Need help with my PEC...

KGBMAN · Apr 6, 2013

I'm finally getting around to trying to replace the PECs in the tone controls of my Grommes E-24 unit. I can't find any information on the PC-1964 modules that came in the system and the schematic from Grommes shows no values.

Can anyone please suggest some capacitor values to start with?

Please consult the attached pdf file for the schematic. Also, to the mods, could you please upload this version to the AK docs and replace my old one up there? I've made some corrections in the PS section.

Thanks.

KGBMAN · Apr 6, 2013

Someone a while ago suggested these values:

Four capacitors:
Pin 1 to pin 3, .0022
Pin 2 to pin 4 .0033
Pin 4 to pin 6 0.1
Pin 5 to pin 7 0.01
All values in microfarads.

I will remove one of the original ones and see what they measure.

s-petersen · Apr 6, 2013

you may find it in this link,there are several links to catalogs with schematics
http://www.audiokarma.org/forums/showthread.php?t=497253

I just read through the links,The highest pec #'s go up to around 350
It may be listed in the Sams for that model

s-petersen · Apr 6, 2013

This amp uses a 7 pin pec,with 1 meg pots,possibly the same one,the internal schematic and values are present

http://members.shaw.ca/pacifictv3/grommes15pg8.jpg

if someone has the Sams for this(folder 432-8 Grommes15pg8 )possibly they can confirm the pec# (k3 on the schematic)

if several pins read 68K for resistance,it would increase the chances that it is the same one

Tom Bavis · Apr 7, 2013

I have that Sams folder, and both Grommes and replacement part numbers are blank... the 207 preamp uses the same one - Grommes P/N S191A or S191B. Grommes amp 214 shows the same one as Grommes P/N TC-190.

KGBMAN · Apr 7, 2013

Thanks for the responses.

I pulled one of the originals and checked it out. It appears to be the same configuration as the #55 unit in the Sprague catalog (see attached).

Values I measured:

C1 = 0.45 nF
C2 = 2.2 nF
C3 = 3.9 nF
C4 = 1.55 nF

R1 = 100KR
R2 = 52KR

I know these values have drifted from the originals but is it worthwhile to try to dissect this thing (a la Scuzzer)?

Scuzzer · Apr 7, 2013

KGBMAN said:
I know these values have drifted from the originals but is it worthwhile to try to dissect this thing (a la Scuzzer)?

I've never had a successful dissection. I have destroyed a few to verify the type of components used though.

jaymanaa · Apr 7, 2013

IMHO, replacing these PECs with quality separates is one of the best bang for buck mods you can do on the older integrated amps. Esp. something like the HF81 which uses them in the active tone circuits. The small rectangular boards that Radio Shak sells (the one you can break in half) are perfect for the job.

KGBMAN · Apr 7, 2013

jaymanaa said:
IMHO, replacing these PECs with quality separates is one of the best bang for buck mods you can do on the older integrated amps. Esp. something like the HF81 which uses them in the active tone circuits. The small rectangular boards that Radio Shak sells (the one you can break in half) are perfect for the job.

That's exactly what I'm using Jay. Some perf board from RatShack with discreet components.

I guess I'll try to mimic that circuit in the Grommes schematic posted earlier and use that as a starting point.

KGBMAN · Apr 7, 2013

Scuzzer said:
I've never had a successful dissection. I have destroyed a few to verify the type of components used though.

I assume you were never able to determine what the original values were?

jaymanaa · Apr 7, 2013

KGBMAN said:
That's exactly what I'm using Jay. Some perf board from RatShack with discreet components.

I guess I'll try to mimic that circuit in the Grommes schematic posted earlier and use that as a starting point.

If you think you have the correct diagram, I'd say build it, then compare resistance and capacitance readings between the original PEC and your new piece. I wouldn't expect them to be spot on due to the age and quality of the stuff in the PECs, but somewhat close maybe.

s-petersen · Apr 7, 2013

Unless you can figure out exactly which other units use the same pec to make/get a cross reference/schematic,like the Eico's use the same pec for many different models,some of which have the internal schematic with values.

jaymanaa · Apr 7, 2013

s-petersen said:
Unless you can figure out exactly which other units use the same pec to make/get a cross reference/schematic,like the Eico's use the same pec for many different models,some of which have the internal schematic with values.

I've found that some schematics break them down and some don't. The Sams Photofacts always seem to.

P.O. · Aug 14, 2023

KGBMAN said:
Thanks for the responses.

I pulled one of the originals and checked it out. It appears to be the same configuration as the #55 unit in the Sprague catalog (see attached).

Values I measured:

C1 = 0.45 nF
C2 = 2.2 nF
C3 = 3.9 nF
C4 = 1.55 nF

R1 = 100KR
R2 = 52KR

I know these values have drifted from the originals but is it worthwhile to try to dissect this thing (a la Scuzzer)?

I do kind of confirm this. Here are values measured from a PC-1964 unit removed from my Precision Electronics S100.
These units were widely used in Grommes and Precision Electronics amps. The configuration matches the Sprague 55 units, but the values are very different.

C1: 490p
C2: 2345p
C3: 4253p
C4: 1760p
R1: 116K
R2: 55K

I'm stull struggling to understand the tone circuit in there: it seems to have feedback components I can't figure out...

Need help with my PEC...

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