This EMP100 Multi-Effects Processor from 1991 came in with a display E2 error message problem, as soon as you switched on. The service manual pointed to some External RAM error which sounds strange since there is no cartridge slot or something for external RAM. Digging a bit deeper reveals Yamaha refers to the user RAM.

Opening up

Opening up is straight forward. Once the bottom plate was layed aside I noticed a missing memory backup battery, someone just had cut off its pins. So I removed all of the housing and installed a 20mm battery clip and equipped it with a new CR2032. As usual, I secured the clip with hot glue to prevent stress for the solder joints. Error E2 remained which makes some sense, since battery faults should lead to error E4.
Note, there is a gap between battery (+) and Diode D202 that should be bridged with a solder drop to operate the device.

Power and board check

User RAM is IC206, a Sanyo LC3664RL-12 8k x8 SRAM as stated in the service manual. However, in this device a pin compatible Toshiba TC5564-PL15 was used. Could this be the culprit? Checking datasheets revealed that this should be no problem.

So after checking power* (regulators for 9V and 5V ok, battery all new) I looked for dry solder joints. As a result, I reflowed all of IC206 and a couple of joints in other places since some of them had small cracks. Plus all jacks and the two voltage regulators, since they receive a lot of mechanical stress by (un-) plugging or by heat.
However, this did not help.

RAM check

Next I took my logic probe, connected it to digital ground and +5V and checked for signals on the address and data lines. I got Toggles, HIs and LOs on all lines but A09. Still, A09 was present on the neighbored ROM and the CPU. So I checked this line for continuity and that was it.

It's not possible to inspect the trace visually unless you desolder the user RAM. So I bridged it on the solder side of the PCB and that solved the issue... error E2 was gone, replaced by error E4.

Here is the error codes explained:

So I had to reset the device and copy the factory programs into user memory by pressing RECALL and BACKUP while switching on. This brought life back to this digital reverb and modulation effects machine.

Sound

The EMP100 reverb sounds very clear, much better than an SPX90 or SPX900, two machines I could compare it to on place. Reverbs include gated and reverse effects. Delays can be tapped by footswitch, the symphonic effect sounds very nice on pads. Chorus and flanger are nice, too, but I prefer Boss pedals here. Only downside of this half 19" rack device is, you can't combine more than two effects. Which is great, I hate clustered and overloaded effects anyway. I found it great on drums or drum machines like TR-707 or TR-505, and on all kinds of synths except for bass patterns.