Spectrum 48k Issues

[PootisSpencer]

So I bought a Spectrum + 48k and I have a few issues that I hope someone can help with.

1) When I plug the power cord into the Spectrum sometimes it won't go to the copyright text. Sometimes it will just stay at the back box/grey border. I have to reset a few times before it will work.

2) Some keys on the keyboard won't work 1 to 6 etc and certain letters in one area also won't work. Can this be fixed or is a new membrane required even though I have checked it and it looks in good shape.

3) Most of the time I get a bad picture till I wiggle or turn the tv input cable to make it clear and not static.

I have opened it up and looked all over the PCB and keyboard membrane etc and I can see anything that sticks out to say what could cause any of the above issues.

The board is an Issue 6A.

I hope someone can help as I really would like to get this working 100%
Thanks

[1024MAK]

1) When I plug the power cord into the Spectrum sometimes it won't go to the copyright text. Sometimes it will just stay at the back box/grey border. I have to reset a few times before it will work.

The code in the ROM does a check to try to work out how much RAM is fitted. If it encounters a problem during this check (which is nowhere close to a full test), it will just stop. It could be an intermittent problem, or you may have a faulty memory chip.

The outside part of the screen is called the border, it can show eight colours, what you describe as grey is in fact normal brightness white.

2) Some keys on the keyboard won't work 1 to 6 etc and certain letters in one area also won't work. Can this be fixed or is a new membrane required even though I have checked it and it looks in good shape.

The plastic tails suffer from going brittle, and then the very thin metallic conductors end up with hairline cracks. This is by far the most common failure with the keyboards. A new membrane from RWAP normally fixes this. On the ZX Spectrum+ machines, some partial keyboard failures can be due to the clamps that grip the membrane not being tight enough. But this is not likely if the membrane is the original one.
If the membrane has been renewed, another problem can be bits of the old membrane stuck in the PCB connectors.
In very rare cases, the custom ULA chip may develop a fault that prevents the keyboard from fully working.

Also note that the two bits of brown goo on the metal heatsink (that strange shaped large piece of metal) and on the TV modulator case are supposed to be foam pads. They were fitted to stop the keyboard tails/ribbons from touching the heatsink/modulator.

3) Most of the time I get a bad picture till I wiggle or turn the tv input cable to make it clear and not static.

Try cleaning the RF/UHF output socket with solvent cleaner (IPA) and with an ink eraser. You may also wish to consider carrying out a composite modification so you can use the AV input (normally a yellow coloured phono socket on most TVs).

I also recommend that the solder joints for the DC input connector be re-soldered.

Mark

[PootisSpencer]

OK so the issue with the Spectrum failing to complete the check to show the copyright text so I can use it is now happening all the time. How can I fix this or is there anyone I can send it to that can fix it for me please?

[1024MAK]

If you can solder including desoldering, then we can try to guide you. You will also need at least a good digital multimeter and I also recommend a logic probe (if you want to build your own, I can point you to a very inexpensive design). You may also need either an expansion that has an on board ROM/EPROM/EEPROM/flash chip that has or can be programmed with a diagnostic / RAM test program. Or alternatively, be able to programme (‘burn’) a diagnostic / RAM test program to a EPROM chip. You will need a suitable programmer in order to do this.

If you don’t want to go via this route, or you don’t have the skills, then Ian of Mutant-caterpillar should be able to bring it back to life.

Mark

[dfzx]

I also recommend a logic probe (if you want to build your own, I can point you to a very inexpensive design).

Without wishing to hijack the thread, yes please Mark! I've been meaning to make a logic probe for ages. Can you give us a link to that?

[1024MAK]

I also recommend a logic probe (if you want to build your own, I can point you to a very inexpensive design).

Without wishing to hijack the thread, yes please Mark! I've been meaning to make a logic probe for ages. Can you give us a link to that?

See this topic over on Sinclair ZX World forums

Mark