Harman Kardon HK770 Thread

[The Plumber]

Sorry if this is the wrong place to ask, but it seems like there are bunch of people quite knowledgeable about the 725/770 here.

Only if you were certain that the 770 wasn't the culprit would this be the wrong place! Which, as it happens, I don't think it is (the culprit). I think it's clear that it's your preamp that has a problem.

The right channel on my 725 plays very low volume all the time regardless of input. It is unaffected by balance or volume control. Impossible to mute or raise the volume. It's the same on headphones connected to the 725 and speakers through the 770. The left channel plays fine and as you would expect.

That's pretty specific. Do you hear any valid signal on Right? Do you hear scratching in either channel when you rotate Volume or Balance? Any level change on Right at all with either one? It's possible that one or both of those potentiometers is dirty or damaged. Or maybe that one of the Loudness or Subsonic switches is crusty. Resistances in that region could all be verified.

If the main output stage downstream of the volume/balance circuits was damaged, I'd expect the volume of even the damaged signal to change with volume and balance settings.

 

[Blue Shadow]

I don't know these pieces but the switches behind the buttons are hard to clean based on my readings of ak. Might have a channel stuck in one of those switches. The Idiot's Guide to Using DeOxit (revisited) | Audiokarma Home Audio Stereo Discussion Forums

Maybe someone has done a how to on cleaning those switches inside the unit, don't know. Did find a how to on disassembling a Marantz switch for cleaning a couple years ago. If you need to take the switch apart, Harman Kardon HK 725 Preamplifier - Repairs (Ep. 46) - YouTube

 

[BS101]

Only if you were certain that the 770 wasn't the culprit would this be the wrong place! Which, as it happens, I don't think it is (the culprit). I think it's clear that it's your preamp that has a problem.

Thanks for the response. I think you're right. I've created a new thread here.

 

[MWH901]

Quick question... I just got one of these off eBay, non-working. It is the multi-voltage version. It came with a Euro power cord with a matching heavy-duty US adapter. The voltage selectors were set for 120V and it had the proper size fuses installed for US voltage. Can anyone confirm if this is how these models came from the factory? It looks like it could be the original factory cord.

Edit: Forgot to mention, it has a standard US 2-prong convenience outlet on the back.

 

[Blue Shadow]

An HK delivered in the US would not have had all that. It might have even been built with just 120V transformer. That is probably an overseas unit with switchable voltage originally delivered to a country that uses the plugs the end of the cord. Owner probably set it up for US power at some point.

Just doesn't make sense to add extra parts to units I the biggest market that are probably not going to be used. Many (most?) brands had North American versions without switchable voltage

 

[MWH901]

Just to update... I've got this one fully restored (excluding relays or other back-panel mods), and I agree that is an absolutely fantastic amp. I've paired it with both an Adcom GFP-565 and a NAD 1600 preamp Leaning toward the NAD at the moment. It just hits on all cylinders... detailed without being shrill, warm, solid bass. Highly recommend!!

 

[sancho22]

Cool
Inspired by this and other threads I could not resist
My pair of HK775, 725 and 715 arrived…
I had the sort of matching TOTL cassette deck CD491
The tuner is already playing (arrived last night) the rest is warming up…

 

[sancho22]

I am about 1 hour into listening. I have to say I am pleasantly surprised so far - especially with vinyl playback. Using my tricked out Bogen 62 and the 725 into the 775 and high sensitivity speakers.
Really nice full sound, detailed no harshness
Will give my Kenwood L series stack a run for its money I think
Thanks guys for all the great Information

 

[sancho22]

Quick pics

 

[bigskyaudio]

I am looking for a really mint HK770. It does not need to be recapped as I can do the work myself. Prefer a 120-volt US model. Please pm me if you have one available.

Rick

 

[jsdc]

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.

View attachment 2438189 View attachment 2438190 View attachment 2438191

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.

View attachment 2438197 View attachment 2438195 View attachment 2438196 View attachment 2438198 View attachment 2438199 View attachment 2438200 View attachment 2438201

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.

You convinced me to buy one of these HK770. Thanks for that. I love it. I am working on getting it restored through one of my friends. He owns a Northwest Audio store in the Washington area. Can you please tell me what is the replacement part for this on HK770?

It's a dual transistor on the main amp P.C. board.

 

[Zackobrien]

Hi there,
Recently came across these 770’s and fell in love. Wanting to give it a try in my system. It consists of:
RSL Elan
APT Holman
SMSL SU-9N

If anyone has one for sale (I’ll drool over a pair of 775’s) at a price a frugal college student can afford, please let me know!

 

[Aymeric]

Hi,
I recently purchased an HK 770 and have some repairs to do on it. I'm new to electronics.
I have to change the 2SD666A (Q413) and the 2SB646A (Q415), i see that some of you already replace them. Do you know a modern equivalent ?
also have to replace a broken diode 1SR35-10 on the output P.c Board (D14), i can't find datasheet, do you have any idea for replace ?

Thanks

 

[joelll]

Hi,
I recently purchased an HK 770 and have some repairs to do on it. I'm new to electronics.
I have to change the 2SD666A (Q413) and the 2SB646A (Q415), i see that some of you already replace them. Do you know a modern equivalent ?
also have to replace a broken diode 1SR35-10 on the output P.c Board (D14), i can't find datasheet, do you have any idea for replace ?

Thanks

Welcome to Audiokarma!

I recommend you ask these questions in our Solid State forum, where there are a number of active audio repair people who could help with your questions.

 

[MWH901]

Hi,
I recently purchased an HK 770 and have some repairs to do on it. I'm new to electronics.
I have to change the 2SD666A (Q413) and the 2SB646A (Q415), i see that some of you already replace them. Do you know a modern equivalent ?
also have to replace a broken diode 1SR35-10 on the output P.c Board (D14), i can't find datasheet, do you have any idea for replace ?

Thanks

Others please correct me if I'm wrong, but I think the closest non-obsolete match to the 2SB646A/2SD666A is KSA1013/KSC2383.

Diode 1SR35-10 is probably actually a 1SR35-100 and would match to a 1N4002.

 

[Aymeric]

Thank you very much for your answer. I will try these references !

 

[Gustav80swe]

Time to give my Silver SA some attention in my garage.

 

[Kevviek]

I've fired up my SA5050 (Silver version of HK770) again recently and noticed 2 things:
1. One channel is scratchy for about 5 minutes until it warms up
2. It shuts down completely after about 20 minutes. If left to cool it will start up again, wither the scratchy channel for a while etc.

I'm going to resolder all the boards as that has never been done, and add a larger heatsink to the driver boards.

Does anyone know where to source the driver transistors: 2SA1111 and 2SC2591?

 

[Kevviek]

I've fired up my SA5050 (Silver version of HK770) again recently and noticed 2 things:
1. One channel is scratchy for about 5 minutes until it warms up
2. It shuts down completely after about 20 minutes. If left to cool it will start up again, wither the scratchy channel for a while etc.

I'm going to resolder all the boards as that has never been done, and add a larger heatsink to the driver boards.

Does anyone know where to source the driver transistors: 2SA1111 and 2SC2591?

Answering my own question. The scratchy startup was the relay in the pre-amp, now fixed.

I have recapped both amps, replaced the driver transistors, added a heat sink and made some value mods based on recommendations in other threads. The only remaining issue is that it will drop out for a second or two once warmed up, but only if the volume is high. Not sure how to fault find that one.

 

[Kevviek]

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