One day I'm going to learn to keep my mouth shut and this was a good example. Once people find out you tinker with old kit the usual "can you do me a favour" question gets delivered more often than not.
16k ZX Spectrum, spent its life in the loft and was looking very tired and unhappy. Owner advised that it powered up ok but the keyboard was not working (surprise) and the picture quality was poor. I decided to take pity on this poor waif and have a look.
Inside the keyboard connector had gone brittle with age and parted where it joined the PCB. I managed to cut back enough to a good section and re-terminate with the result that all of the keys work. A quick call to the owner confirmed that they wanted a new membrane sourced if possible because they wanted it to work correctly.
Moving onto the image quality, what a mess, smearing of the picture and poor colour rendition and no amount of manual tuning of the TV would remove it. Reading the Thorn EMI manual it recommended changing in order the colour quartz crystal, the LM1889 video chip and finally the ULA. Replacement ULAs are sourceable but at a £10 a time they are not cheap. Feeling somewhat flummoxed I decided to look at the problem from a different angle.
The video information is sourced from data provided by the ULA, the LM1889 handles all the assembly of the image and passes it on in composite format to the ubiquitous UM1233 modulator pack. Tapping the modulator with a screwdriver made the screen image occasionally jump but nothing conclusive. Idly wondering if the composite signal feed was clean I ran a lead from the composite input on the TV and touched the end on the feed to the modulator to be presented with a nice clean composite driven image on the TV. The modulator block was starting to look like the culprit but to be honest, from experience, I have never had a modulator block go down, the UM1233s are usually pretty good but I guess there has to be a first time.
Looking through the spares pile I found a suitable replacement and after disconnecting the feeds to the board fitted unit I tacked on the new mod and tuned in the television to be rewarded with a nice clean image on UHF36. So, the modulator was at fault in this instance and after some swearing whilst trying to unsolder the two big earth tags the new mod was fitted and a quality picture attained, all colours where they should be.
To finish off the PSU was re-capped as the thirty plus year old ones were looking tired and unhappy and a good clean up of the case resulted in a nice presentable unit.
Just a new membrane to fit and another machine saved from the scrap pile.
16k ZX Spectrum, spent its life in the loft and was looking very tired and unhappy. Owner advised that it powered up ok but the keyboard was not working (surprise) and the picture quality was poor. I decided to take pity on this poor waif and have a look.
Inside the keyboard connector had gone brittle with age and parted where it joined the PCB. I managed to cut back enough to a good section and re-terminate with the result that all of the keys work. A quick call to the owner confirmed that they wanted a new membrane sourced if possible because they wanted it to work correctly.
Moving onto the image quality, what a mess, smearing of the picture and poor colour rendition and no amount of manual tuning of the TV would remove it. Reading the Thorn EMI manual it recommended changing in order the colour quartz crystal, the LM1889 video chip and finally the ULA. Replacement ULAs are sourceable but at a £10 a time they are not cheap. Feeling somewhat flummoxed I decided to look at the problem from a different angle.
The video information is sourced from data provided by the ULA, the LM1889 handles all the assembly of the image and passes it on in composite format to the ubiquitous UM1233 modulator pack. Tapping the modulator with a screwdriver made the screen image occasionally jump but nothing conclusive. Idly wondering if the composite signal feed was clean I ran a lead from the composite input on the TV and touched the end on the feed to the modulator to be presented with a nice clean composite driven image on the TV. The modulator block was starting to look like the culprit but to be honest, from experience, I have never had a modulator block go down, the UM1233s are usually pretty good but I guess there has to be a first time.
Looking through the spares pile I found a suitable replacement and after disconnecting the feeds to the board fitted unit I tacked on the new mod and tuned in the television to be rewarded with a nice clean image on UHF36. So, the modulator was at fault in this instance and after some swearing whilst trying to unsolder the two big earth tags the new mod was fitted and a quality picture attained, all colours where they should be.
To finish off the PSU was re-capped as the thirty plus year old ones were looking tired and unhappy and a good clean up of the case resulted in a nice presentable unit.
Just a new membrane to fit and another machine saved from the scrap pile.
