[sdiy] ARP 2500 schematic question for 1027 Cocked Sequential Controller

Steve Lenham steve at bendentech.co.uk
Wed Oct 1 23:10:27 CEST 2014

On 01/10/2014 21:58, Travis Thatcher wrote:
> Fixed the stage 8 issue by replacing a PNP in the typical driver
> circuit.  The lamps don't light on the first two stages but I can't
> really test things under power in the ARP. Ordering a few Cinch
> connectors and some 12v lamps.  Any reason I couldn't replace one of
> the lamps with an LED+current limiting resistor?

No, that should be fine.

If the stage 1 and 2 CVs are working OK then there are only four things 
that could be causing the problem - the upper PNP, the 10K base resistor 
to that transistor (they are carbon comp parts so could go bad), the 
bulb itself or a cracked joint/trace. Not too hard or expensive to 
replace all three components.

> As for the dual VCO, I replaced the summing LM301 and still no dice on
> either VCO. I'm going to be rolling my own current source module for
> another VCA that's missing one.  Anyone have and suggestions on
> further troubleshooting of the VCOs not responding to any type of
> control input, including the tune pots?

What sort of frequency are the VCOs stuck at? High or low?


Steve L.

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