ESQ-1 Low Output with 60 Hz Hum - Sound Only Audible at Extreme Amplification (Video Included)

[Pik0]

Hi Syntaur community,

I’m troubleshooting my Ensoniq ESQ-1 (metal case, 1980s model) with a frustrating issue: the audio output is extremely quiet, only audible when heavily amplified (+20–30 dB on my mixer/amp). At normal levels, it’s practically silent, even with the master volume slider maxed out. Additionally, when amplified to hear the notes, there’s a noticeable 60 Hz hum, suggesting possible grounding or power supply issues. The hum is present on both stereo outputs (1/4” jacks), and MIDI triggering works otherwise.

Here’s what I’ve tried so far, with no fix:

• Cleaned output jacks with DeoxIT and tested with new, short TRS cables.

• Performed hard reset (RECORD + PLAY on boot) and filter calibration (RECORD + FILTER; values ~140–160).

• Inspected internal ribbon cables for corrosion—connectors look clean.

I suspect the analog output stage, possibly the TL084 op-amps in the buffer circuit or a power supply issue causing both the low gain and 60 Hz hum. The synth boots normally, passes the auto self-test (no stuck dashes)

I’ve uploaded a video [on TikTok] showing:

1. Boot-up sequence (normal display, no errors).

2. Playing a patch at max volume (barely audible without amp).

3. Same patch through an amp at +20–30 dB, with the 60 Hz hum audible.

Has anyone encountered this combo of low output and 60 Hz hum on an ESQ-1? Could it be the TL084s, a failing capacitor in the PSU, or a ground loop in the output stage? I have a multimeter and a oscilloscope —any tips on what to measure (e.g., DC offset at TL084 pins or PSU voltages)? Schematics from buchty.net point to U40 in the output circuit, but I’m hesitant to dive in without guidance.

Thanks for any advice or similar experiences!

Piko

[Travis]

I have worked on a handful of Ensoniqs with similar issues and these are the steps I take.

Inspect the power supply for bulging caps or caps with a shrunken wrapper. Either of these are evidence of a failing capacitor.

Inspect other PSU components such as diodes and voltage regulators for excessive heat, i.e. scorches on the PCB or excessively oxidized component leads.

Measure the PSU output voltages as described in the service manual with a multimeter.

Once you’re 100% sure the PSU is healthy then it’s time to move on to the main board. There are some higher quality schematics here, note there are a few different board revisions.

High chance it’s an electrolytic cap near the audio output. C116 is highly suspect, especially if you’re getting weak output on both channels. Use your oscilloscope to check the integrity of the signals coming in and out of U38. If the signal drop-off is happening there then the issue is probably C116 or maybe U38.

Check opamps near the output (U37, U38). A healthy opamp’s inverting and non-inverting inputs should have a difference of approximately 0 V. Place one multimeter probe on the inverting input and the other on the non-inverting input, and the meter should read near 0 V. A few mV is fine. Also check that the output isn’t stuck at an excessively high or low DC voltage.

If none of that helps you may want to use an oscilloscope to check for excessive ripple on the power supply lines. Finding a good ground point on an Ensoniq can be a little challenging, the chassis doesn’t work because it is only tied to the primary side of the transformer. I usually find a ceramic capacitor or resistor that goes to ground and clip onto there. They are indicated by a little plus kind of mark around the solder pad. Also there is a separate ground each for the analog and digital power supplies. Check the schematic for any confusion and be careful not to connect the oscilloscope probe to something that isn’t a ground, it can damage the scope or synth or at least blow a fuse.

[Pik0]

Thanks for the comprehensive response. I also noticed in my further testing that the volume control potentiometer has zero affect on output. Going to test that first.

[EnsoniqKT76]

This is very helpful as my KT76 has a similar issue, quiet output.

[Pik0]

Finally found the problem with y low volume!! The write to the volume slider was loose!! Once i resoldered it, the volume worked much better. It’s still a little raspy in the lower 1/3, so I’ll still have to replace it.

Next challenge: getting a sustain pedal to work. So far, a standard pedal has zero sustain no matter which voice I choose!
Andrew