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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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20th Jun 2019, 3:21 pm | #21 |
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Re: Pye Black Box PCL83 version
Service sheet says 13.5V. Has the parallel resistor 220R changed value?
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20th Jun 2019, 3:27 pm | #22 |
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Re: Pye Black Box PCL83 version
The 220 ohm resistor will very likely have changed value, but probably not enough to make the sort of difference seen here - worth checking though.
I've found that many of the resistors in these amplifiers will have changed value, sometimes a little outside their usual limits, but generally this doesn't make much, if any difference to the performance of the amplifier. I'm wondering if there's a valve pentode section down or a leg of the o/p tranny o/c. |
20th Jun 2019, 3:30 pm | #23 |
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Re: Pye Black Box PCL83 version
Check anode and screen voltages then. Would it be safe to pull each OP valve in turn and see what happens to the cathode voltage?
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20th Jun 2019, 3:33 pm | #24 |
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Re: Pye Black Box PCL83 version
I think it would, Graham, but please shout anyone if you think for any reason it wouldn't be safe to do so.
I think I would go with checking the voltages as said. This should soon confirm any issues. |
20th Jun 2019, 3:56 pm | #25 |
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Re: Pye Black Box PCL83 version
Right, I've made a bit of progress the hum got worse because the filter circuit was disconnected due to a dry solder join, I now have 9 1/2 volts across the pentode, cathodes.
Screen and anode voltages are practically identical at 225v. I have got 100 hz at roughly 110mv at the speakers and I'm getting a crackling sound. I haven't pulled a pcl83 out yet, do you think I should? |
20th Jun 2019, 4:02 pm | #26 | |
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Re: Pye Black Box PCL83 version
Quote:
You said you replaced one of these valves at the beginning, but didn't enlarge on why you did this and exactly what was wrong with the original valve. Why not fit a pair of new valves and then see what you get? |
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20th Jun 2019, 4:05 pm | #27 |
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Re: Pye Black Box PCL83 version
Yes, that's the voltage of both valves. I replaced the valve due to instability the amp was whistling and howling.
I think I'm going to have to get a new pair, I was coming to that conclusion myself. |
20th Jun 2019, 4:11 pm | #28 |
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Re: Pye Black Box PCL83 version
To sum up what you've got so far - you've still got low cathode voltage, but from what you've said, the HT is up to normal spec. You've still got an abnormal level of hum - regardless of volume and tone settings..yes?
That instability seems to be a problem that various people have experienced with these amplifiers in the past. The PCL83 amplifier isn't particularly known for output transformer o/c primary problems, but seeing as it looked from the melted pitch as if it had been run with faulty coupling capacitors in the past, then a blown tranny winding was a possibility. Luckily it all checked out as good in that respect. |
20th Jun 2019, 4:35 pm | #29 |
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Re: Pye Black Box PCL83 version
As a last summing up...
The cathode voltage of the output valve pair is a little low. The HT is correct and the voltages on the anodes and screens from what you've said seem to be correct. The cathode resistor value is somewhere not far off from what it should be - yes? Is the biasing correct? Then one or both of the output valves has an emission problem. Edit to add - it could be one of the valves causing the hum. |
20th Jun 2019, 5:35 pm | #30 |
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Re: Pye Black Box PCL83 version
Hello,
Yes, the cathode voltage is a little low, I did a quick check of the cathode resistor when I replaced the cap and I measured closer to 100 ohms than 220, I didn't take it out of circuit though and will do tomorrow. The resistor was caked in gunk and I wasn't sure what exact value it was. Do you mean grid or cathode bias? If I had had some more pcl83's I would have replaced them as a test already but I only had one. I'll get a couple from eBay. Thanks for taking the time to help me. |
20th Jun 2019, 6:03 pm | #31 |
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Re: Pye Black Box PCL83 version
That resistor has dropped in value by an awful long way (they usually increase in value). Change that first and you may find that's the answer.
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20th Jun 2019, 6:09 pm | #32 |
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Re: Pye Black Box PCL83 version
The 100R reading could have been the result of an electrically leaky bypass capacitor. Now that that's been replaced it will be OK the measure the resistor in circuit.
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20th Jun 2019, 6:17 pm | #33 |
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Re: Pye Black Box PCL83 version
Good point, Graham. I was thinking he'd just checked it recently with the new capacitor fitted. If it turns out that the resistor isn't to blame, then that reinforces what folk say about the Plessey capacitor reliability. I sometimes replace them because others have said that they're unreliable, although as I said before, I've not personally found any evidence of this in record players, and those Plesseys are used in quite a few of them.
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20th Jun 2019, 6:27 pm | #34 |
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Re: Pye Black Box PCL83 version
Used to replace them when they were almost new. Not a reliable component.
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20th Jun 2019, 6:31 pm | #35 |
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Re: Pye Black Box PCL83 version
Yes I agree, but exactly what equipment and what application were they found to fail in?
Record players? |
20th Jun 2019, 6:59 pm | #36 |
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Re: Pye Black Box PCL83 version
They seem to fair less well with heat, here's a more extreme example in a cathode bias situation, I've also seen them with a gaping hole in them when close to the cathode resistor when the resistor has been severely stressed:
https://www.vintage-radio.net/forum/...7&d=1481987918 Lawrence. |
20th Jun 2019, 7:02 pm | #37 |
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Re: Pye Black Box PCL83 version
There are at least 8 visible capacitors that will require replacement. It takes just a small circuit imbalance to create problems with this PCL83 version of the Black Box.
I have repaired a number of these over the years and have always discovered the caps to cause endless problems. There is no reliable way of testing them and their replacement cost is low compared with the damage that can be caused to the output transformer, mains transformer and the PCL83 valves. The earlier version with EL42 valves appears to be much more durable. John. |
20th Jun 2019, 8:49 pm | #38 |
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Re: Pye Black Box PCL83 version
I hate to disagree with John as he's "the master", but actually I'm not completely disagreeing with him as he's correct that the old capacitors will show some leakage and certainly a couple of them WILL cause damage to the output valves and transformers etc. Also they are cheap and you could argue that for this reason they may as well all be replaced. I actually thought that unusually it was the earlier amplifier that caused more problems than the later one, although it was thought that the later amplifier wasn't quite such a good overall performer compared to the earlier one.
I agree with Lawrence that heat will not do these capacitors a lot of good and it is rather a bad idea to have a resistor that may run warm or worse, right next to them as in this Pye player. I would concede that this alone could be a good argument for replacing the Plessey, as you'd certainly end up making a mess of things by trying to separate them, probably damaging both parts in the process. I like to deal in hard facts, and below is a picture of the same amplifier as shown in one of my previous posts, but this time after repair by myself. The capacitors that can be seen as having been replaced are the ones that will adversely affect the performance and reliability of the player (in my opinion), and I would agree that perhaps it may have been a good idea to replace the Plessey, if only for the fact that it's against that resistor, but not for any other reason. However, as can be seen in my photo, there's a gap between the resistor and the capacitor. This particular repair was done by me some while ago and was to an amplifier in a Black Box that had never had attention since the day it left the factory and had been unused for decades, so no damage from being used with faulty components. It is possible in the case of the OPs record player being discussed here, that being previously run with a faulty output valve and leaking grid coupling capacitors, that it could have caused that Plessey capacitor to break down due to over voltage, and also excess current possibly damaging that associated resistor - we'll find out when the OP reports back with his findings. I can confirm that the Black Box amplifier shown is still working faultlessly and having had many hours of use playing records with just those four capacitors shown having been replaced. Even the actual value of those capacitors isn't critical, the original ones I think are 0.05uf, so 0.047 would be the replacements, but I've found that the circuit is quite happy with 0.1uf, depending what you have in stock at the time. So to sum up - I actually think that it probably IS a good idea to replace the cathode decoupler in this amplifier if it's thought, or known, to have been subjected to over-voltage due to the unit being run in the recent past with other faulty components in the form of coupling capacitors and valves. Also due to it being wired hard up against its resistor - not a good design by the Pye engineers. As I've said, I've yet to hear any firm evidence that this capacitor fails in this particular position in this particular amplifier type. However, with evidence of previous use with other faulty components causing possible over-voltage, then I will concede that in this instance it may be proved to be faulty. It will depend on whether the OP finds it's the resistor that's gone down in value or the capacitor that had a severe internal leak. |
20th Jun 2019, 8:50 pm | #39 | |
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Re: Pye Black Box PCL83 version
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Most are now 60 years old. |
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20th Jun 2019, 9:05 pm | #40 | |
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Re: Pye Black Box PCL83 version
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