Harman Kardon 930 Short out - need help!
- Thread starter Ulysse
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- 930 harman kardon hk930
No, the speaker fuses look fine. However, the previous owner tried to prevent the blown, and put two 4 amp fuses in the speaker section, and a 6 amp fuse in the power section. I did not try to turn it on until I connect it to my bulb current limiter.
I have not seen anything burnt or loose to my untrained eyes.
I have not seen anything burnt or loose to my untrained eyes.
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NAD80
Super Member
Found the service manual here oh HiFiEngine.com. Just register and download for free. Check out the thread here for checking transistors.http://www.audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/ https://www.hifiengine.com/manual_library/harman-kardon/930.shtml Welcome to AK.
blhagstrom
Mad Scientist, fixer.
Most of the time, vintage gear that shows dead short, it's a blown amp.
Most of the time, an output transistor or 2 or more are blown.
Many times with a blown output transistor, there are other parts blown out too.
Are you able to do electronics repair?
The more rare case for dead shorts is a problem in the power supply.
A blown Bridge rectifier is the most common.
A shorted filter cap or shorted transformed is the rare possibility.
If you do not have the tools and skill, you will need to get them and slowly learn how to fix electronics.
Most of the time, an output transistor or 2 or more are blown.
Many times with a blown output transistor, there are other parts blown out too.
Are you able to do electronics repair?
The more rare case for dead shorts is a problem in the power supply.
A blown Bridge rectifier is the most common.
A shorted filter cap or shorted transformed is the rare possibility.
If you do not have the tools and skill, you will need to get them and slowly learn how to fix electronics.
Thanks for all the help here...
After few youtube videos, I found a blown bridge rectifier. It is part 1S1850, a dual common cathode diode. I removed it from the circuit, and turn on the receiver again. It appears the short is gone, as the light bulb is not lighting up any more, and the front panel of the receivers are lighting up.
Bought a replacement bridge rectifier diode based on recommendation of another forum top. Wish me luck with that
https://www.digikey.com/product-detail/en/on-semiconductor/MUR1620CTG/MUR1620CTGOS-ND/919906
After few youtube videos, I found a blown bridge rectifier. It is part 1S1850, a dual common cathode diode. I removed it from the circuit, and turn on the receiver again. It appears the short is gone, as the light bulb is not lighting up any more, and the front panel of the receivers are lighting up.
Bought a replacement bridge rectifier diode based on recommendation of another forum top. Wish me luck with that
https://www.digikey.com/product-detail/en/on-semiconductor/MUR1620CTG/MUR1620CTGOS-ND/919906
blhagstrom
Mad Scientist, fixer.
Once you install the new bridge, do check with dim bulb.
I don’t know how you verified bad bridge, but removing the bridge breaks all connection to the main power supply and that isolated the transformer from most of everything else. What you proved so far is that the transformer is OK.
The dial lamps must be running AC.
The rest of the system runs DC from the power supply. It may be just a failed bridge or something more further in that cooked the bridge, maybe the filter caps. It’s rare, but I’ve seen it.
I don’t know how you verified bad bridge, but removing the bridge breaks all connection to the main power supply and that isolated the transformer from most of everything else. What you proved so far is that the transformer is OK.
The dial lamps must be running AC.
The rest of the system runs DC from the power supply. It may be just a failed bridge or something more further in that cooked the bridge, maybe the filter caps. It’s rare, but I’ve seen it.
I tested the bridge diode with a multi-meter, and it is shorted out. Checked with both diode check function and ohm function. As this is a twin power receiver, there are two sets of bridges. I found someone replaced the same diode on the other side before me, so I'm guessing this is a part that is prone to failure. Or, maybe there are some other issues causing this part to fail.
Can't wait for the new parts to arrive and put it on...
Can't wait for the new parts to arrive and put it on...
Changed the bridge diode and the receiver turns on OK. However, after running it for 10-20 minutes, right channel fuse blew out. Removed the speakers and measured the output voltage on the speaker posts. One reads 70mv, and one reads 136K mv (the one blown). What's the most likely reason to cause the output voltage to be too high? Thanks.
blhagstrom
Mad Scientist, fixer.
Sounds like a runaway bias situation.
Any suggestion on how to fix this? Thanks!
blhagstrom
Mad Scientist, fixer.
Does it still power up when you replace the fuse?
If it does, run it on the dim bulb. Use 100w, maybe 150w.
If it runs a while and suddenly the bulb goes bright, I would suspect bias runaway and look to the bias system. I never worked on one so I am really unfamiliar. I think I have a junk one in the pile. If I find a minute and remember, I’ll take a look and see what’s where. Watching the bias with meters while it warm up is good too. You need 2 meters or do it twice. If the bias suddenly runs wild and the bulb goes bright, we may be onto something. Hmmm, maybe that will happen anyway.
We may want to try watching the DC offset too.
I don’t know if he 930 is direct coupled.
Usually a bias compensator device, like a transistor or varistors is screwed to the heat sink near the output transistors. When they get flaky, amps act like yours does. But there is a lot of maybe here. You could have a flaky driver or even an output or a bad joint on a critical path. Flaky bias transistors may test ok on a hand meter but fail under power like what your seeing. Replacing them is a best guess thing.
If it uses varistors, we have been known to build replacements out of diodes.
Where are the HK930 familiar guys?
See what happens when I don’t get a broken one to fix? The one that finally showed up here is parts at best.
If it does, run it on the dim bulb. Use 100w, maybe 150w.
If it runs a while and suddenly the bulb goes bright, I would suspect bias runaway and look to the bias system. I never worked on one so I am really unfamiliar. I think I have a junk one in the pile. If I find a minute and remember, I’ll take a look and see what’s where. Watching the bias with meters while it warm up is good too. You need 2 meters or do it twice. If the bias suddenly runs wild and the bulb goes bright, we may be onto something. Hmmm, maybe that will happen anyway.
We may want to try watching the DC offset too.
I don’t know if he 930 is direct coupled.
Usually a bias compensator device, like a transistor or varistors is screwed to the heat sink near the output transistors. When they get flaky, amps act like yours does. But there is a lot of maybe here. You could have a flaky driver or even an output or a bad joint on a critical path. Flaky bias transistors may test ok on a hand meter but fail under power like what your seeing. Replacing them is a best guess thing.
If it uses varistors, we have been known to build replacements out of diodes.
Where are the HK930 familiar guys?
See what happens when I don’t get a broken one to fix? The one that finally showed up here is parts at best.
Yes, it powers on OK after replacing the fuse. I ran it through the bulb for about 20 minutes and no issue yet.
What happened earlier was exactly what you described. It ran for a while and suddenly the bulb goes bright. Then I ran it without the bulb for sometime, and then the fuse blew on the right channel.
I measured the DC offset, left channel 57 mv, right channel 113 mv.
My next step will be replacing the differential pair (Part TR601, TR602, TR621 and TR622), as someone said that will help bring down the DC offset. Maybe I'll do a recap of all the caps in the power section as well.
What happened earlier was exactly what you described. It ran for a while and suddenly the bulb goes bright. Then I ran it without the bulb for sometime, and then the fuse blew on the right channel.
I measured the DC offset, left channel 57 mv, right channel 113 mv.
My next step will be replacing the differential pair (Part TR601, TR602, TR621 and TR622), as someone said that will help bring down the DC offset. Maybe I'll do a recap of all the caps in the power section as well.
blhagstrom
Mad Scientist, fixer.
It is sounding more and more like a bias thing,
At least, I've seen that on other stuff.
At least, I've seen that on other stuff.
If that's the case, replacing those differential pair transistors will likely solve the issue? Thanks
blhagstrom
Mad Scientist, fixer.
I dug out the junker I got (it actually works, mostly)
There is a pair of boards mounted to the heat sinks on either side of the amp board.
Those are the varistors.
Service manual shows them as D601/2 and D621/22
They are KB265
Those I am unfamiliar with but usually a pair of 1N4148 in series is a good replacement for blob diodes.
The trouble is, there are 2 blobs in parallel so measuring them doesn't tell me much.
I suspect one or more of yours are whacky.
There is a pair of boards mounted to the heat sinks on either side of the amp board.
Those are the varistors.
Service manual shows them as D601/2 and D621/22
They are KB265
Those I am unfamiliar with but usually a pair of 1N4148 in series is a good replacement for blob diodes.
The trouble is, there are 2 blobs in parallel so measuring them doesn't tell me much.
I suspect one or more of yours are whacky.
It blew one of my speakers the other day and it is the left channel this time. I changed a speaker and was able to run it for over an hour without issue. However, when I turned it off and then turned it on, the bulb lighted up again. Also, to make it work, every time I have to turn on the receiver with the speakers disengaged for a few seconds, and then push in the speaker button. Not predictable at all.
I'm ordering parts to recap the power supply and driver board, and I'll replace these diodes as well. Let's see what happens...
I'm ordering parts to recap the power supply and driver board, and I'll replace these diodes as well. Let's see what happens...
You've gotten good advice.
The varistors certainly could be the source of the problem. The emitter resistors and the trimpots are also potential culprits. Or a loose / broken connection somewhere. Really erratic behavior has twice, for me, come from a broken wire Lead right at where it connected to a board, was effectively invisible, and connected MOST of the time and disconnected at odd (when moved) times.
Good luck!
The varistors certainly could be the source of the problem. The emitter resistors and the trimpots are also potential culprits. Or a loose / broken connection somewhere. Really erratic behavior has twice, for me, come from a broken wire Lead right at where it connected to a board, was effectively invisible, and connected MOST of the time and disconnected at odd (when moved) times.
Good luck!
