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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 Wed Sep 05, 2018 5:02 pm

Crown GLM100-

On one, added some strain relief on the connector/amp end of the mic. Wires were ripping out.

On another, noise when the head is touched or wiggled a bit. Nothing to do since it's all incased in a rubber mold. I might replace the capsule with another electret element.

Received a shiny new Fluke 287!
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Sep 06, 2018 4:06 pm

Building a new Schoeps 221b power supply.
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Re: GReg/Electrical Tech Journal

Postby greg on Mon Sep 10, 2018 4:52 pm

Parts for the 351 mentioned earlier showed up.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Sep 13, 2018 5:36 pm

Studer A820MCh Blacksides -

Chan 6 muting output when in record (with several other channels) and monitoring input (in Sync playback mode). Signal appears on the meterbridge (signal tapped before output amp) when this occurs, and signal is recorded properly. No failure ever in Repro, Sync or Input mode when not recording.

Problem follows the Line Amplifier board. The problem is intermittent. I shotgunned IC 3, and 7 so far, no joy. The way the board is configured via the DIP switch options rules out IC 4.
Now trying to make it fail in another machine loop recording under similar conditions. I suspect IC 1 (74C374). Want it to fail in front of me with my meter in hand, and the board on the extender. This problem has happened before, and I've never been confident about the source. The weird part, for me is the specific conditions for failure.
There are three controls for muting the output driver amp (that I can see).

1. Powering-on mute, to avoid transient spikes on the output when the machine is turned on or off (IC 7, 201).
2. Buss controlled from the microprocessor for various modes (IC 1, 3, 6, 9).
3. Output Amp overheat condition (R252, IC 7, 3, 6, 9).

Couldn't make it fail in the other machine, now working it in the original machine after replacing IC 1
No failure yet.
Now trying with the original IC 1, and still no failure.



Finished rebuilding a Schoeps 221 psu with a slowish start filament.

Neumann SM-2 noise.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Sep 14, 2018 5:33 pm

Intersound IVP-

No output except for insert sends.

Busted wire from master volume pot to board (and to -10db output). Jumpered insert points while I was in there.
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Sep 19, 2018 4:18 pm

Checking and adjusting Schoeps 221, and Neuman SM2 filament voltages. Paranoid.

Bought one of these to poke around with. I was building a similar thing from scratch like a dummy.
Crazy what's out there and for what price.
"Tube Amplifier Filament LM317 Adjustable Regulator Soft Start Power Supply Board"
https://www.ebay.com/itm/153101615076
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Re: GReg/Electrical Tech Journal

Postby steve on Wed Sep 19, 2018 7:39 pm

That's insane, so useful and so cheap.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Sep 21, 2018 11:19 am

Posted solution to a freelance tech headache at a local studio dealing with a Studer A827

More testing with the Studer, and checking Schoeps 221 and Neuman SM-2 PSU voltages.
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Re: GReg/Electrical Tech Journal

Postby greg on Sun Sep 23, 2018 9:39 pm

Neumann SM-2 NSMa power supply (with active linear regulator for the filament voltage).
Changed R7 to lower the filament voltage to roughly 3.95v
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Sep 25, 2018 5:12 pm

Neumann SM-2(A modded to C) sn193

Noise on low capsule output after being on for a while, or when the mic is gently bumped. I'll update the original post about this.
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Re: GReg/Electrical Tech Journal

Postby greg on Mon Oct 08, 2018 5:11 pm

Neotek Elite-

Removed the armrest for reupholstering.

Neumann SM-2 soft start filament regulator boards.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Oct 09, 2018 9:01 pm

Elite armrest back, and better than new.
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Oct 10, 2018 3:22 pm

Updated all the Neumann SM-2 supplies with the new soft start filament regulator.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Oct 16, 2018 4:18 pm

Film Industries M8 ribbon mic-

Needs an output adaptor.

FM Roddy was nice enough to send me one of the proprietary connector/mount assembly. Trying it out now, in-session.
Thanks Roddy!


Fender Twin (the disembodied head amp)-

Low output.

Before turning it on for a test, I noticed something felt rattly about the power cable. Turns out, the neutral lead was loose in the power connector's screw terminal. Then I noticed the amp had mismatched power tubes. It looks like someone grabbed two pairs instead of a set of four matched tubes. I later found out this is a thing you were supposed to be able to do on this version of the amp with help from the "bias balance" circuit. However, the tube pairs were not in the right sockets to work like this. Anyway, the balance circuit doesn't really allow you to bias the tubes to a desired current, rather lets you equal the current between the two different pairs.

I cut the power cable down, and installed an IEC connector.
Installed a new set of 6L6GCs.
Modified the bias "balance" circuit so it works as a normal adjustable bias,
then discovered one of the tubes wasn't working correctly.
Measured the screen resistors (looked slightly baked), and one was open. Replaced them all, biased, and we're back up.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Nov 09, 2018 4:21 pm

Schoeps M934 B capsule with a "tent" in the membrane (spare from a while ago).

Disassembled capsule housing and sprayed the rear of the backplate (through pattern vents) with compressed air at low pressure, until the dirt worked out. Cleaned all the metal parts that were meant to contact each other. I'll test in a mic when the studio's free.

Cleaned a Gefell M62 and M58 capsules

Neotek Elite

Channel 27 has no signal on the large fader.

Polarity reverse switch dirty. Exercised it vigorously, and life came back. If it happens again, I'll pull the channel and deep-clean it.
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Re: GReg/Electrical Tech Journal

Postby greg on Mon Nov 12, 2018 4:08 pm

Sony C-37 Fet (thought was an early C38)

Week output.

Discovered a tear in the capsule, and the stud mount for the "Y" stand is broken. Ordering parts (thanks for still making them Sony)
12.2019-
Installed the stand stud part "stud stay", but apparently it's a different threading so the thumb nut needs to be replaced. $50 from Sony. Hmm..
The new capsule is in. I had to add a 0v wire between the capsule and amp. Previously, the capsule 0v came from the hoop strap mount, screwed to the grounded body. The new capsule, has an insulator preventing that contact, instead providing a dedicated solder tab (like the C38B) which is better anyway.
I also updated the phantom-to-9v circuit I put in ages ago. The mic was originally powered via 9v battery, or remote 9v supply. It had no provisions to be used with modern phantom power.
It's a simple series regulator circuit dropping phantom to 10 volts, which brought the capsule polarization voltage closer to spec (-140v).
Replaced an electrolytic while there, of course.
Sounds good now!




Neotek Elite Flying Fader channel 6-

Pulley string on the motorized fader broken.

Replaced with a fresh new one from our fishing kit. It took a few tries, but everything's together and tested in the console.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Nov 16, 2018 4:10 pm

Gefell 563-

Crackle and hum on output.
Female cable connectors (on two cables) have poor tensioning. You can push gently back and forth on the connector (unthreaded, but seated) and get it to work.


Used a needle to push in the tabs on the female pins (roughly trifurcated) closer together. This isn't a permanent solution. We'll need new connectors soon.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Nov 27, 2018 2:57 pm

Bassman 6G6B (blond)-

Occasional hum on output today, post preamp stage. Seemingly responds to moving or tapping the power tubes.

One leg of the bias filter capacitor (newish) lifted off of it's post. I could nudge it a bit and cause the hum to come and go. Resoldered, wrapped around the leg of the resistor it's in parallel with.
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Re: GReg/Electrical Tech Journal

Postby drpat on Wed Nov 28, 2018 9:09 am

"Modernizing a Neumann N-61v mic power supply with XLR connectors, IEC power inlet, 120V transformer (from 240v), and recap. This will work with the Gefell/Neumann 582s and 563s."


I know you did this a while back, do you remember the model number for the 120V replacement transformer that you used?

I'm getting 200VAC and 24VAC on the secondaries when the primary reads exactly 220VAC. Just can't find a transformer with those voltages. Been looking for months!
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Nov 28, 2018 10:02 pm

drpat wrote:"Modernizing a Neumann N-61v mic power supply with XLR connectors, IEC power inlet, 120V transformer (from 240v), and recap. This will work with the Gefell/Neumann 582s and 563s."

I know you did this a while back, do you remember the model number for the 120V replacement transformer that you used?

I'm getting 200VAC and 24VAC on the secondaries when the primary reads exactly 220VAC. Just can't find a transformer with those voltages. Been looking for months!



That was probably a custom toroid transformer that we bought for this purpose.
I've had good luck with the Hammond 262D6 in other PSU projects. It doesn't have a dual primary, which is too bad, but the price is right...

Are those voltages read off the transformer connected to the circuit, and mic, or lifted from the rest of the power supply?
You're feeding it 220VAC?
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