New Harman Kardon project

[rossm]

Hi All
Just started new project Hk870 amp hk825 preamp tu615 tuner and cd401 cassette deck. Found on the auction site, was looking for a high current amp to drive my new infinity iiia speakers. Was impressed with the reported damping factor of 125. Plus I have been eyeing the cd401 cassette deck as one of the best around and the seller reported it as working.

First out of the box was the cassette deck and it was indeed working nicely. Next out of the box was the amp and no joy not even power lamps, even though the seller advertised it as working.
Opening the case I found what surely was the problem a burnt resistor

Removed the resistor and found it wasn’t burnt after all. It was the glue used in this equipment had eaten the exterior away. This resistor measured good but didn’t have the -24 volts it was supposed to have. I decided to replace the 2 caps and found this on the back side of PCB-4


I am thinking maybe fedex dropping the box and the blow fractured the ckt board?
Any suggestions on how to reinforce this ?

My search if the missing -24v continues until I find this

Notice how the 2 traces on the top are fractured with the break in the main board PCB-1.
The top trace carries -55 v to Q4 which then outputs -24 v. I bypassed the fracture with a wire and now the amp is working. Next steps are clean glue from the boards and order new caps.

 

[dlucy]

I have a 825 that is quite a mess inside. Corrosive glue in the power supply ate through resistors and diodes, causing catastrophic failure and burned THROUGH board. Still, I want to restore it as the 825 should be one of the nicest consumer-grade preamps HK made. I have two 870's to go with it and they only need recapping and cleaning up. Oh, and the power switch needs to be bypassed since it died. I'll put in the triac switch-saving mod when I get around to that.

Good luck! I'm gonna sit by and watch how you do so I can try similar when it's my turn!

 

[rossm]

I have a 825 that is quite a mess inside. Corrosive glue in the power supply ate through resistors and diodes, causing catastrophic failure and burned THROUGH board. Still, I want to restore it as the 825 should be one of the nicest consumer-grade preamps HK made. I have two 870's to go with it and they only need recapping and cleaning up. Oh, and the power switch needs to be bypassed since it died. I'll put in the triac switch-saving mod when I get around to that.

Good luck! I'm gonna sit by and watch how you do so I can try similar when it's my turn!

Hi Doug,
Thanks for checking in, I was reading old posts on hk repair and came across yours and was wondering if you had sorted out your issues.
My 825 has problems as well. The box powers up and Using headphones I can barely hear signals from the aux and tape inputs. I haven’t spent much time on it yet but guessing it might be the muting circuit

 

[rossm]

Spent some time cleaning up the glue today. Here is a before pic


And an after pic


Thinking the next action will be pulling power transistors and replace the heat sink compound

 

[dlucy]

Hi Doug,
Thanks for checking in, I was reading old posts on hk repair and came across yours and was wondering if you had sorted out your issues.
My 825 has problems as well. The box powers up and Using headphones I can barely hear signals from the aux and tape inputs. I haven’t spent much time on it yet but guessing it might be the muting circuit

I gave up on the 825 for now. Too much damage for now.

I have two PM655 integrateds that have issues coming on (playing sound) after power on. I bet it is caps in the speaker protection board. As those caps start failing, the delay between power on and relay close gets longer and longer.

 

[rossm]

Now the amp is working I have decided to have a look at the preamp, I see 825 is highly regarded but suffers from the caustic glue. Right now my unit powers up the the volume is very low on the non phono inputs, all controls work tone, balance, etc, just very weak. Also the phono relays seem to be working now. If you’re looking to purchase this model, make sure you hear a relay click each time you switch between phono 1 and 2, otherwise the relays might be bad. Has anyone ever found a replacement???
Decided to do some more glue cleaning, any suggestions appreciated, I am using alcohol and toothpick and cotton swabs . The power supply section seems to be the worst
Before


After

 

[rossm]

Looks like the low volume on the 825 preamp was caused by defective relay L1 , looks like the coil side has opened. I removed the relay and now the volume is fine, the default position of the relay is to short the l/r outputs and headphone out puts to ground. The only drawback I can see with the relay out is a couple of clicks when the phono switch is switched phono 1 and 2. I did order replacement on eBay

 

[rossm]

The progress continues with the preamp. Lots of electrolytic caps and toxic glue. It looks like the glue starts of as a light color and then maybe turns dark with heat, the glue on the cables are lighter in color but around the caps (heat) it’s darker, of course it could be different glue , either way it’s coming off


Lots of caps on this preamp, mostly audio quality



Looking a little better now



Found something pretty odd with the headphone amp electrolytic caps c04 and c07 are both 220uf and 6.3 volts ???? Any ideas on why it’s 6.3 v ???

 

[Champco]

Any ideas on why it’s 6.3 v ???

Engineering Circuit design?

 

[ivandezande]

Why is 6.3V odd? I'd say it's pretty normal being a standard value and all.

 

[dlucy]

Found something pretty odd with the headphone amp electrolytic caps c04 and c07 are both 220uf and 6.3 volts ???? Any ideas on why it’s 6.3 v ???View attachment 1441583

That is right in line with my expectation for the voltage sent to headphones.

 

[rossm]

Why is 6.3V odd? I'd say it's pretty normal being a standard value and all.

Hi
Yep you’re correct, I see em on mouser. My first time coming across such a small electrolytic. Never expected something so exact. Going to bump it up to 10v , all I have on hand.

 

[fred soop]

Not sure of the vintage of your equipment but my HK units from around 1990 seem to have electrolytic capacitors that are being operated too close to their voltage limits. I've had a few problems over the years and eventually recapped a Citation 21. Intermittent problems solved with that recap. In other cases, I was able to find the actual cap causing the problem.

 

[rossm]

Not sure of the vintage of your equipment but my HK units from around 1990 seem to have electrolytic capacitors that are being operated too close to their voltage limits. I've had a few problems over the years and eventually recapped a Citation 21. Intermittent problems solved with that recap. In other cases, I was able to find the actual cap causing the problem.

Hi Fred,
Good point, that will surely limit the life of the cap.

 

[rossm]

Hi
Waiting on my Mouser order for more caps !!
I am back on the amp hk870 tonight, decided to replace the dried thermal grease on the big transistors and redo the solder connections, I bet the high current of this amp stress the joints
Before


One down 7 more to go

Only had Time to finish one side,




Power back up and check my work and it’s still working, nice.

 

[rossm]

Was able to finish off the other 4 transistors tonight, went much faster than the other side. Heat sink compound was pretty dried out and solder connects looked pretty bad

Smoke tested and everything works.

Mouser order came in so I can get back on the preamp
I have tone and headphone sections done, tomorrow I hope to finish the power supply



Found the matching EQ today on the bay, should be here next week, all I am missing now is the turntable

 

[rossm]

Finished up the Power supply section with the glue removal and recapping


Man, this has a lot of caps !!
Tried headphone jack and was really impressed with the sound quality.

Connected my turntable and had only the faint signal in the left channel, bummer! Tried scopeing the signal and discovered the phono switch relays were bad, thumping the relay would case the left signal to get louder or just go away, replaced the relay with 2 wires and left channel was loud and clear, still no right channel though. Decided to roll the dice and replace the mm/mc relay will 2 wires, now have both channels, guess I can live with 1 phono input, Also don’t have any MC cartridges so that’s no problem either, but will research relay replacement

 

[rossm]

With nothing to lose I decided to dismantle the mitsumi relay.


Pretty easy to disassemble just remove stamped metal housing, pretty simple inside as well. A plastic plunger moves 4 Metal clips across the terminals.


I removed the clips and soaked them in deoxit gold, cleaned the terminals with deoxit and qtip, assemble and then uses deoxit fader lube on the assembly.



Solder relay back and I now have 2 phono inputs, yaaaaaaaa , makes me one to buy a 2nd turntable haha.
Note, a couple of pins on each relay are were bent over to hold relay in place on the ckt board, be careful they like to break when straightened

 

[rossm]

I rebuild the MM/MC relay, I don’t hear it click like the other relay. But the phono plays and sounds about the same regardless of the switch’s position

I don’t know how many MC cartridges are around today, might just remove it

 

[rossm]

Ran out of caps for the preamp so it’s back to the power amp. Recapped the PCB-2

Time to try and repair damage to PCB-4 looks like amp was dropped to break the tabs off. I have replacements for the big 4700uf 63v caps so I removed them to make the job easier


Going to try a 2 part epoxy with a stiffener , only putting the epoxy on the top side with no traces. With find out tomorrow if it holds