hk 670 problem(the vintage one, not the 2001 model)

[DragonMaster]

I just got hand on a vintage hk 670 amp. The only problem is that there is some intermittent noise(Sounds like noise) on the right channel. I tried moving every switches and pots to see if they created the problem but it's somewhere else.

It seems to be in the amp circuits since if I move tone or volume, it changes nothing.

[DragonMaster]

I just read the Pioneer SX crackle topic. While I don't have the same problem(Me, I have crackle+noise on the right channel, but there is some sound.), I checked DC voltage, and the left channel(the one with no problem) has about -0,1v and the right chan seems to have about 0,6v . . .

I found these with a crappy analog multimeter, since I don't have my digital one under hand. My analog multimeter is sometimes completely inaccurate.

[DragonMaster]

I used a DMM this time and got -0,2v on the left channel and +0.6v on the right channel.

[DragonMaster]

I just measured the voltages, right after playing 4 CDs one after the other they turned to be -19mV for the left channel and 23mV for the right channel.

Maybe the 2 per channel 10000µF 25 years old caps start to fail?

Maybe my crossed coax speaker cabling that activated the short protection (I think)yesterday fixed problems?

I also cleaned the 5 fuses, and changed the 3 burnt front lamps, and removed some dust on the pots on the poweramp boards.

I'm now connected to a surge protectecting bar.

I touched both TO3 transistors at the same time and got a shock.

I plugged the power plug the other side(If I reverse the plug and it reappears, I'll say good bye to the rectifiers)

If the problem reappears, I WILL suspect bad solder joints, if the reversed plug don't change anything.


Should I use my contact cleaner on the bias(?) pots or "If it still works, don't fix it"?

[derguteton]

Hello,

I have had problems sounding like this with my hk750.
Resolding all points on the pre-output transistors, cooled by the seperate Cooling metal (Q421-Q424 in your case) had solved the crackeling problem.

[DragonMaster]

I was about(In a few days) to change the differential transistors.

Also, I don't know if I wrote it has been repaired two times, but the repairs(differential transistors replacement) were done on the left channel instead of the right one.

[DragonMaster]

In the last few days, it was crackling a lot. So I decided to finally fix the problem. It was the transistors like you told. They were quite easy to find because they are attached to an heatsink. 12 screws and 12 solder reworks later, no more problems!

Yay! I got this amp for free because of this problem and it was easy to fix!

[DragonMaster]

Now I've got lots of trouble with the switches. They make the sound intermittent and scratch like hell. Putting "Contact 2000" (Ethanol mainly) got thing worse.