I've worked on probably a dozen of these units and have run into some of the same problems on several of them. Here are a few I remember (please bear in mind that these are from memory only so proceed at your own risk):
|Intermittant Operation||I've run into quite a few of these units that are terribly intermittant.
Initially I tried to debug each of the pieces of functionality seperatly
but because of the tight nature of the enclosure, it was just an exercise
in self torture. As a last resort, I tried reflowing the solder connections
on virtually every connection on the boards. Miraculasly the system worked
Apparently due to board aging (drying up), poor soldering, poor plating on through holes and mechanical abuse, lots of solder connections start breaking up making the units intermittant. Though it seems like a brute force approach, reflowing the solder works very well.
|Unit Dead (no lights)||I found at least one or two units that had the full wave rectifier
in the power supply go bad. The rectifier they use seems like it may be
a little under powered. I ended up replacing it with a fairly overpowered
rectifier. When it comes to power supplys, you're always better with too
much than too little.
Checking the power supplies is always a good place to start with any repair. I usually start at the regulated outputs and go from there. I haven't run into any components shorting out, loading down the supply but I have replaced a regulator or two.
|No Audio Output||On several units, there was no output and one or more of the "Switched Capacitor Filter"s (one at the converter input and one at the output) blew. These limit the frequency response of the signal feeding the AD converter and being output from the DA converter. If either of these is blown you are in trouble as these parts are hard to come by (see Bob's EH16 Parts).|
|Clicks and pops in delay||This is a pretty common DRAM failure. I believe they are socketed so you can either swap them out till the problem goes away or try heating up the parts, one at a time (soldering iron, heat gun, etc). Usually when these parts go, they get worse with heat. These DRAMs should be easy to come by but if you have any trouble finding them, email me (firstname.lastname@example.org). I've got tons.|
|Unit Dead (lights but no control)||If all the supplies are good, the next place to go is the clocks. Start
with the Master Clock (on the analog board) and follow it to the divider
logic. I've replaced at least one of the 5087 transistors in the clock
circuit. Look at the schematics for the expected signals.
The divider logic on the digital board is pretty hard to follow on the full schematics. I ended up re-drawing it (see Redrawn Timing Circuit). If the ROM blows you're screwed but the other parts are pretty standard stuff.