It's the relays clicking out, both channels at the same time. I need to research how the protection circuit works, assuming that is the reason.
Harman Kardon HK770 Thread
- Thread starter Connor G
- Start date
chrisz1957
New Member
Recently retired and am putting my music room together including these purchased in 1981
1981
1981
Blue Shadow
Waiting for Vintage Gear from this century
Hoping someone wise can assist here.. I've rechecked and in one channel the DC offset, measured at speaker terminals, no load is around 1-3mV on one channel, but the other is jumping around all over the place, from 0-70 mV. The trimpot is extremely sensitive, I have put deoxit in it, but blowing air on it will make it jump around and it's very hard to get it close to zero to start with.
wondering if I should change the trimpot (I bought some anyway) or if there's other areas I should look at.
Last edited:
dfoulk
Active Member
I would
jump around like that, it is surely in need of replacing as a first step. It would also be good to make you a dim bulb tester if you don't have one.
You can do a search to get how to make, but very valuable to have when trouble shooting to limit possible damage to the amp when you test out repairs.
replace the trimpots, both while you are in there. Make a note of their resistance setting before you take them out. If blowing on the pot makes it
jump around like that, it is surely in need of replacing as a first step. It would also be good to make you a dim bulb tester if you don't have one.
You can do a search to get how to make, but very valuable to have when trouble shooting to limit possible damage to the amp when you test out repairs.
Thanks. I do have a dim bulb tester that I built when I first started on this amp a while back. I will do the pots, the only reason I didn’t change already was because I wasn’t so sure of the quality of the new ones. https://au.element14.com/amphenol-p...103a2020-s/trimmer-10k-0-15w-1turn/dp/3128480
dfoulk
Active Member
I like the multi-turn trimmers though they cost more, but they are way less sensitive and easier to adjust than the single turn type. They are also sealed
so less immune to dust. Fortunately I seldom need to buy them!
Thanks - I also looked at those but they didn't have the side adjust version in stock. I did get these ones for the 2.2k: https://au.element14.com/bourns/3386h-1-222lf/trimmer-pot-2-2kohm-10-1turn-th/dp/2328540
All trimpots have been replaced but the problem remains. I’m following up suggestions from another site.
dfoulk
Active Member
I don't have the schematic, but I hope that you will find the culprit soon. Sometimes the input pair of transistors can cause offset problems
when they aren't closely matched, or drifted from age. It is good that you have the good channel as a reference to trouble shoot.
I've exhausted my troubleshooting abilities...I am happy to start swapping out transistors but I don't know where I can buy new replacements (I have done all the caps and a few resistors too)
I'm on the other side of the world and have been chasing an issue for a long time of the amp dropping out. I've replaced caps, resistors and resoldered, and. Ow down to replacing transistors. Have you found any common failures on your repairs, and do you know where I can source new transistors ?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:
This is easily remedied by reflowing all of the joints on the driver boards.
Here is a before and after view of the reflow:
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.
Hi team,
I have a Harman Kardon HK770 that I'm trying to fix up and put some real usage into. I've recapped the power and driver boards, and re-flowed most of the solder already.
My issue is that the Left channel runs hot pretty fast, and when I check the output with a multimeter I'm finding that it is outputting ~6v DC with no load or speakers. That is the lowest the POT will go on that channel. Right channel seems to work perfectly fine and can be adjusted to ~0v DC with the POT.
I'm also not getting any voltage when trimming for bias on Lch, while Rch works as intended.
I've tried swapping transistors on the driver boards from Lch to Rch and haven't been able to narrow down the issue.
I have already replaced a few choice transistors but they haven't made a difference.
Any ideas?
I have a Harman Kardon HK770 that I'm trying to fix up and put some real usage into. I've recapped the power and driver boards, and re-flowed most of the solder already.
My issue is that the Left channel runs hot pretty fast, and when I check the output with a multimeter I'm finding that it is outputting ~6v DC with no load or speakers. That is the lowest the POT will go on that channel. Right channel seems to work perfectly fine and can be adjusted to ~0v DC with the POT.
I'm also not getting any voltage when trimming for bias on Lch, while Rch works as intended.
I've tried swapping transistors on the driver boards from Lch to Rch and haven't been able to narrow down the issue.
I have already replaced a few choice transistors but they haven't made a difference.
Any ideas?
swapping the pots is often recommended, did you try measuring resistance across it?
Asmodeus2112
Super Member
Are you still working on this? I have a 770, and a couple of 775's. One of my 775's behaved very similarly. Bias at 0mV and too much DC offset. On my unit it was due to damage to the traces of the driver TR due to shipping. I think you are going in the right direction with the driver TR area.
With some more poking around and swapping the pots, I did get the output to cool down to 0V. I did not find a solution for the Bias as it was still showing 0mv on Lch. I closed it up for now since I fixed the majority of issues. I looked pretty diligently for damage do any traces but couldn't find any.
Back into this again afterward completing a few other projects. My son was gifted a scope for doing well in a Uni exam so I’ve been able to investigate a little bit more.
Symptoms - distortion in one channel at start up, once warm will play ok as long as the volume isn’t turned up too high. After about 50% the protection circuit kicks in and the speaker relay switches off until the volume is reduced again.
I put in a 1kHz sine wave, which was clean until the volume was increased, and then saw it being clipped on one channel at the speaker outputs.
Starting at the preamp, I traced my way through and everything was good until just after the differential amps (uPA74/uPA75).
I understand these are no longer available so will try replacing with KSC1845 / KSA992 as mentioned elsewhere. I guess it makes sense to do both in each channel.
Symptoms - distortion in one channel at start up, once warm will play ok as long as the volume isn’t turned up too high. After about 50% the protection circuit kicks in and the speaker relay switches off until the volume is reduced again.
I put in a 1kHz sine wave, which was clean until the volume was increased, and then saw it being clipped on one channel at the speaker outputs.
Starting at the preamp, I traced my way through and everything was good until just after the differential amps (uPA74/uPA75).
I understand these are no longer available so will try replacing with KSC1845 / KSA992 as mentioned elsewhere. I guess it makes sense to do both in each channel.