Gustav80swe
Active Member
Here is my Silver 5050 series. Love it so far. Going for my tech for some service and lamps.
Harman Kardon HK770 Thread
- Thread starter Connor G
- Start date
avguytx
Lunatic Member
This is the pair I'm ordering based off of his parts list above.
https://www.digikey.com/en/products...TV5-HP-DC24/Z2245-ND/765510?itemSeq=370851179
avguytx
Lunatic Member
@Connor G Do you think this Kemet 0.047uf cap from Digikey would work in place of this Kemet 0.047 cap from Mouser? The main difference I see is that the AC voltage is rated at 250v instead of 300v. Mouser is backordered on the relays and 10uf 25vElna's and won't ship for awhile. Digikey currently has everything else.
That one should work just fine!
Gustav80swe
Active Member
This was a very good combo. Silver 5050 and JBL 4412.
Snowpuppy2
Active Member
I have been an avid proponent of Harman Kardon HK770 power amplifiers ever since restoring one for a friend and hearing it in my system. I can't recommend them enough.
View attachment 2142019
The HK770 was one of Harman Kardon's highest end models in the early to mid 80's, being designed by Matti Otala, with it's design based around his findings on T.I.M. and I.I.M. distortion.
More info can be found here:
http://www.thevintageknob.org/harman_kardon-hk770.html
I have restored 3 of these amplifiers so far, and have somehow managed to convince enough friends to buy them that I currently have 3 more on my workbench for a full restoration.
View attachment 2141979
These amplifiers are DC coupled, ultra high bandwidth, use minimal NFB, fully discrete from end to end, dual mono, high slew rate (150v/μs!), minimal THD/TIM/IIM, protection circuit out of the signal path... Not to mention a breeze to work on and they fly very low under the radar, often selling for less than $200!
View attachment 2142028
Here is a 100hz square wave with the amplifier loaded down into 8 ohms:
View attachment 2141981
Pretty impressive if you ask me.
If you are looking for a bill of materials for restoration, I have one here which includes new output relays, electrolytic caps, as well as the film caps for the input coupling modification:
View attachment 2215148
One of the main issues with the HK770 (as well as most other amplifiers from Harman Kardon of this era) is bad soldering. This is one of many broken solder joints on the (very hot) driver boards of the amplifiers:
View attachment 2141994
This is easily remedied by reflowing all of the joints on the driver boards.
Here is a before and after view of the reflow:
View attachment 2142002
As for the sound, the HK770's are extremely dynamic (even given their conservative 60WPC power rating)
They lay out a very precise and believable stage with great imaging, crystal clear and non-fatiguing highs, liquidy midrange and very fast and authoritative bass. They aren't the last word in absolute resolution, but they are much more than enough for most people looking for a sub $500 amplifier all-in.
I have also personally used HK770's to power high end headphones very successfully, as the noise floor is almost non-existent. They especially work well with power hungry planar headphones. Even electrostatic headphones with a transformer based energizer sound exceptional on the HK770.
Alright, rant over. Hopefully I have convinced at least one person to give an HK770 a shot if they're in the market for a very cheap and very competent "High end" amplifier
Anyone else have experience with the HK770's (or 775's)?
Thanks for the awesome info! I picked up a 700 series set consisting of a HK 715 tuner, HK 725 preamp & a HK 770 amplifier locally around a month ago for basically nothing (less than nothing actually, but that’s a different story lol), and while life has kept me from really being able to sit down and really see what it can do, I have hooked it up and it sounds pretty awesome from what I’ve heard so far. The pre could probably use a little bit of Deoxit on the volume control, but it only acts up a tiny bit once in a while, nothing terrible, so that’s not a big deal, and I have plenty of Deoxit so it’s ok. I also had expected the amplifier to run hotter from what I’d read, maybe it’s just because I haven’t really listened to it at high volume levels yet.
Anyway, since you have experience with these, do you recommend I crack open the amp and check the board and caps, or just leave it alone if it’s running just fine? I’d rather be proactive if there’s going to be an issue, and as far as I know they’re bone stock. I got them from the original owner of them, and based on the impression I got they were well loved and used quite a bit, but never abused. I’d appreciate your thoughts if you have any. Thanks!
avguytx
Lunatic Member
Are you talking about that relay on the 725 preamp? I'm wondering if that may be the culprit on mine as well. I'm on vacation this week so I'm piddling with some stuff around the house so I got the 725 and 770 out and hooked up. I did some basic cleaning and used Deoxit on the balance control but that sealed Alps volume control is rather scratchy. Also, when music is playing, it sounds like the woofers are blown on the small speakers (little Audio Pro's) that would change in intensity while playing. No actual switches or pots seemed to change it either. I know it's the preamp as I swapped it out to a rebuilt Carver C-1 and no more "distortion".
The amplifier "works" but their seems to be a little channel imbalance and it also makes a speaker move in or out, depending on phase, and stay at that spot but still plays. It's 40 years old so I know it's going to need some things swapped out on it as is the preamp. My thoughts are to get them working and use them in the garage. That or maybe find two HK775 mono amps and do a biamp setup on my Klipsch Epic CF-3's with the 770 on the horns and the 775's on the woofers. Hmmm.
But for the moment, looks like I need to get a parts order together sometime soon.
Take it from me who’s working on a pair of 775’s right now. You don’t want these amps. These amplifiers are not for the faint of heart. They are so full of issues that I don’t think I will take any more of them in for service or restoration.
Factory soldering is the absolute worst I have seen after a few years of doing this. There are components that are literally hanging off of the boards. HK also massively undersized many resistors in the driver and pre driver stage, and didn’t raise them off of the board. Lots of lifted traces and heat damage from years of heat stress and thermal cycles. Also, the trace sizes for some of the high current devices is comically small. It’s almost amazing someone greenlit these amps. Not to mention tons of deviations in the service manual, schematics, and board illustrations. Straight up wrong parts called out in the schematic. These are a literal nightmare to work on.
Factory soldering is the absolute worst I have seen after a few years of doing this. There are components that are literally hanging off of the boards. HK also massively undersized many resistors in the driver and pre driver stage, and didn’t raise them off of the board. Lots of lifted traces and heat damage from years of heat stress and thermal cycles. Also, the trace sizes for some of the high current devices is comically small. It’s almost amazing someone greenlit these amps. Not to mention tons of deviations in the service manual, schematics, and board illustrations. Straight up wrong parts called out in the schematic. These are a literal nightmare to work on.
Pre-driver card of one of the 775’s. This is factory soldering after years of heat stress. Absolutely abysmal. I’m surprised these amplifiers even worked to begin with.
After reworking one of the boards and fixing some traces. Also mounted the drivers on new silpads. Much better than the original dry thermal compound.
Before and after reworking one of the main boards. Again, terrible factory solder quality, and boatloads of nasty flux.
Same goes for the front VU control board.
After reworking one of the boards and fixing some traces. Also mounted the drivers on new silpads. Much better than the original dry thermal compound.
Before and after reworking one of the main boards. Again, terrible factory solder quality, and boatloads of nasty flux.
Same goes for the front VU control board.
Dahlquisted
New Member
Connor G - Can you share the part number for bias trimmer pots you used during this restoration?
i’ve got one of these systems too, bought second hand a couple of years ago. unfortunately lost one channel on the power amp followed rapidly by the other and haven’t had a chance to look at what the issue might be.
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.
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.
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.
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.
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.
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.
Gustav80swe
Active Member
Good job!
Really like your dedication to these wonderful pieces!!
I will probably do the same someday with my Silver. Unfortunately I live in Sweden and it is a bit far to send to you.
Happy new year!
Really like your dedication to these wonderful pieces!!
I will probably do the same someday with my Silver. Unfortunately I live in Sweden and it is a bit far to send to you.
Happy new year!
PabloTincho
Super Member
Can the op coments on the HK825 and the HK870 ? are those any good, I know is the next model
I have the HK825 and HK870 driving my Cornwall II's. I Love these amps! They both had very little use and are really performing well for me.
Lou F Quincy
Super Member
Those tuners are cool, too. I love how the lighting changes color when tuned in.
PabloTincho
Super Member
I have been an avid proponent of the Harman Kardon hk770 power amplifier ever since restoring one and using it in my system. I can’t recommend them enough.
View attachment 2142019
The HK770 was one of Harman Kardon's highest end models in the early to mid 80's, being designed by Matti Otala, with it's design based around his findings on T.I.M. and I.I.M. distortion.
More info can be found here:
http://www.thevintageknob.org/harman_kardon-hk770.html
I have restored 3 of these amplifiers so far, and have somehow managed to convince enough friends to buy them that I currently have 3 more on my workbench for a full restoration.
View attachment 2141979
These amplifiers are DC coupled, ultra high bandwidth, use minimal NFB, are fully discrete from end to end, dual mono, have high slew rate (150v/μs!), minimal THD/TIM/IIM, protection circuit out of the signal path... Not to mention a breeze to work on, and they fly very low under the radar, often selling for less than $200!
View attachment 2142028
Here is a 100hz square wave with the amplifier loaded down into 8 ohms:
View attachment 2141981
Pretty impressive if you ask me.
If you are looking for a bill of materials for restoration, I have one here which includes new output relays, electrolytic caps, as well as the film caps for the input coupling modification:
View attachment 2215148
One of the main issues with the HK770 (as well as most other amplifiers from Harman Kardon of this era) is bad soldering. This is one of many broken solder joints on the (very hot) driver boards of the amplifiers:
View attachment 2141994
This is easily remedied by reflowing all of the joints on the driver boards.
Here is a before and after view of the reflow:
View attachment 2142002
As for the sound, the HK770's are extremely dynamic (even given their conservative 60WPC power rating)
They lay out a very precise and believable stage with great imaging, crystal clear and non-fatiguing highs, liquidy midrange and very fast and authoritative bass. They aren't the last word in absolute resolution, but they are much more than enough for most people looking for a sub $500 amplifier all-in.
I have also personally used HK770's to power high end headphones very successfully, as the noise floor is almost non-existent. They especially work well with power hungry planar headphones. Even electrostatic headphones with a transformer based energizer sound exceptional on the HK770.
The hk770 is an extremely versatile amplifier. It has no issues running down into 4 ohms, and have had multiple speakers which dip closer to 2 ohms that run perfectly as well. The hk770 is a class AB amplifier which has an idle current of around 24mV. This means that the amplifier while being able to run in class A for the first few watts, actually runs cool until pushed. I’ve noticed the hk770 is very revealing of the signal chain and is very dependent on which preamplifier you have hooked to it. If you’re using the power amplifier with a passive preamp or plugged directly into your source (DAC, CD player, etc…), you will not get as good of a performance as you will with a preamplifier that has an active buffer and gain stage.
Alright, rant over. Hopefully I have convinced at least one person to give an HK770 a shot if they're in the market for a very cheap and very competent "High end" amplifier
Anyone else have experience with the HK770's (or 775's)?
I have since started offering my restoration services for the hk770 (and any other amplifier for that matter) for people looking to have theirs restored the right way. Shoot me a PM if you’d like me to take a look at yours.
How do I know If my preamps ( Audiolab 8000C and B&K MC-101 Sonata ) have a active buffer and gain stage ?