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Old 22nd Dec 2013, 3:26 pm   #101
kevinski
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Default Re: Pye Black Box

Just remeasured the valves AFTER putting a resistor in series (forgot to put it in before!!)

Pin Valve 1 Valve 2 Valve 3
1 72 39 39
2 0 0 0
3 0 2 2
4 0 0 0
5 0 0 0
6 180 225 223
7 61 16 16
8 81 226 226
9 0 3 3

Cheers

Kevin
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Old 22nd Dec 2013, 4:25 pm   #102
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Default Re: Pye Black Box

The main HT figures look better now, probably close enough not to worry about.

Just remembered that you already have the service info- the table I posted is simply from there.
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Old 22nd Dec 2013, 5:35 pm   #103
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Default Re: Pye Black Box

The PCL83 pentode grids are 3V and 3V according to your measurements. Are you sure that's correct?
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Old 22nd Dec 2013, 5:45 pm   #104
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Yes I've just measured them again and they are 3Vdc each

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Old 22nd Dec 2013, 7:16 pm   #105
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Default Re: Pye Black Box

That's not very good. 3V positive to chassis, on output valve grids is either dodgy coupling capacitors (ISTR they have been replaced?), dodgy valves or possibly dodgy valve holders- pin 9 g1 is next to pin 8 g2 which is at HT+. Either way then the output transformer is at risk from excess primary current. The situation won't be helped by that common cathode resistor in the output stage either.

Just in case it's the capacitors, you can check the pin 9 volts on the output valves with the valves removed. If it drops back to zero, the capacitors are OK, if it doesn't, change them again and see. If it still doesn't try disconnecting the wires to pin 8 to check the valveholders. If the valveholders are bad they may respond to cleaning, otherwise replacement. If the valves are bad, then it's grin and bear it time
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Old 22nd Dec 2013, 9:07 pm   #106
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Default Re: Pye Black Box

It's a bit odd they are both exactly the same. I am looking at the service sheet marked part no 708809 and it definitely shows the grids going to chassis via R23/R24 470K. You might want to check the wiring in case it is different.
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Old 23rd Dec 2013, 12:35 pm   #107
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Default Re: Pye Black Box

PJL - As far as I can tell the wiring is as it should be

Chris - Took out the valves and the voltage disappeared on valve 3 and went down to 0.001v on valve 2.

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Old 23rd Dec 2013, 1:22 pm   #108
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Default Re: Pye Black Box

That does rather sound like tired valves

You could try a careful clean around pin 9 at the bottom of the valves, but the problem will probably be internal.

While the valves are out and power off, check the resistance of R25 (output stage cathode bias resistor), if it's correct, the valves are passing too much current (from your measured volts at pin 7 of each PCL83- 16 instead of 13.5). Enough to provide the extra 3V across the cathode resistor to balance the 3V on the grids.

As a temporary bodge, you could increase the cathode resistor by about 25% to get the current back somewhere near normal, but I suspect that new or at least tested good secondhand valves will be needed fairly soon. The output transformer is definitely having a hard time as it is.
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Old 23rd Dec 2013, 1:29 pm   #109
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Default Re: Pye Black Box

The bottom of the valves have, what appears to be a touch of pitch on them. I think a good cleaning of the pins might not be a bad idea.

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Old 23rd Dec 2013, 2:09 pm   #110
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Default Re: Pye Black Box

PCL83s aren't the hardiest of valves. The anodes on mine were glowing red, leading to a gradual increase in distortion and decrease in volume after a few minutes.

On the basis of a recommendation on this forum I replaced mine with pin-compatible (and cheaper!) 30PL1s.

Worth giving them (and the sockets) a clean first though.
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Old 23rd Dec 2013, 7:42 pm   #111
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Default Re: Pye Black Box

Cleaned the pins and it seems to have worked a bit. The voltage seems to have gone but the volume is no louder. Checked R25, it's measuring 303Ω instead of 220Ω

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Old 24th Dec 2013, 10:42 am   #112
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Default Re: Pye Black Box

I did replace a few resistors when I did mine - a few of them were out of spec and seem to recall that one of them was open circuit. Then again, my mains transformer had previously caught fire, so that wouldn't have helped matters!

Assume you've already squirted some switch cleaner into the volume and tone pots?
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Old 24th Dec 2013, 12:37 pm   #113
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Default Re: Pye Black Box

Yes, both pots and the valve bases have been squirted. Think I'm going to check all the resistors. I'm buying some new valves so it'll give me something to do until they arrive (either that or start on the wife's list of jobs!)

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Old 24th Dec 2013, 1:07 pm   #114
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Default Re: Pye Black Box

If the grid voltages are now near zero, check the cathodes - they should be around 13.5V if the valves are passing the right current and the cathode resistor is correct value. If that's all OK, the distortion problem probably isn't output valves.
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Old 24th Dec 2013, 1:35 pm   #115
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Default Re: Pye Black Box

I don't have a distortion problem, can't get enough volume for that LOL.

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Old 30th Dec 2013, 2:41 pm   #116
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Default Re: Pye Black Box

Hi all, hope you all had a good christmas. I have now changed all resistors that showed incorrect measurements and have acquired some new old stock PCL83's. Volume has again increased but again not by as much as I was hoping. I measured the DC Voltage on the pins with the valves in and they are:

Valve 1 1=63/2=0/3=2/4=0/5=0/6=168/7=52/8=70/9=0

Valve 2 1=35/2=0/3=2/4=0/5=0/6=222/7=14/8=218/9=0

Valve 3 1=35/2=0/3=2/4=0/5=0/6=216/7=15/8=224/9=0.

I'm now running out of things to change! lol

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Old 30th Dec 2013, 8:43 pm   #117
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Default Re: Pye Black Box

Hi Kevin, it is not unknown for the output transformer to develop shorted turns on the primary due to the valve being over-run. This does not cause smoke to pour out, but results in weak and distorted sound. This is not an item that is easy to test, it is better to substitute with a known good unit.

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Old 30th Dec 2013, 10:51 pm   #118
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Default Re: Pye Black Box

Not sure if a complete new cartridge has been tried yet? If you are still using the original, it's likely to be faulty as crystal cartridges absorb moisture over the years and fail giving no/low output. You'll need a medium output type like an SC6M. Make sure you get the mounting bracket with it.


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Old 31st Dec 2013, 11:27 am   #119
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Default Re: Pye Black Box

Quote:
Originally Posted by kevinski View Post
I have now changed all resistors that showed incorrect measurements and have acquired some new old stock PCL83's. Volume has again increased but again not by as much as I was hoping.
To eliminate the cartridge from the equation, have you tried it with a different source? I found a standard line input was a bit on the quiet side, but anything with a headphone output should be more than loud enough on 50% volume.

Mine came fitted with a non-original ECC83(12AX7) in place of the ECC82(12AU7) valve. It sounded quite thin and flat and I was disappointed. Replaced this with a Mullard ECC82 and it was much better. Suspect this was more due to the difference in valve spec than it being worn out though.
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Old 1st Jan 2014, 8:36 pm   #120
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Default Re: Pye Black Box

Hi,

Ed - do you know what I would need to look for if I wanted to find a working Output Transformer?

Behindblueeyes - I have tried on numerous occasions to use a different source. I have tried the MP3 player with the headphone cut off with no success, can't get any sound even out of known working units.

I have ordered an NOS Mullard ECC82 to see if this helps. The two PCL83s glow a warm orange but the existing ECC82 is more like a dying ember.

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