Fixing the Flickering Screen Problem I had when Using a SCART lead on the ZX Spectrum +2

 

This is part 2 of my restoration series of my £5 ZX Spectrum +2. However this was an unexpected modification; I tried out my new scart cable for the +2 after fitting and testing my TZX Duino amplifier. The amp worked flawlessly but the picture quality wasn’t the best on my +2. Well, the picture was ok in truth but I kept on getting a flickering of the screen which was even worse during gameplay.

 

After a bit of reading, I found that my version of the +2 had some transistors put in the wrong way round during manufacture. Surely this was a good place to start? However after replacing them with new ones and in the correct orientation, the picture quality got even worse!

 

I double checked my soldering and continuity between components and everything was good so it was back to the internet for some clues.

 

I stumbled across this page on spectrumforeveryone.com

 

https://spectrumforeveryone.com/technical/making-your-12822a3-rgb-socket-scart-compliant/

 

and it looked simple enough; in fact, I think that the flickering screen may have actually been cured by this mod had I done it first.

 

This entry will take you through the complete process of replacing the transistors to doing the modification found on Spectrumforeveryone.com

 

Now let’s get started!

 

Firstly I will show you some pictures of the original flickering screen problem.

 

Whilst loading Monty on the Run I noticed this.

 

Below is an image of the stable picture before the flicker.

 And below is an image of the flicker

Then it returned to normal

The flicker only lasted about half a second but the +2 kept on doing it during the loading sequence and was even worse during gameplay when there were actual moving pictures on the screen.

I had heard that there were some grey +2’s which had TR4, TR5 and TR7 inserted in the wrong orientation during manufacture so I thought that it would be a good place to start.

 

Below is a close up of the schematic showing TR4.

 

As you can see from the image above, the emitter of the 2N3904 is connected to a 75 Ω resistor and the collector to +12V and the positive of C6

Here is a pinout of the 2N3904

I decided to test this out with my multimeter

The multimeter buzzed meaning that the emitter was indeed connected to the +ve of C6 (the wrong way round)

 

I tested TR5 next; here is a close up of the schematic

Here you can see that the emitter is connected to R57 and pin 5 of IC12, time to test with my multimeter.

 

I connected the probes to the collector of TR5 and to R57 and pin 5 of IC12 and again the multimeter buzzed proving that this transistor was in the wrong way round as well.

 

Finally, I tested TR7 in which the emitter is connected to R64

 

The multimeter sounded meaning that the collector was connected to R64 (the wrong way round)

 

Time to dig out my solder sucker!

Firstly let’s have a look at the position of TR4 on the component and solder sides of the PCB.

 

The above image shows that TR4 is located next to the modulator on the component side of the PCB

And below I have highlighted the middle pin (Base) of the 2n3904 on the solder side of the PCB

Time to desolder.

 

Before I attacked the transistor with my soldering iron and solder sucker, I found that a couple of the legs of the transistor were sticking up slightly after the solder joint.

In my experience, they can be hard to remove when like this as the PCB is plated through hole (meaning the solder will fill the via between layers of the PCB) If the solder cant be removed completely between layers then it can be tricky to reflow the solder and gently pull the component out therefore the shorter the length of leg to remove the better.

It will also make it easier to position the solder sucker on to the via.

Time to get my cutters out.

 

Trim the excess legs of TR4

Then desolder; I tried using my new electric desoldering tool for this job  but found that it just isn’t all that good so I reverted back to my trusty 20 year old solder sucker!

Then place the new 2N3904 in the TR4 position in the opposite orientation to the silkscreen printing (White component profile printing on the PCB) Basically, the flat edge of the new transistor should be placed where the curved line is shown on the PCB as shown below.

 

Once placed in the correct orientation, solder the transistor as seen in the picture above.

 

Now let’s have a look for TR5.

 Component side.

Solder side with the middle pin (Base) highlighted.

Time to desolder...

 

I ended up reverting to my ancient solder sucker as again the electric desoldering tool didn’t really work for me.

Below is a picture of the removed transistor.

 

Fit then solder the new 2N3905 in the TR5 position, again ensure that the flat face of the transistor is orientated on the curved side of the image on the silkscreen.

 

Below is a picture of the refitted transistor.

 Finally change TR7, below is a picture of its location on the component side of the PCB

 

Now let’s find it on the solder side; highlighted is the centre pin (Base) of the 2N3904.

As you can hopefully see, there are thin traces very close to the via’s on either side. All the traces leading to other parts of the circuit board are located on the component side.

Great care should be taken when removing this component as it would be easy to break the traces on the solder side with an accidental slip of the soldering iron.

Also, and if using the melt solder and gently pull the component out from the component side technique it is very easy to lift the traces from the component side.

 

To try and help me with the dsoldering process, I added fresh solder to the original solder joints.

Then I attacked it with my solder sucker.

It still wouldn’t budge so I tried the heat (to melt the solder) and gently pull the component out through the component side.

Eventually it came out; the picture below shows the removed transistor.

 

Time to refit and solder a new 2N3904 in the TR7 position. Ensure that it is placed the opposite way round than the silkscreen advises.

Now that the three video transistors have been replaced in their correct orientation it is time to refit the +2 and test. Firstly inspect the PCB for any solder shards left from the desoldering process. Then when happy refit all the parts in the +2 and test the system.

Firstly, refit the main PCB to the bottom half of the +2 using the six small screws.

 

Then refit the voltage regulator.

Carefully refit the two keyboard membrane ribbons

Then refit the cassette PCB connector

Refit the lid

Then refit the six case screws

Set the system up

My first impressions weren’t good; the screen looked grainy and nowhere near as clean as it was before replacing the transistors.

But my troubles worsened, the following pictures show what happened when I tried to load a game.

 

Basically as well as being of a poorer quality signal, I was now experiencing loss of signal!

 

Was it my soldering? Did I damage the PCB when replacing the transistors? I did not know!

 

So I spent the next hour checking and rechecking my soldering and traces on the PCB. I also checked continuity between the relevant components. Everything came back good! Time to do a wee bit more research on the internet!

 

Luckily, I found a page entitled ‘making-your-12822a3-rgb-socket-scart-compliant’ on the spectrumforeveryone website (link below)

https://spectrumforeveryone.com/technical/making-your-12822a3-rgb-socket-scart-compliant/

below is a screenshot of the page in question with the very simple instructions of how to do the modification.

The instructions say to remove LK4 then link LK2 and then replace the 1kΩ resistor in the position R9 with a 680Ω resistor.

I guess, it was time to open up the +2 again!

Firstly I looked for L4

The above photo shows LK4 on the component side (next to the RGB connector and modulator)

The picture below shows the position of LK4 on the solder side of the PCB.

LK2 can be found next to LK4

The above image shows LK2 from the component side

The pictures above show the solder side position of LK2

Desolder LK2 and LK4

Now let’s find the 1k Ω resistor R9

This was very strange! There wasn’t a resistor in the space for R9, another manufacturing mistake? Or is this standard? I really don’t know!

 

I decided to remove the solder in the position for R9 so that I could fit a 680Ω resistor.

 

Firstly I needed to identify which vias I needed to desolder.

 

Desolder the vias

Now we can fit a new link into position 2 as well as fit a 680Ω resistor into R9

But…

I didn’t have any 680Ω resistors in stock!

And

I was in the zone!

I wanted a repair fix today…

Not in however many days it would take to get a 680Ω resistor.

So

I improvised!

I did have some 1k2Ω resistors, so I paralleled them up and tested with a meter, the actual value came out at 604Ω. Hopefully the relatively small difference in value wouldn’t cause me any issues.

Below are some pictures of how I paralleled the two resistors to fit neatly into the circuit.

 

I used the leg of one of the resistors to make a new wire link as the old one was a little buckled and bent after the removal process. Note that I intentionally left one leg longer than the other to make it easier to fit.

Now all I had to do was solder them into the circuit.

Firstly I fitted and soldered LK2.

Then I filled the vias with solder at position LK4.

Next I fitted and soldered my parallel resistor into the position R9

 

Now all I had to do was to refit the +2, set up and test for the second time during this mod.

Follow the instructions for refitting and setting up  the +2 as shown earlier in this entry then test.

Nervous times ahead!

I plugged everything in and powered up the +2…

And…

…

…

Looking good!

The grainy picture was back to a nice clean crisp picture, now lets test it out loading a game…

No sign of the flickering video or loss of signal… I am impressed!

Even Leo ‘The Terrible’ purred his approval!

Time to play some Monty on the Run

Great game, great picture… all let down by my rubbish gameplay!

 

Below is a youtube clip of the whole process from start to finish.

 

Well, this mod should have taken a couple of hours at most but ended up taking most of the day… but… I am not complaining at all as I absolutely love fault finding and researching solutions! This was geek heaven to me!

 

Upon reflection, I think that I would have gotten away with simply doing the modification I found on spectrumforeveryone.com but that wouldn’t have given me the chance to repair the manufacturing error in the process.

 

This was a great wee project and I am planning my next instalment for the +2 as I write this.

 

Many thanks again for taking the time to read through this entry; I sincerely hope it was of some use to you.

 

If you have any questions or queries, then please feel free to leave a comment or send me an email to

 

Alleged_geek@aol.com

 

Best regards,

 

Alleged Geek