Spectrum +3 to VGA

[1024MAK]

R34 AND R41 work together as a voltage divider, one end of R41 connects to 0V/GND. The output coming from the junction of the two resistors. So if you lift one end of R41, the voltage at the junction will rise to a higher level, as it will no longer be 'pulled down' towards 0V.

The +12V pins on the RGB/Monitor connector ARE at +12V if no, or very little current is being drawn. But there is a 1kΩ resistor (R44) in series, so as more current flows, the voltage reduces.

So if you connect a load draws 5mA, 5V will be dropped across the 1kΩ resistor leaving 7V on the pin.
For a load that draws 10mA, the voltage on the pin will drop to 2V.

I recommend that you replace R44 with a 27Ω resistor or wire a 27Ω resistor in parallel.

Mark

[Magic Knight]

So, I got my +3 a few days ago. Having installed a new drive belt, I was able to plug it in and get it working. Although I couldn't see anything, I was able to hear music playing, so I knew that it was ok. (The sound was even worse than I expected.)

Today, having got together all the parts I needed, I built an adapter for my monitor using the LM1881N. For a few seconds, it looked good. But the menu never came up, I just got the copyright message. And then... fizz... and no more display, no red LED, nothing but a whirring drive. I don't know what happened.

I opened up the machine but could see no damage. I then disconnected the drive and tried a +2A PSU. This was ok - the machine worked again. It looks like the +3 PSU is dead. I couldn't get sync the second time, so I tried another LM1881N, but still no success. Then, the +3 died again. Now it won't come on at all. I don't know if the +2A PSU has died too (I bought it in Japan and have no idea of whether it gave any trouble before) or if the Spectrum's PCB is damaged.

At this time, nearing noon on Christmas Day (I'm working from home), I'm not sure if I should look to get it repaired or just sell it as junk. At least I didn't damage the monitor.

[MrClump]

Have you checked the output voltages on the various PSU pins? Blown fuse?

[Magic Knight]

Have you checked the output voltages on the various PSU pins? Blown fuse?

Yes, I decided to do that yesterday after some thought, and it does look like at least one blown fuse. Since I don't know exactly how the PSUs work, I'll humbly ask you folks what you think:

The +3 PSU gives no voltage reading on the +5V or +12V pins. It is outputting -12V correctly.

The +2A PSU gives 1.7V on the +5V pins, 4V on the +12V pins, and the -12V pin is outputting the correct voltage.

I was planning to get one of those newly manufactured PSUs anyway, and I'll be happy if the +3 is working; at this stage I'm not really bothered about getting a display if I'm going to have more PSU problems. Getting a picture on my monitor was more of a challenge than anything.

As it happens, the PSU somehow ended up breaking the keyboard. One of the keys was smashed off, and I couldn't superglue it back in. So I need a new base for the keyboard (the plastic piece into which holds the keys). It's a real pity because I bought it just over twenty years ago and it's the first time I've ever loaded a disk!

[1024MAK]

In the Amstrad made +2 (for the +2A and +2B machine’s) and the +3 PSUs, a short circuit or overload (too high a current) will cause transistor T1 to fail and self destruct. This sometimes also takes out transistor T2.

Fuse F1 does not always blow. If you have lost the +12V supply, fuse F2 may have blown. Or as T1 and T2 are supplied by the +12V rail, if these are short circuit, this can affect the +12V rail.

In both cases, the -12V supply uses a separate voltage stabiliser circuit, hence it being unaffected (although it shares the transformer and some of the other circuitry).

For both PSUs to have failed, it’s possible that the +3 has a fault that is causing excessive current consumption. So I DO NOT recommend connecting it to a high current PSU like a PC PSU.

Unfortunately despite these PSUs having fuses, they can’t cope with overloads and short circuits without being damaged. And there is no simple alteration that can add this protection.

If you can solder and desolder, repair of these PSUs is not too difficult.

I can’t advise on the ‘newly manufactured PSU’ unless you provide more details.

If I was investigating this problem with these PSUs and the +3, I would use a bench PSU that has an adjustable current limit to supply the +3 to test it.

Mark

[Magic Knight]

Thanks for the explanation.

I don't have a bench PSU with an adjustable current limit, and although I can solder, I expect anything I do is only likely the make it worse.

I checked the transistors in circuit and on TR2 I'm getting 0V both ways between the base and collector. I suppose I could send away the PCB for repair, and get the sound modification done at the same time.

[1024MAK]

In the Amstrad made +2 (for the +2A and +2B machine’s) and the +3 PSUs, a short circuit or overload (too high a current) will cause transistor T1 to fail and self destruct. This sometimes also takes out transistor T2.

Small correction, it’s normally T2 (a 2N2222) that dies, with sometimes T1 dying.

Mark