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.
This entry was posted in Synthesizer Repair and tagged analog, Arp, repair, String Ensemble, synthesizer by Chris Hewitt. Bookmark the permalink.
4 thoughts on "Arp String Ensemble"
Jordan Levantini on May 11, 2012 at 2:46 pm said:
Your site is really great. I am interested in talking to you about a Moog opus 3.
Is there a way I can speak with you about it? I am a tech in SF and would be interested in sending a customer your way.
Thank you again for your rad blog.
Chris Hewitt on May 14, 2012 at 10:13 am said:
Thank you for the positive feedback. I did respond directly to you. Folks that want to contact me directly can do so via email: chris@thisoldsynth.com
Pascal LIEGEOIS on October 5, 2014 at 5:30 am said:
I have a Solina string with a mysterious problem:
When I play more than a key on the left of the keyboard, that lowers gradually the volume of the machine, excepted the sounds of monophonic basses which stay there…. I am an electronics engineer and I try to repair Solina of a friend… You Have an idea where from to look, I changed all the AOP (709, 741), redone the food(supply) but nothing can be done….Cordialy
Chris Hewitt on October 6, 2014 at 10:02 am said:
So this is not the decay controlled by the Sustain Length (decay) slider?
Its a long time since I looked at one of these so no instant solution. You will need to put a scope on it and see what is happening.