c84 rack with oscillation

[fft512]

Hi all
I am completing a rack of 8 C84s. For each of the five channels I built yesterday, I terminated the input to 850Ohms and did attach it in isoltaion to the PS03 in the CH01 and adjusted the supply voltage to +/-16V (to slightly relax on the power dissipation and heat in the U1). Following step38 of c84_asmbly.pdf, I measured approx. 11V. Following step39 I noticed that some channels always exhibit stable DC offset whereas others drifted by lots (200-300mV). Sometimes the channels would start-off with stable DC offset and start drifting all of a sudden.
Next I mounted all 8 channels into CH01 and repeated step38 and step39 - the same observation. I took the oscilloscope and noted strong (up to several hundreds of millivolts) oscillations at approx. 4-6MHz at the junction of R8 and R9. I tried tracing the oscillations and noticed them allover, but very strongly also on the 48V phantom supply. I then gradually unplugged the channels from PS03 to see if I can find the 'one' causing the oscillations. This was not possible, rather it appeared that pontentially all channels can get the oscillations to start. The measured strength of the oscillations depends on the gain settings. The PS03 in isolation (no channel attached) does not show any oscillation.
Question: what may cause the oscillations and how can they be supressed ?

[tpryan]

Interesting, we haven't seen those symptoms.

Did you add IC sockets?
What happens if you terminate the input with 150 ohms?
Do your modules have LM394 or MAT12 transistors?

[fft512]

thanks

I have not mounted IC sockets, the channels run with LM394, all components are as were supplied with the kits. I tried with 600Ohms with the same symptoms. Any more suggestions what I can try ?

[tpryan]

600 ohms across the input or the output? What happens if you terminate the input with 150 ohms? What about a short circuit?