HARMAN KARDON 430 - Oscillating DC Voltage At Speaker Terminal

[JB8]

Hey All,

I've read several threads on this issue, feels like I've checked most of the viable culprits but I need some help, please.

• ISSUE:
  • Wildly fluctuating DC voltage at left speaker terminal, both spkrs 1 & 2
  • With DMM in 600 range, out of necessity, the voltage is quickly bouncing around from what appears to be -24 V DC - 24 V DC
  • If I use a test speaker, and go to connect the speaker leads, I not only get a blast of static, but at one point I could see a blue spark
• CONDITIONS:
  • Running through DBT, getting quick flash to totally dark on power-up
  • Right channel is outputting ~16 mV
  • Right channel sounds fine, FM working well, signal meter working, very clean unit otherwise
• TROUBLESHOOTING:
  • All B voltages are within spec
  • Output transistors pulled and tested fine, reinstalled after replacing thermal compound
  • Main amp transistors replaced thus far (one at a time, testing after every change, not shotgunned - I want to find the issue first before any elective refurbishments are executed)
    • Q410, 414 2SA777 - KSA1220A
    • Q408, 416 2SC1509 - KSC2690
    • Q402, 404, 406 2SC1345 - KSC1845
  • Caps and Diodes on left channel of main amp along with filter caps are all testing fine in-chassis - they're all over spec but not crazy

I know the tone Tone/PreAmp may be the logical next to check but it would be great to know I'm heading in the right direction. Should I just get after the Tone/Preamp board transistors, or am I barking up the wrong tree?

Thanks, as always, for your assistance!

 

[jheu02]

Not quite the same issue, but check out this thread. Since the 430 doesn't have preamp out to main in jacks on the back, this (post #5) would be a way to check the preamp section.

 

[sruchris]

The tone and eq boards both have a handful of 2sc1344s. Those are known to go bad. I'd replace them all with KSC1845s and go from there.

 

[JB8]

I'm on it, will let you know how it goes!

Great to get the hat trick on advice, much appreciated!

 

[jheu02]

Try to isolate before shotgunning. Even if it turns out to be the c1344s, you will have learned a thought process on how to troubleshoot. On the scat it shows LP502 and LP503 the Line out from the Tone Board for the R and L channels respectively that go to LP406 and LP412, the R and L Inputs to the main amp board. You could even add Pre Out - Main In RCA jacks on the back chassis panel if desired.

 

[JB8]

Hey John, and all, great advice here.

I did as you said and the issue indeed moved to the right channel, bingo, preamp it is! My jumper set-up was a bit crude but those leads are super short, thankfully it worked fine. After isolating the issue to the preamp I remained methodical and started by only replacing Q502 to start. Fixed!

Again, I appreciate the council on determining the issue instead of just replacing (shotgunning) everything in sight. Next time I'll do the same to make sure I'm concentrating on the correct board. Thus far I've been prone to swapping everything out then still scratching my head, or hitting up this forum, but I'm learning to slow down and think my way through the process instead of just throwing the kitchen sink at it.

I'll replace the rest of the 1344's on this board then replace the rest of what I've already replaced on the main amp board to balance things, likely get some new caps installed as well, we'll see. In the meantime, really appreciate the assist from each of you!

 

[jheu02]

Glad you found it.