I’ve made a video series chronicling a restoration I just did on one of the ARP Pro/DGX synthesizers I have for sale. In the first part, I show what’s wrong with this keyboard and come up with a plan of attack:
In the second part, I give a technical overview of how the ARP Pro Soloist and Pro/DGX works:
In this part, I install the Synthchaser LED slider kit:
In this part I recap the keyboard and fix some of its issues.
In this part, I rebuild the Pratt Read keyboard and install a new aftertouch sensor, and get all keys working and feeling great. No more clacking keys!
In this part, I troubleshoot and repair the remaining issues in the keyboard. Even after replacing the aftertouch sensor, some aftertouch effects weren’t working. And some of the voices didn’t sound right. So I whip out my oscilloscope and get to the bottom of it.
In the last part, I give a demonstration of the finished product working nicely, and show the features and settings of the Pro/DGX as well as run through the preset voices.
I received an ARP Omni 2 “for parts” that literally was in parts! It came to me as a pile of boards.
ARP Omni 2 Heap Of Parts
Since it was complete, I decided to restore it slowly as time permits, recapping each board, inspecting for damage, etc… This morning I got around to the Upper Voicing Board. Someone had tried to fix a problem (likely a “stuck” note, caused by a shorted tantalum capacitor), but they lifted some solder pads and destroyed a trace while changing some diodes.
ARP Omni 2 Upper Voicing Board Before
To correct the damaged trace between the diode and the resistor network, they soldered a 1W resistor to the board… ARP PCBs are extremely easy to damage without proper soldering techniques. They are not plated through hole, so it’s easy to lift solder pads, and more difficult to solder a component with traces on the component side of the board. With some flux, and good soldering and desoldering skills, it is possible to work on these delicate ARP boards and have them looking pro when you’re done with them.
ARP Omni 2 Upper Voicing Board After
Here’s the same board with the 1W resistor hack removed, the board recapped (using the Synthchaser capacitor kit–pardon the shameless self promotion), and the diodes the previous person tinkered with also replaced. Notice on the capacitors the smooth solder fillets on the component side of the board. Because the boards aren’t plated through hole, it’s important that the solder joints on both sides of the board be solid!
In this video I have a String Ensemble in for repair. The first step is to rebuild the power supply, which is actually spread over 5 different PCBs in the synthesizer! In this video I show you around inside the String Ensemble, explain how the power supply works, what you need to do to rebuild it, and why doing so is important. I show the failed components I pull out of the Solina and get it working again!
These kits are available in the store (pay attention to the revision of your Solina):
We are pleased to offer LED slider retrofit kits for ARP Synthesizers. We currently offer kits for the Omni 1, Omni 2, Pro Soloist, and Pro/DGX.
ARP Omni 1 & 2 LED Slider Kit
These kits are user installable by anyone with reasonable soldering skills. Even when they were new, the original CTS sliders just weren’t very good. Compound that with 40+ years of use, dirt, and oxidation, even the most masterfully rebuilt CTS slider will feel terrible in comparison with these new, modern slide potentiometers. The way these feel literally breathe new life into old instruments.
ARP Pro Soloist Synthesizer Keyboard
For people who just don’t like illuminated sliders, you can simply not connect them to power, and get the benefit of the improved performance without the lights. For “purists” who like to retain the original look, it may be possible to install your original slider caps on the new potentiometer shafts (though we have not tried).
ARP Omni 1 & 2 LED Slider Kit
We have kits for other synthesizers like the ARP Odyssey coming soon, if you have a vintage synth that you would like to add LED sliders to and we do not have a kit listed in our store, drop us a line, we may be able to produce one for you.
The ARP Pro Soloist introduced in 1972 and continued through 1981 as the Pro/DGX featured selectable presets, which used 8 archaic ROM chips to control the routing of the signal path (filter resonance, vibrato, ADSR parameters, etc). The ROM chip used was an Intersil IM5600, a 256 bit (!!) ROM that is long obsolete, and for which no compatible programmers are available today.
It has been long standing in the synth community that the ROMs are irreplaceable–once they go bad, one must be taken from another synth or the patient must be scrapped for parts. This is no longer the case, Synthchaser has replacements available. Contact us if you are in need of a replacement Pro Soloist or Pro/DGX ROM.
Update: ROMs may now be ordered in the Synthchaser store:
This one didn’t go up in a blaze of glory, but the potential was there… The tantalum capacitors in vintage ARP synthesizers were of questionable quality when they were new, and now are disasters waiting to happen. Good quality tantalum capacitors are a fire hazard when exposed to even brief voltages that barely exceed their ratings, but the dodgy tantalum capacitors found in ARPs seem to pose a fire hazard without any coaxing. This failed capacitor on the Bass board of an ARP Omni 2, shorted the +15V rail to ground and caused the synthesizer not to work. Fortunately it didn’t damage anything else when it vented out its magic smoke. It is very important to replace every tantalum capacitor in an ARP synthesizer to protect your instrument. We offer capacitor kits and recapping services for all ARP synthesizers. Contact us to discuss your needs.
The ARP String Ensemble we have in stock rolled off the factory line in 1974. Everything inside was original, including the power supply capacitors. The ARP/Solina String Ensemble uses an unregulated power supply, with the varying voltages being generated by rudimentary resistor voltage dividers. The filter capacitors in the original synth were 25V capacitors, on a 25VDC power supply. This is bad news on many levels, good design practice allows for a nice buffer between the expected circuit voltage and the capacitor voltage rating, to ensure the capacitors maximum voltage isn’t exceeded, as well as to provide for a longer life of the capacitor. A capacitor rated for 25V and run close to the maximum will last less than a capacitor rated for 50V and run at 25V…
ARP String Ensemble Power Supply – before recapping
So, we replaced all the capacitors in the power supply, with 50-63V rated capacitors. We switched to radial capacitors, since the quality of the radials are better than that of the axials, and selected capacitors with a higher 105°C rating. Now this string ensemble is good for another 40+ years!
ARP String Ensemble Power Supply – after recapping
We recently acquired 2 ARP Quadras in nice physical condition, but sadly not working properly.
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#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….
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.
