need help with Harman Kardon 730 popping internal fuses.

JoshHendi

JoshHendi

^ElectroNerd^
Subscriber
Got a H/K 730 receiver with no previous ownership history, and guy that gave it to me to look at said it was popping the internal fuses. Looks like the fuses next to the filter caps (pictured)

I pulled the amp board and checked all four output transistors with a component tester and they all passed and are working. The amp board itself looks totally fine.

The fuse labels read 2.5A-125V S.B. - does the SB mean slow blow fuses instead of regulars? The manual doesnt specify either way.
What/where should I check next?
And would it be okay to power up the unit without the amp board connected to see if the fuses still blow?


EDIT/UPDATE: Issue was the bias diodes on the amp board
 

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is it multi voltage model ?
fuses will be slow blow as before main filters .
all i can guess is one supply blew and someone borrowed the fuses from the good side and blew those too .
.
test those rectifiers
 
Power it up on a dim-bulb tester when you do apply power. I would get some slow-blow fuses, power it up on a dim-bulb, and see what happens, but that's just me.
 
Okay so update... Installed a set of fuses and powered up on DBT... Bulb starts bright them dims (like a normal working amp) but then about 2-3 seconds later, the bulb gets bright again, and the faceplate lights start to dim...

When I powered the unit up without the amp board installed, it turned on fine (bright to dim, then holds dim)

So does that give any clues? Does that 100% mean the problem is isolated on the amp board? (I would think not, since something wrong elsewhere could possibly cause this behavior? )
 
on first thoughts it looks like a biasing issue . ..
could be something shorted ..hard to tell really .
although if shorted it may light the bulb constantly ..
i take it no input or load attached ?
 
Correct no load attached, speaker selectors both off... Bulb goes dim normally, then after about 2-5 seconds gets really bright really fast.
 
So an update on this...

I knew the problem was isolated to the amp board, so I figure'd Id test my luck at a little shotgunning... Replaced the caps, VRs and the bias diode and sure enough, that fixed the issue :)

Allll my money would be on the bias diode being cause of the delayed 're-brightening' of the bulb. But since the amp board was easy to access and knew it would only take an hour or two to shotgun, figured what the hell. (if you are reading this at some point in the future with the same or similar issue, I know shotgunning isn't always advised for troubleshooting, and I tend to agree. But for this unit and problem, I had nothing to lose so I went for it)

Anyways, all is working :)
 
Hello Josh,
I just got a 730 which is blowing left channel fuses.
Your thread is very useful! Just to clarify, are you speaking about the six 1n2076 diodes (d401 to d406)? If yes, did you replace them with 1n4148?
Thanks in advance!
 
AngelAT said:
Hello Josh,
I just got a 730 which is blowing left channel fuses.
Your thread is very useful! Just to clarify, are you speaking about the six 1n2076 diodes (d401 to d406)? If yes, did you replace them with 1n4148?
Thanks in advance!
Click to expand...
So I checked my photos and notes, but I must have done this amp before logging/keeping detailed notes, and can't recall with 100% certainty
According to one of my pictures, I replaced just two diodes which were probably the D407 and D408 bias diodes. However I am pretty sure the 1n4148 can be used to replace both types
If you are comfortable with soldering and replacing parts, you may as well give the board the full treatment if you end up pulling it out :)
Edit: So I have done a couple 730s and A402s (basically the same unit, minus the tuner) and I have replaced those diodes in every restoration, if memory serves, they may have been causing an issue in a A402 I did also. They are the diodes that look like little balls. Get those replaced at the very least as they are 'known issue' types
 
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Comes out mine had a more trivial problem- both end transistors of the left channel were gone. Replaced all four with MJ21193g and MJ21194g and all is well now.
I am slowly advancing through the restoration- recapped the power amp and also replaced the notorious D407 and D408 with 1n4148. The other diodes tested ok. Also resoldered the connectors.
Recapped the sub rectifier board and also replaced D3 and D4 with UF4004.
Recapped the Equalizer Amp board and also reflowed the connectors.

Sound is good BUT I do not get good stereo separation. When using the balance knob there is no muting if one channel when all the way to the other side... Is this common for the model?

I still have to work on the preamp and this might help. Otherwise switches and pots are cleaned, so this should be ruled out.

Any ideas?
 
I've got a 630 and the balance mutes as it should when turned all the way to one side or the other. It sounds like the signal is bleeding over from one channel to the other. Try putting a signal in only one channel and see if it still produces sound in both.
 
Thanks for the ideas nicholas2381 and John!
It comes out that this was happening because I was testing the unit over a DBT. When powered on all is ok.
Anyway, I finished recapping the preamp board. Also replaced here D503 and D504 with UF4004.
Added warn white LEDs and a new dial pointer lamp.
Biased the HK730 and now enjoying some jazz.
I am really impressed (this is my first HK) by the sound of this Harman/Kardon!!!

Cheers!
IMG_20220118_225815.jpg
 
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