Kenwood KA-6000 recap, transistor upgrade = motor boat?!?!

blhagstrom

Mad Scientist, fixer.
Subscriber
Got a pile of gear in to clean, repair, upgrade, restore.

Was just finishing the Kenwood KA-6000. Recapped all E-caps except main monster filter cap. Had all the boards done. Changed all the 2SC871 in there. There were bad ones on the amp and noisy ones everywhere else. Dropped in KSC1845.

Everything was fine. Went for the main coupling caps, restuffed them, fine, good.
Hit the smaller mounted caps, restuffed them. and then bop-bop-bop.
Double checked the work, twice. Yup, fine.

About a 1-2hz popping.

I can get it to stop by engaging the low filter.
While not surging, I checked the center voltage and current draw setting and dialed that all in perfectly.

Separating the pre-amp and amp stops the cycle. Each appears to work fine while separated.

SO, what?
First time I've hit this.

Any hints, tips, tricks? Places to look? Things to check?

Oh, both sides seem to be effected but more on the right than left.

????
 
I've only had motorboating before post-work after dropping in sub transistors for the former ones in a sansui phono circuit. The gain of the new subs was too high ("C" grade, so I dropped down to "B" grade), after that the issue stopped. So I guess in my case, some kind of circular LF oscillation directly caused by transistor choice was my issue.
 
Gain. The 6000 model is stuffed from go to whoa with 458 using the entire range of B,C & D. If I had a shot I'd look at replacing the 1845 with 1815Y on the main amp board.
 
This one had lots of the 2SC458, it had the just-as-bad 2SC871 also.
Thanks for the amp suggestion to go to the 1815Y. Funny it didn't act up before the last cap change.
I did find a service bulletin about the LF switch being wired wrong and that switch does have an effect.
 
Last edited:
Gain. The 6000 model is stuffed from go to whoa with 458 using the entire range of B,C & D. If I had a shot I'd look at replacing the 1845 with 1815Y on the main amp board.

BINGO.

I looked at the amp design and it looks like a series of hot replacements could push things a bit.

I was short of KSC1815 so I ordered a pile. They arrived and I swapped out the KSC1845 and she's behaving now.

Thank you.
 
Damn, not sure where I found that.
I usually go to hifiengine and hifi-manuals.

The LF switch on this one was wired correctly.
 
I'v heard this "motor boat" in an amp with a failed repair, the guy placed a transistor backwards in the power supply, feeding asymmetrical voltage to the preamp.
 
Back
Top Bottom