Harman Kardon 430 Issue

[ivandezande]

Got this guy with both rail fuses blown on a channel with no explanation as everything tested fine.
New trimmer, fuses, and small signal transistors (KSC1845) installed in both channels.

Now I get a nice loud hum (not a small 60Hz hum) out of the broken channel, and it won't bias. Offset is fine.
I replaced D407-1N4004 and the rest with 1N4148, the bias diode with two of course. It got quieter so that I could actually put the headphones I was using to test with on, but it's still there at a volume as loud as music I would listen to.
Pulling the outputs had no effect.

 

[ivandezande]

Anybody? I'm really stumped on this one, even looking at all the joints a month later didn't get me anywhere.

 

[dlucy]

Did you start by validating all power supply outputs were at or very close to their specs?

 

[ivandezande]

Did you start by validating all power supply outputs were at or very close to their specs?

The negative rail of the messed up channel is at -19V instead of the correct -29, something is dragging it down. The positive rail is fine.
I've double checked transistor installations compared to the other channel and I think everything is fine.

 

[mbz]

The negative rail of the messed up channel is at -19V instead of the correct -29.

That's a bad sign, that's the unregulated supply, points to a problem in either rectification, main caps or a downstream near short
that's dragging the voltage down. On DBT does the bulb glow (downstream short, leaky cap?) Consider disconnecting the +/-B
connections to the power amp and see if the power supply normalises (don't unwrap wire wrap...)

 

[dlucy]

Ok, that's where you start: get the power supply outputting the correct voltage for every output (going to another board).

Take a look at your schematic: are there any voltages marked next to components? If so, check each of those, watching for a different voltage than the schematic.

 

[ivandezande]

That's a bad sign, that's the unregulated supply, points to a problem in either rectification, main caps or a downstream near short
that's dragging the voltage down. On DBT does the bulb glow (downstream short, leaky cap?) Consider disconnecting the +/-B
connections to the power amp and see if the power supply normalises (don't unwrap wire wrap...)

No short on DBT, when the channel isn't connected the rail is still at -19-20

 

[ivandezande]

Ok, that's where you start: get the power supply outputting the correct voltage for every output (going to another board).

Take a look at your schematic: are there any voltages marked next to components? If so, check each of those, watching for a different voltage than the schematic.

No such luck, but I can compare with the working board.

 

[dlucy]

You're in luck as the part of the power supply now in question has very few parts to test:



I would suspect the very old 4700 uF electrolytic caps, so test those.

If they are ok (you should consider replacing them anyway), then test the four double diodes that make the two full wave bridges. Any of those individual diodes not working right?

 

[ivandezande]

You're in luck as the part of the power supply now in question has very few parts to test:

View attachment 1444084

I would suspect the very old 4700 uF electrolytic caps, so test those.

If they are ok (you should consider replacing them anyway), then test the four double diodes that make the two full wave bridges. Any of those individual diodes not working right?

I put in new filters already, I bet it's the diodes. I'll go check.

 

[dlucy]

I put in new filters already, I bet it's the diodes. I'll go check.

Ok, if you've replace the reservoir caps recently, then triple check you've got their polarity right to the PCB. Ive flubbed that many times and scratched my head for hours...

 

[ivandezande]

Ok, if you've replace the reservoir caps recently, then triple check you've got their polarity right to the PCB. Ive flubbed that many times and scratched my head for hours...

It passes DBT, so they're for sure in right but thank you, I've done that once or twice and I know exactly what you're talking about.

Bad news: put in a new bridge and same issue, negative rail is 10 volts lower.

 

[dlucy]

It passes DBT, so they're for sure in right but thank you, I've done that once or twice and I know exactly what you're talking about.

Bad news: put in a new bridge and same issue, negative rail is 10 volts lower.

Ok, so here is what you know: the outputs coming of that board at B1+, B1-, B2+, and B2- are not all as they should be.

So, the rest is kinda easy: from those four outputs to the AC wall socket something is amiss. Pull everything and test it. You may have a bad fuse, bad wire, bad solder joint, bad cap, bad diodes, or bad transformer.

By "bad", I don't mean 100% failure. Could be part of one of the secondaries windings in the transformer is bad. A ceramic cap is failing. A joint is iffy.

Good thing is you know where to look, all are easy to test, and ther ain't many of them.