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GReg/Electrical Tech Journal

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Re: GReg/Electrical Tech Journal

Postby greg on Fri Feb 02, 2018 6:52 pm

Studer A820MCH blacksides-

Channel 4 has dc spikes when recording tones, monitoring off the repro head. The spikes are not recorded. When it happens, channels 1-3 are effected in weird ways, muting, changing level, etc.


Problem followed the repro card, when I moved it to channel 8, including the side effects to channels 5-7 suggesting some corruption of the CMOS bus to the set of 4 channels ("bus converter 4 ch" board distributes the CMOS in sets of 4 channels).
Replaced all the chips that are connected to the bus. In the past, a bad chip has caused similar problems- not better
Replaced all the amplifiers, starting with the ones directly related to the signal path- nope
Removed IC 6, which would prevent any possibility that the sync repro signal path could be leaking into the repro path- nope
The board has been recapped already.
One clue, the sine wave starts to distort and triangulate just before the pop occurs.

Working with a spare until there's another free day in the studio to test more ideas.
I'm going to get my heat gun and freeze spray and really torture the board under a scope.


Later-
Other issues probably related to this occurred. Occasionally, channels' outputs would mute while recording. They would still properly record the signal while muted, then playback with no audio missing. This led to me replacing the "Bus Converter MCH 1.820.717" for channels 1-4, and then "Multichannel Bus Driver 1.820.723". The glitches went away.


Since both those boards comprised mostly of logic chips, I decided to shot gun the lot. After rechipping the Multichannel Bus Driver, the transport would hiccup during play as if someone nudged the shuttle wheel. It would also sometimes lose tension at stop, as if someone pressed the "unload' button. I checked the board closely for any cold solder joints or tiny shorts between the traces and pads. I also checked the power supply drop diode (5.6v enters the board through a diode in series to drop the voltage to 5.0v), and the 3 SAL capacitors. The voltage drop on the diode was .15v, which ended up feeding the board around 5.45-5.5v. Even though this seems minor, I replaced the diode, and the supply dropped to a solid 5v. C2 was slightly lower than spec so I replaced that as well. Tested on a free machine, and it worked without issue. When I can, I'll put it in the source machine to see if things are good there.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Feb 02, 2018 6:54 pm

Nate Dort wrote:I figured. I think I've actually posted that part number in another thread around here, but never bothered to order one.

I actually work for Harman, but I have no contact with the AKG folks, so they might as well be a separate company.

Maybe you can get an employee discount!

Email me if you need the guy I contacted.
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Feb 28, 2018 6:16 pm

Collecting ribbon mics for a reribboning shipment.

Digidesign 192HD Studio B-

Input 11 has snappy spike noises.

Tried all the usual things to trouble-shoot. Replaced the AD card (8 channels).

Fighting with a studer reel hold down "adaptor" that has froze. There's a replacement "adaptor" on the machine it came off of.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Mar 13, 2018 3:16 pm

Old e22 prototype-

Noises when mic cable is bumped.

XLR barrel that holds the connector was loose. Tightened the fasteners.
-This isn't a possible failure mode for the production model since the body of the mic is one milled part.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Mar 16, 2018 5:42 pm

Manley/Langevin VTL CR3a-

Noisy when HPF is engaged.

One or Both mylar capacitors that make up the filter circuit were the source of the noise. I tried rewetting the solder joints, and the caps themselves measured ok.
Replaced both with a single film capacitor equalling the composite value of the old ones.
All quiet.



Dyanmite Regulator heatsink broke free when it was shipped for a session. It was held in place by a nylon screw which became brittle over a long life of heat and stress.

New screw. No shorts. All together again and working.


RCA BK11 and 77DX mics off for reribboning.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Mar 20, 2018 6:26 pm

greg wrote:Flying Faders- Channels 16, 32, and 48 are linking in complicated ways, on their own. For instance, if I solo chan 16 in momentary solo mode, no problem. If I solo chan 16 in latching solo mode, chan 32 solos on the down stroke (16 mutes with the rest of the board). As I lift my finger, 16 joins 32 in being soloed. A whole bunch of other anomalies occur between the three channels, all relating to Group, Solo, Link and Rec functions. It is strange that the problem channels are 16 spaces from each other. This might point to a buss driver issue.

We've tried all of the resetting variants. Today, I tried calibrating the fiber optic signal. The rough square wave, which is never square, should have roughly a duty cycle of 50nS on the top excursion. I am unable to change the waveform going from the SEFA (FF card cage) to the computer. There might be something wrong with the transceivers, or even the cable. I will check monday, and get advice from Martinsound. I am not totally convinced that this hasn't been going on since installing the new computer last year.

2/10/10
Got some help for Shea Ako (local smartypants tech/friend). We observed, through the diagnostic switch check window that when any button was pressed on channel 16, a command signal was also read on the same button for channel 32. e.g. when I press and hold down the Solo button, command line is
"16 Solo True"
when I release the solo button, two command lines appear
"16 Solo False"
"32 Solo True"
Only the first should be there.
The same type of interaction occurred between channel 32 and 48, and 48 and 64. Which is significant since we don't have a 64th fader. We do have channels 1-61 for automated faders, which means that we have the electronics for 64 (the fader control boards (FSA boards) control 8 channels a piece, and we have 8 ). We removed all but the first two 8-channel FSA boards, and performed the switch test with the same results. Even though channel 32 was dead, and disconnected from the system, the computer was still seeing commands from it when I actuated channel 16's switches.
We confirmed that the fiber optic signal strength couldn't be changed going from SEFA to the computer. We swapped the fiber optic cable to eliminate that as a possibility. Martinsound says that the signal we are observing should be sufficient, and the problem we are seeing seems more complex than just poor fiber optic communication. He recommends sending in the computer and SECA board for testing. So we are.


2/21/10
Problem located in the computer. Everything is back, and I already had a successful, glitch-free mixing session. More details later...


Pretty much the same thing happened this week sans Shea. Going to see if I can do something without shipping anything.

After some dicking around, the power supply to the computer failed. It was measuring slightly low on all the power rails before failing, and sounded like it was struggling (switcher frequency audible and loud, modulating). Taylor found a replacement power supply and installed it. All the glitches appear to be clear now. Hopefully, for good...
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Mar 21, 2018 3:40 pm

FMR Audio RNC returned repaired.

FF issues.

CR3A mic fixed (above)

Sent two Beyerdynamic M130s out for repair. Both needed re-ribboning, one has a part broken loose inside the ribbon assembly. Ugh.
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Re: GReg/Electrical Tech Journal

Postby greg on Mon Mar 26, 2018 2:22 pm

Traynor Monoblock B-

Smoking in the front by the power switch, lamp.

Power switch was making partial contact in the on position. Replaced, tested...

Replaced the foam in the two remaining Beyer M380s.
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Re: GReg/Electrical Tech Journal

Postby Jared Paolini on Sat Mar 31, 2018 9:39 am

Speaking of the 380, where do you get the foam from?
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Re: GReg/Electrical Tech Journal

Postby greg on Sun Apr 01, 2018 3:07 pm

Jared Paolini wrote:Speaking of the 380, where do you get the foam from?

Beyerdynamic. It's the same as the TGX50 foam.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Apr 05, 2018 4:31 pm

Bryston power amp-

Left channel intermittent.

RCA connector broke loose through twisting action, ripping the pcb trace off the board. I refastened the connector, and spliced the signal to the remaining circuit board trace.


RCA 74 Jr. Velocity-

No Output

The cable strain relief broke loose inside. The free cable yanked down on a transformer winding which had a bare spot. The winding came in contact with the frame, grounding the signal from the ribbon.


Fake Marshal JTM45 -

Blowing mains fuses

Power transformer's filament windings shorted (together). Ordered and received a replacement.

Fixing the individual problems on this amp over the years has become a prolonged game of whack-a-mole. There are enough deviations from the schematic and layout that I'm putting this on the strip down/rebuild pile.


-Elsewhere-

EMI TG12345 MKII/III 24 channel, 8 bus board

Installed and completely tested in a new control room, noted few issues which will be dealt with later. It's crazy how well this 50 year old console has held up. Great quality parts, switches, few pots, few elecrolytics, and a low stress power supply distribution I think are keys to that. Great to play around on.


Complete control room instal with a Neotek SII board, 24-track, pro tools, 2-track, and hoards of outboard gear. There wasn't any legend for the console punchblock, so some time was spent figuring that out.
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Re: GReg/Electrical Tech Journal

Postby scott evans on Mon Apr 09, 2018 9:53 am

EMI TG12345 MKII/III 24 channel, 8 bus board


Whoa.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Apr 10, 2018 5:02 pm

scott evans wrote:
EMI TG12345 MKII/III 24 channel, 8 bus board


Whoa.


Definitely. I've come to realize that with a few modifications, this would be the perfect board. Too bad it's worth more than a nice house.


Klark Teknik DN780 Studio A-

Crackling along with the audio, pre effects engine.

Exercised the many board to board internal connections, and made sure all the supply rails were healthy. Tested on the bench for 2 days. The only time I could reproduce the noise was by messing with the input level pot which eventually cleaned itself.
We'll see.
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Apr 11, 2018 5:43 pm

Lovetone Meatball

No input envelope related effects.

Switches are in bad shape. Wiggling them brings back functionality temporarily. Tried cleaning them, but couldn't get solid reliability.

-Switches replaced, and everything is back up and running.


Studer updates above...
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Apr 18, 2018 5:38 pm

Ampex 351 (preamp #2 located in studio A, position 2)-

1. Signal cutting out when meter defeat switch messed with.
2. Needs phase reverse relay mod, and upgraded phantom supply wiring.



1. Fixed.

2. Done.



Audio Technica 4033 -

Noisy output

Loose ground connection on the xlr output. Tightened.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Apr 27, 2018 3:01 pm

Working on a custom MS Matrix.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue May 01, 2018 3:01 pm

RNC Compressor #2 studio B-

High frequency "tzzzz" on output. Gone in bypass mode.

Noise found the -15vdc rail (ripple).
Recapped the supply section. Noise went away. Recapped the remaining electrolytics.
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Re: GReg/Electrical Tech Journal

Postby greg on Wed May 02, 2018 4:37 pm

Studer A820MCH studio A (older one)-

Taylor discovered the supply reel motor was spinning fast when he showed up today. The only thing that would stop it was shutting the power off. When I turned it back on, there was a quiet whine coming from the reel motors, motor drive amps (MDA) or both.


I removed the two motor drive amplifiers to check them out in the shop.
Both needed to be recapped (4 electrolytics for the ±15v rails C6, 7, 8, 9). I replaced the 5532 op amps (IC2) since they looked like they've been warmed. I checked around for obvious failures in the semiconductors, transformers/inductors, and cleaned the contacts. There were a few hot looking solder joints that I reflowed as well.
When I reinstalled the MDAs, and switched the machine on, all was quiet. I ran through some stress tests and checked the tensions roughly with scrap tape. All seems good. I didn't expect this. I figured there was probably something up with the Spooling Motor Control, or Spooling Motor Driver boards.
So far so good.
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Jun 06, 2018 3:43 pm

Replaced the phantom power supply unit that feeds the four Ampex 351s in Studio B with this module.
I was a little nervous about the low switching frequency. Only getting right on top of the shielded module with a pickup, I could hear the switching frequency, but much lower than the line frequency.
Testing in the application with the preamps at max gain with various different loads and mics, I couldn't hear/see it anywhere.
So far so good.

Cleared out a few suspect microphones that had little or no problems of note.
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Jun 06, 2018 3:57 pm

-Eleswhere-

For Studer A827 savvy techs...

I'm trouble-shooting the machine in this video.

phpBB [media]


"When Play is engaged, tape promptly races away at a faster speed, and pinch roller disengages.
When Stop is pressed the tape halts, but the capstan still spins at correct IPS speed (measured at the capstan MDA board).
From here, if I press Play again nothing happens.
Resetting the MPU boards, specifically the Tape Deck Control MPU the capstan motor stops, and the process can be repeated.
Tensions were correctly adjusted before this condition was constant.
Capstan motor control adjustments and measurements are correct, while testing in the shaft cleaning mode (no tape loaded, play engaged, all speeds tested).
Wind, and Edit modes functioning correctly.
Power supply voltages correct.
Was able to confirm the checksum of all eproms except the ones on the Master MPU board (don't know those so can't confirm 16/97 software).
All tape deck control boards have been replaced while trouble-shooting this problem with no change in behavior, except the Master MPU (no spare).
Replaced capstan MDA board (just in case)...
This machine was built new in 1999.

Any ideas?"
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