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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.

Sep 20,
The mic is back in the shop with complaints of distortion on a moderate level bass amp (not there with another m380).
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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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Re: GReg/Electrical Tech Journal

Postby greg on Wed Sep 11, 2019 5:21 pm

dbx 911 -

One module has a distorted output on the decoder.

The timing cap in the RMS detector measured 1/100th it's stated value. Replaced with a new cap. Works great now.

Cleaned a bypass switch on another module...
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Sep 12, 2019 3:40 pm

Studer A820MCH 16 track

Channel 15 Repro low level rumble/static at 30ips, but not there in sync playback (meaning it's not being recorded).

In edit mode, I hand wound the tape from slow to fast and heard silence to a gradual build of the noise. This sounded familiar.
Head cores need to be re-grounded. Measuring various resistances, but none more extreme than track 15 which was > 250kΩ. The heads look a little worn anyway, so I think it's time to call John French...



When the tone generator is on, signal is coming out of channel 24. This shouldn't happen, because channels 17-24 automatically shut off when the 16-track headblock is mounted. All audio should mute and status indicators go dark on those channels.

I swapped the Line Amp then Internal NRS Controller boards with another channel in that last 8. The problem followed the Internal NRS board. This is odd because the mute function and execution are performed by the Line Amplifier board. The logic signal, "CA-MUTxy" ("xy"= track number) derived from the data buss within the Line Amp board. There's no direct way the NRS Controller could produce signal on the output of the machine without the Line Amp helping.

I've seen bad logic ICs causing problems on unrelated boards before, so I started replacing ICs that were directly connected to the data and address busses on the NRS Controller. Eventually, replacing IC12 (MC14556/CD4556) solved the problem. This chip was directly connected to the address bus and sent the signal "CA-LSOxy" back to the Line Amp board (I don't know what that is or does). Just to make sure it wasn't just a poor connection in the socket, I tried the chip in another NRS Controller board and it did the same thing.
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Re: GReg/Electrical Tech Journal

Postby steve on Thu Sep 12, 2019 5:32 pm

Excellent detective work Sergeant.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Sep 17, 2019 12:21 pm

Sennheiser MD421 N

Weak, thin sound.

One of the wires from the filter/switch output to the DIN connector broke (probably from DIN connector being loose and spinning in place a bit).

Slight hum.

Checked continuity all over the metal case parts of the mic and ground pin. All was good.
There was unequal but less than 3kΩ (DC) between the ground pin and either of the ± output pins, when I lifted the red wire from the ground pin (additional wire going to the metal case left in place), that lifted the output from ground and the mic was dead silent. The HPF still works, so I'm not sure what that red wire was.

Anyone have a clear schematic of the older 421 balanced output model?

This SM-2 s/n 193 again...
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Re: GReg/Electrical Tech Journal

Postby Nate Dort on Tue Sep 17, 2019 12:29 pm

greg wrote:Anyone have a clear schematic of the older 421 balanced output model?


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Re: GReg/Electrical Tech Journal

Postby greg on Tue Sep 17, 2019 4:05 pm

Thanks!
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Sep 18, 2019 5:38 pm

Studer A Machine,

Channel 13 dropped out of record, or was recording nothing all of a sudden.

Traced signal to a dead IC 3 (7524 DAC for level control) and it's following amp IC 10 (5532) was oscillating.

Replaced IC 3, and 10 and the signal came back clean and healthy. This revealed the other two DACs were busted as well. I was able make adjustments from the meterbridge changing the gains of the treble and eq, while witnessing results on the oscilloscope with the card on an extender, probing key spots.

I replaced the remaining DACs and all works on that board now. The board must have been zapped in a recent card swap. It's strange that all three of these DACs died on the same day.
Now that I think about it, the test pins on the label end of the board are the first bit of metal you would touch when reaching for this board. One of the pins, P1/3 is at the input of the first dead DAC. They are sensitive to static charge.


Ground yourself before reaching in, kids!

Reproduce Amplifier of the same channel rolling off high frequencies.

Low and Hi frequency response low, actually. Both the "Bass" and "Treble" DACs (IC 8 and 13) were dead.
Checked the "Repro EQ Standard" adjustment, a buried setting, to make sure that DAC was among the living.


On the bench-

Record Amplifier HX Pro with a bad relay.

This is a board I set aside ages ago because I couldn't source the oddball relay (SM D1012 ITT guy (12 volt)), other than from Audiohouse in Germany. They want $50 for one. No.
We have a parts board I bought as a spare from ebay back when we had a 820 2-track. It isn't an HX Pro board like the other 24 track ones, so I'm not going to use it in them. For now, I'm stealing the relay. If it comes to it, I'll make little custom boards with modern replacements. It's just a DPDT 12 volt guy with an odd pinout).


Repaired instrument cable cross wall ties...
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Sep 19, 2019 3:06 pm

Gefell 563 -

No output.

Busted wire on 5-pin XLR end of the cable.

Seems trivial to mention these things, but maybe it's good to note the frequency.
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Re: GReg/Electrical Tech Journal

Postby steve on Thu Sep 19, 2019 4:09 pm

I think it's good to show that not every problem is a nightmare headscratcher and you can get a lot of things working again by checking the obvious stuff and doing the gruntwork of fixing it. Given the sheer number of things that need to work correctly to get through a session it's incredible it works at all.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Sep 19, 2019 8:09 pm

steve wrote:I think it's good to show that not every problem is a nightmare headscratcher and you can get a lot of things working again by checking the obvious stuff and doing the gruntwork of fixing it. Given the sheer number of things that need to work correctly to get through a session it's incredible it works at all.


I haven't been posting the more mundane, repetitive maintenance regularly for years and years. A busted wire in a fancy tube mic cable seems more important than a busted wire in a regular mic cable, even though, it's basically the same problem with more expensive possibilities... Maybe I've gotten lazy or feel like it would be even more boring than usual. Maybe I'll update the first post again with a list of repetitive obstacles.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Sep 20, 2019 12:22 am

-Elsewhere-

Continuing an old project of modding a broken Ampex 351 channel to our Electrical Audio style preamp. This time using the GAS audio boards, so no hacking the originals.
I don't know if I'll ever do this kind of thing again. It's about as much work as building a preamp from scratch.

Neotek Series II (not ours)-

Chan 3 Dead HF EQ band

Bad TL074. + Channel recapped with "audiophile" electrolytics. 16 of them were the wrong type (polarized capacitors in place of the bi-polar ones). They'll need to be replaced with the right kind... I marked the places, and the owner ordered enough to do the whole board eventually.

Chan 1 No send to bus 2

Poor connection to motherboard.

Echo return 1 and 2 were hard wired to be panned left and right respectively. Owner swears that's the only way they'll ever be used and he could use the pan pots elsewhere as replacements. He was unmoved about my talk of buying new pan pots for $40+.

Chan 26 pop! noise when muting/unmuting

DC on fader buffer amp, replaced

Chan E2 send- no send

Rewet solder joints on molex headers, exercised daughter board connections.


Chan 20 intermittent signal, comes on when it gets loud, then operates normally for hours.

Maybe the source?
P&G Fader wiper off track slightly, and not making great contact. The level would change in steps as opposed to smoothly as the brushes ran over the traces to the track. Cleaning won’t help, and bending the brushes can be tricky. I noted the problem with it for future fixin', and replaced it with a spare fader.
Probably the source?
Signal dies in different combinations of eq and polaritiy switch positions. Was a bad U4 (polarity reverse inverting amp)
Both problems I noticed while searching for problems, since the issue was intermittent.

Chan 19 no pan to the left

Rewet all the joints related to the send to the 2-mix bus

Meterbridge ghost leds

Bad connection between meter ribbon cable and power supply loom at that little distribution board on the VU part of the console.

Patchbay wiring...

Various trouble shooting for ground hum/noise when monitoring from pro tools The reverse side of the punch block (cabling between the punch block and patchbay) had some weak connections that I re-punched. A lot of the hum subsided, but suspecting the power distribution and grounding scheme for more.
The wiring to and from the console was a basin of interconnection don'ts, a nest of all kinds of wiring, tangled together in a web of audio hatred. Trying to remember the particulars agitates as well as the original discoveries, so I'm not going to.
One source of noise was the Mac monitor power supply sitting on top of the snake between the converters and line inputs. Moving it more than an inch or two away dropped the noise an incredible amount. The console has unbalanced inputs, so some attention has to be paid to layout.
This led to reorganizing everything under the console.

Got to play with the Jasper Wasp clone for a bit. The owner bought the kit, a friend put it together, and it sounds pretty right-on. The chassis built for the kit didn't fit right, but you could make something that worked pretty easily. We might buy one...
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Re: GReg/Electrical Tech Journal

Postby greg on Sat Sep 21, 2019 9:27 pm

Neotek Series II / Studio B-

The power supply was removed, recapped, and relocated to the airlock between the control room and common area where the patchbay punch block lives. This involved building a longer PS cable and fishing it through some tight holes in the wall. Thankfully, we had enough spare Belden PS cable from installing the Elite in studio A.
It's sometimes pays off to horde.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Sep 26, 2019 12:01 pm

GML 8400 Mic Preamp -

Channel 2 gain switch intermittently causing noise, or oscillation (maybe). This can be stimulated through wiggling the switch.

Replaced the Grayhill gain switch, tested. All is good.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Oct 01, 2019 10:22 am

All sorts of wonderful things.
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