Harman / Kardon 430 Troubleshooting Help

[BadChile]

A confession: I took something working and broke it.

 

Having picked up a Harman / Kardon 430 last week I decided to "fix it" as there was a bulging capacitor in one circuit in the amp.  In doing so, I got myself flummoxed and after two nights of poking around with a multimeter I'm lost.  I had previously changed capacitors in the power supply and verified the amp was working.

 

I've done the full switching of input cables, output cables, etc. and have isolated that the Right Channel works/measures fine, the left channel is not.

The +/- B2 on the right channel is right at the specified +/- 28.5 V, the left channel (+/- B1) is at 30.3V; this is within stated tolerance of 28.5V +/- 15%.

 

1) The Left channel is not discharging after power down / shut down - zappy zappy happens well after shutdown, whereas the Right channel discharges relatively quickly.

2) The Voltage Drop for measuring idle current - which should be around 25mV and is that on the Right Channel - measures at 0mV across resistor R428.

3) When initially rebuilt I followed the parts list / marckings on the board and used a 470uF capacitor for C408, however, the value installed was 1000uF.  I have since replaced it with a 1000uF cap; and I suspect this is unrelated to the problem as the right channel is fine.

4) All fuses measure zero resistance.

5) Transistors measure so far out of spec I ended up replacing with KSC1845FTA transistors.  I have verified the pinout as being correct.

6) Removed transistors measured below voltage drop with multimeter - see photo, attached, also photo of schematic.

 

So...techs of the Klipsch forum, before I go through swapping parts from left to right, swapping out more transistors, etc ...any ideas?

 

Thanks in advance.  If I get this fixed anyone in the Boston Metro area is welcome to check out the amp as I eat crow.

 

 

 

 

[captainbeefheart]

Both channels rail voltages should match, when they say +/-15% that accounts for fluctuating line power but the two channels should be very close to the same voltages.

 

The fact the left channel is 2v higher than the right tells me that the left channel isn't conducting current, which jives with why you have no voltage across the emitter resistor and also why when you power it down the voltage doesn't drain away, with no load the voltage will not drain the caps down quickly.

 

I know this isn't what you want to hear but if you do not know what you are doing you will most likely just waste a bunch of time and do more damage, no offense.

 

How are you testing the transistors and how did you determine "they are so far out of spec"?  Transistors work or they don't, that's it period, they do not have declining emissions like tubes. Their hfe or "beta" is the same throughout it's life varying only to input bias and base junction temperature.

 

BTW if you end up wanting help with it I am in the Boston area.

[BadChile]

Thanks for the input - I agree, definitely a broken path to earth/ground, and perhaps another clue is the +B1 side remained energized overnight but the -B1 discharged.  I figured the difference in voltages was due to the twin power supply and never once considered the broken circuit issue, so thank you.

 

To date I've been going through this in a poorly thought out and haphazard manner as my mini-lug grabber hooks were repurposed (in all fairness, I did the same to my dad when I was a kid, so I'm happy to see my kids take on that curiosity and 'borrow' my tools) so I was only checking where my alligator clips would fit, but I procured more mini-lug grabbers today and am planning on tracing the entire circuit over the next few weeks to identify the break point.  I'll post those results when I get them.

 

My own fault, this one.  Normally I do one thing at a time but let my ego get ahead and I did a bunch of parts all at once.  Lesson learned, but successful engineering doesn't teach many lessons.  This one did - my normal methodical workflow had purpose.

 

Thanks for the offer of help - I'm going to keep at this, see what I can learn.  Might be a few months between everything else in life!  I'll keep you posted.

[OO1]

15 hours ago, BadChile said:

 

Thanks for the offer of help - I'm going to keep at this, see what I can learn.  Might be a few months between everything else in life!  I'll keep you posted.

I think @captainbeefheart can definitely give you a hand ,  in finding out what's wrong , he's definitely qualified , and more so to avoid any incidental damages  to the amplifier , we all learn from others , and there is nothing wrong in seeking a 2nd opinion  , you're both in the Boston Area , and how lucky can you get ---after all this could could be the start of a Great Friendship --

[BadChile]

Thanks all for the help. After tracing the circuit, checking notes, and pulling all transistors, caps, and desoldering one leg each of diodes and resistors to measure each I have reason to believe I stupidly swapped the positive and negative supplies on the left channel (which still measures high at the PS).  Regardless of if that is what I did I fried D402, D406, and R442.

 

Replaced those three parts and sure enough, she's back up and running better than new.