Nov 302011
 

At the RCM weekend I met up with SirMorris and a guy called Andy Rae and was introduced to the crazy world of the ZX81.
Andy was demonstrating his ULA chip replacement which has the ability to double clock the old 81 and the results were really impressive. After a quick chat I managed to secure one of these ULA chips for myself and quickly set about eBay looking for a cheap ZX81. I eventually got one for £6 which was untested but we all know that means its knackered, and it sure was.

I powered the little machine up and was greeted with nothing on screen.
Straight away I could smell something was not quite right, the Z80 and the stock ULA were roasting. I removed both of them and fitted sockets. At this point I thought it wise to check the voltages on board just in case. Everything was fine.
I fitted a brand new Z80 and the new ULA chip.
The new ULA also outputs a nice composite signal and the mod to utilise this is the same as for the ZX Spectrum.

Still nothing.

As there is not a great deal on the board it was a little pleasure to work on. Rather than removing the ROM and making an adapter up to read it I opted to use the Fluke 9010.

From here I performed a BUS check to make sure there are no shorts, all was fine. I also performed a ROM check which reads the ROM and returns a signature similar to a CRC check. As I didn’t know what the signature was supposed to be a entered a random one and waited to see what it returned in the error. The ROM on the ZX81 is found at 0000-3FFF in address space. The Fluke reported back a signature of 28A8. Using the sigcalc program from Quarterarcade I was able to verify that this was a match for a ROM marked as ZX81a.rom in my collection of BIOS files.

After that I set it off doing RAM checks. The RAM is located at 4000-43FF in address space. This quickly brought up a R/W error, accepting this just kept bringing up more errors right the way through all the address space. Desoldered both of the 2114 RAM chips, fitted sockets and some working RAM.

The ZX81 is now alive once more

As expected this also needs a new keyboard membrane.
Now its probably time to get a ZXpand unit and also wire in the new enhancements of the ULA. Fun times ahead for me!

 Posted by at 8:53 pm

Apple //e repair log

 Computer Repair Logs, Repair Logs  Comments Off on Apple //e repair log
Nov 282011
 

After the recent RCM open weekend I returned home with a boot full of broken stuff, one of which was Panthers Apple IIe which displays garbage on screen at startup.

Originally thought it might be a power supply issue but all voltages checked out fine.
Next step, check the ROM’s.
This version of the Apple II can take a 27C64 EPROM as a direct replacement so I just burned a new set of ROM’s and tried one at a time. The first one I changed was 342-0135 which is the “unenhanced CD” ROM. Replacing this actually fixed the fault.

Pressing CONTROL+OPEN APPLE+CLOSED APPLE+RESET invokes the self test routines. This came up with KERNEL OK which is correct.

I don’t have any software to test this any further but I ran a small basic program which worked fine.
My only problem was the DELETE key prints a character rather than deleting anything but since I’m not familiar with this system this could be right, doubt it though.

EDIT: Confirmed that the delete key does NOT actually operate as delete in the BASIC environment, apparently the key is programmed to be a delete key with certain applications.

 Posted by at 7:14 pm

PLS100 chips

 General  Comments Off on PLS100 chips
Nov 232011
 

Today I received a job lot of 20 x PLS100 chips that I got from eBay recently.
For those that aren’t aware, these are what was used as a PLA in the older breadbin Commodore 64’s and are probably the most common failure in the machine as they tend to run hot.
Ive got the JEDEC fuse map from a genuine C64 PLA and have tested with no problems. These should keep mine and RCM’s Commodore’s going for a while longer now

 Posted by at 4:37 pm

Pocket Gal bootleg PAL added

 PAL Updates  Comments Off on Pocket Gal bootleg PAL added
Nov 212011
 

Dumped a PAL16L8 from the Pocket Gal bootleg board I got.
The PAL was unprotected and I have converted it for use on a GAL16V8.
This has been tested working.

UPDATE: When I get an idea in my head the tendency is to get it done or ill think about it constantly so the second and final PAL on the board is now dumped and converted to GAL. The original device is a PAL16R6 and this was also unprotected.

 Posted by at 6:00 pm