[jan-jaap]

Couple of clarifications:

1. I only have a single L&H Research "Mighty Mite" PSU. It belongs to the Professional IRIS. So I don't have a working reference.

2. When I got this system 10 years ago, the PSU already made the coil whine noises. So I replaced the original PSU with a PowerOne SPM5 model. I did not adapted the wiring harness to supply 'sense' feedback to the PowrOne PSU either.

3. The original L&H PSU would put out correct voltages, under load, even though it would make unbearable coil whine noises.

4. With the new 2N6678 power transistors, the coil whine was gone, but an oscillation on the +5V rail had appeared (and corresponding 'beep' sound from the PSU)

5. With 'sense' feedback disconnected, the oscillation (and the beep) stops. The L&H puts out correct voltages, no noise on the rails beyond what can be expected and basically works. I understand the need for sense feedback in the design here, but since the system configuration is static I think I can get away with leaving it disconnected.

I do have some tools, like an oscilloscope (with isolated probes), a multimeter, a capacitor meter with ESR meter, and have access to quite a few tools from the lab at work. But this is not straightforward to troubleshoot. In the 4th picture of my previous post you can see the -5, +/-12V modules and the 5V control logic with the faulty 2N6678 chopper transistors. Where more modern designs use a single chip like a UC3844 and an optocoupler in the feedback loop, this uses a whole lot of analogue components to do the same. There's half a dozen adjustments (potmeters) on the side. The original 2N6678 transistors were probably matched -- somebody wrote '329' on all of them. But matched for what?

As it is right now, I have a functional PSU, for as long as I leave the sense disconnected. Sure, it would be nice to have everything work 100%, but without schematics or some sort of tech manual I'm not sure it's realistic. Also, it takes a lot of time to remove this PSU from the system and wire everything back up. I do have a set of Motorola 2N6678 transistors on the way -- I did find some in the end. Will they be genuine? Who knows. It was a lot of 4 and they were cheap, if they were counterfeit it didn't bring much. Hopefully it's just old stock. When I get back in I'll take this module to the lab to inspect it for bad soldering joints and can eventually replace any small caps in there. But I'm hesitant to mess with all this analogue circuitry without a way to calibrate it when I'm done.

Next, I'm going to turn my attention to the PowerOne PSUs of the PowerSeries. I have several working and several dead specimen. A much more structured approach will be possible here.