Emulator II Repair part 2

Two days of on/off poking around with a multimeter and pulling each and every socketed IC, de-soldering any capacitor across -15v and AGND and removing a whole group of capacitors that had at some point been (badly) re-soldered yielded absolutely nothing.

The interesting thing is that the earlier repair work had been the filter capacitors on channel 2 and it wasn't the only work on that channel. One of the ICs had been replaced at some point, but typically it was one of the few that were not socketed, which is itself odd as I had thought all E II's had every IC socketed. The replacement IC had been socketed, but it looks as though whoever did the work lifted a pad on the top side of the board and attempted to get the pin in contact with the remains of the track by applying a huge amnount of solder - which ended up shorting two pins - between -15v and AGND.

Previous "repairs" circled

Removing and replacing the socket resulted in me lifting another pad, but I fixed this and the first one with a pair of wires under the board. The short is gone and everything that should be connected is. I'm want to test it, but as I managed to destroy some of the capacitors that I removed trying to trace the problem, I am now waiting for parts. Some of the other ceramic capacitors are visibly cracked and although they have not gone short yet, they likely will in time, so I intend to replace every bypass capacitor on both PCBs - about 150 of them. Oh and that nasty replacement jack just has to go.

Part 3 to follow once I have the parts...

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...