2 ARP Quadras awaiting restoration

We recently acquired 2 ARP Quadras in nice physical condition, but sadly not working properly.

ARP Quadra awaiting restoration

ARP Quadra #2 awaiting restoration

Quadra #1 powers up, but only the bass section works.  The membrane panel isn’t responsive.  ARP membrane panel issues are said to have contributed to the company’s failure…  However, if all the membrane buttons aren’t working, I’d have to suspect an issue with the ribbon connector or the microprocessor board circuitry.

ARP Quadra awaiting restoration

ARP Quadra#2 under the hood

When testing Quadra #2 at purchase, it powered up and the membrane panel was responsive, but none of the 4 sections (bass, strings, poly synth, lead synth) were working right. As I was going through it, I heard some crackling and the LEDs started flickering.  Quickly I powered it off, but was able to spot the problem, a tantalum capacitor on the microprocessor board, specifically one of the decoupling capacitors ARP placed at the connector from the power supply.  Solid tantalum capacitors pose a fire hazard when their voltage rating is exceeded, and the cheap tantalum caps used in ARP synthesizers are prone to failure, either caused by transient voltage spikes or even just sitting unused with age.  Sadly, these capacitors fail as shorts instead of open circuits, and can damage other stuff.  Looking at the power supply board, I noticed the (huge) filter capacitor for the +5V line had leaked electrolyte.   ESR tests OK, but very clearly an under-rated, leaking 36 year old aluminum electrolytic capacitor is not good….

ARP Quadra leaking aluminum electrolytic capacitor

ARP Quadra leaking aluminum electrolytic capacitor

I’ve gone through the schematic, verified dimensions on the synth, and have drawn up a capacitor replacement kit for the ARP Quadra with the following goals in mind:

  • Use the best type of capacitor for the job (e.g. the decoupling capacitor across Vcc and Gnd on the CPU should be a MLCC, not a tantalum!)
  • Use capacitors with the appropriate properties (audio grade, low ESR, ripple current, etc)
  • ARP used 85°C rated caps on the power supply.  Using 105°C capacitors will make them last longer.
  • Use capacitors with higher voltage ratings, where they will fit.  It may cost a little more, but this makes them last longer.
  • Use capacitors that will fit the physical constraints of the synth.  Some boards like the VCF submodules, and the shielded portion of the phase shifter board limit the height of the capacitor that can be used, making a switch from a tantalum to a radial electrolytic more challenging.

The power supply uses axial capacitors, but radial capacitors are orders of magnitude cheaper, and more significantly, have far far better properties.  I feel that it will be worth the extra effort to kludge the radial capacitors in their place.

Once verified, the capacitor kit will be added to the store, and we plan to sell both Quadras once fully restored.  If you have an interest in being notified when either are available (the Quadras or the capacitor kit), please contact us.

Comments are closed.