Roland SDE-2000 repair from hell

Hello fellow techs

I have a Roland SDE-2000 rack delay that I am trying to revive.

This unit has come to an age where the tantalum capacitors should not be trusted. I had a short on tantal C39, which is located near the DAC 6012 chip.

I took the shorted tantal C39 out and replaced all tantal caps. After this all voltages on the PSU stabilized but the audio delay line remains silent. It seems as if no audio reaches the digital section.

I started probing and the dry signal comes through to TP1, after that the signal splits up in a multiplexer and compander. I replaced almost all IC’s with sockets and they are all good. The Supply voltages are all good, the -15V is stable.

At the input of the DAC (pin 19, I don’t get anything). I highly expect that the tantal which was on the -15V rail near the DAC and acts as a buffer cap, might have killed the DAC through shorting.

I would like to verifify this before I order and place more sockets. The problem is I don’t really know what to check on the DAC. Also I don’t find it normal to not measure anything at the input pin 19 of the DAC. It seems as if it pulls my signal down. If I measure before the input resistor of the DAC, I do still have audio, after nothing. The anti parallel diodes seem fine in circuit.

Maybe someone here has experience or advice, to what I should check exactly, thanks in advance.

Hey, welcome to the forum.

If the DAC input failed short to ground then it would be normal to not see anything at the input of the DAC. I’m not sure if that is a failure mode of these DACs but it is certainly possible. You can also measure resistance to ground from the DAC input to see if there’s anything abnormal there. If there’s a short it has to be either a diode or the DAC itself. You can desolder the diodes and DAC one at a time to find which is the problem.

Did you check/replace IC2? IC2 failing probably wouldn’t bring the signal to ground but it’s worth checking. I would also verify continuity between the DAC’s input pin and the input resistor.

Here is an update, I have checked and replaced IC2 the LM311N with an LM311P. I can confirm that this IC is good. I don’t think there is a difference between the suffixes.

I replaced the og multiplexer Hitachi HD14066BP with a motorola MC14066BCP.

Replacing this I have now a small audio signal at pin 19 of the DAC 6012.

The delay line is still dead.
Continuity verified between input resistor and DAC.

I notice that the +15V regulator Q26 of the positive rail gets hot fast but supplies the right voltage. All voltages do seem stable but might indidicate failing under load?

Suggestions very welcome.

I am trying to understand the drive relay circuit, and found out that the audio contacts at the relay are not giving continuity. Both relays are not switching.

You can check if those transistors in that portion of the circuit are conducting properly. There should be a 0.6 V difference between base and emitter terminals on each of them.

There’s also a couple electrolytic capacitors in that area that could need replacing. You can remove them from circuit and measure their capacitance. Also measure their resistance, for a good cap it should start low and raise to infinity or at least beyond the range of your meter. It’s generally good to replace electrolytic caps that are 20+ years old because they are some of the most time sensitive components in electronics.