Occassional LOUD cracking and poppin in HK 430 Twin Power

[da_kube]

Got this gem from a coworker. Sounded like lightning in a bottle when I powered it up. He said a channel was out 30 years ago when he shelved it. Worked fine for me, except that it had very loud cracking and popping coming from all channels and even at zero volume.

So I cleaned it like crazy with Deoxit and it still sounded bad. Months later, I power it on and it's not nearly as bad. It might go 10 minutes without acting up then it starts humming and doing that loud cracking. It sounds similar to dirty pots, and these are a bit hard to get to, so that might be part of it. But it's weird it's so loud at no volume.

Anyone have a place to start troubleshooting? It was in a smoke-filled environment and was/is pretty dirty. Little bit of rust on the transformers and lots of grime throughout. Surprisingly, the meter still works. My HK 330c looks mint and the meter probably died 30 years ago on that. He said a channel died, so maybe the PA transistors might be another place to look. They are larger but easy to change. Also, both 3A 250 volt fuses are dead. Not sure how it's still working like that. I'm just listening with headphones but I assume those are important. Perhaps he bypassed those? I didn't see any obvious signs of that. Yikes.

 

[wmgwizard]

I have seen bad bias and idle current adjustment pots cause similar issues. I usually replace them as cleaning them usually does not good.

IIRC the 430 also uses quick release connectors from the main board. These are prone to dirt & corrosion along with weak solder joints on other the main and amplifier boards.

There may be other issues but these are a couple I have experienced.

 

[ryuuoh]

Agree, clean those connectors and switches, see what you got after that.

 

[SoCal Sam]

Could be main filter caps. Take a non-conductive probe and gently tap the tops. Do this with test speakers.

 

[da_kube]

All easy and cheap checks. I didn't know about the quick connects but that sounds more than plausible considering how dirty the unit was. I'll get spraying and report back. Thanks!

 

[da_kube]

Is it normal that the headphones and everything else works with all three 250v 3 amp fuses are blown?

 

[wmgwizard]

The fuses protect the main amp section that drives the speakers whereas headphones are usually driven by the preamp section of the receiver.

Have you tried replacing the blown fuses, if so, do they blow immediately ?

 

[EchoWars]

Actually, it would be highly unusual to have a dedicated HP amp when it's tons easier and cheaper to simply put a 330 ohm resistor inline from the power amp output to the headphone jack, which is how it's done on 99% of all integrated amps and receivers.

 

[ggregg]

Sounds like power supply caps to me although could be something else. I recently went through a similar issue with a Marantz integrated amp. Turned out being a noisy driver transistor in the PS in conjunction with a bad cap on the tone board. Ended up recapping the whole thing. The main filter caps (usual suspects) were fine. Took a while to trace it out and find exactly what it was.

 

[JURB]

Sounds more like the main voltage amp(s) or the diff (longtail) pair(s) at the front. If the bias circuit opens up the outputs blow in seconds.

 

[wmgwizard]

Using my rudimentary skills looking at the schematic the fuses that the OP mentioned were blown may indicate blown outputs.

 

[da_kube]

I plan to replace those fuses as soon as I get them. That will tell a lot, I imagine. I'll do that and clean the quick connects and move on to the more expensive stuff if that doesn't work. Thanks for the help, everyone! I'll report back soon.

 

[ZOOM]

Replace 2sc458 transistors in power amp. Use ksc1845 instead.

 

[da_kube]

Replace 2sc458 transistors in power amp. Use ksc1845 instead.

Zoom, I read up on these and it does sound like the original transistors are prone to failure, creating lots of noise.

I didn't want to recap this immediately (full disclosure, I broke one channel of my HK 330c when I made a few rookie mistakes, the worst being cracking part of the power supply board and allowed two unclipped leads to touch on a cap.) Also, I found every single one of those Elna caps to be perfectly in spec. That's not to say that another receiver might benefit from a full recap, but it sounds like my 330C would have been better off without me touching it. Positive note: I am a lot more confident working on these things.

Now, I think I'll do a partial recap and replace those bad transistors. If I just use the phone and AUX stage, which boards should I care about? The radio and other channels won't get use, although I remember the radio board being very easy to get to and cheap to recap. I should do the power supply, preamp, phono and controls board (in the front of the unit) Anything else? Can I skip any of those?

Also, what about those big screw-in transistors that look like they belong to the main amp? Should I do those too or just see if they are working properly?