Repairing a ZX Spectrum 128K +2 (serial no U-043583) - Part 7

The tape player was no longer playing tapes at a constant speed and was erratic. This was a quick fix with new tape belts from ebay

Repairing a ZX Spectrum 128K +2 (serial no U-043583) - Part 6

I attempted a mod within the ZX Spectrum to generate a blanking signal so that the display would only show the RGB output. 

Whilst it did not seem to be causing an issue with the RGB display TR5 and TR7 were fitted the wrong way around at the time of manufacture. So I swapped the TR5 and TR7 transistors with new ones and soldered them back the right way around. TR5 generates the RGB vertical sync and TR7 is responsible for generating the ULA IOREQ signal.

I then attempted the blanking signal mod which involves

• Moving the link from LNK2 to LNK4 position
• Adding a 470Ohm resistor in position R9

This mod disconnects the Composite video signal on pin 1 and replaces it with +12V, which through the voltage divider provided by R9 provides 1.5V on pin 16 of the Scart plug which will switch the TV into RGB mode.

Unfortunately, after the mod the display did not work and was blank both on my TV and on the monitor using a SCART to HDMI adapter.

After rechecking everything the issue could not be fixed so I moved the link back to position LINK2 which restored the picture but the ghost image was still there.   

Repairing a ZX Spectrum 128K +2 (serial no U-043583) - Part 5

In part 4, I re-checked the onboard power supply voltages and made some adjustments to the +12V supply which was measuring a low 10.98V but even with +12V on the 12V line the ghost image still remained.

The final fix was found when someone on world of spectrum suggested the TV might be displaying both the composite video and the RGB video signal resulting in the two images on the screen. Initially, I thought the  cable I had bought was only wired for composite but on rechecking (this time measuring for resistance rather than continuity, oops!) it turns out the RGB pins are also wired.

The SCART to HDMI converter that I am using has no means of forcing selection of RGB or composite video in on the SCART input so as an experiment I connected an alkaline battery to the SCART "blanking" pin 16 to force RGB selection and the ghost image was gone.

Finally, it is now fixed and the display looks amazing!

Fixed display with no ghost image

Repairing a ZX Spectrum 128K +2 (serial no U-043583) - Part 4

In part 3, I checked the inputs to the TEA2000 composite video chip and could not see any obvious issues other than the low voltage on the 12V input.

I decided to check and replace some components on the onboard powersupply to see if they were causing the low 10V voltage measured on the +12V supply.

The onboard powersupply uses a ZTX650 transistor TR9 which looked okay but I decided to replace it with a ZTX651 which is an acceptable replacement for the 650 and I also replaced electrolytic caps C41 (100uF) and C23 (22uF) in case they were starting to fail.

Measuring the voltages I still get similar results so TR9 and the caps do not seem to be the cause of the low voltage on the 12V supply.

I measured

• -12.34 (-12V)
•  10.98 (+12V)
• -5.03 (-5V)
• 4.96V (+5V)

I posted on world of spectrum and it was suggested I check resistors R54(1K) and R55(1K8) which form the sense/feedback resistor network that drives TR8. Both resistors checked out okay and I noticed that the two resistors form a voltage divider so I decided to experiment. Replacing R55 with a 2K2 resistor results in 12.3V on the +12V supply. This does not fix the display problem but I have decided to leave it in for now.

Finally, I have decided to order a replacement TEA2000 to see if the IC is faulty. 

Repairing a ZX Spectrum 128K +2 (serial no U-043583) - Part 3

In part 2, I used a CART to HDMI adaptor as my LG TV does not like the composite video output from my spectrum 128K +2 but now I get a ghost image effect which is not present on the RGB output.

Composite video display with shadow

I got the oscilloscope out and started probing the composite video circuitry.

Spectrum 128 2+ composite video circuit

TEA2000

The TEA2000 is the IC in the spectrum that takes the RGB output from the ULA and generates the composite video output. Let's check it out.

The TEA2000 requires a 12V input on pin 11. I measured a fairly steady 10V which is a bit below 10.8V in the minimum spec in the datasheet. I do not think this is the cause of the ghost image so I will come back to look at the 12V supply later.

TEA2000 pin 11 12V input - 10V measured

 
The ramp input on pin 15 looks okay to me.

TEA2000 pin 15 ramp input

I measured the output on pin 6 which looked in spec but I am not sure if this is what the signal is supposed to look like or not.

TEA2000 pin 6 composite video output

I measured resistors R32 and R31 in-situ and they both measured the correct resistance according to the circuit diagram. Diodes D1 to D11 also seem okay when measured in situ and C3 is not shorted.

I can't see any obvious fault so will need to keep looking.

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

In part 1 I fixed the manufacturing fault which resulted in no output on the composite video but the resultant image was terrible.

Spectrum composite video on LG 32LB650V

The screen is covered in dots which do not move and seem to only appear on alternate lines. I do not think these dots are caused by a fault reading from memory as they also appear on the spectrum's border which is not read from memory.

I tried a SCART to HDMI convertor and the dots are gone so it turns out my LG TV (model 32LB650V) does not like the composite video from the spectrum. Maybe it is confused by the progressive PAL signal?

SCART to HDMI adapter ghost image

The dots and lines are gone but now there is a ghost image on the composite video.

Composite to SCART adapte

At this point, I tried another spectrum +2 using the SCART to HDMI adapter and got a perfect image so this ghost image must be due to a fault with the spectrum.

I lashed up a temporary RGB cable and tried that via the SCART to HDMI adapter and the ghost image was not present to I think the problem is something to do with the composite video circutry. Part 3....

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.....