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Incorrect transistor voltages in Kenwood KA-6100 after recap

J

JayWFletcher

New Member
Hello,

I'm hoping someone can help me debug some problems that have come up after I recapped a KA-6100 integrated amp. There appears to be a short somewhere so I am running the amp on a DBT to prevent the fuses from blowing. I have double/triple/quadruple checked the polarity of all of the electrolytic caps, checked for cold solder joints, checked for bridged traces, and everything looks OK.

I have gone through the schematic and checked voltages at all of the transistors and found some issues. For brevity I have only listed the checks I made from the rectifier diodes through the main power amp section below (still a lot, sorry). The voltage at Rm131/Rm132/D20 is 0.64, just too low to make the relay close.

I hope this table of measurements will be helpful. I have highlighted the discrepancies between the schematic and measured voltages in yellow. The nominal voltages are all about 1/10 of normal due to the DBT.

I realize that I should have fixed the amp before I installed new capacitors, but that horse has left the barn. I'm an engineer, but not an electrical engineer, so I can read a schematic, but I don't have a feel for how these circuits work. I can use a DMM and I know how to do a 6-way check on transistors. I have verified that all of the transistors shown below are good by removing them from the circuit and doing a 6-way check.

Thanks in advance for any help you can provide.
Jay

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Qm27 base is .63v this is high enough to trigger the protect circuit.. Check Fuses Fm1 and Fm3...
 
You raw unregulated +/- B is way to low.. I can't believe the DBT is loading it down that much. Should be +/- 43v and your reading 1/10 of that voltage.
 
Should have stuck with your first thread. Instead of creating a second thread for basically the issues.
 
Thank you for your suggestions.

All of the fuses check out OK. They will blow if I take it off of the DBT. I fried some resistors and a transistor the last time I tried this.

I have been assuming that the voltage is low on the DBT because there is a short somewhere. I don't understand how the DBT works exactly, but I believe it is a current limiter. If it is limiting the current due to a short, wouldn't this also result in the voltage being reduced? On the DBT the AC voltage at the input to the transformer is only 30VAC. If I remove the fuses and check again I get 120VAC.

I built the DBT following the instructions to build and test it in this thread https://audiokarma.org/forums/index.php?threads/dim-bulb-tester-build-and-how-to.808399/. Are there any known problems with this design?

I started a new thread because the old one had meandered off of the original topic and the title didn't reflect my current issues. I sent a message to whoever monitors the "Contact Us" link asking if this was OK, but I didn't get a reply.

Please note that I had some errors and omissions in my original table and I have updated it.
 
I should also note that I have made the following transistor replacements:

Qm19 and 20: 2SC1567Q -> KSA1381ESTU
Qm21 and 22: 2SA794Q -> KSC3503DS
Qm23 and 24: 2SA980 -> MJ15016
Qm25 and 26: 2SC2260 -> MJ15015
Qm29: 2SD330 -> MJE15032G
Qm30: 2SA755B -> MJE15033G

I hope these are close enough to be good replacements.
 
avionic said:
Did you use mica and heatsink compound..
Click to expand...
I'm using some things called ThermoPads made by NTE electronics. They are made of silicone and fiberglass and are supposed to provide good electrical insulation and thermal conductivity. I checked the resistance from the output transistor cases to they are all greater than 50kOhms, but not infinite.
 
JayWFletcher said:
I'm using some things called ThermoPads made by NTE electronics. They are made of silicone and fiberglass and are supposed to provide good electrical insulation and thermal conductivity. I checked the resistance from the output transistor cases to they are all greater than 50kOhms, but not infinite.
Click to expand...
50KΩ is not good enough.. Check transistor cases to the heatsink ( chassis ground )
 
avionic said:
50KΩ is not good enough.. Check transistor cases to the heatsink ( chassis ground )
Click to expand...
That is to chassis ground.

I was told by someone else that the resistance from the output transistor collectors to ground has to be infinite, but it is not. I don't see how it could be infinite because the collectors are connected to circuits that ultimately go back to ground.
 
avionic said:
I would agree....
Click to expand...

I replaced the original bridge rectifier diodes with SBYV28-200 diodes and still had this problem. When you suggested that I test them, I tested the originals, and since they tested good, I put them back in. So I have the same problem with the original diodes and the brand new SBYV28-200 diodes. Would you recommend a different replacement diode?
 
I have been installing the diodes with the band oriented toward the cathode, which I think is correct and how they were originally oriented.
 
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