KA-5600 variable oscillation - where do I look?

[tusntuk2]

Still puzzled about finding the cause of the variable oscillation in the Audio board after recapping electrolytics. Low/high tap gets faster with more volume or more bass after unit is warm, no noise when cold. I'll be changing the bias trimmers to multi-turns, and I wonder if changing the transistors Qm7, Qm8 would help. Bias fluctuates pretty high when the amp heats up, then the drumbeat sounds. Please help, it's beyond me! I guess it gives me a new opportunity to learn, right? Thanks so much!

 

[slimecity]

So by "oscillation" - do you mean a regular thumping low freq noise thru the speakers?

You don't mean oscillation that you are measuring and seeing on a scope?

If its the former - does it happen in both channels - what inputs does it happen on (incl phono)?

 

[tusntuk2]

Thanks for the reply! I looked around AK and the best description was oscillation. I do not have a scope, but after the amp is warm it will sound wup-wup-wup (90-120hz?) about 3 in 2s, and I see the speaker cones move (bounce!) in my tiny test speakers. Also matching higher pitched buzz, can cut it out with the treble control. As the volume or bass is increased, the speed of the wup increases. The output this side gets hot, bias goes to 90mv or so and I shut it down. Phono 1, 2 and Aux, no signal. The FM tuner plays fine, sounds great, with no noticeable distortion, but then again I haven't cranked it loud. This side bias varies more quickly, but is it a cause or a response? I installed new KSA992 for Q7,8, and new Bourns trimmers. It helped tame the noise at low volume, but not as the amp cranks up. Voltages I've checked against the s/m have all seemed pretty reasonable, nothing crazy. Not sure where next to look, so I wish I did have a scope!

 

[tusntuk2]

Well, it gets a little more interesting. I have rechecked main PS, checks OK, pin voltages OK. I replaced V/I voltage limiter transistors per Echo Wars suggestion in an old thread. I replaced 4 diodes there as well, and Zener diode, too, per another thread. The FM sounds fabulous, but I get the motorboating when I switch to Phono 1,2, or Aux and start to increase the volume. So I split the Preamp and Power amp, and ran my Kindle in the Power amp. Beautiful sound, no apparent problem. Power amp alone does not motorboat. Haven't tried the Preamp alone yet, need to find another amp. BUT, if the problem is being generated by the Preamp, then how do I check that to find the source? I did not replace caps in the Pre, only the main Audio amp and PS.

The Kenwood KR-5600 is a bugger to get the Preamp! Anybody have any experience with that? thanks so much!