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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 drpat on Thu Nov 29, 2018 8:08 am

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?


Yes, the voltages were read directly off the transformer fully loaded with mic. Being fed from a 220V variac.

With the above conditions:
Node1 measured 217.74VDC (supposed to be 220VDC)
Node4 measured 14.15VDC (supposed to be 14VDC)
Last edited by drpat on Thu Nov 29, 2018 9:50 am, edited 3 times in total.
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Re: GReg/Electrical Tech Journal

Postby drpat on Thu Nov 29, 2018 8:14 am

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...


I'll probably end up with a custom part too... I've got a query in to Edcor for a custom EI core jobbie.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Nov 29, 2018 8:05 pm

drpat wrote:
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?


Yes, the voltages were read directly off the transformer fully loaded with mic. Being fed from a 220V variac.

With the above conditions:
Node1 measured 217.74VDC (supposed to be 220VDC)
Node4 measured 14.15VDC (supposed to be 14VDC)


That's pretty spot-on for unregulated voltages. The 262D6 won't be a drop-in replacement, I see.
I tend to rebuild supplies when the caps are dead, and make them fully regulated w/ solid state parts.

I'll probably delete these posts to keep the log clean, btw. Feel free to start a new topic on this.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Dec 04, 2018 2:45 pm

Shure 94A1670 preamplifier, used for our SM98s, but used in many Shure condenser mics-

Busted pin in the mini XLR connector.


Replaced the connector. Part number, in case it helps, Switchcraft TB3M.

Sorting through Sony and Gefell mics and busted capsules...
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Dec 14, 2018 4:42 pm

Pultec EQP-1a3

High frequency band not working.

Frequency select switch stop tab bent out of the way, so the switch could be rotated 360º. There are only 5 active positions in the 12 position switch. The way the knob was positioned, it looked as though a frequency was selected (double sided pointer, with a faded white indicator line). In reality, nothing was selected, as the switch was 180º from what it appeared to be.
I bent the stop tab back into place.
This is why I make the big bucks.


Skibtronics LA-2 clone

Bouncy meter problem.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Dec 18, 2018 1:18 pm

Skibbe Electronics 5-9c (LA-2 clone) studio B-

A) Meter behaving erratically in gain reduction mode. Meter needle bounces, drifts wildly away from 0 VU. Gain reduction display doesn't track reality, actually reverses movement after a point of further gain reduction.

B) Clipping signal above roughly +4dBu.


A) Replaced the neon lamp (DS-1) with new one - no success.
Replaced the lamp with zener diodes. The lamp works as a voltage regulator, providing the "0 adjust" pot with a reference voltage. This has nothing to do with the audio, just part of the gain reduction display circuit.
This wasn't the solution to the problem, but will keep things running more predictably.
Replaced the "0 adjust" pot after discovering it was intermittently conducting. This was the problem. Gain reduction display follows what's actually happening.
Maybe related, I had to replace the series resistor (R24) with a different value to calibrate the meter to 0dB VU = +4dBu.

B) Replaced V1. All better now. Clean signal up to about +24dBu.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Dec 20, 2018 4:46 pm

Neotek Series II-

Went through all channels to check their buss sends, and correct the positions of the indicator knobs.

Channel 33 had no or intermittent send to Buss 7 (15, 23).

Cleaned buss assignment switch (wiggling it popped the signal in and out). All solid now.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Dec 28, 2018 5:21 pm

Cleaned an older Schoeps M 934 B capsule.

Sourced a new spring pin (for the capsule) on a 221b microphone.
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Jan 02, 2019 5:17 pm

Neotek Elite/Flying Faders-

Channel 24 fader making a grumbling noise.

Pulley cable broken. Replaced, and all is well after tweaking the tensions a bit.


ATR #2, Studio B

Buzzing from take up motor when tape loaded.

Exercised power supply, reel motor, inductor, control panel and PWA connections, and no change.

Replaced Reel Servo PWA- all quiet. Put the original one back, and things stayed quiet. Need the problem to return.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Jan 03, 2019 2:13 pm

Studio B Patchbay Work/
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Jan 04, 2019 5:28 pm

Neotek Elite-

Cleaning various intermittent signal path related switches.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Jan 08, 2019 8:12 pm

ATR #2 in Studio B-

Take up reel motor buzzing, and groaning when tape was loaded and under servo control. This problem was intermittent until the
reel motors stopped responding altogether, and the machine wouldn't load tape.

Maybe related- Control panel leds flickering intermittently.


Some notes on the process-

Reseated all PWAs (circuit boards in the card cage)- no joy
Swapped the Reel Servo board with a spare, and the noise went away. Put the old Reel Servo board back and the machine was still quiet.
Turned on the machine the next day, loaded some tape, and the buzz came back. Swapped the Reel servo again, and no change. Put the original back in, and now the reel motors are dead. Swapped all the PWAs (audio boards pulled for the time being). Motors still dead.
Exercised both the power supply connectors (motor control routs through it, via a relay)
Tested that relay.

Other observations:
Timer display still works, though fades from dark to fully on slower than normal (I think) over a second or two (after the wake up period ends).
Capstan tests normally (when Reel Servo PWA pulled).
Switched the speeds (have had problem with that circuit in the past).
Voltage measurments on the Reel Servo Card-
± 20v is closer to ±27, don’t know remember that’s normal. It’s been a while
± 12v is a little under ±13v.
I measure close to ground on the ground side of the reel motors, < 1Ω.
Signals from the tension sensors correct and smooth throughout their travel.
When I push the tension rollers out to activate the microswitch, and measure TP 3 or 4 on the reel servo, I measure an instant of voltage when I hit stop, otherwise close to 0v at rest.

The dead real motors seemed to be a problem with the “SVO”, or servo logic signal getting to the power supply and energizing the relay (completes the reel motor active circuit). I reseated the edge connector on the rear of the card cage motherboard where this signal is sent to the PS, and the reels gradually came back to life. There’s still a bit of groaning, or buzzing when the tape accelerates in the forward direction. I’m trying to trace it down, but now it’s so subtle, I can’t scope things that might be abnormal.

Day 2-
Today I showed up to limp reels again. Wiggling around the wire harness seemed to change that back to functioning again. I then manhandled the 4 reel motor molex connectors a bit and heard the machine whine and moan. It seemed I could get the machine to behave or fail based on slight tension on the reel motor cables/connectors. I saw a bit of arcing in one molex housing in the process.
I then cleaned and re-tensioned the connectors, and the transport acted normally. Still, some really faint buzzing. Feeling around to see what was mechanically vibrating, I found that one or both of the fuse holders were warm, and gently wiggling their retaining caps caused a change in the buzzing harmonics. After exercising those connections (removing and reinstalling the fuses) all is good and quiet.
Going to leave it threaded and stress test it today to see if it’s good for real. If all is well, I'm going to replace the molex connectors with fresh ones.
Fingers crossed.
Also, the flickering leds never appeared again.

Later...

Reel Motors still groaning, complainin' for a while after power up. Then reels dead, tape won't load.
Tension arm led dead, replaced
Tension photosensor track damaged, replaced
(good thing I had a box of spares at home for my old machine. 15 years later, comes in handy).
Warm 5v supply cap
power supply swap
Tested all power transistors (deck mounted), cleaned their connections, applied new insulators and heat sink compound.
28kHz spikes seen everywhere..

I'll elaborate later.

This one's going to be down for a while.


OK, didn't elaborate, and now forgot some of the details. Basically, the problem was not solved, even with the alternate supply.
Because of the heating 5v power supply cap, I figured I'd just rebuild the PSU, since it's been about 20 years. I just ordered the parts kit from ATR Service since they had everything in one place, caps, transistors, high amp diodes, power switch (already replaced), fresh hardware, and some spare fuses (for just in case?). They even go the extra mile of filing the legs of the TO220 transistors so they sit flush with the board, as the originals did (not a lot of space, and they don't get too hot).
The old 9200uF cap which was being used for the 5v supply, the one getting warm, measured roughly 100uF. Yep.
Now that the rejuvenated PS is in, the machine seems to be ok, but we'll see. I've been tricked before. Going to run it through the motions, check voltages over the next few days.
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Re: GReg/Electrical Tech Journal

Postby greg on Mon Jan 28, 2019 5:45 pm

Digidesign HD 192 DA converter board-

Dead output #1 channel

Burned out negative polarity amp, replaced parts. Working on the bench now.
The SMD output mute resistors were burned out (22Ω to ground from the output connector via a relay when muted). The wattage of those resistors was a fraction of what would have gone through them if someone connected phantom power to the output by accident, so maybe that was a thing...


Drew a schematic for the output amp, so fixing these will be easier in the future. Should have done that 10 years ago. If we weren't on the verge of replacing these old converters, I'd replace the output section with an off the shelf output driver chip circuit from THAT on a daughter board. I kind of still want to...
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Feb 01, 2019 3:04 pm

Digidesign Sync I/O won't power up.

Switching supply is dead. New one is $40. After checking the obvious things, I'm ordering a fresh and new guy.

Later- Received and installed. Working.
For reference, it's a Skynet (yeah) power supply, part # SNP-9034. The Digidesign part had a custom cable loom in place of the male header. You just have to remove the header on the new board, and transfer the old wiring to it.




Sending 2 Lomo 19a9s to Gefell for capsule repair.
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Feb 06, 2019 12:01 am

Studio A's Neotek Elite control room monitor pot noisy.

Replaced
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Feb 08, 2019 6:21 pm

Installed a new solid state drive in my newish mac mini today. Holy shit, what have I been doing up until now!
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Re: GReg/Electrical Tech Journal

Postby eliya on Fri Feb 08, 2019 6:50 pm

greg wrote:Installed a new solid state drive in my newish mac mini today. Holy shit, what have I been doing up until now!


Waiting for it to boot!
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Feb 14, 2019 12:54 pm

Oh my god this is better!

ATR looks solid (updates above).

Cleaned a couple old noisy Noble stereo pots, used as the main control room monitor pot. I'm trying the method I use on the P&G faders, (distilled water cleaning the track, dry, alcohol on the brushes). We'll see if this works, and for how long, if I need to swap the studio b one.

Looking at the Digidesign 192 that gave us problems recently when the fan failed. The symptom was output levels jumping around.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Feb 14, 2019 1:25 pm

greg wrote:FOR SALE:
Listing a bunch of surplus parts taking up space in the shop on ebay.
Going to be selling a lot of Mallory multi section capacitors, XLR, 1/4", and Edac connector parts.
If you want to wire up a studio with NOS connectors, watch our listings...


Update for the Reverb listing of on of our reribboned RCA BK11s. This is the last of these we're selling...
https://reverb.com/shop/electrical-audio
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