Curbside Kenwood

[Hoya Fan]

I found a Kenwood Super Eleven near my place a few months ago. The power cord was cut, and the wood case was in less than stellar condition. I had a good tech fix the cord. He resoldered a few things on the PCB. The receiver had a bad left channel with a lot of distortion, though. He didn't pin that problem down, and this went unrepaired.

I took the monster to another local shop in late March. The problem persisted, but it became intermittent.

I got the Eleven back from this shop with the tuner and lights in much better shape, at least. I cleaned the balance and control pots as best I could, several times. Still no joy.
In disgust, I opened up the case to visually check for bad caps or other easily identifiable problems. That's when I noticed the variable resistors on the main PCB. I figured: if I botch this, it cannot sound much worse than it already does.

I spent 20 minutes adjusting two of them. At one point, the Kenwood's protection circuit kicked in and stayed engaged. I unplugged the monster and put that resistor back to Last Known Good Configuration, and when I plugged it back into the AC and turned it back on, it worked again. Major improvement in the sound now, about 90% of the problem is gone. There is still a small amount of distortion in the bad channel at medium/high volume, say around 11:00 or 12:00 on the volume knob.

My question is... is the volume pot still dirty, or is something else causing that last bit of the problem?

[nightanole]

Id say you still have bad caps, a dirty volume pot should not cause distortion. Just because they're not swol doesnt mean they're still good. Your pots are probably bias and transistor matching. So one pot sets how much current is going to run through the transistors, and the other makes sure equal current goes to each transistor in the pair.

[ostripper]

An amp that old most likely has every electrolytic "on the edge" if not outright leaking. I posted the whole schema in "solid state pix". My 20 YO Nikko had the big caps leaked all over the board... The small HV supply caps bulging. If I was to fix it ( I gutted it instead - reused the trafo/heatsinks/case) I would of replaced ALL the caps. Caps are consumables , 10-15 years MAX. It is a nice 120W amp , worth restoring.
PS - looks like .22R resistors (big square ones near heatsinks) read 13-15mv across one of them. you might have a bad spot on the cheap bias control - that why it went into protect mode (opened up - too much current) replace them with 25 turn precision trimmers. Put heatsinks on them poor drivers , too.
OS

[Hoya Fan]

"PS - looks like .22R resistors (big square ones near heatsinks) read 13-15mv across one of them. you might have a bad spot on the cheap bias control - that why it went into protect mode (opened up - too much current) replace them with 25 turn precision trimmers."

I am a newb at working on audio equipment (I build computers, which is MUCH easier than this), so some of what you write zooms well over my head.
The two pair of vertical white squares should be replaced with precision trimmers.

Like these?
3296Z-1-253LF Bourns Trimmer Resistors - Multi Turn

Thanks for the help. This Eleven had me stumped.

[Hoya Fan]

Originally Posted by nightanole

Id say you still have bad caps, a dirty volume pot should not cause distortion. Just because they're not swol doesnt mean they're still good. Your pots are probably bias and transistor matching. So one pot sets how much current is going to run through the transistors, and the other makes sure equal current goes to each transistor in the pair.

I'll look around for some now caps. The big power filter ones will have to be replaced first, I think.

[ostripper]

Originally Posted by Hoya Fan

"PS - looks like .22R resistors (big square ones near heatsinks) read 13-15mv across one of them. you might have a bad spot on the cheap bias control - that why it went into protect mode (opened up - too much current) replace them with 25 turn precision trimmers."

I am a newb at working on audio equipment (I build computers, which is MUCH easier than this), so some of what you write zooms well over my head.
The two pair of vertical white squares should be replaced with precision trimmers.

Like these?
3296Z-1-253LF Bourns Trimmer Resistors - Multi Turn

Thanks for the help. This Eleven had me stumped.

white square things are the emitter resistors from the outputs to the speaker. When the large output transistors are biased correctly... 15mv- .015/.22 = 68ma per device. do not replace these with a trimmer. The trimmer should be the only control close to the big output transistor/ emitter resistors , it is 1/2w (small power) but critical to the thermal stability of the amp.

If you have it working now , be thankful. With it's advanced age , you are lucky it has a protection circuit .... between the bad caps and old trimmers , it could go up in smoke at any time. I build PC's, too. My PC knowledge compliments my amp knowledge. Actually , a switching power supply in a PC is 10 X more sophisticated than these kenwood amps. But if you just "swap" PS's , that's a different story. I only bother with repair of 700-1KW PC supplies ... all the rest - to the garbage.

(Below 1) is a basic amp like the kenwood . "B" is the emitter resistor and should have 10-20mV across it. "A" is one of those precision trimmers , clockwise makes the mV go up , counter clockwise makes it go down.


OS

[Hoya Fan]

and changed the speaker wire from spade terminations to bare wire. So far, so good. The monster is running very warm though, much more so than it normally does. Hopefully, the protection circuit won't be needed (and nothing will burn up).

I'll leave the local classical FM station going for a while longer while I get some things done (they have been playing opera all afternoon now).

Thanks for the download, ostripper.

[Hoya Fan]

Anyone know how to accomplish these tasks? I don't want the big Kenwood to end up resembling a certain Deep Purple song, sans the water.

Any help on this is appreciated.

[ilimzn]

Mucking about with trimmer pots for bias current may end up in dead output transistors, as in most cases extreme settings are really extreme - one is zero bias (lots of distortion), the other is WAY too much bias = too much heat and dead transistors.
STOP twiddling those pots without knowing exactly what you are doing or your curbside find may well end up on the curbside again, in even worse shape. Honestly, i really don't understand where people get ideas that if it can be adjusted, it should, without the faintest on what is adjusted and shy. Would you open up the engine bay of your car and randomly screw or unscrew bolts and screws just because you can obviously do that?!

[dickl]

Good luck.....I found a pair of Advent speakers in the trash on Mulholland drive one day and picked them up.

One woofer was gone.....the other destroyed. I replaced with some equivalent Vifa woofers and been using them for 29 years......great tweeter/midrange....wonderful speakers....very strong, solid boxes