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Old 17th Oct 2019, 12:15 pm   #26
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Default Re: Westminster S470M power supply problem?

Thanks again for the advice. Since my last posting I have also suspected that it is the output valve rather than the rectifier (I thought I might temporarily substitute a diode for the UY85 just to verify this, any thoughts?) Anyway, I will try the resistance check on pin 3 of the UCL82 as suggested. Also, re the 0.047 capacitor, I will try another. On the same subject, the original cap was a green "Eire" item. Not knowing its type, I replaced it with a polyester; reading up on cap replacements the general rule seems to be that these are the ones to go for if in doubt, would others agree with that? Thanks again.
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Old 17th Oct 2019, 12:46 pm   #27
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Default Re: Westminster S470M power supply problem?

Original cap "Erie" perhaps?


Anyway, yes, polyester is fine, 400V rating plenty.
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Old 17th Oct 2019, 8:30 pm   #28
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Default Re: Westminster S470M power supply problem?

Quote:
Originally Posted by julie_m View Post
It's also not altogether unheard-of for valve holders to go faulty and become conductive, especially if they have been overheated in the past.
That's an interesting idea, presumably due to carbon tracking between the pins? Had that once on a Leslie speaker connection to a Hammond organ many years ago, will certainly check that out too...
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Old 18th Oct 2019, 12:31 am   #29
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Default Re: Westminster S470M power supply problem?

Measure the voltage ACROSS the smothing resistor. If the current is low then the rectifier could be at fault Do not put a diode in parallel with the rectifier. Other components are under stress and you may kill them.

Use an analog voltmeter iff you do not like digits randomly changing. You do need high accuracy.
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Old 18th Oct 2019, 10:44 am   #30
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Default Re: Westminster S470M power supply problem?

I should have said "you do NOT need high accuracy"
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Old 29th Oct 2019, 11:14 pm   #31
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Default Re: Westminster S470M power supply problem?

Thanks for all recent advice. Had another session tonight and I've had a bit of a breakthrough. The general feeling was that if it were a faulty valve it would be the UCL82. As a belt and braces approach I ordered a UY85 as well. First tried substituting the UCL82 - no change. Then the UY85 - success (fingers crossed). The amp seems to run smoothly and the dropper resistor no longer gets hot. The voltages on the circuit are stable enough to be measured. I've done the following readings and I'd appreciate comments. I'm still running it through a bulb just to err on the side of caution, and the voltage on the mains in is about 200V. The 2 voltages are firstly without any signal applied and secondly with a signal (by which I mean touching the input and making the speaker buzz....) - here goes: across smoothing caps resistor: 2.4V (1.9V). Pin 3 of UCL82: 11mV / -7V. Pin 9 of UCL82: 72/79V. Across 470K resistor: 5mV / 6.6V. Should I be cautiously optimistic or is it getting ready to destroy the new rectifier as well?
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Old 30th Oct 2019, 10:25 am   #32
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Default Re: Westminster S470M power supply problem?

Sounds like there may have been a heater-cathode leak on the UY85. That could cause a modicum of mayhem as the valve heated up!

The unstable voltages could have been dc with a load of unwanted ac superimposed.
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Old 3rd Nov 2019, 11:04 pm   #33
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Default Re: Westminster S470M power supply problem?

Unfortunately the assumption of success was premature, the fault is still there. So far the smoothing capacitors, decoupling capacitors, the mains dropper resistor and both valves have been changed (interestingly enough a new UCL82 made the voltage drop at a much faster rate than the old one). All the resistors are reasonably within their stated values (those fitted are all 20% tolerance items so I'm assuming values aren't that critical) This has got me completely baffled. After all it's a very simple circuit. If the HT to the amp section is disconnected and the radio tuner section is operated on its own it functions perfectly through a separate amp. I'm beginning to think it can only be a fault on the output transformer. It shows no signs of overheating or arcing but it seems the only thing left to replace.... any further input greatly appreciated.... many thanks again for all the advice so far.
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Old 4th Nov 2019, 10:49 am   #34
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Default Re: Westminster S470M power supply problem?

Is the surge limiter the correct value?

Lawrence.
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Old 4th Nov 2019, 3:35 pm   #35
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Default Re: Westminster S470M power supply problem?

The original surge limiter, which was open circuit, assumed overheating failure (the reason for the set being completely dead), was 200 ohms. I replaced it with a 220 ohm 10-watt; I think the general opinion was that the value wasn't that critical and 220 is within 10% tolerance...
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Old 4th Nov 2019, 3:56 pm   #36
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Default Re: Westminster S470M power supply problem?

The first thing to try would be to replace the transformer primary with a resistor. Measure the anode current and the voltage across the primary, then use Ohm's Law and the power law to select a suitable resistor.

If the voltages behave themselves this time, then the transformer is suspect.
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Old 4th Nov 2019, 4:30 pm   #37
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Default Re: Westminster S470M power supply problem?

Quote:
Originally Posted by lesparapluies View Post
I'm still running it through a bulb just to err on the side of caution..
The lamp should start bright (heater resistance is low when cold), go dim (heaters hot), gradually brighten (output valve conducting). You should expect the HT voltage to reduce as the output valve warms up.
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Old 6th Nov 2019, 9:04 am   #38
lesparapluies
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Default Re: Westminster S470M power supply problem?

Thanks for recent postings, will follow them all up. Just had what may be a significant breakthrough. I originally removed the complete chassis from the cabinet and have had it connected to an old surplus speaker while on the bench, leaving the set's speaker in the cabinet. Today I went back to the radiogram cabinet to check the actual speaker for impedance, for the purpose of finding a replacement transformer should it be needed.....it appears to be open circuit! This would possibly suggest that either the speaker failed and the open circuit on the secondary caused the transformer to overheat, or the transformer failed and high voltages on the secondary caused the speaker to fail..classic case of overlooking the obvious perhaps...
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Old 6th Nov 2019, 12:21 pm   #39
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Default Re: Westminster S470M power supply problem?

Update - took the speaker out of the cabinet and found dry joint where the flexible wire from the cone joins the terminal panel...could be significant or could have happened when I unsoldered the amp cable to remove the chassis... more to follow
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