I bought a ZX Spectrum +2 gray (toast rack) for my retrocomputer collection and it was the fail. The video output was bad and also there is a serious problem with RAM or ULA. But this article is about making perfect video output without fancy tuning. This article is a bit rude- I was searching the internets for more information and almost all data I found is copy paste of people without any deep analysis. Also, I found that about everything for this computer is deeply commercialized compared to Atari ones. Also, schematics and documentation are shitty.
So here is the starting PCB of ZX Spectrum +2, marked Amstrad 0500 ISS3 (Z70500):
(press on image for bigger)
ULA (with heatsink) is removed for inspection. Metal RF modulator’s cover also is removed. Also, modern polymer electrolytic capacitors are added on 5V and 12V rails. I have noticed, that sometimes “recaping” is done in very stupid way- keeping old strange values of the capacitors. Believe it or not, ancient engineers would place bigger capacitors on these rails if they were not limited by technology, size of the device and the price of the components. No need to place a 16V cap on the 5V line when industry is making 6V caps for this purpose. If it will be 16V on these rails, all your chips will be dead before these caps will blow.
There are several parts for forming a PAL UHF (antenna) signal in this computer: RGBI and SYNC from ULA are sent to the TEA2000 composite video signal generator. Somewhere there must be taps for RGB connector (PERITEL/SCART).
There is PAL (NTSC) output from TEA20000 on pin 6. The levels are bit too high and too weak for normal TV/Monitor, so there is chain of diodes (D4, D8, D7), transistor TR4 working as emitter repeater with 75R output.
All “web pages” are complaining about bad placed transistors (TR1-TR3), reducing noise from the audio circuit and etc.
But there is one important thing- if you will not use RF output (who has so shitty TV?) no need for these transistors. There is a very easy fix for all the problems… We will not use that all crap, so here is finished PCB:
(press on image for bigger)
As we can notice, there is nothing inside RF can. I am using this can as a sturdy video connector- RCA video out is here. There is one resistor placed inside- it is a ~470R bleeding resistor to discharge posible capacitors in the TV set. Also, I do not like floating emitters of TR4. Small green kynar wire is directly connected to the video out- emitter and 75R resistor in the LK4/LK2 jumper.
Here is close up:
I noticed that my particular TEA2000 chip is not very stable at 12V. Maybe it is damaged, or it needs fine tuning. But it is very stable and produces perfect video at +9V or a bit less. Maybe this fix is not suitable for you, but I want to share this information with everybody. Just to reduce voltage in TEA2000, I removed the 10R resistor (R8) and replaced it with 3V3 zener. Now voltage is always 3V lower than 12V rail and my TEA is working fine.
And now the last stuff, that I never seen in webs:
This is a sound subcarrier generator, (FM modulator) MC1376P. We do not have RF output anymore, so we do not need this shit mixed to video output. Audio subcarrier is 6MHz, it is way higher than the video carrier in PAL, but due to bad schematics, it is interferencing with video quality. So remove this chip. It is useless in this configuration. There are lots of components around it, they can be removed, but without a chip they are passive and they are not worth soldering job.
Also, we can lift or remove D3 diode, isolating all old video amplifier for RF modulator. This will give us some theoretical improvement.
Now just plug the video RCA jack of your monitor or TV set to the RF modulator’s socket and enjoy crisp video without fancy DIN connector.
