In the QL, the AC feed is immediately rectified into DC. This derives +12V and -12V. The DC feed is immediately regulated to 5V for running the system. There's nothing in the QL that relies on the AC frequency. A QL will happily work on AC from about 38 Hz up to around 225-260Hz without problems. Ch...

My SGC draws 320ma under load and 260ma idle.

Glider felt is not anti-static. It will build up a static charge in use which can create noise in the head, and read/write errors.

Sorry about that. Nobody seemed interested so I dropped the project.

Yes.

Scuffing and corrosion not on the line taken by the tape doesn’t matter. Very light corrosion or scratches in the tape path can be polished out with jeweller’s rouge on a lint free cloth wrapped around a small dowel.

What did you see when you checked the power to the DRAm area? Was it nice and smooth, or dipping? What about at the 8302?

The inconsistency of this is similar to a QL I repaired many years ago where the DRAM wasn't refreshing properly, and it turned out to be a power/bounce problem.

Are you using the EPROM directly? EPROM chip select is active low.

Would it be reasonable to do a flash adapter and IC to go there? Would it be in system programmable?

What is the situation with termination resistors on the drives? If they're terminated incorrectly, they may accept "motor on" signals but not recognize data properly. On 5/25" drives, usually only the data lines were terminated on the last drive, with a resistor pack.

You could have a physical witch to connect /WR. If you tie it to 5V it can’t be written. A mechanical pull-up.