Loud, violent popping in one channel HK 430 Twin Powered

[Drumcan]

Hello all,

Long time lurker, first time poster. I have an HK 430 that I have had for a couple of years and has recently become unusable. It's one of my favorite amps, so I'm hoping it can be saved. I have a video rundown of the issue that explains what I am seeing here, but I'll also write it out as best I can. If you have any troubleshooting tips, please send them my way!

A few months ago, I started having intermittent pops in the right channel. I Googled it and found a thread on Audio Karma about the same issue for another HK 430. People on the thread recommended getting some freeze spray and spraying the transistors on that channel individually. As many on the thread assumed, it turned out to be the voltage differential transistors (Q401 and Q403) as seen in the service manual here.

I replaced the two transistors with KSC1845's, per the recommendation on the thread, and I replaced them in both channels. I did NOT adjust the voltage regulator (could that have burnt something out ?!)

This worked for a little while, until it started to violently pop at start up. It would go away after 30 seconds, so for a while, I would disengage the speakers when I turned it on, and would only engage the speakers after 30 seconds,

Then after a little while longer, it just never stops popping. It's so bad I cannot test with speakers, so I have been monitoring with the DC offset with my multimeter at the speaker terminals. Its oscillating violently between 100's of MVs in both the positive and negative direction. It does this with the volume turned down completely and with all of the dials at a mechanical north (neutral position). It also does so on both sets of terminals. For a minute, I thought the problem lessened when I engaged both the high and low cut filters, but not really.

I removed (literally desoldered) most of all of the transistors from the circuit in the main amp board and tested them with the diode setting on my multimeter, and they all seem to be good - no shorts that I could find. I spot checked voltages across a variety of components (resistors, transistors) from each channel, they match very closely. I did not remove the power amp transistors (the big silver screw in transistors), as they aren't easy to remove.

I have exhausted all of my troubleshooting knowledge of electronics and now appeal to the AK community. Please help!

 

[Coytee]

I have no clue but can empathize with your situation. You're already over my head with knowledge.... if it were me, I'd get it to a shop and have it completely gone through.

My understanding is those are absolutely fine sounding units....so it would be worth fixing verses replacing. Good luck.

 

[Drumcan]

Thanks. When this first start happening, I took it to the shop, and they wanted to charge over $200 for it, and honestly I can buy another one for that kind of cash... Also I don't mind learning more about troubleshooting these types of things.

 

[infullview]

Do you have a schematic and know how to read it? If not, download it from https://www.hifiengine.com/manual_library/harman-kardon/430.shtml It looks like tone control board is separate with clearly marked outputs, you might try disconnecting them from the power amp to see if this is where the noise is coming from. I wouldn't rely on your measurement of DC offset if there's popping noise on the output as this would tend to read all over the place on a meter.

 

[dlucy]

A few pointers:

1. Gone-noisy transistors will test perfectly ok in a tester, but will throw random noise into a signal. In a preamp this noise will vary with the volume control, but in the power amp circuit it will be really loud and risky. Transistors that haven’t gotten in the gone-noisy list can still go noisy like this; they just haven’t done it enough yet to become famous.

one path would be to replace all the small signal transistors in the power amp.

2. The 430 preamp and power amp circuits are nearly identical to the 330c. The big difference between the two is the dual power supply in the 430.

so, when looking for prior advice, you can search for 330c, too, knowing most all those threads and posts about a 330c will apply to your 430, too.

 

[rickl]

@Drumcan, since you are new (Welcome to the list!), you probably don't have dlucy's HK330c restore bookmarked like I do. here it is.

 

[Drumcan]

Hi all! So I was able to solve this, and it turns out this had nothing to do with the main amp board.

First, I replaced all of the small transistors in the right channel on the main amp board that I hadn’t already replaced (Q401 and Q422), but it still didn’t work. I was about to give up when I decided to look at the pre-amp board.

And that’s when I noticed transistor Q505.

It’s hard to describe what it looked like, but basically the transistor was sitting high above the board, and all of the legs were bent inwards to the point where the collector and emitter legs were touching. It looked almost like it had one leg. I took a screwdriver and wedged it between the legs and then bent them apart. I attached an “after” picture that gives you and idea of how far it was sitting up.

I have no idea how or when this happened. I’ve never been anywhere near that part of the amp, and it’s actually a pretty hard area to get to. Also this just started happening out of the blue, no shifting of the amp caused it.

but whatever the case, this is solved! Thanks for the tips.