Synth Repair: Ensoniq ESQ-1

Hey team. I picked up an Ensoniq ESQ-1 over the weekend, and as it turns out, it’s in need of some TLC.

There’s the usual suspect: replace battery. That one I can handle.
The tricky part: The filters and the vca don’t appear to be fully operational. Notes play, but there’s no envelope. I reinitialized and ran the filter tuning, and the values I received were approximately half of what they should be. I tested the power supply, and all voltages check out okay.

So, I’m turning to the Shruthiverse for help, as some of the original builds had the CEM3379 inside.

Any suggestions?

Thanks for your help!

This is very helpful. Theres also an active ESQ/SQ80 Mailing List you want to join. If one of the CEM3379 (Filter) or CEM3360 (VCA)s would be bad you would miss some Voices. So try triggering 8+ Voices and hear if every Voice is Playing. If so the Precious CEMs are OK and you need to look a stage before in the Digital Circiutry that drives these.

Thanks, Frank!

Yes, all eight voices sound fine, and the filters make SOME noise, but definitely not like they’re supposed to. I’ll have a look at some more schematics and work backward from the CEM3379s.

Hello Silverfish,

did you figure out the problem with your ESQ1.
Mine developed the exact same problem. I had no luck finding the source of the issue yet.


Sorry Andre, I have not resolved the issue. In fact, I made one problem into two problems. I shorted the display board, and now I’m really screwed. All hope is not lost. I’m tracking down what parts are needed. I’ll get back to you with more info soon.

Thanks for the info and good luck with your repair.
I’ll also try to get to the bottom of this.
Will let you know for sure when i find out what is wrong.


Okay, the lowdown: Most likely, some components aren’t getting the correct voltage and thus, aren’t operating correctly. There are a few chips near the end of the signal chain that convert the digital signals to analog signals (control voltage, basically). I’ll direct you to a thread on gearslutz where Waveform Blue offered lots of helpful advice. If you’re handy with a multimeter, you can probably work out what the problem component is. Just don’t short out your display board like a rookie schmuck.

Thanks for the heads up.
There is great info in the Gearslutz thread.

Will enjoy the weekend doing some music and look into the ESQ again next week.

Hi guys,

I’m working on an ESQ1 with the exact same problem as Silverfish & moelhoven. Have you gained any new insights? I’m currently looking at the path of the CEM-outputs through the 4051 multiplexer to the ADC for calibration purposes. My ESQ1 has slowly developed this problem, going back from working to non-working several times. I remember accidentally shorting the display PSU for a moment and suddenly having it back to a good filter calibration (and after that it stayed ok for several years…as long as i didn’t try to recalibrate). So I’m a little concerned it is a stability issue somewhere in the PSU or digital control signals, very hard to debug…


Hi guys,

Good news, I found and fixed the problem! It was a bad DAC (AD7524) in the CV generation path that was causing this. In the test procedure that waveform:blue described here the DAC seemed fine, but the malfunction was concerning only one of the switches inside the DAC. These switches (8 of them, each flipped by one of 8 bits) determine the final output of the DAC and when only one of them fails, the DAC output looks almost normal but is shifted (especially when a switch on the MSB side fails). This caused all the symptoms. I read somewhere this DAC is very sensitive to PSU failure, so that has probably caused it in our cases. In my case, there was some corrosion on the powerconnector to the mainboard which led to unstable supply to the mainboard.

For general info: I’ve replaced the DAC (AD7524) with a PM7524F which works fine. A TLC7524 should also work. Different brand, same function.


ps. Also posted this in the gearslutz thread, as the info might be of value to others

Hi Rutger,

thank you so much for posting this info. Took me some time to find it but i fixed my ESQ1 yesterday.


Mine, when calibrating the filters, is giving me values of straight 0 or showing all voices but one are in between 140 and 180 and the voice that isn’t is at 0. It started happening after I left it on for a few hours by accident. Figured I’d ask if you have some advice since this is the only thread I have found pertaining to filter problems with the esq 1 on the internet.

Did you happen to follow the link to the gearslutz thread? Lots of good info there. If you can press a key eight times and get sound each time, then you can rule out that a filter chip is bad. It sounds like a similar issue to what has happened with others - the chips aren’t getting the correct control voltage. Wish I could help more, but I ended up just getting a different board.

This mail list can be useful:


This is very bizarre and irritating but I think it’s one of my outlets in my home. I have now taken it out of the home to the venue I work and it’s fine and also just plugged it into a different outlet and it’s fine (this after receiving funky filter issues again while plugged into this outlet)

Maybe the PSU needs a recap? Sounds like a bad power supply to me.

Well actually it’s still slightly funky but not as bad. Weird

I think maybe you could be right. Also a metal with 3.5 OS

Thanks for the help, no help to be found over at vintage synth boards on this one.

Weird behaviour in old equipment will often come from electrolytic capacitors. They age quickly and can dry out. If I was you, I would open the unit up and take a close look at the power supply. Can you see a cap that has leaked or one with a deformed top?
It would be very helpful to have some oscilloscope shots of the power supply voltages. Do you have access to a scope? Photos of the power supply might be helpful as well.