Kenwood model 600 bias problem

[Chrisr3521]

So i have restored my model 600 back to its former glory i.e completely recapped, new output the 2st5949's and 2st2121's new adjustment pots, zeners on poweramps etc.

Only now i have wondering bias, on switch on it goes higher than the 25mv it should, not a problem it quickly drops to around ~30mv leave it for a half hour and it drops some more to around ~23mv, adjust it to 25mv as per manual and it just drifts between ~20mv and ~26mv.

Is there any obvious cause for this? Schematic attached.

Thanks
Chris

 

[sgrossklass]

I'd guess thermal feedback is not unconditionally stable. There's a considerable delay in the circuit depending on where on the heatsink the thermistor is mounted, and delays in control systems always are potential trouble.

Thermal feedback in amplifiers is a somewhat obscure topic. I'm aware of Douglas Self discussing it in his power amp book, other than that not sure. It seems to be advantageous to track the drivers rather than the finals in a number of cases, but I forgot when and why. (I can see it making sense when the drivers are not mounted on the big heatsink. Their temperature would change far more quickly.)

 

[Chrisr3521]

I thought this too, but surely being kenwoods statement amp at the time you would think this is very poorly implemented. Do you think its anything to be concerned about?

 

[electrochap]

Kenwood 600 ??? I will try not to help you (to buy it... well too much shipping...).

No... Seriously bias is not an exact science. It can fluctuate a bit, more at startup. Between 20 & 26 is not much a variation. But starting higher is weird though. Usually it should start at 0 or pretty low and raise slowly to 25mv. I would look for bad contacts or bad soldering. Wasd it like this before your restoration?

I'm banging my head working on a Kenwood L-07M and the amp circuit is almost the same as this one. Also works at 25mv measured bias but has 3 O/Ps on each side.

I'm also waiting for my brother to get me back the KA-8007 I lend him years ago. This little brother to the 600 is also playing quite well. I would absolutely love to run a 600A and 600T in my life. If my shellf can support those 2 beasts.

Great amp! Don't ever leave it! And get it's tuner!

 

[Chrisr3521]

Do'es anyone think there's a problem with this? i'm not 100% sure there is, the thermister works as it should its just that little niggle of the voltage not staying constant for long.

 

[Marmotus]

I discovered such variation and found that I had the 600 standing by side when I set bias, being so precise that in normal position bias became higher. After this conclusion I set bias in normal position (at the edge of the table...)
and it remains stable at 25mv. BTW very little heat generated, mainly by drivers and PS card, in normal and past normal use.
Hope this helps.
Cheers,
Marcelo

 

[Chrisr3521]

So stood on its side the bias drifted but in its normal position i.e like your picture the bias was stable?

Thanks

 

[Marmotus]

So stood on its side the bias drifted but in its normal position i.e like your picture the bias was stable?

Thanks

Yes, that was my experience. It works as it should, now.
BTW, it's 100% original, low hours count use when it came to me, 35 years of masterpiece sounding specially. I found every component I tested in good shape so I had to stop myself in my long practice of recapping, I think two times this happened to me, the other with a Pioneer SX-1050... nothing to do inside...

 

[Chrisr3521]

Well the advise was good voltage now stable.

Thanks