HARMAN KARDON 330C - Bacon Frying Popcorn Static On Cold Start

[JB8]

After a long hiatus from working on receivers (many of which were fixed due to Audiokarma help, thank you) I picked up another 330C. The power supply board had been removed, it was missing the main filter caps and one cap on the main amp board (C407). Whoever worked on this guy last replaced a ton of parts, so I was guardedly optimistic. After rewiring the PS board and installing the missing caps I fired it up under dim bulb and it came to life, sounded great, sweet. But then after listening to it for a half hour, turning it off to let it cool, I powered it up and got that wonderful blast of static you'd hear with bad transistors. It was 90% on the left channel but some bled through on the right channel as well. I repeated the test - listened for a bit, power down, cool, power up - and got the same crazy-loud static (across all functions at zero volume) that would eventually dissipate and go away. You'll see from the pics of the main amp board there was a fried transistor on the left channel (Q408), I pulled it to test it and it must have been installed during the rebuild, tested fine so I reinstalled it. One of it's pads was cooked so one of it's legs is jumpered. Looks like all of the transistors on the main amp have been replaced and I'm assuming this board will be the source of the problem. I pulled and tested Q601 on the PS just for the heck of it, it was fine so I reinstalled it.

It's been a while and I feel like I've lost most of the chops I had developed to track down the source of the static. I'm hopeful that the "warming the unit up stops the static" condition will be helpful but I'm also happy to work the problem based on your advice, just like I used to years ago.

Update: It no longer dissipates, just snap crackles and pops, violently...

 

[JB8]

I think I'll just start in here and if my path seems wonky feel free to hop in.

Pulling and testing the output transistors may be helpful, seems there are those of you who don't particularly like the TIP41, I think I have some MJE15034's that would work as a replacement if the TIP's are no bueno, please advise otherwise if that's not the case. Once the output transistors are checked out that should send us in a specific direction, will keep the thread updated along the way.

 

[tarior]

Outputs don't generally fail noisy. Small signal transistors do.

 

[avionic]

This maybe your issue. Scrape all signs of carbon away from the board around that transistor. Carbon is a good conductor of electricity. Also eyeball the solder connections for cracked solder pads or joints.

 

[JB8]

Thank you both. @avionic, nice to see you're still out there, you used to help me a ton several years ago, apologies in advance for any bloviating.

I'll get to work and report back along with providing updated pictures.

 

[avionic]

Old audio techs never die. They just get re-capped.

 

[JB8]

I removed the transistor (Q408) to get better access. Should I try to remove more of the remaining carbon? What is the goal in how it should look once enough of the carbon has been removed? Lastly, on the underside of the board, should I attempt better purchase further down the trace for the collector leg due to the half pad?

 

[Mumbles]

I powered it up and got that wonderful blast of static you'd hear with bad transistors.

A few months ago I picked up a 330C for next to nothing and it did the very same thing on power up. Doing that twice for no more than one or two seconds each time was enough to convince me this thing needed some love.

So, after removing the cover and studying the schematic, I saw it had only one 'shoot on sight' 2SC-458 on each channel along with three 2SC-1345's, which can also get noisy. Replacing the 2SC-458 and one of the 2SC-1345's with KSC-1845's was straight forward but the other two 2SC-1345's were the differential pair and had to be replaced with matching KSC-1845's. Thankfully I have lots of 1845's on hand so matching them was no problem.

After replacing these transistors on both channels, I crossed my fingers and powered it up again with no DBT or Variac and yee-haw, it was quiet, dead quiet! I was over the top happy and so were my speakers. No more frying bacon noise.

 

[avionic]

What is the goal in how it should look once enough of the carbon has been removed?

Ideally the same color as the rest of the phenolic PC board.

That area between the Base and collector pin holes. Looks pretty deep. Maybe totally compromised.

https://audiokarma.org/forums/attachments/330c-carbon-removal-after-jpg.3825203/

 

[JB8]

@Mumbles , thanks for sharing, I wish this one was that easy. Whoever worked on it before me replaced nearly all of the transistors on all of the boards, thankfully they look to be the proper replacements as well. The C2690A pulled from Q408 - where the great blast took place - tested the same as a brand new one, so I can't blame the part. I think the damage had been done, maybe even before the last person worked on the unit. If all else fails I think I may have a donor main amp PC board I could swap in.

Thank you, @avionic , I figured that might be the goal. I may just pop the transistor back in to see if anything has miraculously changed and if not I'll regroup and go from there. And of course I'll update the thread as well.

 

[Mumbles]

The C2690A pulled from Q408 - where the great blast took place - tested the same as a brand new one,

Well, I guess it should test like a new one because it probably is.

Q408, a current regulator, would have originally been a 2SC-1509 so someone must have replaced it with the KSC-2690. Something is causing it to burn like that unless the original one failed and fried.

 

[JB8]

Yep, good points.

I just worked on it a bit more then reinstalled the transistor. Before installing it I pulled some insulation from a small copper stranded wire and pushed it onto the collector - it was initially missing, in fact, it was the only transistor on the main amp missing that little yellow insulation they all seem to have from the factory. I fired it up - no pun intended - and I got a couple small pops initially then it quieted down. I've been listening to FM and content from my phone via the AUX input and it's been quiet. Now I'm just listening with the speakers engage but with no signal, quiet as can be. There have been a couple random pops over the past 20 minutes but nothing like it was, not sustained or angry at all. I'll keep testing it and see how it goes. Thanks again.

 

[avionic]

and I got a couple small pops initially then it quieted down.

Tap around on that board to induce vibration. Maybe want to use some test speakers. Just in case.

 

[JB8]

Tapped all over the board this morning, no issues, will still do some test listening to see if anything pops up. And yes, test speakers were deployed early with this guy. I'll update this thread if anything notable comes up.

Last night after reinstalling Q408 and letting it warm up a bit I checked the idle, it was spot on across both channels.

Thanks again, always appreciate you having my 6, pointing me in the right direction. I have a couple more 330C's and a couple Yamaha's still to fix, eventually, so I'm sure I'll be back with more questions.