Looks like I'll be getting a Mega ST4 soon

[rubber_jonnie]

@PaulJ Yes, it was always 6 chip TOS, the only change I made was to remove the CPU and fit a socket. The machine had been working before, but had an intermittent problem when the board got a lot of handling.

So I went back and focused on the ROMs, no shorts to either GND or VCC on the data lines, so for the hell of it I went back and rechecked the pins on the ROMs for other problems, and there it was. LO0 had no GND, so the ROM was effectively powered off.

Since it had been working before, my earlier test of GND and VCC at all the chips had been good then, but something has clearly shifted in the meantime whilst doing the CPU socket fit.

Therefore garbage or nothing into the CPU and it falls into a halted state.

I then put a bridge between GND and the bad pin on LO0 and booted. Instead of a plain white screen, I have borders top and bottom, and eventually the desktop appeared. I'll do a proper repair later this week!

I have to thank everybody here for their help and persistence, plus the fantastic troubleshooting advice I've had from everybody who's joined in, you're an amazing bunch of very patient people!!! I'm very lucky to know you all.

I have had an idea for a test tool based on the advice from @exxos , and when I've built one I'll put an article up on here about it and how to use it.

It's been quite the odyssey this one, and it goes to show that faults can appear at any time, so keeping an open mind is really important.

The best thing is that I'm now armed with a lot more troubleshooting tools going forward. Not just physical tools, but tips and tricks!

I'll post some pics of it working again when the repair is done.

[DoG]

Great find. Really glad you got it working again.

[rubber_jonnie]

Great find. Really glad you got it working again.

@DoG Thanks, me too!

[exxos]

:bravo:

I have been thinking about a test tool for the H4s . though its another project I don't have time for :(

[Tomswork]

Excellent news about the st


Tom

[PhilC]

Glad you got there in the end RJ :dualthumbup:

[rubber_jonnie]

@Tomswork @PhilC @exxos

Thanks fellas!!

What a marathon!!

[rubber_jonnie]

So, quick repair done in the first 15 minutes of my lunch hour, just a bloody dry joint.

Left it soak testing with YAART, but I'm confident it's going to be OK.

[rubber_jonnie]

Here she is in all her glory:

IMG_20200301_180611.jpg

I think it's been worth the struggle:

Replaced missing AY chip and replaced broken tracks with wire.
Removed CPU, fitted socket and added IDE interface with TOS 2.06/1.04.
Added CF adapter as hard drive (HDDRIVER 11 now in use).
Fixed damaged floppy power lead.
Replaced missing floppy drive with a Gotek.
Replaced missing Blitter.
Made new RTC wiring harness.
Replaced faulty RAM chip. (Hidden under the Gotek!)
Replaced PSU with Meanwell unit.
Had a massive troubleshooting effort (Thanks for everybody's help!!) which led me to a bad GND connection on the LO0 ROM which prevented boot.
Along the way fitted replacement DMA and Shifter sockets to replace the cruddy original ones.
Last but by no means least, I found out it's really difficult to find a Mega ST keyboard. Since I also have a Mega 1 which is sans keyboard as well, finding two is nigh on impossible!! Like rocking horse sh*t as a good friend once said to me.

All in all it's been a ton of frustration, a lot of fun, I've learned a ton about troubleshooting the ST and I'm happy to have a good working stable Mega ST 4.

There is one last thing to try, and that's SMonsons DVI shifter adapter, which could be very nice fitted internally here.

[Icky]

Nice work RJ. Looking good.