ARP 2600

ARP 2600 circuit board work
We forgot to take a picture of it when it was done, so here’s a picture of it when it was dirty!

The owner of this ARP 2600 got it in an insane trade in the late 80s… in exchange for a Peavey keyboard amp and a TR-505! A lot of it had never worked in the entire 30 years he had had it.

When we do restoration of an ARP that’s in bad shape, we’ve learned that there’s really only one good way to approach it. We basically strip it down to its bones and do everything we possibly can in one fell swoop before even trying to test different systems. It ends up being so much more efficient that it actually costs less than taking a more step-by-step approach.

PCB work in progress, CTS trimmers and polarized capacitors replaced.

Typically this consists of rebuilding the power supply, replacing all tantalum capacitors, replacing all CTS trimmers (the big blue ones), reflowing the solder on every connector pin, deoxidizing and flushing all mating female connectors, replacing certain op amps and CMOS chips if they are present, and disassembling, cleaning and lubricating and then reassembling every slide potentiometer. Any and all slide switches or rotary potentiometers in the synth are cleaned as well.

Disassembled ARP 2600 sliders in the process of being cleaned.

This phase also includes a thorough rebuild of the Pratt-Reed keyboard which also is stripped to its bones. Someday I’ll share photos of this process, but not today! The bus bars are removed to be thoroughly cleaned and polished, every key contact is deoxidized, straightened and then cleaned with a solvent, the keys and key brackets are washed and keys whitened if needed, the keystops are leveled and straightened, the remainders of the old bushings are removed from the keystops and replaced with new ones lubricated with Dow Corning 7.

On a 2600, frequently most of the dozens of 3.5mm switching jacks are so tarnished that they don’t conduct and these are polished with metal polish, treated overnight with Deoxit, and then washed clean with a solvent. Many end up also needing to be rewired due to the brittleness of the wire used. once a wire is broken off of one of them, I’d rather just replace the whole piece rather than stripping and continuing to use the same brittle wire.
Also, when ARPs have switched 1/4″ jacks, we often end up just replacing them because they are so tarnished and dirty that it would take an unreasonable amount of time to get them clean.

extraordinarily, and yet not extraordinarily tarnished jacks in the 2600

Only after all of this is done does the troubleshooting begin, but usually after all of this is done almost all of the original issues are gone. This one had a couple of other special challenges to be addressed. First off, some broken-off long connector pins. Since these are riveted to the boards they had to be drilled out and replaced with entirely new pins of the same length. Since the new ones are gold, I replaced the original tin female contacts in the mating connector with gold ones as well, as mating gold and tin connectors can speed up oxidation and lead to a less reliable connection in the long run.

drilling out the remnants of the broken off connector pins
replacement gold pins and female Molex contacts in place

The owner of this one also requested to have 1/4″ jacks installed in parallel with the normal left and right output jacks so the top edge of the far right PCB had to be delicately filed so that the new jacks could be placed nicely above the others and clear the board.

When all this was done, the only remaining issue in a previously almost completely nonfunctioning synth was that the reverb didn’t work. This was traced to a failed reverb driver transistor 2n6076. Some ARPs use 2n3904 and 2n3906 for all their transistors, while others use 2n6076 and 2n5172. Of all of these, I have found for some reason the 2n6076 seem to fail the most often by a considerable margin.

The final step was just to calibrate it… and spend some hours playing it myself to test it. It’s such a wonderful synth. All synths should have built in reverb!

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