----- Original Message -----From: synthjoeSent: Sunday, August 19, 2012 11:28 AMSubject: [vintagesynthrepair] Re: ARP 2601 (2600) 4072 VCF problem...Even though I have not seen the circuit diagram, but for the 3900 I'd think you can substitute any bipolar opamp, but the pinout is different. A small perf board or some wires might do the trick.
--- In vintagesynthrepair@yahoogroups.com, "desneed" wrote:
>
> Hello everyone!
>
> I've been working on my ARP for a little while - fixed the power supply and tested all of the modules. I'm having a problem with the VCF though - it's a 4072 that's not encapsulated. It doesn't pass any audio and if I turn up one of the mod CV inputs - like the OSC 2 input, I can get the VCF to make some noise...not really useful noise, but it's noise, depending on where I have the initial freq. slider at. I'm using a bench supply to run the ARP until I have it working properly, then I'll reinstall the original supply and calibrate the synth.
>
> I pulled the module and attached wires between it and the main board to gain access to the component side. There were 5 10uF/35v Tant caps that did not show bad, but I replaced them anyways with some Panasonic electrolytics I had purchased in bulk a while ago. This didn't fix the problem.
>
> If I look at source audio - say from OSC 1, I see ~10vp-p hitting the slider. When I look at the input of the module it is down to a fraction of it. This is the same whether it's OSC 1, 2 or 3. The resistance to ground is ~ 220 ohms which is what the input resistors to the module are.Anyone know what level p-p I should be seeing at the module input? DC voltages look good on the VCF module. I'm not sure if the problem has something to do with the level shift circuit (the LM3900 op amp is running between ground and -14vdc and inputs need to be adjusted accordingly) or if the input xsistor pair (2n5087 xsistors) may be dragging the input down. I looked at the opamps input and outputs - basically nothing getting thru it. Of course there might not be enough of anything getting past the input stage to get to it..
>
> Any help would be greatly appreciated! I looked thru the old posts and 'Ethan' said he had a problem with a VCF on his ARP but no replies were made to the initial thread. I was thinking of just pulling the input pair and testing them out of circuit. Was going to do that to the LM3900 but i didn't have any in my collection and no one local had any..oh Radio Shack how you disappoint me these days.... :)
>
> Tnx
>
> Dave
>
Message
Re: [vintagesynthrepair] Re: ARP 2601 (2600) 4072 VCF problem...
2012-08-19 by jammie
the lm3900 has low impedance inputs when standard
bipolar opamps have high impedance inputs
so you should really change it with a 3900 opamp or
an equivelent based on the same circuitry
national semiconductor
or texas instruments did an equivelent
try jameco
or utsource
i had some from jameco before a few years
back
but get a lot of my stuff from utsource now as they
seem to have a massive inventory