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Kenwood KA-7100 fuse blows

TheCodeDawg

TheCodeDawg

New Member
I recently acquired a dead Kenwood KA-7100. I opened it to discover that [Fe1, Fe4] and fe2 were blown. I tested the output transistors and found that Qe27 and Qe29 were bad. Qe28 and Qe30 tested good. I replaced Qe27 and Qe29. Now Fe1, Fe4, Fe3 survive, but after 20-30 seconds fe2 still blows. Re20 and Re77 get very hot, but both test at spec. The speaker protection switch closes as it should and stays closed until the fuse blows. I also replaced Qe28 and Qe30.

I have a full set of new caps and heat-sinks on their way from Mouser and I have a PDF of the service manual.
 
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Well, it's about time I actually built a dim bulb tester. I have all the parts already, I just haven't taken the time to assemble it yet :(
 
Update: Last night, I assembled the dim bulb tester, re-capped the amp, and tested it. The dim bulb tester does a nice job of eating the excess current, and the resistors that were heating up stay cool. Without the dim bulb tester, the resisters that were heating up get very hot quickly and I don't leave the amp on long enough to blow the fuse. So now I will use the dim bulb tester configuration to make sure the bias and offset are correct and get back here if there is still an issue. It was past midnight, so I didn't get to this step yet.
 
TheCodeDawg said:
Qe28 and Qe30 tested good. I replaced Qe27 and Qe29. Now Fe1, Fe4, Fe3 survive, but after 20-30 seconds fe2 still blows.
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A simple diode test is lucky to catch maybe 70% of transistor failures. The fact that you are blowing the fuse for the other channel indicates a problem still...perhaps the outputs and/or drivers on that channel, or the rectifier diodes (De20, 22, 24, 26), or Qe32 (but if Qe32 was shorted, I'd expect Re78 to burn up).

Re20 and Re77 get very hot, but both test at spec.
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Perhaps the Fe2 isn't where you think it is...maybe you're losing Fe1 or 3 instead. I just said that if Qe32 was shorted, you'd burn up Re78. But you're losing Re77 instead. So some triple-checking here is needed.

Re20, under normal conditions:
I = (47-28)/1.5K = 13mA
P = 13mA² x 1.5K = 240mW (so a 1/2W resistor is called for)

But, if, say, the 28V zener were shorted, the whole 47V supply is dropped across R20.
P = 47²/1.5K = about 1.5W

Something to consider.
 
Awesome! I will check the diodes. Fe2 is silk screened on the board both front and back. Of course, it's no guarantee that its screened right.
 
I'm curious how the dim bulb test went - did the bulb start bright and immediately go dim, or did it stay bright? What wattage bulb did you use?
 
gort69 said:
I'm curious how the dim bulb test went - did the bulb start bright and immediately go dim, or did it stay bright? What wattage bulb did you use?
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Good question. The 100watt bulb starts bright as the caps charge, and then dims and stays dim. However it's not as dim as other receivers that I have.
 
Update: I have carved out some time work with this amp again. First, let me begin with an observation about the schematic for the amp's Power Amp circuitry. The numbering for the components is split between the two channels. Even numbers for one channel and odd for the other channel. The channel that is bad is even numbered. I'm not sure if this is the right, or left channel, but it really doesn't matter.

I have ~58-60v on the even side's 47v rail. I dont leave the amp on long enough to get a stable reading, but the voltage is usually somewhere in this range.

I have replaced all of the "standard" even numbered diodes since replacements from a local electronics store were easy to obtain. I tested all of the existing diodes with my Fluke MM, and they all tested okay, but i swapped them anyway, just in case they fail under load. Getting 28 volt zener diodes requires an internet order, so I ohm tested them out of circuit. They seem okay. I swapped the even zeners (both 24 and 28 volt) with the odds. I still have 60v on the even side rail and the odd side is around 45v. I am now in the process pulling and testing all of the transistors where the 60v shows up on any of the pins.

Also, the resistors that have 60v on one side and 23v on the other the ones burning up. As one might expect...

Any other advice would be helpful...
 
"47V rail"

Are you talking about the unregulated high-current supply for the output devices? The only thing having an influence here is the voltage supplied by the transformer and the ability of the rectifiers and filter caps to do their job. There's only one way for that voltage to be reading so high, and that's if your home line voltage was way the hell too high, or if this is a multivoltage model that is set for 100Vac and you're supplying it with 120Vac. But the only multivoltage model I'm familiar with has two settings: 220~240V and 110~120V.
 
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