HK730 help

[Redox600]

Good day all. Newbie here and looking for some direction from this knowledgeable community . I have recently taken an old HK730 Twin Receiver out after decades of being in storage. I will try and list where I am at in steps:

1) Opened unit and found 2 slow burn fuses on the main rectifier board (right channel I believe) were blown, no other fuses blown. Ordered new SB fuses and installed. Unit now produces sound through the speaker outputs and headphones.
2) Deoxy the pots and switches, replaced lamps with LEDS. All good, unit sounds great and I use it daily with no issue for 3 weeks at various volumes and various inputs.
3) Week 4 I turn on the unit and no sound. Open unit and find the same 2 slow burn fuses are blown. Visually no caps are swollen or leaking, nothing obvious.
4) I build a Dim Bulb tester, put in 2 new slow burn fuses, power on unit without speakers attached or volume up. The dim bulb lights up quickly then goes out, wait 4 seconds and bulb becomes bright again and stays on.
5) I remove the power amp board and try the dim bulb test again. This time bulb stays off. So I think my issue is with the power amp board.
6) Start testing diodes and transistors on the amp board with a multimeter. Notice the transistors (Q409, 410 Bias stabilization) on the heat sink are slightly discolored. I test these transistors and get results that suggest they are bad (passing some current in both directions). I remove/de-solder the transistor on the side that has blown the SB fuses and test it with a multimeter, and the results show it is good and only passing current in the proper directions. So I assume my failing tests while the transistor was still attached to the board were a result of the current finding other paths on the board.

So this is where I am at now, and looking for some advice on how to narrow down my search for the trouble. I was hoping to avoid removing components from the board willy nilly, at least without asking for advice first.
- Does anyone know what kinds of trouble would cause the slow burn fuses to blow after weeks of use? They blew immediately week 4 when turning on the unit, so I don't think it is an over heating issue.
- Maybe my assumption of removing the power amp board, and the dim bulb test passing was wrong? Maybe not the amp board?

Any suggestions are appreciated.
Cheers!

 

[jheu02]

I would check the outputs and drivers for drawing excess current. Also the trim pots can fail open

 

[Dave1027]

On the DBT and new fuses, compare currents between the channels to isolate cause.

 

[Redox600]

I checked offset bias (on the DBT) at the speaker outputs, and I found swinging mV, past 200mV at startup, eventually down to 50mv or lower. Even got negative mV at some point. Dim Bulb (100w) glowing bright during operation.

Removed the power amp board again for another round of testing. Eventually found some discrepancies between the 2 channels which lead me to remove the bias transistors (Q409, 410), and the associated varistors (D407, 408). The right channel varistor 1S1211 position does not pass current in either direction. So next step I will get replacement diodes 1N4148 installed and do further testing.

 

[Redox600]

Replaced the defective diode, and the matching one on the other channel, as well as both trim pots. Put it back together and tested on the DBT. No problems, bulb did not come back on. Ran for 30 mins and watched offset bias. Right channel is nice at around 10 mV, but left is in the negatives around -25mV. Ran the unit off the DBT and no blown fuses. So it's still needs bits replaced to get bias sorted, but it's running for now.

 

[Dave1027]

May need to match the diff pairs to get the DC Offset down but those numbers aren't bad. Bias is set to 35 mV between collectors of the outputs.

 

[Redox600]

Should I ever have bias in the negative mV?

 

[Dave1027]

It depends on how you are measuring. Where did you measure the -25 mV? Positive lead of DVM on what and negative lead on what?