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Quad 33 desoldering PSU

Yes, but that guy's problem turned out to be, and was fixed by changing R130.

Those two caps are brand new Nichicons, which measured fine, at least before soldering them in. So I'd be surprised if it was one of those.

As my R130 also measures high I'll start there, then move onto freeze spray and the transistors.
 
Hi guys:

When you are problem solving it’s best to change only one thing at a time. R130 being high at the moment is the most likely cause, change that first then have another go. Also don’t get too obsessive about the 35-volt value, its not that critical, within a volt either side of 35 and you will be fine.

I would suggest replacing R130 in both channels however purely for symmetry and consistency.

LPSpinner
 
I agree, and will see what happens after replacing R130. Seems to be a reasonably common fault with that resistor after doing a bit of Googling around.

Would a 0.6w 250V rated metal film be OK to use here as an interim measure? I have a pair from Maplins I bought earlier today - they look rather tiny/fragile though... But if I have my ohms law correct, R130 should only 'see' 0.02993 watts. If they turn out to fix the problem, would then order a pair of 1/2 watt carbon films to use to keep the appearance 'vintage appropriate'.
 
Well I went ahead and replaced those resistors. The good news is they bought the voltage back within the 33.5V adjustment range again on the 'bad' board. The bad news is the voltage is constantly wandering around from anything around 33.10V to 35.80V. Sometimes it's a slow wander, sometimes it's quite fast.

Voltage on the right-hand channel is pretty solid & barely moves.

Bloody frustrating I was looking forward to having it all sorted by the weekend. What I don't want to do is randomly keep replacing components, I'd like to methodically work this one through.

Having said that, I think I'll try replacing the RV100 pot first - it's travel feels looser than the one on the good board. Failing that, I guess the freeze spray is the next port of call? Though if I could narrow it down from measuring voltages at various points I'd prefer to do it that way.
 
Did you get the transistors?

I would not worry about this bother that you have had. I went through all four of my 303's much less thoroughly than you many years ago. They have been perfect since.
 
No, because I don't know which transistors to get - though I think DaDa have a list of modern subs. I'll get some spares of each type for the driver boards. But to be honest I'm trying to avoid randomly replacing components to find the fault if I can. As those tracks are so easily damaged, and the amp is in as-new condition I really want to narrow this problem down logically.
Your idea of trying the freeze spray on the transistors, and seeing if the voltages temporarily stabilise seems like its worth a try. I'm just a tad wary about spraying it and applying power - assume its a totally non-conductive chemical of some kind?

I'm afraid I disagree there - it might well suddenly worsen and risk one of the output transistors which are all original, so I'd rather work out why one channel is drifting and the other is fine.
 
The idea was to have the transistors in stock, should not be expensive. Yes, not a good idea to replace components randomly, but replacing a transistor or two around the problem area is reasonable.

Never had a problem with the spray being conductive. It was years ago when I used the stuff at work, was a standard fault finding method.
 
Good to hear that the output transistors are original. I have some new old stock 2N3055H as spares for my 303's.
 
Radfordman said:
The idea was to have the transistors in stock, should not be expensive. Yes, not a good idea to replace components randomly, but replacing a transistor or two around the problem area is reasonable.

Never had a problem with the spray being conductive. It was years ago when I used the stuff at work, was a standard fault finding method.
Click to expand...

I've sent off an order direct to Quad for a complete set of driver board transistors, just to be on the safe side. At least I'll know they're 'Quad approved' substitutes that way.

Yes checked the output transistors and they're all RCAs, untouched since the day this 303 left the factory. Hopefully I can get this sorted without destroying any of them & sulking for the rest of the year :p

The elation of my bargain find has been tempered a little, but hoping I can get it up and running and stable over the weekend. It's my 42nd next Tuesday so it'd be nice to have the 33 and 303 up and running by then for any LPs/CDs I might have winging my way ;)
 
Arkless Repairs said:
It will probably be fine as it is.
Click to expand...

Hmmm but there must be a reason why one channel remains stable (after warm up) at 33.5V and the other is constantly fluctuating around 33.10 - 33.75V; it just won't settle. Seems to suggest a dodgy transistor or diode somewhere? I'd rather trace it down on the off-chance it could suddenly get worse and take out one of those driver or output transistors.
 
Wait until it is varying, use freezer spray on transistors until you find culprit?
 
Dowser said:
Wait until it is varying, use freezer spray on transistors until you find culprit?
Click to expand...

It varies without any warm-up time, straight from switch-on, or if the unit has been on for a while, so not sure changes in temperature would show any fault/cause, but I'll give it a go.
 
Is the voltage on C101 stable?
I would suspect C100, C101, C104 and C106 before changing any transistors if you get music
 
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