Model 1072 right channel distortion

[alfizta]

Hi all!
New to forum.
Trying to fix my dad's Model 1072. A gorgeous low power amp that is showing lots of distortion on right channel. Hoping to get some feedback from you people before I quit or damage it trying to replace all caps.

Distortion is quite big, whichever ever speaker system output (1 or 2) speakers are connected to, with whichever speakers I connect, with any input source. Left channel sounds clean. Can't hear any distortion through headphones, though.

Thank you!

 

[ton4eff]

Can you check if R806 is open?

 

[alfizta]

not open, but it is reading 350 ohm where it should be 220 ohm, according to the color codes and service manual. could this be the issue? multimeter is OK.

 

[rBuckner]

I'd unsolder one end then and remeasure as some electrolytic cap may be causing a false reading. My first thought was one or both output transistors are blown. These can be checked with the diode function on most meters these days. Details are in a sticky post in the DIY forum.

 

[alfizta]

My first thought was one or both output transistors are blown.

where can I locate them?...

 

[alfizta]

well, been investigating... should be those four flat ones bolted into the heat sink, right?

 

[rBuckner]

That's quite likely especially if it's a larger heatsink. I'm not familiar with this model, never having worked on one. You can post a picture of the area and I bet someone will reply.

 

[alfizta]

well, having some holiday time to spend on this repair, been «thinking»...
I'm thinking of inputing sound from a RCA source only on right channel and connect only the left speaker, then bridge one right power transistor (B,C and E ends) to the equivalent left one (respective B,C and E ends), and then check the left speaker output. if there's distortion, then the right channel power transistor is not the problem. if distortion is gone, then right power transistor is the culprit.
what do you think of this procedure?

 

[Bert 1100]

bridge one right power transistor (B,C and E ends) to the equivalent left one (respective B,C and E ends)

sounds like a bad idea to me... the only way this would work is if you disconnect completely the transistors from the good side and disconnect as well the suspected ones of the bad side.
Maybe better remove them and swap them, bearing in mind that you might damage the transistors from the good side that way, in case there are other bad components...