McCurdy Talkback Mic Pre PC861-2-B

I bought this thing on a fire sale and thought it looked like it could be a fun project. I'm not sure what kind of desk it came out of - the seller didn't have any info - it looks like something from the 80's or 90's. It would certainly by useful to have a schematic, but I think I can make due without.

The questions I wanted to put out there are:

1. Does anyone have any info on this, or know what it is?
2. How do I figure out what voltage this runs on? I'm assuming I should go through data sheets of the IC's, transistors, and voltage ratings of caps and come up with a number? I believe I've seen most of these chips in consoles that run on +/- 15-18v.
3. Can just I swap the bal/comp/gain variable resistors with panel mount potentiometers?
4. Anyone know what the THLD variable resistor is? You can't see the "THLD" label that well in the photo, but it's the one right in the middle of the board.

Photos:





Any help would be much appreciated! I'm planning on getting started on this thing next week, but thought I'd put this out here in the event that there are any McCurdy experts out there!

Cheers,
Erik

EWalter said:

1. Does anyone have any info on this, or know what it is?
2. How do I figure out what voltage this runs on? I'm assuming I should go through data sheets of the IC's, transistors, and voltage ratings of caps and come up with a number? I believe I've seen most of these chips in consoles that run on +/- 15-18v.
3. Can just I swap the bal/comp/gain variable resistors with panel mount potentiometers?
4. Anyone know what the THLD variable resistor is? You can't see the "THLD" label that well in the photo, but it's the one right in the middle of the board.

Click to expand...

Those are 15 volt +/- regulators in the photos, so feed it at least +/-18VDC so they can do their thing.

Yes, you can put in pots of the same value and taper for any variable resistor.

I would guess that is a Threshold control for the built in compressor.

mjrippe said:

Those are 15 volt +/- regulators in the photos, so feed it at least +/-18VDC so they can do their thing.

Click to expand...

Ah yes that totally makes sense. Will do!

mjrippe said:

Yes, you can put in pots of the same value and taper for any variable resistor.

Click to expand...

Figured as much. How might one determine the taper?

mjrippe said:

I would guess that is a Threshold control for the built in compressor.

Click to expand...

Duh! Not sure how I didn't get that...

Thanks for all of the help!

Taoer on 99.9% of all trimmers will be linear.

Bri

Brian Roth said:

Taoer on 99.9% of all trimmers will be linear.

Click to expand...

True.  A quick test is to put the trimmer at it's center position.  Measure from the wiper to each leg.  If the values are close to the same (say 4.5K and 5.5k for a 10k pot) then it is linear.  If they are very different (2.3k and 7.7k) it is log.

i ain't no expert but find my post of a couple weeks ago tittled "mosfet limiter?".

It is probably similar topology. My
talkback limiter card was McCurdy ap-276.  Yours seems more recent but i see a lot of similarities.  Look at my schematic and manual for clues.

Yours has the same Beyer transformer (1:10) , also same "dual matched opamp" for the balanced out. (mta1537 or something).

Look fo a mosfet or jfet for the limiting, mine was a mosfet.


i just started to experiment with sound through it on mine. The threshold control don't seem "traditional", It seems that the threshold is fixed and the threshold trimmer control the amount of signal controlling the fet, it kinda acts like the input control on a 1176.

The attack and release seem to be controlled by a single cap (100uf on mine), lowering that cap seems to reduce release time.

Very weird circuit but fun.

Let me know what you find.

mjrippe said:

True.  A quick test is to put the trimmer at it's center position.  Measure from the wiper to each leg.  If the values are close to the same (say 4.5K and 5.5k for a 10k pot) then it is linear.  If they are very different (2.3k and 7.7k) it is log.

Click to expand...

Interesting! Never knew that this was such a simple test. I'm going to pull them out soon and test them. I found the data sheet for these, but it doesn't seem to give any hints about the taper:

https://www.ttelectronics.com/TTElectronics/media/ProductFiles/Trimmers/Datasheets/91.pdf

The value printed on mine are 20k - so that's an easy gimme.

diggy fresh said:

i ain't no expert but find my post of a couple weeks ago tittled "mosfet limiter?".

It is probably similar topology. My
talkback limiter card was McCurdy ap-276.  Yours seems more recent but i see a lot of similarities.  Look at my schematic and manual for clues.

Yours has the same Beyer transformer (1:10) , also same "dual matched opamp" for the balanced out. (mta1537 or something).

Look fo a mosfet or jfet for the limiting, mine was a mosfet.


i just started to experiment with sound through it on mine. The threshold control don't seem "traditional", It seems that the threshold is fixed and the threshold trimmer control the amount of signal controlling the fet, it kinda acts like the input control on a 1176.

The attack and release seem to be controlled by a single cap (100uf on mine), lowering that cap seems to reduce release time.

Very weird circuit but fun.

Let me know what you find.

Click to expand...

Very cool! Thanks for the info! I'm gonna look into it and will report back. My plan for now is just to get power on it and pass audio through it and listen. Modding attack and release would of course be extremely handy, so that would certainly be my next move. Thanks for sharing!

I finally got around to doing some work on this thing. I got it powered up and it works as a pretty good sounding little mic pre. The compression circuit doesn't really seem to be working though. The "Comp" pot seems to only function as a secondary gain at this point, sort of like Diggy Fresh's explanation of the 1176 input control, however I'm not hearing any compression at all.

I'm very much a novice, so just following a schematic is a challenge for me and I'm certainly not able to just look at a PCB and follow exactly what's going on, so I'm doing a lot of shooting from the hip here

I noticed an in/out switch on Dougy Fresh's schematic and found  a 3 prong jack on mine labeled TP2 that looked like it could be for a switch. The center pin isn't connected to anything and the two end pins look to me like a break in a circuit where one could implement a switch, but I didn't have any luck when jumpering them together. See below:



I'm sure there's something obvious that I'm missing here, and I'll keep plugging away.

One other note: There is a second input on this thing, labeled (with a sharpie) #2. This looks like the transformerless input and I thought it was potentially a line input, but it was extremely distorted at line level, and didn't work at all as a mic input. Any thought as to what this could be?

Thanks for all of the help!

Cheers,
Erik

I think you'll find the MTA1537 is a VCA and not a balanced output driver!

Nice spotting Walrus ! i was confusing with a mc1437 (dyslexia..).


So Erik, it looks like unlike me, you have a vca compressor!

diggy fresh said:

Nice spotting Walrus ! i was confusing with a mc1437 (dyslexia..).

So Erik, it looks like unlike me, you have a vca compressor!

Click to expand...

Good to know! I'm going to try to do a little more work on it this week and this will certainly be helpful. I still need to figure out how to engage the compression circuit though! 

Cheers,
Erik