Just had an interesting repair.
Customer had a Prophet 600. The unit powered up correctly, ran through the tuning cycle, and appeared to be perfectly fine – for about three minutes. Then the control panel would lock up and pressing buttons had no effect. If you turned it off and back on, the same thing happened. Obviously, unless you’re in a band playing REALLY short songs, this rendered it pretty much unusable.
The symptoms would generally lead me to think it was either a power or thermal issue. I dug up the schematics and checked all the voltages; they were all fine. The main chips – including the processor – are all socketed, so I popped them all out, checked for bent or mis-aligned pins, and reinstalled everything. No difference. It didn’t appear to be thermal – I got the same few minutes of normal operation whether it had been on for a minute or an hour.
In my day job I work with computers so at this point I’m thinking it’s a bad chip. The consistency of the failure, though, is unusual. In talking to a friend about it at some point in the conversation I was explaining that this unit is microprocessor based…which got me thinking (not as dangerous as it sounds). Once the unit finishes tuning, it has to monitor the status of all the front panel controls, not just the switches but also the rotary pots. Suppose one was dirty? It was conceivable that the CPU could read that as constantly changing, and hang up attempting to deal with it.
I removed all the knobs, removed the two control panel PCBs from the case, and cleaned & lubed all of the pots & switches. Plugged it back in, waited three minutes and then…it kept working. The first time I let it on for a couple of hours, randomly changing patches or moving controls every time I walked by. The second time, it was on for five hours – still no lockups. Re-loaded the factory patches, made sure the oscillators were properly scaled, and it was back to operating like it just rolled off the line.
First time I’ve run into this, but it’s conceivable that any programmable knobs-and-switches synth could develop this issue.
This was a recent repair job. When it was dropped off, the owner said it had a bunch of non-responsive keys and he wanted to see about getting the battery replaced. Testing it, however, showed an additional problem. The patches were all really, really wrong, and it would hang up randomly. I let the customer know; he just said “fix it’.
One of the things I absolutely love about older equipment is how easy a lot of it is to work on. Four screws, fold the control panel up, everything is there in front of you. The date of manufacture – 10/19/83 – is clearly visible here. I was in high school when this rolled off the line.
I’ve got keyboard rebuilding down to a (tedious) science. As expected, it took about an hour. Also as expected, all keys worked. I checked the battery; despite it being the original I was still getting the correct voltage. Since there’s no way to know how long that would continue, however, I went ahead and replaced it. Although it’s mounted from the top, I had a holder that was perfect for it.
Replacing the battery, of course, completely wiped out the memory. I was unable to get the factory preset WAV file to load, so I used MIDI-OX to push the SysEx files over. One bad thing about this unit is that there’s no indication that it’s receiving, so had to unplug it, schlepp it downstairs, and hook it back up to the amp to see if it worked. And it did! The memory must have been hosed from the start, because all the patches sounded fine and the locking up problem went away. But…
Two of the voices were really, really off; the oscillators were nowhere near each other in pitch. Since the unit tunes itself, that meant that the main voice board needed to be rescaled. Fortunately, SCI built in a relatively painless procedure for doing this. It took about twenty minutes, but after that it was all working perfectly. I replaced a few missing screws, touched up the stain on the end panels, vacuumed the inside, cleaned the outside, and let the customer know it was done.
He got back a fully-functioning synth, and I got to mess around with a classic piece of vintage gear. And got paid for it, too…everybody wins!
No major structural changes. More models and a few new vendors added.
As always, the “Range” column represents the year-to-date average taking into account the availability of that particular hardware along with the difference between the high and low prices. A wide range indicates either low sales volume or large gap between the high and low (or both).
The price displayed is the average selling price for that particular model for the month of February. The percentage is the difference between the current and year-to-date average, which is an indicator of the direction the price is going. As per the last column, that direction is either “-” for down, “+” for up, and “0” for staying (relatively) the same.
All of the “classic” synths have much lower sales volume, so the direction and percentages may not be as accurate. There were no sales of the Moog Source and the SCI MAX during February.
The Prophet 600 and SixTrak are still pretty good deals, if you have to have a piece of genuine vintage kit. The MicroMoog, surprisingly, is also quite reasonable. I’d take it over the MG-1 if only for the pitch ribbon.