I worked on two ARPs most of the day for the last few days, this one and an Omni 2. The filter, ADSR and VCA on the Axxe are very close to identical to those in the Omni’s “synth section.” While I was initially skeptical of the Axxe (as I am of any synth with only one oscillator), I really appreciate the fact that it lets you route the envelope generator to control PWM, which you don’t often see. Pulse width modulation is underutilized in most vintage “budget” synths, but it’s very powerful, and not at all hard to implement on almost any pulse oscillator. I like the nubby little colorful slider covers too.
This is a very simple synth, one of the simplest I’ve ever worked on, but it still has what I’ve come to think of as a sort of arbitrary contrariness that seems to be shared by all early ARPs. They are finicky and they want you to fail, and I came to feel like every time I put the cheap, rickety connectors back onto the board, it was anyone’s guess whether the synth would make sound. I’ve got three more ARPs coming down the line… starting with two Avatars today from the same client, so I guess I better get used to it!
The Axxe is also one of those synths that you open up from the bottom, which is kind of annoying too. You have to take the keyboard out to work on the PCB.
Service Tips: I am skeptical of ARP’s decision to use 250 uf capacitors in the power supply for this one. I was getting clicking sounds from the key trigger voltages (which are the -15V and +15V from the power rails via a couple of transistors) leaking into the CV even after I rebuilt the PSU and replaced all the capacitors everywhere. I decided to try replacing the PSU caps (already replaced with 270 uf) with 470 uf ones, and it seemed to somewhat improve.
I finally learned how to desolder ARP boards without damaging the traces: don’t use any suction of any kind, use braid only and a temperature of around 650 degrees (F).
Work done: Cleaned jacks, key contacts, buss bars, cleaned and lubricated sliders. Replaced all tantalum and electrolytic capacitors, put in IEC connector to replace built-in power cable, reapplied thermal compound to PSU heat sinks. Replaced power supply regulator IC and transistors to correct bad PSU output voltages. Replaced S+H polypropylene capacitor that wouldn’t hold CV, replaced 2 failed transistors for correct trigger voltages. Resoldered bad wiring to interface connections. Modified 4075 filter board for higher cutoff. Tuned, scaled, calibrated, VCO, VCA, and VCF.