Kenwood KR-5600 Strange Tone Problem - Know what this is?

[tusntuk2]

I'm rebuilding a Kenny 5600. Original description was "one side out", and all the lamps are out. This improved some with deoxing the controls, but there were two anomalies. The FM had terrible distortion on one side, and while checking a signal thru the AUX, the Treble control, when turned to full open, suddenly made a weird high noise in the headphones. Other than that it seemed to be sort of working.

Inspection revealed a badly burned PS board, a documented design problem. I totally repopulated a second board and installed it. (that's a real bugger, boys, I don't recommend it for the faint of heart!) I recapped the AUDIO board, and changed diodes and transistors for the relay circuit and the bias circuit.

View media item 21721View media item 21722
Passed DBT, to my delight. FM shows same weak side, bad distortion, so I hope thats in the FM. I switched to AUX and things sounded great! For a minute. I had powered back up with all controls down, so I was testing controls. Bass, fine, Loudness, fine, Balance, fine, Treble - was working until about 4 o'clock on the dial. Heard a tiny pop in the headphones and that sudden high pitched noise when I went to the max on the control. The DBT glowed bright and I quickly turned the Treble control back. Fuses did not blow, the noise stopped, and the amp sounded great.

What do you think causes that sudden surge at full treble? As a musician, I would equate it to Feedback, that sudden squeal. What is happening to the amp? Some kind of oscillation?
How do I find it, where do I look, what can I measure to get off the hook?

As usual, your fine help and insight is greatly appreciated!

schematic available

 

[audio3000]

You mentioned you had recapped the audio amp board.

1. By any chance that Cm11 and Cm25 were replaced with polar ones?
2. Make sure the Miller caps (Cm5, Cm7, Cm27) are installed with the correct values.
3. Make sure Rm57 and Cm35 are good.

Items 2 and 3 about are added to create sufficient operating margin for the amp to prevent self-oscillation.

Also, any transistor replaced on the amp board?

 

[tusntuk2]

Thank you for your quick response! I am a bit perplexed with #2 and #3, because these are parts I did not replace, and they have corresponding parts in the other channel as well. They are small ceramics.

1. I replaced Cm11 and Cm25 with Non-Polar caps. I often raise the voltage spec on electrolytics, but on these two caps (actually 4, Cm11-12, Cm 25-26) I used exact replacements, a Nichicon UES I believe.

2. I'm not sure of the term "Miller Caps". On the KR-5600 the caps Cm5, Cm7, Cm27 and even Cm 35 are all small ceramics. These were not replaced. They also have correponding alternate channel ceramics, Cm6, Cm8, and Cm28. Hope we have the same schematic!

3. Rm57 (and Rm58) is a 4.7 ohm 2 watt to ground on the output. I tested those in place as good, but I had bought the parts to change to a 3 watt resistor.
Because of the PS board difficulty I had reconstructed the amp to test my work at that point, which is when I found the problem. I still wanted to recap the muting circuit and the preamp boards, then consider the FM board.

Besides Qk1, the voltage regulator, I also replaced Qm7 and Qm8, the bias transistors, with KSA992, and I replaced the relay resistors Qm25 (KSC 1845) and Qm26 (KSC2382Y). These seemed to function correctly. I have not replaced or tested any others on the main board, but it does play.

Thanks again for your help! Hope we can figure this one out, it's one of my favorite amps of all time.