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Old 13th March 2013, 10:58 PM   #11
Champ is offline Champ  Canada
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Originally Posted by Mooly View Post
Tants are favourite failure items. They often go noisy and leaky or even short. Just pop and ordinary electro in its place.
Mooly Would I need to put in an Electrolytic Bi Polar? If not where would I put the neg lead?
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Old 14th March 2013, 08:24 AM   #12
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You should be able to tell from the circuit but if not just measure the voltage across it with the amp on and fit accordingly. Just fit an ordinary cap of suitable voltage.

Is that a carbon composition resistor lurking around there too ?
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Old 14th March 2013, 01:00 PM   #13
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You should be able to tell from the circuit but if not just measure the voltage across it with the amp on and fit accordingly. Just fit an ordinary cap of suitable voltage.

Is that a carbon composition resistor lurking around there too ?
There are a number of carbon resistors in there. When I get this noise issue settled I am going to replace them.
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Old 14th March 2013, 01:02 PM   #14
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There are a number of carbon resistors in there. When I get this noise issue settled I am going to replace them.
The noise issue could possibly be a carbon comp playing up
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Old 14th March 2013, 01:12 PM   #15
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The noise issue could possibly be a carbon comp playing up
mmm, but it also could be a transistor, no? Where to start Mooly? Yanking the resistors and a few tants are easy and probably should be done anyway. Would this be your first approach? I have taken voltage readings across most of the transistors that I can get my probes into and they seems to be no shorts, they are all reading voltage.
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Old 14th March 2013, 01:26 PM   #16
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Input diff pair transistors are the usual culprit.
Try swapping these transistors from channel to channel.
While you are at it, inspect these transistors for black legs....this is a usual sure sign of noisy transistors.

Regards, Dan.
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Old 14th March 2013, 01:41 PM   #17
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Well, everyone keeps saying it could be the front end transistors so that has to be considered... they could be a type prone to this kind of failure... it happens. So perhaps do as others suggest and swap them over to prove.

Where would I start... replacing any tants simply because they are notoriously unreliable. Carbon comps are in the same category really. There is no really scientific way to prove where the fault is... a scope could be a good guide but not definitive for this kind of problem. Go for the most likely suspects first. That tant for one and perhaps the diff transistors as everyone seems to know something I don't about those
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Old 14th March 2013, 01:47 PM   #18
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Input diff pair transistors are the usual culprit.
Try swapping these transistors from channel to channel.
While you are at it, inspect these transistors for black legs....this is a usual sure sign of noisy transistors.

Regards, Dan.
Silly question I am sure, but which ones are those?
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Old 14th March 2013, 01:54 PM   #19
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Do we have a circuit for this amp ?

The differential amp transistors will be the first "pair" with the emitters connected (or closely connected) together. Those metal tabbed driver types would be my suspects but none of this is helping is it

Be methodical, and it should take very little time to pin the fault down. Ultimately anything could be causing the problem but in practice it will be one of the "usual" suspects. Old preset pots can go intermittent too, not just the wiper but the rivets that secure the leadouts to the carbon track.
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Old 14th March 2013, 02:02 PM   #20
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The ones in red are the differential amp. The one in blue (the VAS or voltage amplifier stage) would be a suspect purely on the grounds that it runs hot (ish) continually and would be suspect for that reason and the type of device it is. That's very unscientific I know, just experience points the finger
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