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Kenwood KA-9100 oscillating

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destructor8

New Member
Hello everyone, first post, I'm from France and passionate about renovating old audio gear, from turntables to tuners and of course vintage amplifiers. I started my journey in this passion about 5 years ago with an SU-7700 from Technics which is still working beautifully to this day.

In the meantime i stumble upon the beautiful Kenwood KA-9100 which i knew nothing about and the seller just told me that there was some smoke and that's all.
And the long walk began:
- i decided to do a complete check-up without even turning it on and followed the thread of hopjohn religiously, so everything is renewed in this amp
- upon completion, i turned it on and i had the complete dc rail on the left channel output... so turned out that the left channel TA-100WA was smoked
- i ordered a new module from pyramid-audio, hooked it up and all was fine, no dc offset, clicking relay, power coming from the left output (tried with an 8ohm ballast resistor)

And then a detail caught my eye: the left vu-meter was going upward to 0.01W when hot. So i connected my multi-meter and got 2.5-3MHz on the left output directly on the amp board (granted this handheld multi-meter has a 4-400KHz bandwidth) so it's oscillating.

Then i stumble upon this thread: http://audiokarma.org/forums/index.php?threads/ka-9100-impossible-problem-has-me-stumped.604218/
which seems to describe the same problem so i tried replacing the Qe1 and Qe3 on the board (2SC1775 to KSC1845F) and now the left meter is showing 0.01W to 0.03W when hot so worse.

I contacted pyramid-audio and he suggested that the Cz3 0.1µF cap on the diode board could be faulty so i swapped to a new one and still the same oscillations.

What i also tried:
- swapping the filter caps from left to right: no change (small precision, i haven't changed them, but they tested good)
- freezing the transistors with cold CO2: got a good response on the Qe3, the left vu meter returned almost instantly to 0 (swapped to two new ones on my reel and no change).
I tried freezing the µPA63H module and got an erratic response, sometimes the meter goes up sometimes down.
Also tried to freeze the Ce6 mylar cap and the meter also got to 0.

So that's where i'm at, i'm beginning to think that ether a mylar cap is bad somewhere or the µPA63H module is bad and introducing noise into the channel ?

Beginning to be a bit tired of this thing, not gonna lie. I was hoping someone could have an insight ? Thanks, JD

ps. some pictures of the beast, and the good/bad module:
IMG_6611.JPGIMG_6604.JPG
IMG_6607.JPGIMG_6606.JPG
 
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No first hand experience with the 9100, but have seen bad compensation cap causing oscillation.
µPA63H is a prime candidate also.
High frequency compensation caps: Ceramic - any one open could cause problem.
Ce11 (12) 100 pF, Ce17 (18) 33pF, Ce5 (6) 100 pF
Good luck.
 
Thanks for your feedback
So, i tested the compensations caps and they all tested good without shorts, tried to replace some with new one i had on hand and still no difference.
This leaves the µPA63H so i found a µPA68H from a good unit and NOS µPA63H on ebay japan. Now waiting on delivery.
If these replacement aren't working i'm planning to order some LSK489 in TO-71 package and bend the leads in place.
 
Change the other transistors and caps while you're at it.
I usually redo the whole board when I change the TA-100.
I have tons of 9100 boards and parts if you need any.
john
 
Update: got the supposedly good uPA68h in the mail today, still the same oscillations problem at the output.

One thing to add, when setting the dc offset to 0 the oscillations started going up, is this normal ?

Will update with plan b and c as stated above when receiving the other components
 
Also, while playing with the dc offset trimpot, i found out that imputing a negative dc offset (around -0.3V) completely stopped the oscillations and L vu meter returned to 0.

Is this related to the bad µPA68h or something else ?
 
rjb5191 said:
check wire dress. Check 2sk jfets in the control amp.
Click to expand...
I disconnected the pre-amp with the separate switch and still got oscillations.
Also tested the NOS µPA63H from japan i received and i have the same behavior : oscillations when DC Offset goes to 0v, stop when setting it to -0.3V

What do you mean by wire dress ? I checked continuity for all grounds and they are good. Also for all wires on the amp board and no discontinuity

Maybe i badly tested ceramic compensation caps and one is bad.
 
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Hello everyone, little update,
- tried changing every ceramic cap with new Kemet C0G : still oscillations
- tried LSK489 : still oscillations

This thing is kicking my a** and i'm contemplating throwing it in the trash every day i look at it dissembled on my bench...

If someone have a little insight or a bright idea i'm all ears

Maybe the next step is looking for an oscilloscope, i tried using my LMM frequency function to search for these oscillations on the power supply/diode side but haven't find any, is it even possible ?
 
Okay small update : for my final fight with that amp, i tried to re-solder back the old transistors and miracle ! 0v DC Offset and no oscillations !

Now, if someone more knowledgeable than me could explain why the TA-100WA+ from Pyramid-Audio is entering in self-oscillations with the new replacement transistors that would be amazing because i still have the original power pack on the Right side amp and everything is perfect with new transistors.

To recap: i replaced the bad power pack with an aftermarket one, as well as Qe1+Qe3 (2SC1775 to KSC1845F), Qe5+Qe7 (2SA912 to KSA1220AY), Qe9 (2SC1400 to KSC1845F) and Qe11 (2SC1885 to KSC2690AY) the new transistors caused self-oscillations on the left power amp, the old one not
 
These forums are such a godsend. I'm having this exact problem after putting Pyramid TA-100's in a KA-9100. Only on the channel where I replaced most of the transistors (long story, they probably didn't need replacing). At this point I'm just glad to know that my symptoms, running hotter & 5Mhz oscillation, have happened to someone else and there is a known resolution. I'm not sure which ones to try swapping first, Q1&Q2 with KSC1845's per the older post, or put all my original ones back in, but at least I know there is light at the end of the tunnel. I'll post back later once I have it solved and say what worked for me.
 
destructor8 said:
Okay small update : for my final fight with that amp, i tried to re-solder back the old transistors and miracle ! 0v DC Offset and no oscillations !

Now, if someone more knowledgeable than me could explain why the TA-100WA+ from Pyramid-Audio is entering in self-oscillations with the new replacement transistors that would be amazing because i still have the original power pack on the Right side amp and everything is perfect with new transistors.

To recap: i replaced the bad power pack with an aftermarket one, as well as Qe1+Qe3 (2SC1775 to KSC1845F), Qe5+Qe7 (2SA912 to KSA1220AY), Qe9 (2SC1400 to KSC1845F) and Qe11 (2SC1885 to KSC2690AY) the new transistors caused self-oscillations on the left power amp, the old one not
Click to expand...
KSA1220A/C2690A aren't good replacements for A912/C1885, IMO. Cob is way high by comparison. They're also a little slower, but I doubt that would make much of a difference. KSA1381/C3503 or something similar would be much better. Good through hole VAS transistors are getting scarce these days.
Also, where did you source your parts? If they came for ebay, Aliexpress, or the like, there is a very good chance that they are counterfeit.
ETA: IMO, there is no good reason to replace any of those transistors if they aren't faulty. They're all good parts.
 
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