KAT88 playing up circuit diagram needed

[gilbodavid]

hi.

I have a Kat88 power amplifier that seems to be going into oscillation with all speakers.

My repairer, Henry, needs the circuit diagram to do the repair.

Does anyone here have one? Thank you!

David

[gilbodavid]

some closeups pics of the circuit board

[Richard]

Hi David,
Looks a nice build from a while ago. There were one or two changes early on but yours looks to have included them. Has it just started playing up? 1 or both channels, HF or LF or a hum?
Send me a PM with your direct email and I'll send the info you need.

[Richard]

Thanks David, info sent

[shunt54321]

I built one of these back in 2000 I think. I have lost the schematic too so a while back I took it apart and draw out the circuit with full intention of rebuilding it.
Sadly it is now defunct as there are a number of quite serious design issues, one of which finished it.

The main theme is almost everything seems operated right on its maximum rating, like a 3W resistor has 3W dissipated in it etc. and then put in a hot box. The tubes are biased far too hot. An attempt at "pure class A"!

I rebuilt this amp many years ago after it red plated a tube, resulting in it's cathode bypass capacitor exploding, stinking out the dining room for a few weeks!

About a year later after repair it ate some more tubes with a little firework display so gave up on it for a decade or so.

I now have it stripped to find one of the output transformers has it's primary open cct. EOL!

When I draw out the cct. I realised just how many serious errors there are in this design. Like 8 Volts of ripple on the screen grids. This explains why I was never impressed favourably with its sound quality. That will have modulated the audio with 100Hz.

So put 1KHz in and out comes with 900Hz & 1100Hz sidebands.

Also as far as I can tell the preamp HT supply over volts it's supply filter caps on power up, until the filaments warm up. Couldn't quite believe that so had to triple check my cct.

It's all fixable but not in my one. So if you want any spares let me know.

If you want advice on how to make it sound better and make it safe to operate then let me know. It needs the preamp supplies sorting. It needs its screen grid supplies smoothing. It needs a re-bias to class AB to stop it lunching tubes.

But if nothing else IT NEEDS HT FUSES !!!!!!!!!!!!!!

Regards
Stu

PS. If anyone has an original cct I'd be intrigued to see how accurate my scribbled cct. is?

[bob orbell]

A freind of mine built one of these in 1995, it is still going and sounds very good with a few mods that Richard has spoken about, may be you just got a bummer or it was poorly assembled. Bob