October 2011 – Page 2 – JAMMArcade.net

ZX Spectrum composite video mod

Guides, Mods Comments Off

Oct 152011

There is a very simple mod that you can do to bypass the RF modulator and use the same connector for a composite output.
It takes a few minutes to do this and works really well.

First job, remove the lid on the RF modulator.

There is a resistor connected to the centre pin of the RF output socket, remove this and bend it up out of the way.

On the outside of the RF module there are 2 wires leading from the modulator to the PCB. These are the VIDEO IN on the left and +5v on the right.

Remove both of these from the PCB.

There is a second insulated hole in the the side of the modulator unit that is unused. Get some wire and strip enough insulation off the end so you can feed it through and then solder it onto the centre pin that the resistor was originally connected to.

As im never going to be using the RF again I went ahead and totally removed the old VIDEO IN and +5v wires. I also removed the old resistor. This step isnt necessary but if you leave them in make sure they are insulated so they can short against anything.

Fire it up and check out the Spectrumy goodness.

ZX Spectrum repair log #1

Oct 152011

Got myself a broken ZX the other day and it arrived with a spare motherboard. The spare is a later revision so I started with that one.
This is my first Spectrum repair so was a little unsure where to start.
This started up with garbage on the screen in the form of many vertical lines.

As the Z80 was socketed I tested that. Failed and replaced.
Looking around the board I saw a transistor that was a little damaged. Removed TR4 and it was shorted internally. It is a ZTX650. I robbed one off the other board and fitted it.
The Spectrum now works but the picture is really really bad quality.
The keyboard also doesn’t function properly, half of the keys don’t work. Ive read the keyboard membranes are usually shot so ill have to order one up.

Commodore 64 ‘breadbin’ repair log #5

Computer Repair Logs, Repair Logs Comments Off

Oct 132011

Not so much a repair log but a bit of information.
The system booted to either a blank screen, to a blue screen with a couple of bits of garbage or to a screen full of random garbage

Pulling the SID chip let the C64 boot normally.
I always pull the SID chip when starting out testing for this very reason. Not sure why it does it but it can cause problems like this.

Psycho Soldier PALs

PAL Updates Comments Off

Oct 102011

Dumped some unprotected PALs from a broken Psycho Soldier board.
There are several empty PAL sockets on my board set and not sure where any of them go. There was also a PAL marked up as A6003-1 which MAME shows as being from Chopper. Not sure if this is also used in Psycho Soldier too but and info on this will be appreciated.

The PALs are:
A6003-1
A6004-2
A6004-3

As always with non working boards, these dumps are assumed working until im informed otherwise.

Homemade C64/C128 power supply

General Comments Off

Oct 072011

I recently won a Commodore 128 on eBay. It had no power supply and was therefore “untested”. In eBay terms this means its broken.
I know I shouldn’t have tried but in my haste I attempted to use my C64 supply. This resulted in one dead C64 power supply.
As the original C64 supply is pretty poor and a C128 supply is impossible to find for a decent price I thought I would build my own.
For a total cost of around £35 I bought a 5v power supply and a 9VAC transformer. Both of these are massively overspecced compared to the original and they will both fit into the case of my old “lightweight” Amiga power supply case.
I used the old Amiga lead as the connector for the C128 and reused the C64 lead from my newly burnt out supply to give me 2 separate outputs and it is capable of running both at the same time with room to spare.

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