Harman/Kardon HK6500 restoration - need advice

[DaveK]

Hi,
i recently bought a cheapo but working HK6500 amp from the early 90s.
When i took it apart i noticed it was somewhat badly repaired, one of the channels' power BJTs had most likely blown up and were replaced by low quality knockoffs, not original Toshiba transistors. I checked DC bias which was OK, but the idling current was way off and even after i tried to adjust it, i couldn't get the right values (pot maxed out). Power supply voltage was OK (more on that later), nothing was overheating or burned out. Speaker output was hissy, crackled when adjusting the volume and had constant (not changing in volume) mains hum.
Issue number one is the power. When first measuring the voltage, 49.2V rails were at 54V. The amplifier was set to 220VAC, so i set it to 240VAC and measured my wall, which read 235Vrms. Then the 49.2V rails read the correct voltage. Could anything be damaged from that higher supply voltage the amp ran with before?
The hum could point to bad PSU filtering capacitors, which are both 13000uF 63V. I tried looking online, but i can't find exact replacement locally. I would get 12000uF 80V cap and put 1200uF 63V cap in parallel on the bottom of the PCB, would that be a good solution? I think that would provide even bit better filtering, not sure. Also, the big capacitors are glued down to the PCB. What is the best way to get them off? Acetone?
For the spare parts, i ordered full set of 4 NOS Toshiba transistors for both channels and i listed out all of the electrolytic capacitors and ordered replacement ones (All Panasonic caps).
Is there anything else i should do?
Appreciate any help or advice.

 

[BobK]

Sounds like you are doing the right stuff. The filter capacitor is not critical, I wouldn't bother paralleling two. If you can fit it, a slightly higher value would be better than a lower one. 15000 uF?

Bob

 

[DaveK]

Sounds like you are doing the right stuff. The filter capacitor is not critical, I wouldn't bother paralleling two. If you can fit it, a slightly higher value would be better than a lower one. 15000 uF?

Bob

I looked what kinds of caps are available from atleast somewhat reputable sources, i had option of 15000uF 63V or 12000uF 80V, both Nippon Chemi-con. The 15000uF cap was 40mm in diameter, that wouldn't fit on the PCB, so i bought 2 12000uF 80V and 220uF 63V that i will put in parallel. I'll have to extend the capacitor leads because the old caps were obsolete panasonic 4pin TH-A series with proprietary lead spacing. Still haven't manged to unstick the old caps from the PCB though, even desoldering them is hard, the solder wick can't pick up all of the old material. I also noticed some of the driver bipolar transistors were replaced, well, i hope they're not chinese knockoffs of the original toshiba ones.

 

[Harald Kapp]

desoldering them is hard, the solder wick can't pick up all of the old material.

Add some flux. If you lack separate flux, add some new tin with a flux core. This usually helps melt the olf tin, too.

 

[DaveK]

Add some flux. If you lack separate flux, add some new tin with a flux core. This usually helps melt the olf tin, too.

I luckily got them off. The solder joint was loose enough, i just had to rip off the plastic jacket and take the metal can out seperately. Got new ones 12000uF 80V, measured 11.50mF each, so i'll put some smaller caps with these in parallel.

 

[Harald Kapp]

so i'll put some smaller caps with these in parallel.

Imho that is not necessary. Electrolytics have a wide range of tolerance (+- 20 % is common). That means the original 13 mF capacitor could have been as low as 10.4 mF and still be within specs. A practical power supply design (I suppose Harman Kardon is not on the el cheapo side here) will work with even less capacitance to take into account ageing.
A single new 12 mF capacitor will work in this position for a long time, especially as you are operating it a good bit below the max voltage, even more than the original one (which was rated at 63 V only).

 

[DaveK]

Imho that is not necessary. Electrolytics have a wide range of tolerance (+- 20 % is common). That means the original 13 mF capacitor could have been as low as 10.4 mF and still be within specs. A practical power supply design (I suppose Harman Kardon is not on the el cheapo side here) will work with even less capacitance to take into account ageing.
A single new 12 mF capacitor will work in this position for a long time, especially as you are operating it a good bit below the max voltage, even more than the original one (which was rated at 63 V only).

Well, i read an article on amplifier power supplies and it was recommended to use more (usually 2 like 6000-8000uF) smaller caps because they provide better filtering than a single massive one. I got backup 1000uF 63V high quality caps that i'd like to put there.
Also, i measured the original capacitors and they are 12900uF 0.1ohm both. Should i keep them in or change them for the new ones? I'd rather keep them because they have proper mounting for the PCB, but ofcourse if new capacitors will improve the power and sound quality i will swap them.