Emulator II Repair Part 1

As mentioned in the last post, I have acquired some new toys. The DEP-3 looks band new despite being 25 years old, works perfectly and sounds great. The early '80s Soundtracs mixer has some dead op amps, including one that appears to have got hot enough to scorch the inside of the front panel. The other item is an E-MU Emulator II. With problems....

I knew the E II was dead before buying it. The power supply had failed (a common problem) and supposedly one of the voices had also expired prior to this. The first thing to do was get the power supply working.

The PSU in the E II was not designed by E-MU. It is an early switch mode supply made by a company called Compower and supplies 5v for the digital circuitry, 12v for disk drive motors (although this isn't actually used in my early E II, they share the 13.4v rail), +13.4v and -15v for the analogue circuitry. The latter two rails are linear regulated after the switch mode supply.

PSU before

and after

Initially the PSU was completely dead and blew the fuse on switch on. A couple of resistors were scorched and one tantalum capacitor had pretty much vaporised. The chopper transistor was shorted and I suspected that most of the electrolytic capacitors were past their prime. As such, I replaced the damaged components and all capacitors. This had no effect on operation - the fuse blew again on power being applied.

I had intended to replace various other aged components later on o make an effectively new PSU and as such had replacements for the various rectifier diodes. This lead me to remove and test the diodes on the mains input on the off chance that they were the problem. Two of them turned out to be shorted! Replacing these solved the fuse blowing issue, but the output rails were still dead. Replacement of the remaining transistors solved this and I now have a working E II PSU.

I still intend to replace the remaining rectifier diodes and also the two linear voltage regulators, but that can wait for another day. Unfortunately I have found an unexpected problem on the E II itself - a short from -15v to ground on the (huge) output PCB. This board is covered in tantalum capacitors and these are my initial suspects. Time to dig out the desoldering gun again...