General Stuff(tm)

[stephen_usher]

The only other thing I can see on the data lines is the ULA. Or of course the pull-ups and the rom as you know.

Has that board got a reset switch?

No. No reset switch. Pulling down the /RESET line on the CPU does reset the CPU but you don't see any change as the memory isn't cleared, but that's normal.

[PhilC]

Ok, I can dig out a working issue 4 board and diag cart if you want to borrow them?

[stephen_usher]

It's OK. I should have a ULA replacement board arrive early next week. That is at least a reliable ULA. The one I'm using is from an Issue 2 and shouldn't work at all, so that could well be the problem. The later ULA I have has a mostly dead video output section, so I can see on pin 15 there's something being displayed, possibly even correct but I can't tell. It looks like it may be doing the memory test and then clearing the screen.

[stephen_usher]

Well, well, well...

I decided to desolder the ROM and put an EPROM with the diag ROM on it... with a bit of a hack.

IMG_3909.jpeg (99.63 KiB) Viewed 332 times

Oh, would you look at that!

IMG_3910.jpeg (49.96 KiB) Viewed 332 times

I then did a proper bodge to allow 27C128/256 EPROMs to be used. Basically, NANDing the /ROMCS and /MEM-REC lines so that the ROM is only enabled if it's a memory access and the ROM is enabled.

IMG_3912.jpeg (75.4 KiB) Viewed 332 times

Burning a 48K ROM and putting that into the socket I get....

IMG_3911.jpeg (34.6 KiB) Viewed 332 times

Yay!

Now I have to wait for the replacement ULA equivalent to arrive and then get a new case.

I've even managed to put the OKI upper RAM back in even though it fails testing in the RAM tester it passes the ROM tests. Strange.

[PhilC]

Great to see it working Stephen.

The ram tester I have can indicate whether and upper bank or lower bank is faulty. It likely thinks those 4132s are 4164s, which they are but with a faulty bank on each one. Cheap and nasty chips. I think Memotech did similar with the MTX500 didn't they?

[rubber_jonnie]

[stephen_usher]

Great to see it working Stephen.

The ram tester I have can indicate whether and upper bank or lower bank is faulty. It likely thinks those 4132s are 4164s, which they are but with a faulty bank on each one. Cheap and nasty chips. I think Memotech did similar with the MTX500 didn't they?

Well, the tester showed lower bank OK for five chips and the MIDDLE OK for three others.

Yeah, there were quite a few manufacturers who used the half dead chips as they were sold at a massive discount, less than a quarter of the price of the fully functional 64K chips.

[stephen_usher]

Anyway, so, the list of faults was:

1. Dead TR4 transistor (+12V & -5V DC-DC circuit)
2. Dead TR3 clock amplification transistor.
3. Dead ULA
4. One dead 4116 RAM chip
5. Mostly dead ROM
6. Dead modulator. (How can you kill a flippin' modulator?!)

I've ordered a complete new case from Retro ReNew, which should arrive next week, and the ULA replacement board should arrive on Tuesday. Once that's done another Speccy will be ready to face the World again.

[PhilC]

Now if I could get a case in purple I'd be very tempted.

Btw, when you find out how you kill a modulator then please let me know, I've got a C64 one someone killed

[stephen_usher]

Now if I could get a case in purple I'd be very tempted.

Btw, when you find out how you kill a modulator then please let me know, I've got a C64 one someone killed

I'm afraid that I've gone very traditional.