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ka-7100 problem

N

nomis_82

New Member
Hello some month ago i get an broken Kenwood KA-7100 from a friend.
The first thing I found was that one fuse blows because of a metal piece on the chassis that was short to the circuit board. When i wiped out the piece of metal the fuse works again but I do not get any sound from it. IMG_20180823_211253.jpg
I have found that relay does not click due to any problem in the power amplifier.
I have measured some components and measurement points from the service manual.
There are many incorrect voltages in the power amplifier but I have not found any defective components so far. The components marked with yellow on the electrical diagram have removed and measured voltage drops on diodes and transistors eg. On the components I have not found any errors as far as I knew to measure.
The voltages I measured in the amplifier are in red text.
It seems to be the same problem on both right and left channels.
I do not really know how to proceed so if somebody has any idea let me know.
Edit: link to picture with higher resolution. https://drive.google.com/file/d/19gyBClS6__2fCtHdzW44WYaaIQzhLT8P/view?usp=sharing
and document with component measurments
https://docs.google.com/spreadsheets/d/1pO0go2BCLcF4MonyaShucrfwplr7TruUUc9Zutyi-Zc/edit?usp=sharing

ka7100 poweramp_M5.jpg
 
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Negative voltages are missing, I would try replacing Qe11, outside chance maybe Qe13.
While these tested OK, sometimes they need to be under load before they fail, so if it passes testing, it
still may be faulty. Nice work on the measurements.
 
roger2 said:
@mbz you can see those red numbers? I can just barely make out a small percentage of them on my 19" monitor
Click to expand...
Had to save image on laptop and open with paint.

Scratch Qe13 (and 14), my money is on Qe11 (and 12).

SM has them listed as 2SC1940 or 2SC1885, anyone chime
in with a suitable replacement...
 
It is unusually good information. I doubt that most can read it. I tried saving the image to Paint and increasing the size but still was not able to read (especially) the red voltage values. I wish that AK allowed 1200px width images.
 
Thanks for all helpful answers. Now i have update the first post with links to an higher resolution picture and an document with some of my component measurements.
When i measure Q11 i get 0,69 voltage drop wit my ut61e and hFE 171 with an simple circuit tester. I do not really know when one can say that one component is bad. Is there any other way to measure? I was hoping to find a clear problem before I started ordering components. When i measured Q34 as roger suggested i got a low hfe value of 29 and voltagedrop 0.66v maybe this is where i'm going to start.
 
Suggest you give the others (@roger2 ?) a chance to analyse the measurements
before ordering parts. Your last measurements suggest Qe11 is ok. Qe34 is part
of the protection circuit. The channel has excessive dc voltage so the protection
circuit is doing its job.

Looking at the power amp and your measurements, signal flow is from left to right.
Things go pear shaped in the area/line Qe9,11,13. The voltages are all near positive
rail, suggesting the voltage from the negative rail is "missing", this negative
voltage normally cancels out the positive voltage giving near zero (1-2...Volts)
The negative voltage should appear on Qe11c but you have +45V, that's why I suspect
Qe11. The other fault scenario is that Qe9 has failed short c-e (infact your
voltage measurements suggest this (47.8V@E and @C), start with Qe9.
 
The higher resolution schematic is excellent:thumbsup: It is rare to see such good information and so clearly presented.

I do not have the knowledge to troubleshoot unfortunately. But I agree that Qe34 is not causing the primary issue. It should be replaced though. I see you also measured the caps in the relay circuit and those appear to be reading OK.

Looking at the voltage readings in red I do have one question: are you certain that negative voltages are all indicated correctly with a minus sign?
 
Hello again. Yesterday I sat down and did some measurements and I'm quite sure that minus the characters I measured are right. Unfortunately I made a mistake and shorted Q25 (2sa794) so now I need to order a new one before I can continue. However, I'm somewhat unsure if I'm going to order a 2sa794 on ebay or make a bigger order from mouser and switch to KSA1220A that seems to replace it.
 
When I asked about the minus signs on your red voltage markings, I was wondering if perhaps you had omitted some minus signs where the voltage readings were indeed negative.
 
Further to post #8, the Qe9 collector and emitter voltages are (about) the same, possibly Qe9 has failed short c-e.
This would "flood" the amp with positive voltages. Thinking Qe23b (+48V) is just a mains bounce, Qe23e (+47.2V)
fits in with the Qe9 c-e short. Suggest replace Qe9.
 
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