2364 replacement

 General  1 Response »
Dec 082011
 

So my ZX81 ROM is apparently too slow to take advantage of my ULA so that means I needed an alternative. Unfortunately, as with the breadbin C64’s there is no EPROM alternative for the 2364 ROM but there is also no room in the ZX81 case for an adapter to fit properly.
Enter the ROM-EL 2364 from Retro Innovations.

This is a modern day solution for those old ROM’s and can be made switchable with a little extra work.
The only problem is an adapter is required in order to actually program the replacement. As the one I made was a complete hash I wont be taking any pictures of it but it served its purpose and now I have a switchable ZX81 BIOS.

My 81 is starting to look less like the original and more like a modern rehash but its working just great.

 Posted by at 8:58 pm

Commodore VIC20 in FPGA

 General  Comments Off on Commodore VIC20 in FPGA
Dec 072011
 

For some time now I’ve been watching the fpgaarcade website and have even bought some of Mike’s replacement C64 PLA chips.
Recently I got my hands on a Spartan 3E FPGA development board and thought id try out Mike’s FPGA implementation of the VIC20.

Following the instructions provided it was a breeze compiling and transferring the VHDL.
Plugging in the VGA cable and setting the on board switches to 31khz I was greeted with

On my monitor here the screen is offset slightly but im told another monitor may yield better results.
The Spartan development board also has a PS/2 port which when I plugged in a keyboard was able to use the VIC20 as I would a real one.

This thing is so good that I can even upload 4 converted cartridges into the on board flash memory and run them at the flick of a switch. Once I get my serial cable built ill try that out too.

Now, on with learning VHDL, its not going well.

All credit goes to Mike at FPGA ARCADE

EDIT: Got a serial cable made up and flashed over the Jupiter Lander game as described with the VIC20 VHDL source, works pretty well.

 Posted by at 8:06 pm

Amiga 4000 repair log

 Computer Repair Logs, Repair Logs  Comments Off on Amiga 4000 repair log
Dec 012011
 

Its been a good week for repairs this week. Got Jimbob’s Amiga 4000 here which would constantly reset with any RAM in bank 0 and would not detect any RAM at all with bank 0 empty.

The board has had some repair work done before but its been done to an excellent standard. The new SIMM sockets looked flawless so that was my first suspicion down the pan. I checked the on board battery which was showing the first signs of “furring” on the contact. The battery was completely dead so I replaced this while I had the board out. There doesn’t look like there has been any battery leakage but I cleaned up the area anyway just in case.

With a bit of internet searching and a bit of probing I found a knackered 74F245 transceiver at location U891. I ordered some up and replaced it.

Fired up the unit and checked the free RAM

Played a game of Blood Money for good measure.

 Posted by at 5:07 pm

ZX81 repair log

 Computer Repair Logs, Repair Logs  2 Responses »
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
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