Thanks! Hope everything goes smoothly for you.
DO NOT CUT R1 AND R2.
I’ve done some initial testing and while it seems that the Traco SMPS substitute DC-DC converter works in place of the LM7805, cutting the resistors that we_robot suggested killed the Vbb supply pin (should be +45 to +55 VDC according to the service manual, reads 0 after those resistors are cut.)
Upon further inspection of the board, the LM7805 that was replaced is marked for the Display, not the main Digital line which seems to be handled by the LM2925 regulator.
I’ve also noticed that on my PSU, the Vu line is running at about 12.7VDC, which is above the +8 to +11VDC indicated in the service manual.
Well, I’ve discovered we_robot’s mistake.
He’s using a PSU schematic for the ESQ1, not the SQ80.
My fault for not checking first. Do your research, folks.
Well, the SQ80 has been on for about 26 hours straight and hasn’t overheated. It did crash once after flexing the keybed, but flexing it back solved that issue. The ambient air temperature is about 25 degrees cooler now than it was during the summer, but this is still the longest it’s stayed on without hanging in a while, so at the very least its mitigating the problem, if no outright solving it. I’ll provide an update if it starts acting up, and I plan to inspect the keybed to see if there are any bad solder joints, but for the time being seems the refurb works. I’ll follow up this post with the Digikey BOM since the forum doesn’t seem to allow file uploading. It contains all the parts I used EXCEPT for the LM2925, which is no longer made and thus will need to be found on ebay or something.
Thanks for the list of parts!
Wish you well with your SQ-80
Going to tackle the SQ-80 cap replace on the PSU.
On this video for an ESQ-1 PSU he said he had to remove the keybed to
get to the PSU.
Is this also the case to get to, and pull out the PSU on the SQ-80?
Thanks
It’s been a few months since I opened mine up, but as far as I know the case designs are identical save for the Disk Drive bezel/cartridge slot. I don’t think you necessarily HAVE to remove the keybed in order to get to the PSU, but it will make it a lot easier to remove if it’s out of the way.
So I was able to get my SQ-80 back and running quite by accident as I was in the process of scoping the
5Vdigital voltage and dropped a needle into the PSU.
Naturally the power was off at the time, but I could not find said needle, so I had to turn the SQ-80 upside down with the top open and bang it like burping a baby to get the needle out.
In doing so I must be reseated a loose connection.
It has gone well over a week with no crashes, but before it would go 5-20 minutes before becoming unresponsive.
After I got the needle out and used it properly to connect to those dinky scope probes I found very little ripple on the Digital5V.
Can anyone tell me if the keybed circuit on the SQ-80 is 2 piece as in the ESQ-1 or if the SQ-80 has different way to connect left and right contact boards?
I never got that far as I believe in if “it ain’t broke, don’t fix it”
Somewhere I “burped” a connection back to health, and I was wondering what the loose parts can be.
I have heard socketed ICs, edge connectors.
Thanks