Wire Wound Resistor Failure?

[jjai]

I have been in the process of troubleshooting a solid state guitar amplifier (Ampeg SS-140c) for the past month.

I seem to have narrowed the problem down to the chorus circuit. When the chorus is off, the amp works fine. With the chorus on, all you get is a putt-putt noise.

When I feel the board, I notice that the two wire-wound resistors on the board get burning hot (180 ohm, 10 watt). When I test them cold they test to 180 ohms, with voltage in them, they are completely open.

Other symptoms are: There are several J112 transistors on the board that I've replaced twice. There is also a power LED that is attached to the board that does not light (I have tested off the board and it lights when voltage is applied).

Can anyone tell from this description if I need to replace the wire wound resistors? Is it possible they've failed? Is failure of wirewounds likely or even possible?

Thanks a lot--

jjai

[Netlist]

Originally posted by jjai
When I test them cold they test to 180 ohms, with voltage in them, they are completely open.

This tip will probably not solve your problem but keep in mind that you can't measure resistor value with voltage applied.
In fact to be absolutely sure you measure correctly you need to take it out of the circuit. Or lift one leg.

The 2SJ112 are probably the output mosfets.
Since the amplifier tends to play fine with chorus off, they won't be broken.

That's about it for the moment, a schematic would help.

/Hugo

[JohnW]

Or - if the J112's are in a TO92 type cases then they are constant Current sources.....

How do you measure the resistors with a voltage applied - would this not upset the resistance range on your meter?

It’s very unlikely a wire wound resistor would have an intermittent fault - once these fail - there normally gone for good....

John

[john curl]

J112's (American type) are analog switches that can be used for current sources, switches or cascodes. Their Idss can be just about anything above 5ma.

[JohnW]

Originally posted by john curl
J112's (American type) are analog switches that can be used for current sources, switches or cascodes. Their Idss can be just about anything above 5ma.

Yep, Johns quite correct - See attached link for data sheet: -

http://www.chipcatalog.com/Fairchild...heet/15394.htm

Sorry for any confusion,

John

[Netlist]

Confuzing numbers: j/2sj...
Anyway, I still think we need more info.

/Hugo

[jjai]

Thanks so much for the help -- I'm a total newbie (as you can tell, I'm sure) - and didn't realize that I can't test resistance with voltage in the circuit.

That did, however, prompt me to check the voltage in the circuit at various points, though which I discovered a bad power resistor (although not the one that I originally suspected).

Thanks so much to everyone!!