Finishing old projects: MC76 limiter kit...seeking help

[DrFrankencopter]

Hi Gang,

It's been a few years since I've been here. Lots of activity I see! I'm trying to get a few DIY projects that I started a few years back off my plate. 1st up is the Purple Audio MC76.

The schematic is here: http://www.purpleaudio.com/pdflib/PurpleMC77v34.pdf on page 13

I bought the kit way back when it was still the MC76 board (not the MC77). I've got all the PCB components (I think), & transformers but need some help sourcing the pots. The schematic calls for:
Output pot: 250K 2W Audio taper (I suspect these will be pretty common...as it's a typical guitar part value)
Attack: 25K 1/2W no taper specified, but I imagine it's linear. The original had a switch output at the end of the travel to turn limiting on or off (not quite bypass, as the audio sill goes thru the audio path and Xfmrs)
Release: 5M 1/2W no taper specified (I assume linear)
Meter zero adj: 2K (linear?)

Any part #'s (digikey?) would be helpful.

Also, I see in the meter driver section of the schematic (lower left corner)  there is a resistor R44 specified as T&C with a value of ~1K...I guess this means 'try and see'? The same nomenclature appears on the ratio switch for a resistor going between R19 and the common to all the switches. Any guidance here would be great...

Finally, on the schematic there are 3 LED's in series leading from the +30V to Q22...what are these, and where do they go?

Thanks

Kris

 

[indigom]

Hi Kris,

I recently re-opened my MC77 partial kit which came with a few diodes, the power supply transformer, the input and the output transformer as well as the PCB (purchased in 2007, the last of them?).
I assembled what parts I had on hand at the time, been recently filling in what was missing on it.    A few resistors and caps, and racking it hopefully within the next few months.

The pots that I found were from http://www.studioelectronics.biz/ Studio Electronics in Burbank, California and are _rather_ expensive but are supposed to be an excellent replacement for the real UREI1176 (certain versions).


250K Audio Taper Output Pot; UREI Rev A-E and UA 1176LN

1 $25.00 $25.00
New 25K Attack Pot with switch for all UA UREI 1176LN

1 $27.50 $27.50
New 5 Megohm Release Pot for all UA and UREI 1176LN

1 $26.00 $26.00

At least one seems to be made by PEC in Canada, you might be able to find them cheaper elsewhere.

I still have to figure out how to hook in all the transformers and the mounting plus design a frontpanel and get a meter.

 

[edanderson]

i would not recommend the PEC pots, and certainly not at those prices.  digikey has the k series from PEC for better prices if you want them.

output:
95A1D-Z28-EA0/301L-ND
digikey - this is a very nice 250k audio pot, good match to the attenuator used in the mc77

attack:
you have plenty of options if you don't need the switch.  if you do, you can either get the alpha pot from mouser (not the greatest quality, but cheap), bug andrew at purple, or the studio electronics part.

release:
53C35MEG-ND
digikey - probably overkill, but this is a very nice clarostat pot, should last til the end of days

the zero adjust can be any decent linear 2k pot.

the correct value of r44 will be around 1k, usually in the range 750r to 1.3k.  you can try various fixed resistors, or use a jumper and a 2k trimpot, which is what is on the mc77 pcb.  i believe this is described in the urei manual.

the LEDs are the meter illumination.  a better/easier solution is to get the to92 version of the lm317 and wire it with a fixed resistor to create a CCS and let that power your LED string.  the lm317 has thermal protection, etc.  

ed

 

[DrFrankencopter]

Many thanks Ed, just the info I was looking for!!!

Cheers

Kris

 

[indigom]

i would not recommend the PEC pots, and certainly not at those prices.  digikey has the k series from PEC for better prices if you want them.

Thank you Ed; too late.  : )    on the MNATS version next time.

cheers
Jay