Some of the steps in the repair of a 48K ZX Spectrum that did not produce any video output. It was sent to me by a member of the worldofspectrum.org forums who had been unsuccessful in his attempts to repair it.
Also, I very highly recommend using a bench power supply to power a Spectrum under repair. Set a current limit (700-750mA is about right for a 48K Spectrum with no add-ons) and if the limit operates for more than an instant when you switch on, switch off immediately. In most cases, this will prevent any further damage from being done.
so would you suggest that when connecting a bench power supply that the current should be limited rather than the voltage? when I connect mine to my game gear having set 9v, it powers on and then immediately winks out again
I meant the comments I originally put above to be a reply here, so I'll move them... That diode is normally D15. A likely cause of failure is something drawing too much current from the voltage converter's 12V output. There's a decent chance one or both of TR4/TR5 have been damaged too. Replace D15 and try the tests described in the Initial Tests video. What results do you get? If they are not as expected, DO NOT power on the Spectrum.
Hi! is it ok if TR4 NPN shows 0,9 and 0,6 on the N legs? Also on IC13 pin 1 =0v; pin8=9.4 V. I have changed all caps excluding С46 ,what I should to do next?
It isn't particularly critical. Most displays contain an input DC blocking capacitor anyway; I just think it's slightly nicer to remove the arbitrary offset that's a result of the output buffer circuit before the signal exits the Spectrum.
+Sony Trinitron Your computer almost certainly has one or both of the following faults: - Failed transistor TR4 or/and TR5. - One or more failed lower RAM ICs that overload the power supply. It's quite possible that one of the above led to the other and the same will happen again if you don't make sure both faults are not present before connecting power again. The tests described in the 'initial tests' video usually indicate whether any of the above faults are present. There are not many physically small transistors that'll work for TR4. If you need to replace it, you should use a ZTX651/ZTX652/ZTX653. These are fairly readily available from many suppliers of electronic components.
JoulesperCoulomb I get scrambled symbols and colours on the screen, someone said try piggyback the lower 4116 DRAM IC's which I will do, I may just check the transistors first.
+Sony Trinitron Sometimes putting a working IC in parallel with a faulty one can make a system work, but it isn't a particularly reliable method and I don't recommend it in most cases. Certainly don't do this with a 4116 if all power supply voltages are not present and correct, otherwise it'll probably damage the new IC.
Nice vid sir! Really intereting. I have a clone after ZX Spectrum named HC-90 made in Romania from 1985 till 1992 or something. I have found it in basement and tested after so many years of inactivity. I have a problem with it. A video problem. It worked fine until one day when the colors went black and white and now the loading stripes/lines it goes over the paper. It is going right in rest only this problem. I have try to conect it with the RGB cable a Db9 pins to scart and the colors now appear is not black and white anymore but still the loading lines go over the paper. The border influence the paper I mean it doesn t support all combinations of borders and papers. If I try BORDER 0, the paper automatically goes magenta which is PAPER 3 and so on. Even the starting logo is written in green color not black which supposed to be. Is frustrating because in games u can not play good because it not translates all the colors corectly and combinations between them. Iis interpretate black as green and white as magenta somenthing like this. The defect that occured both influnced the lines problem and the black/white video coaxial module proble. What can be the problem? What should I check? Maybe a cold soldering point, should I measure all the pieces? I m trying to find a circuit diagram for it, still no luck so far. THX in advance!!!
I haven't worked on one of these and without being able to examine it for myself, I don't think I can say much that'll be of help to you. So far as you can tell, does it work properly other than the display fault?
Yes is working properly it starts without problem,is loading the game corectly, I am able to play to make programs in BASIC, only that video problem. I even tryed to switch RGB colors on the cable among them but still the same problem. This type of clone is a pale pale copy of ZX Spectrum made in that period with russian parts. They have improvede many characteristics. It has a regulator of 5v, on a radiator in front just like on ZX but from 9v source although i have eliminated that and I am powering him from a pc source.If I find any documantations I will send it to you.
OK thanks. So didn't he put them in and think "errr the speccy isnt working correctly". Sorry Im genuinely just trying to understand if it's possible to have the transistors the incorrect way and the effects of that in case I find a similar situation. So transistors the wrong way will not work, period.!?
I think the owner had replaced the transistors, but not done so correctly. In general, a transistor connected incorrectly in a circuit won't work and in some cases, would be damaged by being so. However, a BJT connected with its collector and emitter transposed will work as a transistor to some extent, but its performance will be terrible and most circuits won't function if this is done.
Hi. My Spectrum 48K won't show any colors, only black and white. When i first got my Spectrum the screen went yellow on first startup. black and white after that. Do you know where to start? It is an Issue 2.
+Emil Åkre In the case of an issue 2, it's quite likely you simply need to adjust VR1 and VR2 to bring back the correct colours. Sometimes it's necessary to clean the tracks of these variable resistors and occasionally replace them altogether.
+Emil Åkre You are very unlucky as VR1 or VR2 are almost always the cause of problems related to the display's colour in issue 2 Spectrums. The 48K Spectrum service manual contains some instructions for setting VR1 and VR2 by adjusting for particular voltages between pin 3 and pins 2 and 4 of IC14. Are you able to adjust VR1 and VR2 to achieve the correct voltages? Beyond that, if you want to proceed other than by blindly replacing components in the hope of effecting a cure, it'll be difficult to continue without using an oscilloscope. The service manual gives examples of the signals (table 4.1) that should be found at various pins of IC14. Checking these would be where I'd start. Sometimes it is possible to diagnose faulty components in the video circuitry by taking DC resistance measurements between various points while using the circuit diagram to work out whether the readings are reasonable or not. It is very rare for the passive components in this circuit to cause problems, though I recently repaired a Spectrum that had a faulty 100nF capacitor that caused loss of colour in the display, so it can happen.
František Fencl I've never attempted to use a ZX Spectrum with a modern Samsung TV. It is the case that some televisions, mainly newer ones, reject the video signal from a Spectrum. Have you been able to test your Spectrum with another TV? It could be worth asking about your specific model of television in the forums at www.worldofspectrum.org to see whether you can obtain a report from someone else who has tried the same model.
