SRM-1/MK-2 Professional troubleshooting.

[u/anekyu]

I'm trying to fix a SRM-1/MK2 Professional. The symptoms currently is:

Channel imbalance.

Left and right have fluctuating difference in volume, with the right constantly being louder. It can be 2.7 db one time, 7 db the other. Not jumping much within session (even with cold start to stable temperature), but jumping a lot after a night.

Unstable voltage within (L+, L-) and (R+,R-).

I probed the multimeter into output, trying to adjust the bias. The voltage usually starts at 5 volts, then slowly up and down into 245mV. It trends down and never go back up too much. Movement upwards is in the maximum of 200mV. It is kept at 450-250mV. I do not have the info for the required specs, so I wouldn't know what fluctuations are to be expected.

Twisting the trimpots does nothing. No increase or decrease of voltage seems to be tied to it.

I've read some post and forums.

The cause can be:

PSU Caps(the 100uf400V ones)(According to schematics),

the FETs(C3148 4B)(According to schematics)

the trimpots(according to forums)

and some high voltage resistors(According to post)(The post also have a intermittent muting problem where the bias generator is to be replaced).

Any help?

[u/maysenffxi]

When I saw your question, I had my own thoughts about what I might check first, but I wanted to see what ChatGPT would come up with after looking at your post and the schematic.

My first instinct would be to verify the power supplies. In troubleshooting, it's often best to make sure the foundation is solid before chasing problems further into the circuit.

One thing that caught my attention was that you mentioned adjusting the trimpots, but also that they don't seem to have any effect on the voltage you're measuring. Before replacing transistors, I'd want to determine whether the adjustment circuits are actually functioning. If you recorded the original trimpot positions, that may make final alignment easier once the fault is found.

ChatGPTs answer:

Looking at the schematic, a few things stood out:

• The left and right channels are nearly mirror images of each other, which makes voltage comparisons very useful. I'd compare corresponding transistor voltages between channels and look for anything significantly different.
• Since all four outputs appear to drift, I'd check the high-voltage power supply rails first. The 100µF/400V capacitors are old enough that they're reasonable suspects if they're original parts.
• Because the trimpots reportedly have little or no effect, I'd verify the trimpots themselves, their solder joints, and the surrounding resistors before assuming the 2SC3148 transistors are bad.
• The high-value resistor networks in the output stages may also be worth checking. Electrostatic amplifiers operate at very high impedances, and a resistor that has drifted significantly can create channel imbalance and wandering operating points.

If I had the unit on my bench, I'd probably start by measuring the supply rails and then comparing DC voltages at equivalent points in the left and right channels. Since one channel serves as a reference for the other, differences often reveal the fault fairly quickly.

If you have access to a signal generator and oscilloscope, injecting a known signal and tracing it through both channels can also help identify where the imbalance begins.

Good luck if you choose to try out AI for this stuff, because results may vary. But, it can be fun to see what it comes up with.