MORE SC200 amplifier unstable bias voltage across output transistor resistors troubleshooting

Hi I am still having issues biasing the Sc200 power amplifiers a ccurately. Biasing the output so that 4.4mv - 5mv appears across the ouput transistor resisrors appears to be an impossible task. When I try to adjust the bias on one channel in particular i wind the trim pot VR1 until the mV across t increases upto about 2 mv then as i wind the trim pot further the mV valve drops to zero. This is when the heatsink is at room temperature. I wind the trim pot further and the mV increases further then drops to zero again. The mV values seem to reach a value of 3 millivolts  and then reduces to between 1-2 mV. The mV value seems very unstable. If I set the mV value to 3mV and let the heat sink warm up then and then turn the trim pot further slowly to increases the mV drop  does increases but then is reduced over time, sometimes to almost zero. I have also observed the mV value across the 0.1 ohm resistors increases but the voltage between TP1 - TP2 decrease. The heat sink temperature was quite hot at this point. The mV drop reached about 20mV so I wound the trim pot back to zero again. I have both amplifiers connected to one heat sink. I have set the mV value of one amplifier to about 3mV across each transistor more or less because setting a higher vale close to 5mv has proved very difficult and has produced unstable result.I tested a typical 1kohm trim pot and i set set the trim pot valve to within a couple ohms fairly easily.
The biasing cirecuit on the SC200 amplifier seems impossible to set accuratelyHas anyone else had similar problems.
ThanksHeath

Hi,  I have copied Editorial in on this and will try to get advice from Silicon Chip.Thank you for your patience,
Alan W

I have 2 more boards which I am going to build and see if I have the same issues biasing the amplifiers.
RegardsHeath

I have got round to building the other 2 amp modules which were absoluely fine, The bias circuit is completely stable and eaily set. I tested unused cermet bias pots months ago becase i thought that these were the problem but the samples i tested were fine. So i decided to swap out the bias transistors and this made no difference to bias issue. After virtualy swapping all the transistors on one board this made no difference. I then removed the cermet pot and discovered it was terrible and completely nonlinear, the resistance jumped up and down as the screw was turned in a nolinear fashion jumping 15 ohms with a tiny adjustment. I installed working cermets after testing which were completely linear. I can now adjust aand set the bias circuit accurately. I  initialy bought the cheaper suntan cermts which failed at £0.3. Next time i will try the more expensive bourns cermets at £0.83 hopefully these should be better.Regards
Heath

Some folks in the DIY audio forum report recent success in building this amplifier, and you might find other constructors who can help there. DIY Audio.

Heath: Thanks for sharing your sleuthing process and success with us.

Bourns are well known for quality products, especially potentiometers and  resistors.  Good choice.

I had the same bias issue as Heath, impossible to adjust bias.  I only built one module and regrettably destroyed the output transistors in the process. I pretty much experienced exactly the same symptoms but bias ran away on me before  I could turn off the power
I will be replacing the cermet as well it seems...

Yes I nearly had the amp nearly thermaly  runaway because of the
biasing thermet problem. The bias voltage increased to 30mv before i new it but i caught it in time and powered of. Just make sure the thermets you install you
test first and the resistance you can set accurately to a tolerance of
an ohm . The ones i installed i can set the resistance to a toleranece
of an ohm  easily and linearly up and down in resistance  as i turn the
thermet screw.
RegardsHeath