Arp String Ensemble

I have had three of these come into the shop in the last 6 weeks, and the last of these was picked up yesterday by the owner. I’m seeing failures of rare and expensive divider chips such as the SAJ110 (QDIP pin layout but the only NOS parts I can find are DIP so an adapter socket needed to be made), and SAH190 can. The Mullard C280 series “tropical fish” striped capacitors are falling apart on touch (you see cracks appearing at the ends), but wherever possible I leave them alone in the signal path as newer mylar types may have a different tone (the C280’s are sought after by vintage pedal builders for there tone, which I guess means they are non-linear!).

An SAJ110 on one machine was sucking many amps (it measured 12 ohms) and had caused the power supply to fail, on another it was a missing output. I recapped the power supply on two units, and for one of them we did a mass replacement of electrolytic’s as part of a restoration (and a sample of physically tired looking capacitors showed they were drying out with severely reduced capacitance.

The third unit had one dead key, and the owner had already changed a TDA0470 to no avail. Looking with a scope the replacement chip was bad, and in fact it was ordered over Ebay from a vendor in China so who knows what it actually was. The original was put back in, and after much probing and deciphering of component layout the bug was traced to a bad resistor that had oxidized where the leg was cut off (they seem to have used some sort of pin flattening cutting tool in the factory which makes it hard to get components out), and then it had been re-soldered with a blob of solder over the top of the joint hiding the issue.

These keyboards sound great, but there are a lot of supporting components and rare chips that went into it so reliability is an issue.