zx spectrum 128K +2 (grey, issue 1) rgb problem

[cerror]

Hello everybody!

I have a problem with my Spectrum 128K +2 (grey, issue 1, board Z70700 (3886)).
I get a greatly distorted image with RF, RGB->scart and a black screen with a RGB->composit cable (coolnovelties.co.uk 128+2 grey model cables). I recorded the image of the screen via the RGB->scart to USB capture device.

Untitled 4.jpg

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As you can see, the computer starts and the familiar welcoming screen is there. It's just majorly distorted.
I know there is a problem with the 128K +2 models (some?) that have to do with the TR4 transistor, wich is the wrong way around.
Though I read, through horrible translation, some russian chatter, that the transistor problem is also on this revision.
According to some docs it says the issue 1 doesn't have the problem, and if it is the problem, the transistor from the pictures is not in the same location on my board.

I am pretty useless when it comes to electronics on a deep level like schematics etc. and I am worried that if I flip the TR4 transistor on my board I might break the whole thing.
Also, my multimeter can't handle the low voltages so I can't measure the 12v on the TEA2000 to make sure it is the problem :/

Please see pictures for board version and the problem.
I really hope someone can help me :'(

Here are some pictures of the board:

15541388_10154615552317911_425918250066663022_n.jpg (77.28 KiB) Viewed 7151 times

15622770_10154615547217911_7538297815177525341_n.jpg (100.93 KiB) Viewed 7151 times

[balford]

That's almost definitely TR8 and TR9 that have failed, since you get the same results over RF as well as RGB.

On the Z70700 Issue 1 boards, TR4 and other transistors are the correct way around, although you do have to relocate C31 to remove the audio mix (this is only a problem if you're using composite from pin 2 of the RGB socket, however). Details are here: http://www.benophetinternet.nl/hobby/va ... cables.pdf.

You can get replacements for TR8 (ZTX213, but a ZTX751 is more readily available) and TR9 (ZTX650, again ZTX651/653 are more common) here:

http://store.retroleum.co.uk/zx-spectrum-transistors

Lastly, if doing the soldering yourself, be very careful as the Amstrad PCB's don't tolerate heat very well. Snip the old parts off and desolder each leg one by one - don't use more than 300C if using a temperature controlled iron.

B

[1024MAK]

A multimeter that can't measure low voltages? Is it a clamp meter? Only every multimeter that I have used can measure low voltage DC.

Mark

[cerror]

Thank you!
I've ordered the same transistors via a local electronics shop, since shipping from the UK will take a while.
Will try it out, and I see that TR 8 has a little bit of it off. So big chance that is the problem.

[cerror]

A multimeter that can't measure low voltages? Is it a clamp meter? Only every multimeter that I have used can measure low voltage DC.

Mark

I have a pretty crap one (velleman DVM831) and I can't get it to work the way I want it.
Apparently it's not suitable for low-voltage distribution boards it says in the manual. I tried the transistor mode on it but it won't give any reading on any of the resistors so I guess the damn thing is either broken or not suitable.

[balford]

Must be broken, looks from the product description that it should do the job.

Just to check, are the probes plugged into the correct sockets (middle and bottom for voltage measurements)?

B

[1024MAK]

TR8 and TR9 along with the coil (transformer), and various resistors, capacitors and diodes form the DC-DC converter (also known as an inverter). It is this circuit that generates the +12V supply. In turn, the colour encoder and various other parts of the video circuit need this +12V supply.

Mark