Having completed the breadboard for a Buffalo III - dual mono, I'm now ready to replace the trimmer pots for voltage and shunt current regulation on both of the Placid HD BPs and both of the Placid HDs with fixed-value resistors. The voltage and shunt currents on the Placid HD BPs remain very stable, as well as the voltage on the Placid HDs.

The problem is regulating the shunt current on both of the Placid HDs. I allow the entire set-up to warm up very well, usually overnight, before trying to adjust the power supplies.

I feel like a dog who is chasing his tail. This morning I measured the shunt values and found that the left Placid HD was reading 134.8 mA and the right was measuring 0.00 mA. I reset the values to ~50 mA and remeasured them 1/2 hour later. The left was measureing 196.8 mA and the right 34.2 mA. Without stability in setting shunt current I'm concerned about two things: 1) Damage to the circuits, 2) The inability to select the correct fixed-value resistors to replace the variable ones.

I believe that the problem is heat from the heatsinks, which are very close to the shunt regulation trimmer pots. As a precautionary measure, I have installed 2" heatsinks in place of the 1 1/2" heatsinks provided in the kit. However, I hasten to add that the temperatures of the transistors measured with either size heatsink was well within the limits spelled out by Russ elsewhere on this forum.

I would be sincerely grateful for any insight into the resolution of this problem. Thanks!

Leon,

Thank you for your excellent suggestion. I will measure the actual resistance at the trim pot each time that I set the calibration; in that way I'll know what the resistance is at the shunt calibration. Hopefully, I'll be able to determine the value of a fixed-value resistor to substitute.

eseal