2270 with Failed Outputs but No Apparent Reason

[hirscwi]

I'm working on a 2270 for a friend of mine. Both output transistors on one channel are blown.

This is a very early serial (1072 - champaign faceplate). This receiver was completely recapped about 6 months ago and operated just fine until very recently when the outputs failed and blew a fuse (on turn-on I think). The bias transistor was replaced with a KSC3503 as per Catrafter's suggestion.

I cannot find a reason for this failure. I have checked all the transistors on that amp board and they are fine. I'm reluctant to replace the outputs without knowing the cause of the failure.

Any suggestions will be appreciated, thanks.

 

[bberkom]

You didn't mention it in your post, but have you tried firing it up on a dim bulb tester? Does the bulb dim or stay brightly lit? If it's still bright, then there is a short somewhere, you just have to track it down. Have you measured the voltages in and around that channel and compared against the schematic to see if anything looks off?

Are you testing the transistors on the amp board in circuit or out of circuit? They can sometimes act differently in versus out of circuit. It also isn't necessarily a transistor that caused the fault, so you should be checking resistors, diodes, etc. to see if perhaps one of those failed on you.

 

[rBuckner]

I have seen just one or both outputs go bad with no other parts going bad along with them. Several times at least. You can listen to the output through headphones to see if there's audio with the outputs both out, making sure the socket can't short to the heatsink or back of the pc board.

 

[hirscwi]

Thanks Randy and bberkom. I'll try those suggestions. Transistors were checked out of circuit using a DCA55. The bias diode was checked also. I'll report back.

 

[hirscwi]

OK it passed the dim bulb test so I connected headphones. They worked fine with no apparent distortion and balanced output. I will replace the outputs and bet that solves the problem.

Thanks, Randy for the observation - I didn't realize those outputs failed without provocation. I'm wondering if my friend accidentally shorted the speaker output?

I'll see if I have the correct replacements (MJ21193 and MJ21194 or MJ15024 and MJ15025) and give it a try.

 

[blhagstrom]

I've found a lot of units or all makes with a single or both outputs on one side blown and no other damage.
I figured age and stress finally caught up.

 

[hirscwi]

Apparently just the outputs failed. I replaced them, set the offset and bias and they've run for 10 hours without a problem. A happy ending.

I hope age and stress don't catch up with me in the same fashion...

 

[blhagstrom]

Apparently just the outputs failed. I replaced them, set the offset and bias and they've run for 10 hours without a problem. A happy ending.

I hope age and stress don't catch up with me in the same fashion...

Well, there's always hope.