Vintage Simmons Drums (UK) Users Group

I’ve recently had a couple of SDS-9’s in the workshop.  Both exhibiting the same problem, power it up, LED’s light, 7-segment display shows the software version, then nothing, everything blank and not working.

It took a long time to get the first one working, but once I knew what was happening, and found the test ROM, the next one was much easier.

The technical description says that the SDS-9 has a testmode built in.  Unfortunately it doesn’t have the full testmode – it does the tests but doesn’t tell you what the fault is if it fails :-(  So on boot, the normal ROM does a ROM checksum, RAM check, A/D check, fast interrupt (1KHz clock) check and serial receive check.  If any of these fail, it seems to just hang waiting for the test to pass, which never happens!

I found on the first SDS-9 that it wasn’t getting an end of conversion (EOC) from the ADC IC54.  The CPU waits for an EOC before attempting to read what’s on the data bus, and if it doesn’t get it, it waits forever...  Replacing IC54 fixed the problem.

For the 2nd SDS-9, turned out the fast interrupt from IC1 (555) wasn’t there.  The 555 was completely dead, no fast interrupt signal nor sawtooth.  Replacing it brought the unit back to life :-)

The test ROM will show you (via the panel LEDs) where the fault is likely to lie, and makes troubleshooting so much easier.  Otherwise check that the above bootup tests (ROM/RAM/A/D/interrupt/serial receive) are doing what they should.