Pye P224: Advice Please.

[Joe]

There's a good inch of clearance between the panel lamp brackets, as circled by Rob T, and it sounds bad with the chassis pulled out to the limit of it's wires.

It could be something nasty under the dust cap, though that itself isn't loose. I'll investigate further tomorrow as by back is already very cross with me for bending over it so long earlier.

By the way, the circuit board IS bent and appears to be by design. It's held by screws at either end and has a guide on the chassis in the middle.

[Paula A]

I don't usually stick my twopennorth into valve output stages, but don't worry too much about the 9v on the grid of the el84.. measure the cathode.. 16v above chassis is spot on in those circumstances.. as long as the overall bias is right with the grid 7-8v negative with respect to the cathode everything is fine.

[Joe]

Thanks for taking the trouble to reply, Paula, I'll check it later - providing I can identify which pin the cathode is. This is the first valve thing I've ever done so am still feeling my way somewhat and the schematic I'm working from doesn't show the pin numbers.

To be truthful I'm not bothered what a meter says as my ears are more than happy, or they will be when I sort the speaker out.

All the best
Joe

[AlanBeckett]

Joe,
http://www.r-type.org/pdfs/el84.pdf Page 4
Alan

[Joe]

Thanks very much, Alan.

[AlanBeckett]

Joe,
Now I've come to life .....
Paula is entirely correct in that it's the Grid-Cathode voltage that's important. However, I don't see from the schematic why there should be any (Positive) voltage on G1.
Alan

[Joe]

I think the darned thing is trying to drive me off my onion.

Last night I tried connecting a tranny to the speaker and that sounded rough as well. After reading Chris's advise I tried this again but with the speaker sitting on the bench and it sounded fine so it seemed like it was some dirt or something that was moving about.

I put it back together this morning as it still sounded fine and while I was at it changed C27 which was a Hunt's with a cracked case. I'd previously left it thinking that it was only something to do with the tuning indicator. The old one did prove to be slightly leaky.

When I tried it the speaker sounded OK but there was a hell of a general hum, which had been completely absent yesterday. Altering the volume makes no odds to it, though obviously the higher that is the less it's noticeable.

I've just put new smoothing/reservoir caps in and tried a different one in C43 but these modifications have made NO odds at all.

I've also taken some more careful readings around the EL84 with my AVO:

Black lead on chassis and red on pin 2 = 5V
Black lead on chassis and red on pin 3 = 12V
Black lead on pin 2 and red on pin 3 = 2V

Besides these I connected my megger to pin 3 and went 'round the others but that didn't seem to show anything, but I don't know if it would or not.

It's thoroughly maddening as it sounded fine until putting it back together yesterday. I'm inclined to think that there must be a bad contact somewhere but I'm damned if I can find it.

It just shows that the simple-seeming jobs never are.

Yet again, thanks for the help.

Joe

[Joe]

I think I'm onto something but my back's playing hell so I'll report later.

It is also Tuesday when I look after my mate's greyhounds so have better things to do:-

[attachment=817]

My leg is in the bottom left corner.

Cheers
Joe

[AlanBeckett]

I can see that it's hard work looking after them
Alan

[Joe]

My earlier hopes have been dashed. I'd realised that the current symptoms are very like the trouble I'd had when I switched it on for the first time and that had been down to R20 having gone high. I checked that again and it looked like it was reading over a meg, as opposed to 680k, so I changed that again. This made no difference and when I tested the 'bad' resistor out of circuit I found it was spot on.

I can find no broken tracks or dry joints and have had a poke around between the tracks with my megger and can't find any conductive bits either.

I'm loathe to do much more as repeated de-soldering and soldering isn't going to do the PCB any favours. Is it possible that the EL84 itself could have an intermittent fault? I'm loath to buy another without being sure as they're not cheap and I don't have another radio that uses them so it wouldn't be that likely to be of use on another job; it isn't as though I'd dropped it or anything.

So far I've been philosophical and thought how for all that in some ways it would have been nice to have changed a couple of capacitors and then plugged it in and it have worked perfectly I wouldn't have had as much entertainment or really learnt anything. It is now though starting to cross the line into becoming a pain in the rear.