exxos's Atari, Amiga & retro forum

Well, well. It seems that I'd forgotten that I'd already swapped out the memory controller and the new one has exactly the same date code as the one in the machine!

I found the original TMS4500A and put it in and the Apricot FP now passes the RAM test! No more lock-ups and my March test program errors out only above the 512K memory mark.

However, the machine can't read disks until it's warmed up.. and I think that's a PSU issue.

Apparently there's one more low voltage cap in the PSU which is important, which is part of the voltage feedback circuit, and I've not replaced that one. I've got one in stock, a 220uF 25v, which will be able to replace the original 220uF 16v with ease. However, I've not really got time for this until after Saturday's event.

Replaced the 220uF 10v capacitor with a 220uF 25v capacitor in the PSU and it made no difference to the voltages. Oh well.

Anyway, seeing as I now have enough 41256 chips and the schematics suggest that filling all the banks with RAM would bring the system up to 1MB.

So I removed the solder from the holes, fitted sockets and then some RAM...



Unfortunately, when powered on the system gives a memory error and shows either 256K, 258K or 320K. Switched the RAM in the high banks with some I got from @Steve yesterday, which checks out in the tester, the machine detects 512K, as it does if one of the banks is left empty.

The new memory is faster than the original memory but slower than my other replacement memory. It's a strange one.

Found a jumper, J7 which seems to change the memory access.

*IF* I have only the 150ns Motorola 41256 RAM that I got from Steve in the high bank and not the Motorola 120ns 46256 RAM and change J7 then I get 832K of RAM reported and my memory tester checks up to 768K fine (it doesn't test any higher). If I put the 46256 RAM then I get errors even though I've tested that RAM and the lower bank has those same chips installed. I'm guessing there is at least one faulty chip and the RAM tester isn't testing for that failure mode.

The machine's still not completely stable, but that may be the PSU's voltage. I need to investigate that further.

Well done getting it fixed

Opened the machine up again to get the specs. and sizes of the two large PSU capacitors as they're the last electrolytic ones in the power supply and they could be causing the power issues. When I first turn on the machine the floppy drive doesn't behaves as it seems to not be able to read the disks.

Just in case I replaced the capacitors on the floppy controller board as I had spares from when I did the same to the original F1 floppy drive (which doesn't work).

After putting it all back together I have the same behaviour so I'm pretty sure that it's the power supply.

Now, getting replacement 150uF, 200V capacitors was difficult! RS don't have any at all. Farnell had one type in stock, which is not quite the right size but it will fit. Again, everything else out of stock. Anyway, they should arrive on Wednesday hopefully. Then I can see if changing them changes the voltages on the PSU.

I haz capacitors. Power supply hacking tonight!

stephen_usher wrote: 28 Sep 2022 12:07 I haz capacitors. Power supply hacking tonight!

Reminds me I need to revisit the PSU in my A5000 (Despite already fully recapping it), it intermittently fails to power up the HDD and I get a bare desktop.

Well, I replaced the two "large" capacitors on the high voltage side and there was no change. At least I now know that all the electrolytic capacitors in the PSU have been replaced now.

So, time to adjust the output voltage.

It seems the voltage adjustment is made by changing one or other of the resistors R26 and R29, which are in parallel. The initial values were 22K for R26, which is what is in an earlier spec for this lineage of PSU, and R29 was 100K. Removing R29 brought the unloaded voltage down to 4.8V, so the wrong direction. Replacing it with a 27K eventually got my target unloaded voltage of 5.2V.

Putting the machine fully back together and the +5V at the far RAM end of the board was 4.85V, which is acceptable. Far above the TTL minimum of 4.7V at least and far better than the 4.63V it had been previously. Without the CPU board (just the I/O board) the voltage was 5.18V, which shows just how much current the CPU board is pulling!

The floppy now works from first power on and the screen does look a bit better.

OK. Floppy issue isn't fixed. :-/