[sdiy] (sdiy) Stuck in an ARP 4034....

[Bill Keys]

Help!

I'm confused... Hope this isn't too far OT.....

I decided to cannibalize my old ARP Explorer I; I can use the 4034 VCF and 
the 4027VCO and enhance them to start off my modular.

All I have had for the past 20 years or so are the main boards (no case, no 
power supply) and the schematics (except for the Power supply). I bought a 
PAiA +/- 15v PS to use for power.

The VCO works fine, emitting a ramp waveform. I can use it to throw a signal 
into the VCF. My problem is the #$%#!! filter. Since it was not working, I 
took about 8 hours to carefully remove the oven/potting material. After 
exposing the circuitry, I started testing. In short, I eventually had to 
replace all 4 3086's and the 301 opamp with new parts. I also replaced the 
FETS (2N5459's) but had to use MPF102's, since I had no 5459's (admittedly 
an error on my part; the original FETs were fine). However, the MPF102's are 
working for now.

The problem seems to be in the output opamp. I think it is oscillating. The 
weird part is that the signal from both FETS is fine (see
http://www.straightupband.com/sdiy/problem.jpg). The signal into pin 3 of 
the 301 is fine. The signal on the left side of R27 is fine. The signal on 
the RIGHT side of R27 _isn't_ fine. It is apparently being modulated by an 
internal high-frequency sine-wave oscillation of the 301. Pins 2, 6 and 8 
(the compensation?) of the opamp have the same
modulated waveshape (although at different amplitudes).

I thought at first it was coming from the ps. It isn't. The waveform is 
clear everywhere else on the circuit.

I had the output going into an old PC sound card amplified speaker system. 
Thought maybe that was the problem. Disconnected that, and the modulation 
signal is still there.

I don't know what else to do. I have about 15 301's. I hate to keep popping 
them in until one works. As I mentioned in a previous post, the 301s on this 
unit have 100% failure rate. Is this just a bad design? Did ARP not use 
301's? The original IC is house numbered, I guess. I'm using a 301 because 
of the schematic from J. Verghese on the web at 
http://www.netcontrol.fi/~jocke/arptech/

Maybe I should increase R23? or R24? Use a different opamp - TL082? 741?

Any help would be appreciated.......


+_+_Yes, honey, I know there are 25 boxes of electronic junk cluttering up 
the attic. They will, one day, magically be transformed into a modular 
synthesizer cluttering up the attic.... www.mp3.com/OriginalSimulacrum _+_+


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