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GReg/Electrical Tech Journal

All your geeky questions about electronics, microphones, tape machines, etc.

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Re: GReg/Electrical Tech Journal

Postby greg on Sat Jul 20, 2019 9:05 pm

Neotek Series II-

Mute leds on unmuted tracks flickering cyclicly causing quiet "tick-tick-tick" noises on the 2-mix bus.
This could be stopped by turning off the Mute Group/Solo logic feature (mod).


I narrowed it down to channel 26's Mute Group/Solo isolate function switch (there's one on every channel). When I set it to the equivalent of "solo defeat" (not muting when other channels solo), all was good. Pulling the channel out, I saw that a diode was barely touching the leg of the mute relay transistor (both mounted on the switch itself). I simply separated them and tested the channel. Works now, but the problem went away before, then came back after a day or two. I don't know why this problem just now showed up, other than maybe it was warmer in there at some point and things expanded? The room is pretty well climate controlled, and mostly cold. I'll go with ghost.
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Re: GReg/Electrical Tech Journal

Postby greg on Mon Jul 29, 2019 5:24 pm

AB International Model 600LX Power Amp-

No or really low output. This was being used for the coffee table set of speakers in St A's control room.


The ±15 power rails were being loaded to less than 1 volt. These power rails supply the input op amp, and discrete input section of the power amp. They are derived from the ±80 volt unregulated DC rails by a simple zener clamp regulator circuit (no pass transistor). Using the bench supply to power that ±15v path, I could see a large draw of current, which I isolated to the input board. This board had only one dual amp (4558), which was f'ed. I replaced it then tested things out.
Incidentally, the cooling fan (brushless 12v DC) was frozen. I disassembled, cleaned and lubricated it. Now it's as good as new(?). Never did that before.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Jul 30, 2019 12:17 pm

Neotek Series II-

Replaced noisy control room monitor pot.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Aug 01, 2019 5:57 pm

Yamaha Model P2050 power amp-

Left channel (A) intermittently loosing volume/distorting.

The failure seemed to be only at certain gain settings. I was able to cause the problem by putting pressure on the back of the volume pot. The pot is actually a rotary switch made by Alps, and is pretty easy to disassemble. I cleaned the wiper-to-pcb contact area with alcohol, then applied contact lubricant (MS-738) staying clear of the carbon resistance areas. After a bunch of testing, all is good. It's a unique "pot" (33.3kΩ pot equivalent).
Let's see if this treatment lasts.
I also replaced the 4 electrolytics in the signal path while I was in there. They measured fine but this thing is 30+ years old, and I had the parts...
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Re: GReg/Electrical Tech Journal

Postby greg on Mon Aug 05, 2019 5:43 pm

Beyer M380

Distortion when used on a bass drum.

Poking around carefully, looking for garbage in the air gap. I couldn't find any obvious obstructions, so I saw it as an opportunity to try a replacement driver/capsule from a pair of headphones sent to us by FM FransFromBavaria. The capsule I'm using was marked dead and measured open-circuit. He threw it in with the "good" ones just in case. I'm glad he did, because I managed to reflow the dead joint of one of the windings to a terminal post successfully.
The capsule diaphragm was covered in hair (surface tacky) and the center dome was inverted and stuck to the magnet (equivalent looking to the dust cover on a speaker). I pulled the dome out with some tape (peeling gently while supporting the diaphragm with a flattened q-tip stick). The hair was cleaned off with tweezers, again supporting the diaphragm with the stick. It's certainly possible to reuse the humbucking coil, but I thought I'd test the capsule in action first. As noted in one of the Beyer 380 threads here, the HP capsule had an identical form factor and fit perfectly in the M380 mounting frame. The build of the HP capsule looks generally similar to the original except for the shape of the vents around the magnet's perimeter, circular PCB for the humbucker/solder connections and no hole in the center of the magnet (not present on all our mics). The back of the HP capsule also had a black nylon mesh dust cover, as opposed to the original's naked back (which is covered by a dense cotton disk when put together). After some voice testing, I put everything back together. The front and back sounded significantly different (back having less high frequency response). I removed the cotton disk covering the back side of the capsule, and listened again. That helped match it a bit. I realized that this completed setup looks like what Beyerdynamic did with one of our M380s when we had them repair it years ago. This was after they ran out of original capsules, and said they'd be using an alternative. I'm guessing this was they did, or something similar.
Let's see what we get!


Several days later and after a couple of sessions in use, the thing seems to work as expected. I haven't tried a head-to-head comparison yet. I'd be surprised if they sounded identical, but close enough would be good.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Aug 16, 2019 12:41 pm

Studer A820 MCH Blacksides (studio B)-

Channel 23 output level low.

Problem follows the Line Amp "Trafoless" board. There is an output trim adjustment (need the extender board to do the calibration), but the random, all of a sudden nature of this made me suspect the board, which looks like it has the original caps. This must have been a replacement someone put in to troubleshoot something since the machine was recapped a while ago.
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