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Improving the E1 Circuit in a 370/12RM

Posted: Mon Dec 17, 2018 12:52 am
by Very Occasionally
I just got the RM signature guitar and am finding some fault with the circuit design. It's basically the MXR Dyna Comp CA3080 circuit, but without an output (volume) control and a sensitivity control. For those interested, the IC is a CA3080 or NTE996 and the xstrs are 2n5088's. I've attached the schematic - hat tip to Mark Tefler who posted it in 2008. My guitar is not exactly the same, but close enough to make no material difference. Also posted are 2 MXR DynaComp schematics.

The 1st issue - decreased overall volume compared to the non-compressed settings - can be remedied with a 100k, or 50K, pot where the 82k resistor sits, or by simply jumpering that resistor (R16) and lifting the ground on R17 (On the circuitboard of my guitar R12 is not present).

The 2nd issue is there is no way to control the amount of compression in the circuit. Although the instructions that accompany the guitar tell you it can be adjusted with a trim pot (1k variable resistor on the PCB, R23), that pot really is a "set and forget" that works best in the 12 O'clock position and does nothing to vary the amount of compression. And there is an insane amount of compression in this circuit.

If you compare the E1 schematic to the DynaComp, you see the E1 has a 33k resistor from the Emitter of Q5 to pin 5 of the IC, where the Dyna has a pot (in front of a 27k resistor on electrosmash schematic, and no resistor on the GG schematic) going to the same pin. I suppose Roger liked a super-smashed sound, so the designer left out a real sensitivity control, but I find I prefer to be able to reduce (or increase) the effect, depending on the song, etc. So then the question becomes how to implement the mods without compromising the integrity of the pickguard and circuit board.

Any suggestions?

~ Steve

Re: Improving the E1 Circuit in a 370/12RM

Posted: Mon Dec 17, 2018 6:29 pm
by Tube_Fan
If you want to add a sensitivity control without drilling more holes in the pick guard, you could try putting in a stacked dual concentric pot and use one pot for the control that was there and the other pot for the sensitivity control.

CA8080 datasheet
https://www.intersil.com/content/dam/In ... 3080-a.pdf

Applications
https://www.experimentalistsanonymous.c ... AN6668.pdf

Re: Improving the E1 Circuit in a 370/12RM

Posted: Tue Dec 18, 2018 5:39 am
by Very Occasionally
[quote="Tube_Fan"]If you want to add a sensitivity control without drilling more holes in the pick guard, you could try putting in a stacked dual concentric pot and use one pot for the control that was there and the other pot for the sensitivity control.

Hi John, thanks for your suggestion; much appreciated. Take a look at the pic and you'll see the controls are hard-soldered to the circuit board, so it's not possible to install a stacked pot.

What I am considering is having a spare lower guard made so I can mod without destroying the original. In that case I may use a couple mini-switches to give me some more options as presets for higher output and stepped levels of compression.

I did reduce the compression about 25% by inserting a 150k resistor in series between the 33k resistor and pin 5 on the 3080. And I increased the output of the E1 with a 10K in parallel with the 82k to the output, leaving R17 (10K to ground) in the circuit. Totally easy to remove these mods to put it back to original spec. So far I'm finding it more useful than stock.

~ S.

Re: Improving the E1 Circuit in a 370/12RM

Posted: Thu Oct 27, 2022 10:35 pm
by gtoira
Where do you check the setting of the trim pot?
Is it adjusted with the guitar plugged in and compressor in one of the 4,5,or 6 positions?
The spec for it is 2.5v from the factory. Where do I put the voltmeter leads to check this?

Re: Improving the E1 Circuit in a 370/12RM

Posted: Fri Nov 04, 2022 6:42 pm
by Tube_Fan
Some interesting information here
https://antiquesci.50webs.com/CompExp.htm

Re: Improving the E1 Circuit in a 370/12RM

Posted: Sat Jan 14, 2023 8:40 pm
by gtoira
OMG I can't read a schematic, especially one from the mid-fifties.

Does ANYONE know how to adjust the variable resistor. Send a picture showing where to put the meter. I assume it vaies the voltage to 2.5 volts between what and what.
When I go from either leg of the resistor to ground, it starts at 2.49 then immediately starts to drop.
One would hope Rickenbacker would be more forthcoming, but nooooooo........

Re: Improving the E1 Circuit in a 370/12RM

Posted: Sun Jan 15, 2023 5:37 am
by kennyhowes
Very Occasionally wrote: Mon Dec 17, 2018 12:52 am I just got the RM signature guitar and am finding some fault with the circuit design. It's basically the MXR Dyna Comp CA3080 circuit, but without an output (volume) control and a sensitivity control.
...
Any suggestions?

~ Steve
I suggest doing like I did and getting an old MXR DynaComp. :D
gtoira wrote: Sat Jan 14, 2023 8:40 pm OMG I can't read a schematic, especially one from the mid-fifties.
Is there one here from the ‘50s? I’m confused.
gtoira wrote: Sat Jan 14, 2023 8:40 pm One would hope Rickenbacker would be more forthcoming, but nooooooo........
About what?

Re: Improving the E1 Circuit in a 370/12RM

Posted: Thu Nov 16, 2023 9:54 am
by jdogric12
Antiquesci link above seems to be long gone now, so here is what i captured off the internet wayback machine just now:
Volume Compressors and Expanders.pdf
(2.09 MiB) Downloaded 41 times

Re: Improving the E1 Circuit in a 370/12RM

Posted: Fri Nov 17, 2023 7:26 pm
by Tube_Fan