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Harman Kardon HK770 Thread

Connor G said:
I have since started offering my services to people who would like their hk770 repaired or restored. Since creating this thread, I have serviced and restored over 15 hk770’s for friends, community members, and even fellow AK’ers. They seem to be one of the most common pieces to come across my workbench. The funny part is that almost every single one of them has performed perfectly even without preventative maintenance and restorative work. There are however many issues that plague the hk770, including corrosive glue and terrible factory soldering quality. In my experience, these issues have affected many of the hk770’s I have worked on enough to pose a threat to their stability and working condition.

Here is a severely corroded trace which I had to repair. This was caused by the corrosive glue they used in various parts of these amplifiers.

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As I type this, I have 2 hk770’s on my bench, and have more on their way to me from friends and other clients of mine. It’s becoming a tad bit redundant at this point. But ensuring these amplifiers will work for the next 30 years is very satisfying.

For the HK770, I use Elna Silmic II and Panasonic FM/FC for their ESR, impedance, and ripple characteristics, as well as high reliability. For the input capacitors, I use films, so no need for Nichicon electrolytics there. One place I use Nichicon audio grade caps is the power supply for the output stage as well as the driver stage. I use extremely nice Nichicon Gold Tune caps which I have used extensively on every HK770 I have worked on so far.

Here are a few photos of an hk770 I recently did a full restoration on, including binding post and input terminal retrofit.

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If you would like me to take a look at your hk770, just shoot me a PM and I’ll be more than happy to help you out, or even answer any questions you may have.
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Hello Connor. This is Randy Wilson in Vermont, Illinois. I purchased my HK 700 Series Triple stack 770 amp/725 pre-amp and 710 analog tuner back in 1981. I still have these units and will never part with them. They made my Heresy III Speakers really sing. HK Citations were some of the best units made, especially the Citation XX amp and matching pre-amp. My units were restored back in 2008 by a tech named Wayne in East Sautuckett, New York (Long Island). I would like to have my amp and pre-amp gone over and, more than likely recapped with inspection of the heat sinks, solder joints. This 770 amp was the baby brother for the Citation line. Hats off to the late/great Dr. Matti Otala !!! I'm currently using a newer Jim Fosgate designed Black Ice fusion/integrated F-22 Tube amp/pre-amp and Fosgate's F-159 Phono Stage. Fosgate's units are pretty good, but can't quite beat the performance of a dual mono-block amp like the 770 ! My HK units all function, but need a check up. I still prefer the performance of my solid state HK 770 and 725 units. Too bad you and I are not closer distance wise, as I would like for you to take a look at my HK amp and pre-amp.

Thanks !
Randall S. Wilson
e-mail address: prorec48@hotmail.com
 
I have a pair of H/K 775 monos that I had not listened to for a couple of years (too much gear!). But came across them in the basement the other day, and finally got off the dime and hooked them up. All I can say is wow. I had forgotten how good they were. Mine are still original, and work fine. Highs, mids and bass all there and balanced. Detailed but not fatiguing. Works well with budget and higher-end speakers.

I would be interested in recap / tune up, just to keep them going for a long time.
 
And one of, if not the best, performing cassette decks ever till date; are from this HK in black, silver or gold look available model range years. From the then entry level ones in this look, including the most expensive cassette decks. Neutral, dead quit and undistinctable from origional CD, recording copies they made ( with use of a type IV metal cassette tape and no Dolby engaged of course during recording and playback )!
Great succesfull components and receiver line from HK! And highly sought after, athough still affordable(ish).
 
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Hi!
These are my HK 775 monoblocks and the rest of the series except the preamp. The power amps were restored and upgraded with non electrolytic caps where possible. I really like them. The volume pot of the preamp drifted it's value and caused a very nociceable channel imbalance so I replaced it with a Parasound P3.
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Just got these 2 from my audio tech

Very good preamp, the tech had problems with the muting relay so he bypassed, distortions numbers went down, better than the original specs
Is a really nice set up

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I am going to test the HK725 against the best HK preamp the AP2500, all discrete circuit, the headphone too

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PabloTincho said:
Can t I bridge 2 770 ? is even possible ?
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Why not just use a single to differential adapter externally? Even a good quality line-level center-tapped transformer should do just fine. Since the 770 is reputedly comfortable driving 4 ohms, should be quite capable of driving 8 ohms when used in bridged mode.
 
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