VIC20 repair log

 Computer Repair Logs, Repair Logs  Comments Off on VIC20 repair log
Jan 072012
 

Got Stiggys other VIC20 the other day, it wont load anything via the serial port and just reports back “DEVICE NOT PRESENT”.
According to Ray Carlsen this can be caused by a dodgy 7406 IC and he wasn’t wrong.

I checked all the states of this chip with a logic probe and all seemed to be as it should be so I went through each gate with a logic pulser and checked for its output. The outputs on pins 4 and 6 were stuck.

Removed the chip and eventually found one on a C64c motherboard that was scrap.
Replaced and tested with a demo and a couple of games.

I also took this opportunity to try out the VIC20 JiffyDOS (PAL).
My original test with JiffyDOS on this failed, nothing would load up. Turns out there is an NTSC and a PAL version, the later being significantly harder to find but we got there in the end.

 Posted by at 5:18 pm

Jumping repair log #1

 PCB Repair Logs, Repair Logs  Comments Off on Jumping repair log #1
Jan 042012
 

Got this Rainbow Islands bootleg yesterday.
The game worked when I tested but was told about a dodgy ribbon connector before buying it so decided to take a look. Not sure what happened after that but the board refused to boot.
After finding a few stuck address lines I decided to check the program ROM’s. Found 3 that would not read properly so I replaced them.
In MAME it lists 2 of the program ROMs as being from the genuine Rainbow Islands set but these are double the size so needed to be split into 2 files and burned to 27512 EPROMs.

Probably worth mentioning that if you use the older 2005 ROMIdent program then the dumps from Jumping actually show up as 2 separate ROMs. Must have been like this at one time.
Booting up now gives me a screen of garbage

Every now and then I could make out a “SCREEN RAM ERROR” message. The screen RAM is located on the second board and suspected this is what happens when the ribbon connector got a bit wrong but couldnt get the game to boot again no matter what I did so I decided to socket the 68000 cpu and see what was going on, kinda wish I hadnt now as I got this

Accepting the error just kept flagging more “tied bits” errors. Turns out this was a red herring as these lines are controlled by a PAL20L8 chip and everything passes if this is removed. Wasted a little bit of time on this.

After taking a careful look at the connector it turns out the plastic housing that holds the ribbon cable in place was damaged and this allowed the ribbon cable to come away from the pins. I properly reseated the cable and tested, the board now fires up in all its crazy bootleg glory

The game plays pretty much like Rainbow Islands does but some of the sprites and names are different and I think the secret rooms don’t work too well.

This game has 3 PAL chips on it, unfortunately all are security locked and I am only able to read 1 of them (might be able to read the one on the video board but never tried).

 Posted by at 7:11 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
 Older Entries  Newer Entries