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Eclipsed

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This part is in the output section(I think) of an audio amplifier, it has no markings what so ever, its made from glass(?) like a normal switching diode.The silkscreen print on the PCB implies it is a resistor, and it has a 1.1k resistance in both directions.Any ideas on identification? One of them is broken in half and needs replacement.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
It looks rather like a diode, it could be used for bias compensation, quite commonly two diodes are used in series to do this. It could also be a small thermistor, again, used for bias compensation. If so they are crucial, failure of them is almost certain to destroy the output stage. If there are no adjustable presets, it's absolutely vital that they are replaced with EXACTLY the same parts - preferably obtained as a spare from the manufacturer (as they are often specially selected).

Your best bet is to draw out the relevent part of the circuit and post that, I suspect these components will probably be between the bases of the two driver transistors.

BTW, I'm presuming you don't have a circuit?.
 

Eclipsed

New Member
I believe it is a thermistor(which is quite obvious now that I think about it), resistance lowers with heat, and it is connected to the bases of 2 transistors.I have found a close match(1k at 25c), should I replace both of them to make sure both channels have the identical value? I also have a fizzing noise coming from a large torroid transformer, any idea what would cause that? Finally any hints on testing the output stage?
 

coyotesden

New Member
post:

The transformers windings are old, and that sound you call fizzing is arcing between the windings Thats not a good thing.I know I lost a transformer too old age in my dynaco amp. :(
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
Eclipsed said:
I believe it is a thermistor(which is quite obvious now that I think about it), resistance lowers with heat, and it is connected to the bases of 2 transistors.I have found a close match(1k at 25c), should I replace both of them to make sure both channels have the identical value? I also have a fizzing noise coming from a large torroid transformer, any idea what would cause that? Finally any hints on testing the output stage?

As I mentioned previously, if these are for setting the bias of the output stage, it's absolutely crucial that you replace them with exactly the correct components - if not you are likely to kill the amp again. Either that, or be prepared to redesign that part of the amplifier - personally I always use a Vbe multiplier for setting the bias, as it gives an adjustment.

Diodes also vary with heat, the voltage drop across them falls with increasing temperature, so they are commonly used for bias compensation.

As for the transformer, it's quite common for a toroid to buzz, although I wouldn't describe it as 'fizzing'.

As for checking the output stage, if the amplifier has 'blown up', with the output transistors S/C, I would recommend replace ALL semiconductor devices in the DC chain. Almost any of the devices can cause this to happen, and they could be intermittent, replacing them all is the safe way, particularly as the smaller devices are a lot cheaper than the expensive output transistors.
 
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