Help trying to resurrect an SQ-80

[Mr_Time_Waster]

[Ensoniq] [SQ-80] – [No Audio and Garbled display]

Hi all! I recently purchased my first synth (apart from a stylophone), an Ensoniq SQ-80. The Good: it was only $100 and came with a SKB case. The Bad: there’s a reason it was only $100

At some point this poor thing had a bath in some kind of liquid, as almost every single piece of metal has some sort of rust or corrosion on it. The keybed is super yellowed and almost looks like it has fake wood grain on it. Also it does not work (I know, what a shocker). No sound output and nothing on the display. Now although I’m new to synthesizers, I’ve been repairing retro game consoles for quite some time and am currently studying electrical engineering, so I’m not a complete newbie.

I started off with checking if I was getting power. No continuity on the power switch let me know it was broken. I ordered and swapped in a replacement (thanks for the switch Syntaur) and checked my fuses which were all good. Then using Buchty’s service manual, I was able to check the voltages of my power supply and they were all within spec (despite how gross my transformer looks).

I then started diagnosing the display board. I unplugged the 4-pin connector so that it wouldn’t receive any data from the main board, which should have put it in the test mode where button presses should display random characters on screen. This did not work and no characters appeared. HOWEVER, I also had ordered a set of replacement faders for my volume and data entry sliders that were rusted in place (thanks again for the parts Syntaur), and when I soldered the new ones in and plugged everything back in, my display was on and displaying random segments across the entire display. I don’t know if this was because the heat from the soldering iron reflowed a cold joint or just from me unplugging and replugging connectors.

I still can’t get any sound to output. However when powering down the synth, it makes a noise that sounds like a frequency sweep going from low to high (Almost exactly how Antilles describes it in his post Ensoniq SQ-80 DOA ).

I have unplugged/replugged every ribbon cable/connector and the two OS EPROM’s to see if I just had a dirty connection somewhere, but that has not fixed anything either. I also tried resetting the RAM by shorting R1 to to the lower left pin of U50, to no effect.

I think my next step is to replace the OS EPROM’s incase mine are corrupt (plus it’d be nice to have the updated v1.8). I believe I will also need the KPC 1.5 EPROM, but I’m not sure.

I did get ahold of one of Rainer Buchty’s Display Panel Kit’s, but haven’t installed it yet. (https://www.youtube.com/watch?v=Pas89x2WEC4) The ESQ1 screen in this video does look similar to mine, however the one in the video still produces sound.

Also I recently rechecked the voltages on my power supply and on pins 16 & 10 (Designated as VU in the service manual) it is supposed to read between 8–11 Volts where was getting ~12.5V. I don’t think this is too big of a difference, but am not entirely sure

If anyone has any tips or helpful info for me I’m all ears. I really hope I can get this thing up and running.

[sargfowler]

I’m in complete awe that you’re taking on such a project, but the satisfaction in the end will be immense!

The caps on that power supply don’t look great. Do you have a scope to check how smooth the voltage is?

I think lots of continuity testing on buttons may be required if they are in bad condition. Make sure none are stuck down.

In my experience, ROMs can go bad when synths are this age, also RAM too.

Check the clock circuit is working too, they’re sensitive to dampness.

[Mr_Time_Waster]

Good call on using an oscilloscope! Most of my DC voltages were initially looking pretty clean. I got to that one voltage I had mentioned was a bit higher than specification and it had a little bit of variance to it compared to the others.

+VU (Pins 16 & 10)

Then I got to the voltage for the display (Vbb), which is supposed to be between 45-55V DC. A multimeter confirmed this, however looking at it through the scope showed a sawtooth wave. (I had the channel on AC mode so this is a variance of around 0.275 Volts)

Vbb (Pins 25 & 23)

But perhaps the worst one (if the service manual is correct), is between pins 27 – 23 for ‘Display Offset’. It’s supposed to be between 8-11V DC, however measuring it on the scope showed a full on sin wave. The measurement listed prior to it (‘Filament’) in the manual is an AC signal and measures as such.

Display Offset (Pins 27 & 23)

So yeah, definitely looks like a recapping is called for. Will have to get those ordered and replaced.

[sargfowler]

At least you have some readings on the PSU rails. Good work.

The caps that look the worst are the 2 green ones. They’ve bulged at the top by the looks of the picture. Are these on the display offset rails?

[Dansaudio]

Good eye on those caps, sagefowler. It takes some experience to catch that from a photo.

I don’t think this had a bath in some liquid. I see things in this condition often that are located near the coast from the salt air (or are stored outside under a tarp, again near the ocean).

[Travis]

It’s normal for there to be a good amount of ripple on the display voltage (Vbb) in my experience. I don’t think yours is necessarily out of spec. 0.275 V out of 50 V is less than a 1% ripple so should be fine.

The display offset should be a 9 VAC sine wave with a +8 to +11 V DC offset. So with the scope on DC make sure the middle of the sine wave is between 8 to 11 V. Or use a multimeter set to DC.

Power supply schematic

[Mr_Time_Waster]

Thanks for the info Travis! I was a little startled thinking the display offset was supposed to be a DC signal . I used a multimeter and got a DC reading of 12.7 V for the display offset, so I’ll still replace all the caps to see if it’ll drop to between 8 &11 V.

EDIT: Although now that I’m looking at the PSU schematic, there are no capacitors on pin 27 (9VAC+), it just comes straight from J1

[Mr_Time_Waster]

The more I thought about it, the less likely I realized it getting some liquid dumped on it was. All the circuit boards and the case were relatively clean compared to the metal, so this definitely is a more likely reason

[Mr_Time_Waster]

C11 is on Terminal 11 (+5 Display) and C18 is on Terminal 22 (+12 Analog) which I believe goes to the floppy drive.

Travis pointed out the display offset is supposed to be a sin wave ( Help trying to resurrect an SQ-80 - #6 by Travis ) but my DC offset was measuring at 12.7 V when it should be between 8 &11 V. And referring back to the PSU schematic he posted, there aren’t any capacitors on that +9VAC line, it comes straight from the transformer

[Travis]

What does your mains voltage measure? Could just be where you’re at the mains is high. Since there’s no regulation on the 9VAC line it’s directly proportional to input voltage.

As far as I know aging can’t really cause a transformer to increase in secondary side voltage output over time. Corrosion would cause the opposite effect, a decrease in output voltage. So it’s likely this transformer always measured high but still passed quality check from Ensoniq back in the day.

At a high voltage your display might have a shorter lifespan or it might be okay. There’s no data sheet available for the exact display so I can’t say for certain. But this 9VAC is mainly used for lighting the display, so the fact that your display lit up means it’s at least functioning despite being a bit high.

SQ80 Display Schematic and Others