Kenwood L08M circuit discrepency

[BSST]

Sounds encouraging. Fingers crossed!

 

[pwdiya12]

more progress?

so amp is now biased and off dim bulb,it passes signal, however there is a low level hum and noise on the audio (and the hum can be seen on the scope). I am hoping it is one of the many solder connections in the input stage and not one of the dual transistors.....

So I am testing the other amp and getting it to the same stage (biased and tested with signal) and then will use that as a reference whilst I find the hum and noise

One done, I will try and write a summary for any others who have these units and have problems with them

Again many thanks

 

[Mooly]

Can't see any hum being a transistor, its going to be a wiring issue most likely.

 

[pwdiya12]

@Mooly that is my hope, maybe a dodgy solder joint, though the noise is reminiscent of transistor noise....

 

[Mooly]

Noise or random noises can be a faulty transistor but hum would normally not be. There is no mechanism for a transistor itself to cause hum beyond such things as shorted power transistors pulling a supply right down etc.

 

[pwdiya12]

to no one's surprise, the the amp had more than one problem...

the hum, was mainly solved by physically connecting all the various metal components of the chassis.
the noises were solved by replacing Q1 and then Q12

I now have the amp having the same THD (0.001%) as the other one...at last.

I will try and write up a summary of the problem and solution steps for any others that have similar issues.

Thanks so much for all your assistance and guidance

Peter

 

[Mooly]

Well done 👍 Stuff with hidden histories and multiple issues are often challenging at best. This one has been a real challenge 😉