fixing a dead Amstrad PCW9512+

I'm on a bit of a "i wish I had a PCW as a kid" kick. So yeah, I have a working out of the box PCW8256 that has been upgraded to a PCW8512 (but with only one floppy drive for now. Stay tuned.) I got lucky; that one was in the US and didn't need any modification to work here. (And, amusingly, although it's setup for US power - and shipped with locAscript instead of locOscript, it's still configured for 50Hz UK and 256 line display!)

But yes, finding PCW's in the US - well, any Amstrad really - is super difficult. So when I saw a PCW9512+ for sale by someone in Florida, I jumped at it.

... unfortunately although the seller was listed in Florida, it came from Cairo! So would it survive the trip? who knows. It was sold as "didn't power on", the internal photos didn't show any evidence that the tube was cracked, so .. maybe I'd get lucky.

Coming from Cairo meant that it would be wired for 240v power, and indeed it was. So, up on the bench it went. And yes, it was very dead. So, I pulled out the boards and started testing the switch mode power supply.

First - start by discharging all the AC side capacitors and EHT side capacitors - and the tube, just in case! - and then start checking components from the AC line input all the way to the big switching coil. If you're lucky, it'll be something stupid like the fuse. If you're unlucky, the coil is open or the switching transistor is dead. (Or in the case of the PCW, a big honking integrated switchmode controller hybrid IC which is unobtanium in 2024.)

Here's what I started on:

This covers the AC input, EMI filtering, switching and AC -> DC rectification. The output of that rectification from 240v AC RMS is around 330v DC peak. So be careful, that stuff will definitely kill you dead!

Basic continuity tests of each of those components in circuit didn't show anything obviously wrong. But since there's a rectifier there, it's not always obvious that something's bad. So it's best to lift the parts out to test. So I did that, and .. oh look. R5001 is very, very dead.

I spoke with Jaz (https://mstdn.social/@coregaze) about it a bit to figure out why it's there. They pointed out it's very likely a damper resistor to stop any ringing that may happen between the filter coil on the left side, the capacitors in the circuit around it and the diodes switching on and off. It likely would get hot in normal use, so the fun question was - did it die because the whole unit was just running for too long in a hot room, or is something else dead / shorted and the power supply was being over-taxed?

In any case I wanted to test the rest of the power supply with that fixed, but I didn't (yet) want to light up the monitor circuitry. If there's a problem with the monitor circuitry - eg a shorted power transistor or hybrid IC - I didn't want to risk further power supply damage. So, back to the circuit diagram I go to see how the monitor side is powered up.

On the power supply side, there is a separate 12v DC supply to the monitor, called B+:

And on the monitor side, there's a few places it branches off of that B+ net:

Ok, so off to the layout diagram!

Now, see the track there with W2? That goes off to the horizontal deflection circuitry and flyback input to feed the tube EHT.  The rest of that W2 track/net is all of the other monitor circuitry. So how's it hook up into the Q5003/VR5002/C5026 net? The actual 12v monitor B+ line?

The answer - a solder blob! Here's how it looks after I was Very Intentional about cleaning around it after desoldering it.

So, with that blob removed, the 12v B+ output from the power supply doesn't feed the B+ input into the monitor, so none of the CRT related hardware is energised!

Well then! Off to test the rest of the power supply circuitry I go!

(Stay tuned for part 2.)