Just scored my first ESQ-1 but there's a problem

I love this thing so far. It’s exactly what I needed to take my music project to the next level. Everything is going great and then after a while of playing, and very intermittently, the volume across all voices will duck down. The volume dips by different amounts but never really to zero. Sometimes it’ll duck out in little bursts and sometimes its a one shot and then stays low for 10-15 seconds. Also occasionally a single note will get stuck and ring indefinitely or until I do a keyboard reset, but this could be related to bad sysex when transferring new patches. I’m not sure if the two problems are related at all.

So, here’s what I know about this unit so far:

metal case version with ESQ-1 120v transformer w/ *5601 rev 1 PSU
os 3.53 (hidden waveforms)
has sequence expander cartridge
battery replaced (reading strong enough 2.9v)
it appears that the power supply has had a fairly amateur recap job done but I touched up the soldering and checked connections and it seems ok enough to check levels
5v digital (pins 10 and 11) showing +4.83v under load
+V unreg +10.9v
+12 Analog +11.8v
-12 Analog -12v
filters all between 140 and 146

I’ve reseated all connectors, removed the display and keypad board, cleaned/checked all sliders and buttons. The problem occurs through both output jacks.
System reset has been done a few times. Established a sysex connection and checked midi settings. I’m pretty sure the main board is a used replacement because it has $98 written on the lower left corner. I have yet to start pulling chips and reseating them because I want to check in with those more experienced than I am to make sure I get the correct procedure down. I’m fairly convinced the problem is somewhere on the main board, possibly a cold solder joint, but where to start…

I greatly appreciate anyone with some experience chiming in with tips or words of encouragement. Help me save this gem.

We have seen several instances where bad slide pots cause this sort of thing. If the data slider is bad, you can watch the value of whatever parameter is currently selected start changing on its own, usually switching between one value and another. If the volume slider were faulty like this, it could cause the volume to drop seemingly at random. The fact that all voices are affected equally points to a more ‘global’ issue like this. Replacing that pot (or both of them) is easy and not expensive, and seems like a good place to start. The slide pot (both Volume and Data are the same) is here:

If that doesn’t do it, then I think you are looking at an electronic issue. It could be the CEM3360 VCA chip, but a failure there is usually total and final. More likely, it is the circuitry telling the VCA how loud to play - and you can see, this is getting more complicated quickly.

Good news, bad news.

The good news is that I took the sliders apart and removed some tiny little pieces of paper from inside. They are reading 5k again after cleaning.
The bad news is that when reassembling I plugged the display board ribbon cable in the wrong way when first turning it on. It powered up briefly then shut itself off. I reversed the cable and although it starts up and sounds good many of the soft buttons don’t function properly - where patch selection goes to env3 or seq selection…it’s all wonky. I’m guessing I sent the 12v or 5v through parts of the controller chip that really didn’t want to see voltages like that any time soon.
I disconnected it from the main board to run self-test mode and even in that mode several of the soft buttons were sending incorrect messages which seems to suggest the problem is isolated to the display or control boards.
I’m a bit crushed at the moment because it seems as though very few of these chips are available these days if at all. Any suggestions now? Should I just sell off the parts and buy another fully working one?

Did you check the fuses on the power supply board to make sure they are all still good?

All the fuses are good on the power supply. Everything seems to power up fine and gets appropriate voltages.

I think what happened by reversing the ribbon cable between the display and keypad boards was that it sent +12, via the data slider center tap (val), to pin 5 on the 21121-11 chip. That could explain why some functions of the matrix are fine and others are behaving incorrectly if only one pin got fried. I don’t really know a lot about microprocessors. Are they made up of huge numbers of diodes and transistors, etc.?

I also went through all the switches to check that they were passing signal when pressed and were disconnected when not. All but one of them were working correctly. The switch that was no longer working seemed to be in the path that would’ve received +12v. I wouldn’t think that would be enough to take out the switch but perhaps there’s more current there than I realize. Or maybe its a coincidence.

It seems as thought the ic’s on the mainboard that are connected directly to the 4 pin jumper (J1) are fine as well - as far as I can tell. I was hoping putting the display in self-test mode would sort of isolate the problem to that board but perhaps not?

I went ahead and ordered a few things from you and managed to track down a fully functioning display board. I may refurbish the one I have with buchty’s chip kit if in fact the R1121-11 is the problem and not just the one or more of the TL4810 or TL5812 chips. I haven’t quite figured out yet how to test those.

I’m starting to think that my digital supply voltage is a tad low for the esq-1 120v transformer version, it drifts down to about +4.65 after a few minutes. My power supply has R1 and R2 at 68ohm to bypass Q3 7805. Wondering if I should change those closer to the stock 51ohm and up their wattage a bit while I’m at it since those look a little discolored. Would there be any reason that I should change the 1ohm resistors across the 14vac lines back to fuses?

A little update just in case anyone might find this useful. After replacing the sliders and a couple switches the volume dropping out problem was fixed. I got a new display board and now the keyboard is in perfect working order again. If anyone has an extra microcontroller chip we can make a working board out of my old one. Perhaps I’ll buy buchty’s kit.

I dug in deeper with the power supply and realized someone had replaced the caps but swapped a couple values and installed one backwards. After fixing these mistakes the digital voltage supply was too high so I lifted one side of R1 and R2 which brought it down to 4.85v. My PSU has a 1ohm 5w resistor in series before the regulator (not included on any of the schematics I could find), I’m guessing its just there to pass off a little heat before the regulator, I think R1 and R2 would’ve helped in this regard as well but according to datasheets and sample regulator schematics I decided they could be omitted even though I don’t believe ensoniq would’ve chosen this layout needlessly. Not sure what else could cause the psu to go out of spec like that but it seems to be holding stable as is and relates more to the expected voltages of other typical 7805 based circuits.

hi see my post elsewhere on this forum regarding replacing the 7805 for a TSR Traco switch mode regulator and getting rid of the resistors which are just to shunt heat away and get more current out of an overworked 7805