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GReg/Electrical Tech Journal

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Re: GReg/Electrical Tech Journal

Postby greg on Thu May 18, 2017 12:26 pm

Nate Dort wrote:
greg wrote:Traynor TS-50b had a tiny amount of 120hz hum on its output with nothing connected, and the volume all the way down.

The amp was recapped already. I tried chassis grounding the center tap of the power transformer's secondary (also negative point for the speaker output) and the hum disappeared. This isn't true to the schematic, but it worked. School me on why this is wrong please.


Greg!

According to the schematic:

Image

...the PT secondary center tap (ground 1) should already be connected to chassis. It is connected to ground 3 at the speaker connectors. Ground 3 is connected to ground 2 at the master volume pot. Ground 2 is connected to chassis at the input jacks. I'm guessing there was some sort of ground loop that you short-circuited by directly connecting the center tap to chassis, or one of those ground connections was broken somewhere in that chain.

Thanks,
I overlooked that. Funny how circuitous the ground path is on the schematic.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu May 18, 2017 5:28 pm

Altec 175 dead

A jumper was shorting against the inside of the body. Replaced the jumper, which was bridging a gap in the pcb trace with a flat piece of solderbraid. My guess was the body was flexing enough from a mic clip to short it out. There isn't much space in there.

Altec 525a power supplies-

Replacing selenium rectifiers with modern diodes, and figuring out the right value for a voltage drop resistor for the B+.
I discovered, while testing the above mic that a couple of our supplies were providing only 4 volts to the filament. Replacing the selenium fullwave rectifier with silicon diodes brought that up to 6-6.3 volts. The B+ was way to high with the new diode. I have to add a series resistor to it to bring it down. Working on the best value to get the voltages specified in the schematic.
More later.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue May 23, 2017 4:39 pm

ATR Lifters stuck out

Altec 175s
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Re: GReg/Electrical Tech Journal

Postby greg on Wed May 24, 2017 3:38 pm

Ampeg B15n-

Low output, no headroom.

Someone wired the output incorrectly, grounding the external speaker jack, when it needed to be floating (sleeve connection carries the top winding of the centertapped output transformer). This meant both the top and bottom windings of the output transformer were grounded, including the feedback path to the output tubes. The original wiring allowed for the additional speaker to be wired in series with the native cabinet speaker, using both sides of the o/p transformer's secondary. With the external speaker jack disconnected, the bottom winding drives the cabinet, while the top winding is loaded with a 250Ω resistor, and provides the feedback signal.
When we originally bought this amp, the speaker was bust, and we had it reconed. Ever since then I incorrectly attributed the crappy sound to a bad reconing, while searched for a decent replacement.
This discovery was because the ground pin broke off the power cable, so I opened the can of worms to see why else it sucked...
This thing has power now!
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Re: GReg/Electrical Tech Journal

Postby tallchris on Wed May 24, 2017 6:55 pm

Nice! Had wanted to use that when I recorded there last spring but it sounded totes farty.
Bass, minivan: [BRACKETS]
Guitar, vocals: BURN PERMITS
Bass, vocals: POLICE TEETH
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Re: GReg/Electrical Tech Journal

Postby greg on Thu May 25, 2017 8:16 pm

Neotek Elite-

Stereo channel #7 makes a bit of noise when unmuted and fader all the way down. Noise goes in and out when you wiggle the fader. I saw this on two other faders as well.

Disassembled, cleaned and lubed the faders.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri May 26, 2017 3:58 pm

Studio B's Echoplate
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Re: GReg/Electrical Tech Journal

Postby greg on Wed May 31, 2017 3:38 pm

Altec 150 (M11) noisy

Cleaned tube socket and capsule connector.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Jun 01, 2017 6:11 pm

Neotek Series II

Talkback intermittently going to Cue 2 bus... or Cue 1, I can't remember now. "2-mix to Cue" also intermittent on whichever Cue bus it was. Problem has been getting worse lately.


Ribbon cable between the Cue busses (mother board) and monitor module (where the talkback and 2-mix to cue signal source is) was mangled and barely making contact on some solder points.



Work on the 2-mix buss master module, and Aux Master module.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Jun 13, 2017 3:10 pm

Beyer M380 windscreens decomposing.

Just received the replacement foams. They are used in the TGX50s and fit perfectly. Part number 915149 for anyone's reference. I think a set (2 pieces) cost $4-6 ish
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Re: GReg/Electrical Tech Journal

Postby eliya on Tue Jun 13, 2017 9:22 pm

greg wrote:Beyer M380 windscreens decomposing.

Just received the replacement foams. They are used in the TGX50s and fit perfectly. Part number 915149 for anyone's reference. I think a set (2 pieces) cost $4-6 ish


That's from Beyer US or Germany? The people I talked to in the US office had no idea what I was taking about and I got some foam from McMaster Carr following your recommendation.
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Jun 14, 2017 11:51 am

eliya wrote:
greg wrote:Beyer M380 windscreens decomposing.

Just received the replacement foams. They are used in the TGX50s and fit perfectly. Part number 915149 for anyone's reference. I think a set (2 pieces) cost $4-6 ish


That's from Beyer US or Germany? The people I talked to in the US office had no idea what I was taking about and I got some foam from McMaster Carr following your recommendation.


Hey Eliya,

I just emailed parts@beyerdynamic-usa.com and asked them. Looks like the right person responded by luck? Their number is 631 293 3200.

Looking at the invoice, it was $1.39 per screen (2 needed for one mic).
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Jun 15, 2017 12:18 pm

Sennheiser MD421U -

Body broke apart at the mic clip-on slot. Ordered a new body.

6.19.17- Replaced. It couldn't have been easier to do. hopefully this will last another 15 years.


Studer A820MCH (studio B blacksides)-

While recording a band, monitoring in Repro mode, large DC spikes on the output of Channel 24. There was no input signal to channel 24 (dead patched). The DC spikes were in rhythmic timing with the music. The noise wasn't being recorded, and couldn't be seen on input (while recording), just Repro.

Replaced the Record, and HF Driver board, and the problem went away. Testing to come...
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Jul 05, 2017 5:04 pm

Lomo 19a13 -

Aftermarket, 2 channel power supply (built sometime in the '80s, I think) failed on just one channel.

Filament regulator failed (LM317 circuit).
While probing around, I discovered the B+ was a little high at 145vdc. From what I can gather with the notes we have, this should've been 100-120vdc.
The B+ circuit consisted of a pass transistor regulated by a zener diode. Both the diode and transistor were blown (zener- open, transistor - shorted collector to emitter). The result B+ voltage was just the unregulated DC appearing at the collector.

I rebuilt the power supply, replacing all the active parts, zener, and aging caps for good measure. All is working now.

For reference, I replaced a Russian KT940A transistor with an MJE340.


Thunder noise on the output when the mic cable is wiggled.

The ground/shield wire was fraying at the mic end of the cable. I trimmed back the cable and replaced the connector with one that has slightly better strain relief. These connectors were meant for thicker cables than the one we have, so it isn't an ideal match.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Jul 07, 2017 4:53 pm

-Elsewhere-

EMT 140/162 Plate reverb, broken motor controlled reverb time dampener.

The motor was frozen, so I had it rebuilt. I replaced the torn rubber coupler with a home made one cut from a similar sheet ordered from Mcmaster Carr. Built a temporary 24v supply out of an old wall wart. Going to build a more solid supply when I finish the remote control box.

-Electrical-

Neotek Elite

Left VU meter (2-mix) occasionally stuck at around -2 to +1db (not mechanically).

Removed the amplifier board and bench tested it with another meter.
The drive amplifier was oscillating. The rest of the circuit was operating correctly. Replaced the amplifier... all good so far.

Calibrated both meters while everything was pulled out.
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Re: GReg/Electrical Tech Journal

Postby greg on Mon Jul 10, 2017 5:24 pm

Neve B002 mixer-
Various problems related to the gain switch (gain intermittently jumping, large DC thumps, etc.).

Found some broken "C" clamp retaining rings on the rear of the switch, shafts. Ordered new ones. This probably won't solve the problems, but needed anyway.

Discovered poor connections between the contacts of the Elma switch.
Completely disassembled the switch and thoroughly scrubbed a contact cleaner or some similar solvent off the contacts with alcohol. There was a green residue all over. Then I reconstructed the switch, applied some diluted Stabilant (my new favorite raw contact conductive lubricant) on the contacts. This will be a test for some of the more gummed up switches. The resistance from switched pins dropped from 30+Ω to <.2Ω. Tested great in circuit.
N.B. The pressed fit nuts for the cover plate mounting screws all popped out. Whatever adhesive they used is now done doing it's work. Some of them are in hard to reach spots. I used a drop of rubber cement to tack them in place long enough for me to screw the fasteners in.
On to the next one.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Jul 14, 2017 4:22 pm

greg wrote:Soundfield Mk IV project-

Discovered some dead solder joints in the microphone head amp(s) after going through the electronic checks. After fixing them, the mic started to behave like it's supposed to. The collapsing capsule issue was related to this. Going to do the rough acoustic adjustments when I have access to a good, dead space (as close to an anechoic chamber as I can find). Conducting the listing tests in the manual, this thing is closer to right than I remember it being for years.


Used this the other day with weird results. It was in "Blumlein" and the eq balance between the left and right output didn't seem right, as if I used two different mics in a non-coincident pair.

Finally did a proper acoustic setup. This is a 80+ step process that I had to create a rig for. The goal is to match the eq curve and level of the four capsules to a standard in the tech manual, then the gain structure of the B format matrix. For this, we needed a speaker that would reproduce 500hz and 50hz at roughly the same level, and from the same point source in an anechoic environment. Luckily, we just finished repairing the Big Reds for sale, which has an Altec concentric woofer/tweeter arrangement. Are closest thing to an anechoic chamber is the dead room in studio A, so I set it up there (in a places where no direct reflections could occur from the doors or window). I set up an earthworks QTC1 coincidentally as a meter-sick for frequency response assuming there would be irregularities with the speaker response. This was important initially, because the speaker happened to have a slight dip in response at exactly 500hz, which was the specified "midrange" frequency to refer to for most of the alignment process. I used 550hz to get clear of that.
There was a lot of rough adjusting, then fine tuning going through all the gain stages of the capsules and B format amps. Almost every step required repositioning the mic so the individual capsules were facing forward in exactly the same spot as the last. In retrospect, I should have removed the windscreen cage around the capsules so I could see them easier.
Most steps concluded with more checks to confirm individual functions of the matrix were working correctly. The instructions leave a little to interpretation, with definitions of terms sometimes explained downstream. This lead to a lot of time consuming second guessing and redos (that reminds me, I need to make notes in the manual before I forget (why did I just write that)).
Anyway, the process corrected a lot of the inconsistencies in response and pattern. The thing sounds much more uniform. I'm looking forward to using it now.


Neumann SM-2 -
Unusual results when using it though the MS Matrix. Swapped the mic, same results. Swapped the Power supply, same results.

Should have swapped the cable. The pattern select wire for the top capsule broke off its pin in the cable connector. That capsule, which is normally used as the "side" mic in MS, was locked in omni. Resoldered the guy, and tightened the strain relief clamp ring. The cable was being twisted when unscrewing the mic cable from the power supply.
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Re: GReg/Electrical Tech Journal

Postby greg on Wed Jul 19, 2017 10:37 am

Echoplate in Studio B-
Ground loop, hum on input.

Installed a differential line receiver on the input of the amp. Although the noise is gone now, I'm going to install a driver on the send to it on the control room side. The plate is about 80-100 feet of cable away.

Neve B002 gain switch-
Elma style. Intermittent contacts.
Disassembled the switch, thoroughly cleaned the parts.
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Re: GReg/Electrical Tech Journal

Postby greg on Fri Jul 21, 2017 11:26 am

Digidesign 192HD-

Odd Ball problem.
When setup to transfer from the 24 track to pro tools, tape machine stopped, a static noise would appear on input 10 (pretty regular with several seconds in between). When recording in pro tools, similar noise would appear on the inputs (and record on) tracks 9, 10, and 12, but only when music was playing from the machine. I ruled out tape static since the noise wasn't there during quiet passages. I played with the clock sources, cycled the power on both the converters and computers, swapped the input db25 connectors, reconnected digilink and loopsync cables, nothing significantly changed. When the 24 track, which is plugged directly into the inputs of pro tools (not through the patchbay) was disconnected from those inputs, the noise stopped. OK, so it's the machine, right. No. Monitoring the outputs of the 24 track directly, there was no noise. The only thing that reliably got rid of the noise was by using a different sample rate. We were operating at 96kHz, with the noise. When I set up a similar session at 192k, or 44.1k, there was no noise (at rest and recording audio). WTF?

In summary,

Conditions for noise-

Must be a session at 96kHz (or 88.2 I found).
Inputs must be connected to the 24 track outputs

Conditions for silence-

Must be a session at 44.1k or 192k
If at 96k, or 88.2k, tape machine unplugged.

I have seen a bad connection of the digilink cable be responsible for progressively worse noise at higher sample rates, but the fact that things are all good at the highest, 192k rules that out, right?

Any ideas?
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Re: GReg/Electrical Tech Journal

Postby eliya on Fri Jul 21, 2017 3:31 pm

greg wrote:I have seen a bad connection of the digilink cable be responsible for progressively worse noise at higher sample rates, but the fact that things are all good at the highest, 192k rules that out, right?


I mean, you never know. It's an easy swap if you have an extra on hand.

greg wrote:Any ideas?


Nothing concrete, shooting in the dark:

Moving PT/Digidesign preferences to the desktop (better than deleting and losing your prefs in case the problem doesn't go away) is easy enough to try even though it doesn't really make sense. However, I did see odd problems solved by removing preferences and reinstalling a piece of software. For instance, this odd problem I had with VLC and its solution. EDIT: these links don't work. It's the second post on page 52.


Have you tried swapping the input modules between the two HD192 units to see if the problem moves with one module? If one unit has two 8 input modules, then maybe swap between the two (since the problem is with inputs 9, 10, and 12).

I assume there's no static when Pro Tools' inputs are connected to the patch bay and not directly to the tape machine?
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