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Another Leak Stereo 20

We'll start differently.
On your picture there is a bottom row of tags with components on, Yes?
Lets call that row A, with the leftest tag being 1, then 2 and so on (so we have A1, A2 up to A6)
Next row up is row B, it has 3 tags, so B1, B2 and B3.
Next row up 5 tags, so C1, c2 etc.
As an example R3R is soldered to B2 and B3, gottit?
Right now I want you to measure between A2 and B2, what reading do you get?
 
Actually looki9ng at your last post it appears you have done that correctly.
Right, Now we can with fanfare, church bells and grand music, diagnose the fault.
There is a wire underneath the board that connects tags A2 and B2, which is not connecting for some reason, could be it has broken underneath or become unsoldered during the original component removal. If you can't see a problem then you will need to add another bit of wire between the two tags.
 
Like Mike MA, I'm bating my breath until Tony S comes back with a reply to the above. Meanwhile, my Mike P restored ST20 is sounding great right now, if that helps.
 
So....where to start?

First off, many thanks to @toprepairman , your diagnosis was absolutely spot on. Couldn't see any obvious breaks but, of course, all the wirings hidden under the board so I soldered a piece of wire between the tags of R4R & C2R. Checked the resistance between all the points discussed above, all now measuring as they should.

So of course I plugged it all back in & turned it on...

Good news first. Ear to the speaker with no music playing, RH speaker as quiet as I've heard an ST20. LH speaker getting a hum, louder than I would like & louder than previously. Went ahead & pressed play on my CDP. Music, from both channels, yippee!

Bad news...about a minute in, a very loud bang and smoke emanating from the underside of the amp, oh dear! Closer inspection has revealed C8L has deposited its contents all over the underside of the amp.

So more detective work required. I am suspecting a dodgy valve base & will look here first.

Although disappointing, in a way I am quite reassured. At least now I know for sure that all the transformers are OK so there should be nothing here that can't be sorted with a bit of detective work.

TS
 
TonyScarlett said:
Bad news...about a minute in, a very loud bang and smoke emanating from the underside of the amp, oh dear! Closer inspection has revealed C8L has deposited its contents all over the underside of the amp.
Click to expand...

You're braver than me Tony - that would have had me immediately carting the amp off to a professional fettler!

Good luck getting it all sorted - very much enjoying this thread.
 
Hi Tony.
A bit unfortunate re the exploding capacitor.
It could well indeed be the valve base.
The good news here is that if that is the case, you should be able to identify the broken pin. They are of a forked construction and do occasionally break.
Rather than replacing the whole base you can remove a spare unused pin from another socket and just replace that.
 
TonyScarlett said:
So more detective work required. I am suspecting a dodgy valve base & will look here first.

Although disappointing, in a way I am quite reassured. At least now I know for sure that all the transformers are OK so there should be nothing here that can't be sorted with a bit of detective work.
Click to expand...

I’d be tempted to pull the board out again as from what you are describing it sounds like there may well be a problem on the top-side. The fact you had to jump a wire below suggests something has got loose or is incorrectly connected there. I’d want to see where the break was and to check nothing else was shorting or had the potential to. The capacitor has to be telling us something too: you had unacceptable hum and then a cap blew, that suggests that cap was seeing something it shouldn’t. What was it seeing and why? I’d want to inspect every joint and if necessary beep continuity on every wire. I’d not be happy jumping wires below without understanding where the real problem was and where any potentially stray wire may be.

Obviously none of this rules-out broken valve base pins etc!
 
toprepairman said:
Any progress/news on the runaway valve problem? Was it the socket or something else?
Click to expand...
I've had a couple of busy weekends so I haven't had a chance to have a proper sit down with it yet. A quick inspection hasn't revealed anything obvious with the valve sockets.

I suspect I may end up having to admit to my own limitations & seek some professional help, not quite there yet though.

TS
 
I've now removed the board again to have a proper look. There are no breaks in the wiring so the fault causing the the problem with the RH channel must be a dry joint on the component side of the board where the wiring is joined to the solder tags. I should be able to just reflow those joints & remove the link wire I installed.

Can't see any broken pins in any of the valve sockets so now I am at a loss to explain what the issue is. I am beginning to think I may just have got really unlucky & a valve hadn't seated properly when I reinstalled it. Remember that the first time i fired up the amp after the rebuild the LH channel was working fine for several minutes with no hum.

Waiting for some resistors to arrive so I can tidy up the area around the preamp socket. Can't see any issues here, I'm just not happy with the neatness of my work so want to redo it before I reinstall the board.

Once I've reinstalled the board I'm unsure of how to proceed. Would like to recheck all the voltages again before I hook it back up but don't fancy the idea of the same fault occurring, this time with my face in the firing line of an exploding capacitor! Any suggestions?

TS
 
TonyScarlett said:
I've now removed the board again to have a proper look. There are no breaks in the wiring so the fault causing the the problem with the RH channel must be a dry joint on the component side of the board where the wiring is joined to the solder tags. I should be able to just reflow those joints & remove the link wire I installed.

Can't see any broken pins in any of the valve sockets so now I am at a loss to explain what the issue is. I am beginning to think I may just have got really unlucky & a valve hadn't seated properly when I reinstalled it. Remember that the first time i fired up the amp after the rebuild the LH channel was working fine for several minutes with no hum.

Waiting for some resistors to arrive so I can tidy up the area around the preamp socket. Can't see any issues here, I'm just not happy with the neatness of my work so want to redo it before I reinstall the board.

Once I've reinstalled the board I'm unsure of how to proceed. Would like to recheck all the voltages again before I hook it back up but don't fancy the idea of the same fault occurring, this time with my face in the firing line of an exploding capacitor! Any suggestions?

TS
Click to expand...
Keep plugging away at it Tony,its worth the effort and journey ;)
 
No updates on the amp, I've been very busy plus just had a holiday so haven't had time for it. I have been trying to get in contact with Graeme at AmpRegen to help get it over the line without success.

In the meantime the madness continues...

20231027-132547.jpg

Really wasn't expecting to win this, i was sat outside a restaurant in Malaga having a lovely meal when the auction ended, I impulsively put in a really low speculative bid fully expecting a flurry of late bids that never materialised.

20231027-132712.jpg


It's a little scruffy with some small rust spots & several, fortunately light, scratches. Will probably look presentable after a good scrub & polish. It's a 1961 version, looks like it has the later output transformers. Obviously had some work done previously although it was advertised as spares or repairs.

Only came with two ECC83s & two EL84s,
20231027-133004.jpg


The ECC83s are Seimens, not sure if they are considered decent? Can't identify the EL84s, can only make out Tun (Tungstram?). Can't make out any date codes on them.

Not sure what I'm going to do with this one yet, got to get my 1959 example over the line before I can even think about it. I do have most of the parts necessary for another rebuild, I doubled up on most of them when I ordered for the 1959 amp, including the K42s.

TS
 
Those valve pins are very badly bent. Do not attempt to plug them in before they are correctly straightened, you'll likely break socket pins as well as crack the valve glass.
 
No idea how they managed to get that bent. Will gently straighten them before trying them.

I'm quite keen to hear how those Siemens ECC83s sound. The information I've managed to find suggests they are Yugoslavian made.

TS
 
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