Harman Kardon 430 woes continue

[jpreza]

Good evening everyone,

After resoldering all the joints on the amp board, I rechecked the voltages on the replaced transistors to make sure the solder is good. Most of the voltages read between 450mV and 550mV.

After turning it on, the audio is worse than ever. A loud humming sound coming out of the left channel; volume on the right channel is barely audible. If I turn the volume up or adjust the treble or bass, a high-pitched sounds comes out of the right channel.

After turning the unit over to look over the connector side, I noticed that the FU3 fuse was blown, so I said, what the heck?! I had previously replaced it; thus, I put a new 3A fuse again. After turning the unit back on again, there was no change to what I described above. I also noticed that the FU3 fuse was getting hot (i.e. red in color) as if it was gonna go again, so I turned the unit off.

Also, the output transistors for the right channel are hot to the touch and the thermal grease is almost steaming after maybe 3-5 minutes of the unit being on.

And of course, there is still no audio at all coming out of the left channel (except for the humming sound previously described).

Any ideas what to do next?!

TIA...Joe

 

[cademan]

Make sure that the transistors you replaced have the proper BCE orientation. Some new replacements are different from the originals.

Also check the drivers, emitter resistors and pre drivers.

 

[avionic]

Also, the output transistors for the right channel are hot to the touch and the thermal grease is almost steaming after maybe 3-5 minutes of the unit being on.

Check/Adjust the bias..

 

[jpreza]

Make sure that the transistors you replaced have the proper BCE orientation. Some new replacements are different from the originals.

Also check the drivers, emitter resistors and pre drivers.

Thanks, I did double check the transistors - the Q401-Q406 transistors were replaced by BC550Cs, which I know have a different orientation than the original. Also, these were replaced a while back. At least before this "heating" issue started.

 

[jpreza]

UPDATE: While trying to adjust the bias for the output transistors, I've discovered additional issues to work on and beyond my expertise:

1. Upon turning on, the FU2 fuse next to Transformer T1 pops right away
2. FU3 fuse gets red hot after less than a minute and then pops unless the unit is turned off

From reading other AK postings, it may be that one of the transformers has gone bad. I'm leaning towards chucking/parting it at this point. @EastPoint will be my guide if it's still salvageable.

 

[EastPoint]

Just bring it over.

 

[jpreza]

Just bring it over.

Thanks! I'll drop it off tonight or first thing tomorrow

 

[EastPoint]

Two blown fuses. One incorrect 3A fuse underneath in place of a 0.5A fuse. The KSC1845s were in backwards. Some burned traces. I replaced all the transistors on the amp board and the trimmers as well. Fixed the traces as best I can.

It’s still doing the thing where it hums on both channels. Dim bulb lights but not super-bright. It does this on both channels despite me pulling pretty much any fuse. Very odd. Will keep working on it tomorrow.

 

[jpreza]

Two blown fuses. One incorrect 3A fuse underneath in place of a 0.5A fuse. The KSC1845s were in backwards. Some burned traces. I replaced all the transistors on the amp board and the trimmers as well. Fixed the traces as best I can.

It’s still doing the thing where it hums on both channels. Dim bulb lights but not super-bright. It does this on both channels despite me pulling pretty much any fuse. Very odd. Will keep working on it tomorrow.

Thanks for the update!

 

[EastPoint]

Two bad diodes on the amp board. A surprising number of capacitors on the amplifier board were extremely bad as well. One of the 1000 µF caps measured 167 µF. One of the 47 µF caps measured around 20. Still have a hum issue but hopefully I am narrowing it down somehow.