Harman Kardon hk870 power amp restoration

[dlucy]

I finally got to the hk870 sitting in my to-do stack and couldn't remember what was wrong with it when I bought it. The kind AK'er who'd sold it to me said "well, the transformer was delivering power but it wouldn't turn on" when I sheepishly asked "do you remember why you sold it?" Ok, no problem. That's a fine start.

This one has a HUGE, HEAVY, toroidial power transformer as it is 100W/channel and uses the HK dual-mono design. So, there are two separate transformer windings buried under a few pounds of wax inside the big can of a transformer. Connecting the primary to 120VAC through a fuse gave me 85VAC (unloaded) on both secondaries, so the problem isn't in the transformer. Whew! Having to buy a new or even used one at $150-$200 would have stopped this project dead. BUt the hk870 doesn't disappoint and I get to continue checking it out and restoring it to working order.

 

[dlucy]

So, if the power transformer is working as it should, but there is no power getting to the power supply... next place to look is the power switch.



Reading across the X2 safety cap I get a correct 125VAC when it's plugged in, but no power when the switch is depressed. Hmm, that probably means....



A well-scortched switch and horribly-pitted pair of internal contacts. Sigh. It makes sense on a high current high reservoir capacitance power amp that got a lot of use over decades.

Ok, I guess I get to add a new rear panel power switch and that fancy TRIAC power switch bypass mod....

 

[slimecity]

Hmmm - power switches normally switch the primary - if this is also the case with this amp then I would have thought if there is power at your trafo, then the switch is working OK.

Is there an inrush current suppressor for this amp? If so, I would look there next, that would fit in between the trafo secondaries and the powersupply.

Have you downloaded the service manual to confirm? Its free here - just register:

https://www.hifiengine.com/manual_library/harman-kardon/hk870.shtml

 

[dlucy]

Is there an inrush current suppressor for this amp? If so, I would look there next, that would fit in between the trafo secondaries and the powersupply.

No suppressor in this design. Thanks for the tip and the pointer. I always count on hifiengine for the manuals.

The problem, well, the FIRST problem in this unit was the scorched innards of the power switch. There was an overheated voltage-regulating transistor that needed replacing and a half dozen fusible resistors that had blown or at least overheated. More on that tomorrow.

 

[lightcapture]

I had that amp at one time. It is very very good as well as the next versions that came after it. I don't believe the 870 used a toroidal transformer but a potted transformer.

 

[NAD80]

Going to keep an eye on this one. Great pics dlucy.

 

[dlucy]

The 24V voltage regulator (transistor at Q4 2SB649 C) had obviously gotten way too hot and maybe damaged the nearby Zener. So, the transistor got a beefier upgrade (KSB546 Y with heatsink an silpad), the Zener got replaced with a beefier one, and the electrolytic caps got replaced.




Original device 2SB649 C

PNP
Power dissapation max Pcm 1W
Collector current max Icm -1.5A
Collector-base breakdown voltage V(br)cbo -180V
Collector-emitter breakdown voltage V(br)ceo -120V
Collector-emitter breakdown voltage V(br)ebo -5V
Transistion frequency fT 140 MHz
Package TO-126C
Pinout ECB
DC current gain hFE classification C 100-200

My repacement choice KSB546 Y

PNP
Power dissapation max Pcm 25W
Collector current max Icm -2A
Collector-base breakdown voltage Vcbo -200V
Collector-emitter breakdown voltage Vceo -150V
Collector-emitter breakdown voltage Vebo -5V
Transistion frequency fT 5 MHz (I don't think this matters for voltage regulation)
Package TO-220 (means I can add a decent heatsink and avoid some of the overheating problem evident on PCB)
Pinout BCE (so I'll have to remember to turn the new transistor around)
DC current gain hFE classification Y 100-200

 

[dapbmw]

I am working on the same amp now I have an exact replacement switch that works and I still get 0v when I turn in the switch but get 120v on not terminals on the power switch at off position so I am stumped what could be wrong as I do not get any power at all.

 

[Bigyank]

I am also working on this amplifier and would love to see where your switch rework looked like post and if testing completed. Mine works but definitely needs recapping.

 

[EngineerNate]

Here I came in expecting new juicy @dlucy content, only to find a latent project stuck in the queue.

 

[EngineerNate]

 

[dlucy]

I spent the last two weeks sequestered at my parents' house where the hk870 had been languishing... as @EngineerNate so aptly pointed out , so, I recapped six receivers and opened the 870 up to see where I'd left off.

I found I'd bypassed the front switch. So, I plugged it in to see what I'd left out and blew up... and dammit if it didn't come to life. Ok, I don't remember that but looking at horse teeth and all... plugged in a signal source, Schiit Modi, and some speakers... and it works. Huh. Did I finish this thing and not remember?

Opened it up... ah, I replaced everything, fixed a over-heated transistor... and stopped at replacing the bigger caps.

I think I was planning on replacing the transistors, but there sure are a lot of them... and they seem to work.. at least at working-at-moms-house listening levels.







I'm pretty sure I was planning on replacing them with MJL1302AG + MJL3281AG (EDIT: these are wrong), but they were on order... so I waited... and forgot.

Looking at the package, that cannot be the right replacements. I must have looked that up on a heavy DIPA night.

To do list:

Replace the power LED upfront



Order the two sets of bigger caps:

 

[pjrengineer]

Hi Dlucy. What are your thoughts on the sound of this amp? I’m either going to get one or the 770 or just use my 725 with a tube amp sometimes.

 

[dlucy]

It is still in the To Do stack, so I cannot attest to how it sounds.