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Harman Kardon hk770 Upgrade Project

Back in the '70s I have never been a fan of HK solid state amps. IMHO, they had a very analytical presentation. I bought this HK770 out of curiosity after reading this thread.
I didn't bother listening to it before the restoration, so I can't comment on its original sound.
Sound-wise, this is a high-end amp: balanced presentation, natural soundstage with life-like dynamics.

My main system consists of 805 SET monoblocks. Listening to the HK770 driven by a Luxman DA-250 Dac/Preamp in the second system, gives me equal satisfaction (all being relative).

The next upgrades are to modify the heatsink on the driver board and fit better rectifiers. I am not too sure about the multi-turn trimmers, I might
revert to the single turn.

In the restoration pipeline: HK Citation 22, another HK770, AU-717

Thanks for your contribution to our community.


IMG_3809.JPGIMG_3810.JPG
 
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Thanks for the comments about the main system.
I have rebiased the amp at 10mv and found the sound more textured. At 24mv, the voices seem compressed,
There might be a sweet spot in-between, the future will tell. I’ll leave it at 10mv for the time being.

Re: AU-717, i have saved the link.
 
Hi Leestereo, can you suggest a modern replacement for the output relays? There's no marking on them. Cheers
 
I haven't replaced the relays in my hk770; nevertheless, IIRC, the relays are labelled on side; with the relay installation in the hk770, the label is facing the bottom (they may have to be de-soldered to view the label).
 
Al Casey said:
Hi Leestereo, can you suggest a modern replacement for the output relays? There's no marking on them. Cheers
Click to expand...
I just started working on a HK770 myself, and have done some cross referencing to the original relays used. It seems that Omron G4W-2212P-US-TV5-HP-DC24 will be a direct drop-in replacement. I will be ordering them here soon and update when I get them.
 
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Omron G4W-2212P-US-TV5-HP-DC24 is a direct drop-in replacement. Works great with no issues. Same footprint as well.
It was incredibly hard to remove the old relays from the dual layer PCB, and I ended up pulling a couple of traces through the board which I had to repair.
I have never had such a hard time with removing relays in my life! :mad:
 
Al Casey said:
Back in the '70s I have never been a fan of HK solid state amps. IMHO, they had a very analytical presentation. I bought this HK770 out of curiosity after reading this thread.
I didn't bother listening to it before the restoration, so I can't comment on its original sound.
Sound-wise, this is a high-end amp: balanced presentation, natural soundstage with life-like dynamics.

My main system consists of 805 SET monoblocks. Listening to the HK770 driven by a Luxman DA-250 Dac/Preamp in the second system, gives me equal satisfaction (all being relative).

The next upgrades are to modify the heatsink on the driver board and fit better rectifiers. I am not too sure about the multi-turn trimmers, I might
revert to the single turn.

In the restoration pipeline: HK Citation 22, another HK770, AU-717

Thanks for your contribution to our community.


View attachment 1838250View attachment 1838251
Click to expand...
My hk 770 is getting tired and the left channel is not good anymore. Since I don't have the service manual nor the electronic schematic, I don't dare diving into it. Anyone can suggest me someone qualified to help me restore its beauty?
 
theophile said:
The circuit looks incredibly simple.
Click to expand...
I'm no expert, and I sent mine to an expert, after a year of chasing him for updates he told me he fixed it but then it blew up again in testing, so I got it back broken. I've had a poke around and found a transistor shorted (Q425) and two nearby resistors were cooked. One fuse had blown but there was no sound in either channel. Before I fix and power it up, is it likely that might be all that is wrong or should I keep searching?
 
taddeus said:
After some tweaking hour on hk870s, the only leak ring I have found was on the back to back EL input capacitors (2x Elna for Audio 220uF).
After getting rid of them (first shortening them) the sound opened a lot.
For safety ( :D ) now there are placed some MKP capacitors that, more or less, don't impact on sound as as much as the old ELs did.
I did not see the hk 770 electrical diagram but I suppose may be similar to hk870.

On the other hand, I didn't hear any change by recapping (and soldering additional by pass capacitors) on power supply unit.

Now some picture that can be helpful for someone.
back to back input caps
http://img683.imageshack.us/img683/8088/img0491o.jpg


by-pass caps
http://img860.imageshack.us/img860/6812/img0495a.jpg
On the bottom left you can see one of the (new) blue input MKP capacitors.
Click to expand...
Hello,
Thanx for the explanation, which values of mkp capacitors did you use for substituting
2x Elna 220uF?
Regards
 
Kevviek said:
I'm no expert, and I sent mine to an expert, after a year of chasing him for updates he told me he fixed it but then it blew up again in testing, so I got it back broken. I've had a poke around and found a transistor shorted (Q425) and two nearby resistors were cooked. One fuse had blown but there was no sound in either channel. Before I fix and power it up, is it likely that might be all that is wrong or should I keep searching?
Click to expand...
Just checking back in, I replaced the transistors and all good now...
 
I have one of these units and I've been very happy with it. Recently, I started getting pops in the left channel only. I had this unit repaired two years ago and I seem to recall the technician saying only the one speaker relay needed replacing, and I swear it was the right channel. Now, I am having the left channel popping. Is it possible this relay for the left channel could be causing this? I don't have a voltmeter to test what is going on with the speaker output on the right channel. The front display lights up on the left channel up to 50 watts, so I've disconnected the speakers to avoid any damage. I'm positive the technician replaced most if not all the electrolytic capacitors.

Any help with this would be greatly appreciated. Also, is the relay difficult to replace? I'd rather just do this myself at home as I have a soldering iron.
 
Hello:

It's been a few years since I did my refresh...trying to recall everything involved:

I started getting pops in the left channel only. - What do you mean by pops? Does this channel stay silent until you turn up the volume?
Many times the relay will break free with some volume...

I seem to recall the technician saying only the one speaker relay needed replacing - this never makes sense to me... why not replace them both?
Just like headlights on a car - they are going to fail within a short amount of time of each other.

Do you have some pictures of the interior?
(To confirm the extent of the work done by your tech)

The relays are on the speaker output board... some disassembly required.
 
There are 2 output relays in the hk770 for speaker A or B; each relay serves both the left and right channel. The popping noise may be due to failure of the input differential transistors.
 
traderitch said:
Does this channel stay silent until you turn up the volume?
Many times the relay will break free with some volume...
Click to expand...
Very well described. I have a 770 that exhibits exactly this behaviour - on one channel only. It was re-capped quite thoroughly within the last year or two, and the unit sounds great once that 'ear' pops... I've been pretty confident a relay is to blame, but haven't ordered replacements yet.
 
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