Syntheholics Anonymous

Keyboard Speed Problem (S/N 76 and below) Due to a problem with CD4503s not being able to respond fast enough when warm, we have replaced U308 and U309 with 74HC367s. These parts are pin-for-pin compatible with the 4503. Failure symptoms for this problem will be dead notes or notes droning.

Nylon screws on power supply regulators (S/N 95 and below) On early units, the three main power regulators on board 4 were attached to the heat sinks with nylon screws. Unfortunately the screws cannot be tightened down tightly enough without stripping. This causes poor heat transfer between the regulator and heatsink causing thermal shutdown. Replace the nylon screws with metal screws and nylon shoulder washers as insulators.

Backwards capacitor (S/N 252 and below) Due to an error on the silkscreen on PCB 4, C401 a 1.0uF 10V [tantalum!] capacitor was installed backwards. The capacitor should be installed opposite from the marking on the board. Failure symptoms for this problem will be that the unit will work fine for a while then the audio will cut out [or the CEM5530s freak out and cause horrible sounds]. This problem should be corrected on all units.

Noise on audio output (S/N 374 and below) To reduce overall noise on the audio output we have removed two 1N34 diodes, D903 & D904 on the audio board, PCB 9.

Have any of you have ever turned on a M6 and got the following symptoms: strange sounds out of the voices, droning, no key response besides popping occasionally, calibrate shows that EVERYTHING fails (BAD OSC 7, BAD WAVE 63, BAD RES 63, BAD VCF 63) even if you test the chips and get good signals? 

Hint: You can do Master Edit -> 'D' -> "Ready?" -> Hit "yes" and hold down the 'C' button under the display.  This will print out any errors found during the calibration routine.

Good news is that it is probably just one pin of one chip that caused the whole thing.

So... the CMOS ICs (4051, 4053, etc) get their power from a 10k resistor and diode divider setup between +12v and +5v, however the schematic erroneously shows -5v instead of +5v. 

Because of the resistor this supply will have a small delay before it reaches full 5.6v.  Now, on one particular IC (u711 4053 triple multiplexer) there is an unused channel that has its input tied to +5. 

The +12/-12/+5/-5 will all switch to full power much faster than the divided 5.6v.  What happens is for a short time the 4053 sees a higher voltage at the input than the power rail supplying the chip.  The IC latches up and sinks current, causing the power rail to be pulled down.  The chip will get very warm or even hot.  On the M6 I worked on the 5.6v output read around 3.1v and, because of this, all other CMOS would not function correctly. 

I replaced the 4053 as the current sink would have damaged or killed the chip anyway.  Then I cut the trace to pin 5 which supplied the +5v and connected pin 5 to 6 (GND).  The pins were shorted on the underside of the board.

After doing this modification, everything calibrated right away and plays perfectly. 

What looks to be an original ARP designed CPU board before Rhodes took over the project and updated the layout.  Battery corrosion has eaten away the traces and someone attempted a sloppy repair...  They also tried to update the board to the Rhodes revision but didn't quite get it right.

I'm not sure if this happens to many people, but I've read about this same problem on some other web posts.  The problem is that the LCD screen will sometimes freak out and go black, too light, or basically just have irregular voltages going to the contrast input of the screen.  Recently working on one that had coffee spilled on it, I found that any slight voltage leaks will make the screen go crazy.  After cleaning the circuit board as well as I could and unsoldering the LCD board to clean the connections underneath the problem still persisted.  The flickering wasn't as prominent or as rapid as before, but was still annoying enough to prohibit long term usage of the machine. 

SCI liked to put little jokes or messages on all of their circuit boards.  These are from the boards inside a Prophet T8.

Enjoy!

Star Trek:

Butterfly?:

Terrible C&K coded rotary switch used in the Crumar Composer.  Plastic shaft that likes to break off and leave you worrying about a replacement.

They are still available and listed in the C&K datasheets from 2-12 positions, but the easiest way to find them is through Mouser.  Part No. RTAP32S06W25NS is the only one usually stocked, though the number of positions is six and not four/five as in the Composer.  If installed as-is, the extra stops in the switch do nothing or shut off the sound; the other stops work perfectly fine.