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GReg/Electrical Tech Journal

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Re: GReg/Electrical Tech Journal

Postby greg on Thu Feb 21, 2019 6:05 pm

thecr4ne wrote:Hey Greg, do you guys use the 4665 Transformers in the Altec 525A psus for the 175s? Thoughts on this vs. going without them?

We did have one transformerless supply, but I put one in so they'd be all the same.
I think it sounded fine without it, but can't remember.

Beyerdynamic M500 -

Loose part inside.

The diffuser/filter assembly fell off the ribbon magnet. Glued it back on, and replaced the decomposing foam in the windscreen.


Schoeps 221b -

Broken capsule pin on the amp.

Replaced with a springed gold probe pin.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Feb 28, 2019 10:05 pm

Neotek Elite-

Cleaned some dirty switches.

Channel 5 Aux AB always sending to A.

I pressed the ground leg on the offending stack, and the problem disappeared. I suspect it's the same as some of the other pots in that the rivet securing the leg to the pot's pcb (or wafer) is losing connection. I tried a number of different pressure points, and the problem didn't come back. It will eventually. Unfortunately, you have to desolder the entire row of aux switches to get this one out. If it comes to it, I might disassemble the pot, in circuit, and do whatever work I can that way.

Schoeps 221b-

No output.

DEAD AC701! The grid is internally shorted to the cathode. A smarter person than me said the mic was probably roughly handled. It doesn't happen electrically. So, I guess let that be a lesson for everyone. Treat like crystal!


-Elsewhere NY-

Neotek Series II
Cleaned a bunch of faders, fixed some channels, some by reflowing some broken solder joints on the channel headers, replaced some pots...

Lexicon Prime Time delay-

Delay would work for a few seconds after power up then go quiet, while the display looked the same (though no input metering).


5v regulator was making bad contact with the socket it was sitting in. I cleaned it all out, and bent the to220 legs towards the pins in the receptacle. Worked the rest of the day. Their plan is to recap the main board, so I recommended to them that they solder the regulator directly to the board later.

AKG BX20-

Noisy Channel 1

One of the fairchild dual op amps was oscillating. All the components around it measured correctly, and the oscillation occurred if the channel PCB was in either socket. They plan to recap and replace the amp when they get parts.

Slappy delay sound on Channel 2

One of the "record" coils measured open when the DC resistance should have been under 300Ω. The fault description in the manual was consistent with what we were hearing too.
They're ordering a new one from a guy in Germany


Otari MTR 90 CB120 Remote-

Display and functions freak out when buttons are pressed hard.

Discovered I could reproduce the problem by wiggling a four pin cable connector on the board with the buttons (on top of the display board). I cleaned and checked the tension on the female socket pins, but the problem could still be there with some force.
This cable went to a 5v switching regulator module. The four wires were in two pairs, high voltage-0v send, and 5v-0v return. The two 0v conductors were tied together at the regulator, but not on the pcb. I tried jumpering them together at the pcb header, and the problem never came back.


Cleared up some other ground and power distribution issues.
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Re: GReg/Electrical Tech Journal

Postby thecr4ne on Sat Mar 16, 2019 11:15 am

Hey Greg,
RE: altec 525A

1:When replacing the selenium rectifier, did you just put in 4 diodes in a bridge configuration? I have one of these that just has 2 diodes soldered onto the old rectifier, wondering if this is sufficient.

2:Do you also put a new diode in place of Rect.2? should this be left half wave or bridged?

Thanks
greg wrote:Altec 175 dead

[color=#004000]
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 Thu Mar 21, 2019 11:21 am

Neotek Elite Channels

Channel ?
Intermittent switches in the signal path cutting out, or distorting the signal in various modes.


I previously hit the suspect switches with an alcohol cleanse, exercise, then stabilant routine (via syringe). This had mixed results, probably due to the fact that the dirt can't escape, so it took it to the next level. I pulled the channel and took apart the switches. This way, I was able to scrub the parts individually with q-tips and apply the conductive lube in a clean way. I lost 3 of the 36 contact tension springs in the process (about 1 x 3mm in size), but had some spares to steal from. It's a little tricky to get them in there. A slight misjudgment sends the spring 15 feet off into oblivion (as far as my eyesight is concerned). I found my rhythm using a tiny flathead screwdriver (flathead wedged between the last turns of the spring, pressed the other end into the back of the wiper piece, compress and twist into place, drawing out the screwdriver with my finger holding the spring in place). This all took more time that I hoped. The faceplate had to come off and there were some stubborn nuts to remove.
This problem is becoming more frequent. It would certainly be quicker to replace the switches. I'll wait to see if this channel gives us any more grief.
Installed in Channel 2 position now.


Channel 2-

Pad and Phantom power switches intermittent.

Pulled. It's worth noting that the microphone signal goes through the phantom power switch (as opposed to the switch just handling the 48 volts). It's just as important for it to be perfectly working as the pad switch.
Replaced the switches with a pair of new gold contact versions. This took 5 minutes to do, mainly because they can be removed without taking the faceplate off.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu Mar 21, 2019 11:36 am

thecr4ne wrote:Hey Greg,
RE: altec 525A

1:When replacing the selenium rectifier, did you just put in 4 diodes in a bridge configuration? I have one of these that just has 2 diodes soldered onto the old rectifier, wondering if this is sufficient.


Just replace the selenium rectifier with two diodes (as it appears in the schematic). It's not meant to be a bridge. The old rectifier should be completely out of the circuit if you do. It sounds like yours might have two new diodes in parallel with the old?
We have converted a few supplies to use a regulator circuit for the filament. In that case, a bridge rectifier is good for getting the most out of that transformer winding.

2:Do you also put a new diode in place of Rect.2? should this be left half wave or bridged?


It should be left half wave as it is in the schematic.
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Re: GReg/Electrical Tech Journal

Postby thecr4ne on Fri Mar 22, 2019 12:21 pm

Awesome, Thanks Greg.
Yes, mine has the diodes paralleled to the old rectifier, just soldered to it's terminals. I have 5 of these I'm trying to recap and standardize, figured I'd update the rectifiers along the way.

I finally got a 29B capsule, but it looks like its' got some crud in the screen around the base. I've seen some posts indicating you've cleaned these before, and even taken some apart. Do you have any advise for taking care of this?

Thanks again. I really appreciate the help.
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Mar 26, 2019 12:55 pm

Not sure exactly where you're talking about as far as dirt goes, but in general, be extremely careful. Don't use compressed air to blow things out. Particles could end up where you don't want them (wedged between the diaphragm and back plate, for instance).
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Re: GReg/Electrical Tech Journal

Postby greg on Tue Apr 23, 2019 5:24 pm

RCA 74 Jr Velocity-

No output, ribbon in good condition.

Primary measuring a short (roughly .2Ω)


Altec 436c-

No audio getting through...

Hi voltage secondary is on the power transformer is dead, as is the selinium rectifier.
Replaced the original power supply caps, and power cable. Cleaning up some other wiring.
Ordered new transformer.
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Re: GReg/Electrical Tech Journal

Postby greg on Thu May 09, 2019 3:28 pm

-Elsewhere-

EMI TG12345 MKII/III 24+ channel 8 bus console.

Random sputtering noise on the outputs, annoyingly intermittent (once every few hours annoying).

The channels are made up of multiple PCBs tied together by solid wire jumpers. Some of these have oxidized, degrading their solder joints, causing this kind of noise. Also, a number of electrolytic capacitors have been bulging, and falling out of spec (50ish years old). In the past, one of the cassette power supplies was causing a similar noise all over both the channels in the cassette. The problem was traced back to a failed electrolytic cap.
The decision was made to replace the electrolytics , and reflow all the board to board solder joints, replacing any really oxidized jumpers. I cleaned some other hard to get at switches and connections while I was at it.
So far, the console has been quieter.
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Re: GReg/Electrical Tech Journal

Postby steve on Sun May 12, 2019 3:25 pm

I've read about the potential for oxidized leads to make a semiconducting junction with connector contacts, like a cat's whisker or a diode, do you think that's what happens inside the solder joint or the solder joint breaks and becomes intermittent?
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