Repairing a ZX Spectrum 128K +2 (serial no U-043583) - Part 1

I bought a ZX spectrum 128K +2* from ebay (serial no U-043583) which whilst it seemed to be mostly working had a terrible picture on the UHF output and no output on the composite video. So I set about trying to repair it.

Issue 3 of the ZX Spectrum 128K +2 had a number of transistors fitted the wrong way around during manufacture. So let's check those out.

* Not the black amstrad ZX spectrum +2A and +2B

Incorrectly fitted TR4

TR4 is often fitted incorrectly which prevents a composite video signal being present on pin 1 of the RGB connector. Sure enough the TR4 on this +2 is fitted the wrong way round. Note the silk screen is incorrect for this transistor.

TR4 fitted the wrong way round

TR4 is a TN3904 transistor. The collector should be connected to 12V, the base to diode D7 and the emitter to composite video out. If the transistor is fitted the right way round then testing for continuity with a multi-meter will show the collector connected to pin 11 of the TEA2000 IC (12V).

2N3094

If the transistor is fitted the wrong way round it is very easily corrected by unsoldering the part and putting it back the right way round.

TR9

TR9 forms part of the circuitry which generates the 12V required by the TEA2000 composite video IC. The TR9 is a ZTX650 NPN transistor and when fitted the right way round the emitter should be connected to ground. I have read reports of this being often fitted the wrong way round but on this +2 it is fitted correctly.

ZTX650

TR9 fitted right way round

Spectrum 2+ TR8 and TR9 schematic from service manual

Incorrectly fitted TR8 

TR8 also forms part of the circuitry which generates the 12V required by the TEA2000 composite video IC. TR8 is a 2TX213 PNP transistor and when fitted the right way round the emitter should be connected to +5V and the collector to resistor R29.

TR8 was fitted the wrong way round in this spectrum and this was corrected by unsoldering the transistor and fitting it back the right way round. Again, the silk screen is incorrect and indicates the wrong way round for the part.

TR8 fitted wrong way round

Incorrectly fitted TR5 

TR5 forms part of the circuitry for generating the Vsync signal which is required when using an RGB output. TR5 is a TN3904 transistor and when fitted the right way round the collector is connected to 5V and emitter to resister R57.

TR5 is fitted the wrong way round in this spectrum and can be corrected by turning it around but given this problem should not affect the composite video signal I have left it as is for now and will fix it later when I want to use the RGB output.

TR5 fitted wrong way round

 

 

TR5 Spectrum 2+ schematic from service manual

 

How to check a transistor is not faulty

The diode test mode on a multi-meter can be used to check a transistor. The transistor should be unsoldered from the circuit prior to testing.

A good diode should display a voltage drop ranging between 0.5 to 0.8 volts when forward biased and OL when reversed biased.

Composite video 

With the transistors swapped round I now get a picture on the composite video output but it is rather poor! Part two.....