A little help with a H&K 730

[Jackassrock]

I have a 730 with a problem. One of the channels keeps blowing a fuse. Sometimes a new one will last a second, sometimes a week.
I've taken it to a couple shops, and they will think they fixed it. They will test and everything, but within a week, it always blows the fuse again.

Any ideas what might be causing this?

 

[blhagstrom]

Been a while since I worked on one.

Does that one have the snap out amp board?
Check the solder joints on the connector pins, both main board and amp board. Reflow the solder to make sure.

The other thought is the bias circuit. Either the trimmer is wonky or the bias component is wonky. I’ve seen the behavior you describe when that situation occurs.

 

[Soundork]

Yes, the bias potentiometers are bad news. I would start from there as blhagstorm suggested.

 

[xrayspex77]

I had a problem with mine that had a similar issue. Long long Story Short.
I still don't know why but the Main Caps in one channel weren't discharging at shutdown. They kept the Cap in Slow Start Circuit charged so it would send 40V (not mV) to the Speaker Terminals.
If I left it off long enough for them to fully discharge, it would start normally. Turn it off and back on, normal. Off for a minute then back on, 40V.
I ended up adding bleeder resistors to the Main Caps.
If you have a Multimeter test to see what voltage you're getting at the Speaker Terminals, turn it off for a minute or 2 and turn it on again and see what voltage you're getting.

 

[Soundork]

I had a problem with mine that had a similar issue. Long long Story Short.
I still don't know why but the Main Caps in one channel weren't discharging at shutdown. They kept the Cap in Slow Start Circuit charged so it would send 40V (not mV) to the Speaker Terminals.
If I left it off long enough for them to fully discharge, it would start normally. Turn it off and back on, normal. Off for a minute then back on, 40V.
I ended up adding bleeder resistors to the Main Caps.
If you have a Multimeter test to see what voltage you're getting at the Speaker Terminals, turn it off for a minute or 2 and turn it on again and see what voltage you're getting.

Hi Xrayspex,
Did the symptom you describe develop with time, or after addressing other issues with the receiver?
Thanks in advance.
Best
Soundork

 

[wmgwizard]

Does the fuse blow if you disconnect the speakers and turn the unit on ??

Had a similar situation where a kitten got behind the couch and chewed the speaker wire causing a short......check your speaker wires carefully

 

[xrayspex77]

Did the symptom you describe develop with time, or after addressing other issues with the receiver?

It was Ok until I had a problem with a TT with bad RCA cables, LOUD noise, blew all the Fuses, killed a speaker. Started happening after that.
It would sometimes blow the fuse for that channel with a speaker attached.
It drove me nuts, I'd think i'd fixed it, tested without speakers, I would get mV, turn it off/on get mV, turn it off for minute or more and get 40V or 30V or 16V and then drop to mV after about 60 secs.
It finally dawned on me that the 40V, 20V, 8V reflected the state of charge of the Main Caps and they were keeping the Slow Start charged so that there was no Slow Start in that Channel.
If I started it with the Speaker Switch off for at least 60 secs, it was OK, but I got tired of waiting so added the Bleeder Resistors to the Main Caps.
If you don't have a Multimeter, maybe have one of the shops that thought they fixed it check to see if the Main Caps for the L Channel are discharging as fast as the R.

 

[Jackassrock]

So, Yes the boards snap out.
I should add that I am not actually working on this myself. I've had it to two different guys already. They have checked the solder, replaced the bias pots, disconnected the boards and switched sides. Same story, it blows one of the fuses within a week or less. Mostly less.

Xrayspex77, this is something I'm going to look into. ^