exxos's Atari, Amiga & retro forum

Thinking about this I may replace R36 with a combination of a 720 ohm resistor and a trim pot. This should allow the +5V rail to be tuned both up and down as necessary.

stephen_usher wrote: 20 Feb 2024 09:11 Thinking about this I may replace R36 with a combination of a 720 ohm resistor and a trim pot. This should allow the +5V rail to be tuned both up and down as necessary.

I really don't like PSU's that can't be trimmed, so that sounds like a good plan to me.

The trim pots I had were too low current it seems and anyway, whatever resistance I used I could only ever get the loaded +5V down to 5.15V, which is still too high. I'd have liked something like 4.9V to be safe.

At the moment I've got it bottoming out at 5.30V with fixed resistors.

It turns out that the bulb on its own wasn't loading the power supply enough.

The bulb and the hard drive together load the PSU enough that I have full control of the voltage via my trim pot.

So, what did I change in the power supply?

Replaced R36 (2K2) with a 1K8 resistor.
Removed R37 and replaced with a 2K potentiometer.
(R32 wasn't fitted, looked to have been removed after manufacture.)

The AY-5-8116 chips have arrived so I can see if it fixes the Kaypro serial clock issue.

Seeing that both the SIO and 8116 were bad, who's betting that the crystal's OK?

It works!


Even Drive A booted first time... But copying to Drive B failed at track 7 (reading from A, not writing to B!)

Bother, the Z80 SIO/0 I ordered from Little Diode turned out to be a Z80 DMA when I opened the packet. *sigh*

More shenanigans with the Kaypro drives this morning.

Drive B's motor decided to run full speed whatever I did, even replacing the LM2917 on the servo control board.

I'd already reflowed the solder on the connector, which had cracks. So it was a case of buzzing out traces.

Found that an old bodge to repair where a capacitor had been pushed through the board and pulled the trace off the board wasn't working any more. It was extremely ugly anyway. So I removed it, put a proper bodge wire across the board, secured by hot snot, and resoldered the leg of the capacitor to a better piece of the trace.

That worked.

Put everything back together yet again... and Drive A started playing up. *sigh*

@stephen_usher you have my sympathies. I’ve been working on an Opus Challenger 3 in 1, got the ram working now but the drive spins up but doesn’t step whilst trying to format or read.