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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 Thu Nov 02, 2017 4:28 pm

Schoeps 221b RTF pair and stereo power supply-

Upgraded the power supply and cables with 6-pin XLR connectors. The old ones were worn, and binding when screwing together.
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Nov 08, 2017 1:20 pm

Schoeps 221b RTF (part of a pair)

Slight wind noise.

Replaced shattered ceramic capacitor between filament and ground, rewet solder joints, and cleaned connections between capsule, amp, and cable connectors (whatever I can think of to avoid replacing the $1k+ tube).
Ending up having to replaced tube. :u
Note: The NOS AC701 tube I used was initially noisy, which, as you would guess, bummed me out. I decided to leave it on for 24 hours, and the noise diminished to nothing, equaling the other mic. We have a dwindling stock of these tubes, and lots of mics that use them...

Replaced transformer coupling caps in the two mics with matching film ones (.47uF). They had dissimilar caps before, an old mystery cap (.47uF), and a newer electrolytic (1uF).



Beyerdynamic M88

No output
Open circuit between pins 2 and 3. No severed wires that I can see between the XLR connector and dampener cylinder in the middle of the body, same between the capsule assembly and center section. Capsule seems to be functioning.
The way to free the capsule is to scrape the sealer off the wires and rubber plug at the hole in to the middle of the mic from the connector end. Remove the plug, then the wires will be free to allow you to pull the capsule assembly straight out. Note the 4 lengths of yarn, and where they are in relation to the wires. Fortunately the yarn was still tacked to the wires with some leftover glue.
The solder pads that had the windings from the capsule and hum bucker were easy to see and had all the wires still soldered to them. The hum bucking coil measured no resistance (open). It's in series with the capsule windings, on the way to pin 3 on the XLR. I pulled the insulating tape off the coil and found a damaged section. Peeled up the winding end until the break, then found the other end and gently dug that out with a pin. I solder spliced both ends together, then covered the uninsulated splice with rubber cement as a crude insulator. Tested for a signal on the output leads, then pieced it back together, wrapping new tape around the coil to protect it.
I fished a solid wire through the center section and soldered it to the output wires. I applied a bit of rubber cement to the ends of the yarn to keep them stuck to the same point on the wire, where I found it, then fished everything back though the center tube. Apparently, if the yarn bunches up or is compressed in the tube, the low frequency response of the mic changes. After resoldering the XLR connector, I did a little audio test to make sure it sounded normal. All good.
Why did this happen. Well, there was a dent in the top of the grill, which was probably from someone using the mic as a door stop while loading in gear (my guess). The capsule housing has 4 rubber pads that come in contact with the inside of the grill. So when the door swung close on the microphone, the grill caved in, forcing the capsule assembly against the other side of the interior frame. The outer edge of the capsule assembly is the hum bucking coil, so that took the trauma.
This is why we have carpeted bricks, guys!

Thanks to nice folks at Beyerdynamic for fulfilling my primum non nocere tendancies.
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Nov 15, 2017 3:59 pm

Wrestling with schoeps mic.

Broken tuners, cables, other small stuff.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Nov 16, 2017 5:42 pm

Schoeps update.

Fake Marshal JTM45 amp

No signal on "Normal" input.

Cold solder joint between resistor and volume wiper wire.
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Re: GReg/Electrical Tech Journal

Postby greg on Mon Nov 20, 2017 1:33 pm

Shellac travel pod-

Boss Tuner Tu-12

Intermittently working.

1/4" connectors completely stressed though tour action.
Replaced both in and out connectors.


AB 105 amp -

Not turning on after big fall.

The main filter caps that were mounted on a small circuit board shot out of its clamp severing wires to it.
Signal meter pcb also broke loose.

Removed the meter pcb and installed a simple led to show when the unit is on.
Resoldered the wires to the capacitor board, and sandwiched foam between the caps, chassis and clamp to add traction. Tested.



Intersound IVP-

Chassis bolts that mount the box to the faceplate were missing in all but one spot. The main part of the chassis fell loose destroying the master volume pot.

I replaced the pot, and the bolts (adding lock nuts). Nothing else was obviously busted. Will clean the pots after a quick check to make sure nothing else is wrong with the setup.
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Re: GReg/Electrical Tech Journal

Postby greg on Mon Nov 27, 2017 6:02 pm

Designed a little phantom power supply board for the 351.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Nov 30, 2017 6:12 pm

U67-

Noisy sputtering wind style.

Noisy tube. Replaced with Valvo EF86. Burning in...


Hiwatt custom 100

Busted speaker output connectors. Someone did something bad.

Replaced all three output connectors.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Dec 05, 2017 5:56 pm

Lomo 19a19





Random tidbits
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Dec 07, 2017 1:13 pm

Neotek SII

Talkback signal to Cues intermittent

Suspect switch attached to the "talkback to cues" control. Temporarily bridged it with a jumper to see if it cuts out again.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Dec 12, 2017 4:04 pm

Fender Deluxe model 5C3

Extremely scratchy pots and rattling noise.

Tightened up a loose nut on the fuse holder and a few other loose parts.

Ordered replacement pots.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Dec 15, 2017 5:31 pm

Studer a820 Blacksides

Calibrated the "Generator Unit" which handles the internal tone generator and reference level used to calibrate the machine.
This is often an overlooked check on these machines. If it isn't functioning properly, all the calibrations can be off.

16 track headblock-

More than expected crosstalk between channels in sync mode (recording on one channel, monitoring the neighbor channels).

This is happening since replacing the record head with a Flux Magnetics built head. It could be a normal property of the new head. Exploring possibilities. There is a crosstalk compensation circuit, but it only applies to the repo path (in the repro peramp). Incidentally, the capacitor mounted on the bottom of the headblock related to this was out of spec, so I replaced it.


Finished Neumann u67 tube replacement burn in and tests. Sounds good.......

Designing new configurable tube microphone power supply circuit board with a soft start for the filament voltage. The last one I made had a delayed standby-like start for the B+, which would slowly ramp up the voltage after a minute after the filament voltage was supplied.
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Re: GReg/Electrical Tech Journal

Postby greg on Mon Dec 18, 2017 5:36 pm

Gefell 582 not working because of a busted mic cable.

Fixed.

Replaced Fender Deluxe pots, and tightened all screws in and out of the chassis to reduce any sympathetic rattling. Only a little bit of buzzing at certain low frequencies at high volume. Going to see if it's a problem in real world recording. If so, I might have to do some dampening on the cabinet, unless it's the ancient speaker (in which case we'll recone or replace).
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Dec 20, 2017 6:07 pm

Prepping EAPreqs for sale.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Dec 21, 2017 6:10 pm

Tube mic psu design, and other lovely things.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Jan 02, 2018 4:40 pm

Studer A820MCH-

Recapped the MPU basis board, and power supply in the meter bridge on both blacksides machine in Studio B, and the machine in Studio A's "A" position.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Feb 01, 2018 5:03 pm

ATM450 noisy through several Ampex 351 preamps.

Phantom power supply in Studio b (discrete power supply that distributes 48 volts to the four 351s) failed. The pass transistor and zener diode were busted in the regulator circuit. I modified the design a little bit, since there was an under specified trimpot (wiper burned up).
Should last another 20 years, hopefully.


Various travel related damage to a mic and two direct boxes repaired.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Feb 02, 2018 1:09 pm

AKG 414 B-ULS -

High pass filter switch actuator broken.

Replaced.
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Re: GReg/Electrical Tech Journal

Postby Nate Dort on Fri Feb 02, 2018 1:17 pm

greg wrote:AKG 414 B-ULS -

High pass filter switch actuator broken.

Replaced.


Where did you get the switch? Direct from AKG? I've got the same problem on one my B-ULS's.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Feb 02, 2018 6:37 pm

Nate Dort wrote:
greg wrote:AKG 414 B-ULS -

High pass filter switch actuator broken.

Replaced.


Where did you get the switch? Direct from AKG? I've got the same problem on one my B-ULS's.


AKG/Harmon still sells the parts. I forget the part number, but it's the same as what's on the exploded diagram on the manual. There are two types, 3 and 4 position switches.
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Re: GReg/Electrical Tech Journal

Postby Nate Dort on Fri Feb 02, 2018 6:49 pm

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.
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