Tricky ARP Quadra issue - need a sleuth!

Synth Repair
1

Hi group, just thought I would throw this out there as it is a tricky one:

Totally refurbished ARP Quadra is creating a noticeable distortion when the gain sliders are moved in the mixer for the POLY or STRING section only.

The distortion is only present when the Phase Shifter is in use and is only present on the stereo output jacks, not the MONO OUT.

It is also interesting that the distortion is only present as the gain slider moves through a certain range of the fader - usually about 30-70% up.

I’m puzzled. Any ideas where to look? Thanks. - jimmy

2

The problem likely lies in the compressor, expander circuit on the phase shifter board. My guess is the compressor circuit. You will need to probe with an audio type probe where the volume drops off and is boosted at each stage of the circuit. I’m having the same problem and finally installed two resistors on the output to control the distortion. I can send pictures and specific details if you like.

3

Check z12,13, 16 to start. Z16 is supposed to compress the volume. Replace these ic’s first.

4

Kevin, thank you so much for this information, it is extremely helpful! I’d take any and all additional info you could send gladly. Those IC’s were replaced in the upgrade. Perhaps resistors related to those IC’s are at fault? Much appreciated! - jimmy

5

Hi Kevin. If you have any time in the next two days, I have my tech coming over Thursday afternoon and would love some pictures or more info about your fix for this problem.

Thanks in advance if you can send anything that may help! Much obliged! - jimmy

6

Bump! Hoping to still hear from Kevin! TIA!

7

Still waiting to hear back from Kevin - hope you read this sometime soon! Thanks. - jimmy

8

Kevin! Help! Still stuck on this but I know we are close zeroing in on that Phaser board. Any info you can send would be a Godsend. Thanks! - jimmy