536
by greg_Archive
Beyerdynamic M500Plastic filter/diffuser piece broke loose.
The glue looked like it simply dried up, so I used a little contact cement to tack it back in place. Also replaced the foam in the windscreen.
Finished the SM-2 power supply (waited for new caps).
SM-2 s/n 193
Lower capsule output noisy. Sounds fine for 2 minutes or so, then there's an explosion-crackle-washing sound, and the sound is thin and noisy. Incidentally, the anode voltage drops at the same time, thought the B+ is solid.
Looked around for culprits other than the tube, reflowed solder joints for the tube and other components. No joy. Replaced the tube with a spare, and the mic sounded right. I put the old tube back just to make sure it was the source, and it worked. More probing... Left the mic on for another 5 hours. No evidence of the noise returning. I'm going to leave it like that. Not stoked when I can't be sure of the thing that was causing this.
Later-Discovered the output would fail, with a tonn of noise if the mic was tapped semi-firmly. Up until that, the output was identical to the upper output. Dug in.
I flayed the microphone looking for rotten guts again, with a little more focus on the passive components. This mic was partially modified to the "C" version schematic, which includes bypassing the cathode, and some passive component changes. Of the original high value (30-150mΩ) resistors that were in there (from the factory), about half of them measured 50-100% higher than their labeled value. More than a few of the 150mΩ resistors were especially bad. I replaced all of them. R15 and R16 were leftover from the old version (voltage divider for the capsule back plate(s) voltage). They measured correct and provided the required voltage, so I left them alone for now. C5-8 were 6.8nF, a value which isn't in any of the schematic versions I've seen. They are tubular ceramic capacitors (which look like mini power resistors). They measured correct to their labeled value. I'll probably replace them with modern guys of the "C" schematic some day, but didn't have any on hand. A key discovery in doing the work was finding a hairline fracture in the connection between the tube's plate and R18 (plate resistor). (:uTested a bunch on the bench, and in session. Let's hope it stays working.
-Elsewhere-
More EMI TG12345 MkII/III work.Mostly relating to cleaning and rebuilding Painton rotary switches (pan pot, and level related). I had to replace a resistor in one. That was a slow process.
Modified the echo returns with input pads to match the +4dBu world with the lower nominal level of the console (the line inputs had already been done).
Greg Norman FG