*** M E M O R U M ***

Hi all,

I have just received my CFX-II and after it sitting for two days awaiting installation I managed to have a crack at it last night.

Whilst the CFX appears to work as it should (thanks Dave, Martin and Bill), I thought I would switch the TV to the VDP output to see what was being output and compare with the VDP shadowing.

To my surprise, the VDP output was monochrome, with a slowly strobing (maybe the wrong term?) white band moving over the screen. It was working fine, so I figured I may have damaged something. First, I tried my other TV and where I had colour output there, witnessed here I got a brief monochrome screen, with the same rolling artifacts, then synchronisation is lost and has never returned.

I've opened the machine up and check the grounding strap, continuity checks out. I have removed the CFX to simplify the troubleshooting and the issue remains. Back to the old TV, this is what it looks like:
Any ideas what I've done?

It could be the TIP2955 power transistor. If the 10 ohm resistor at R62 is hot then that's an indication the transistor has failed. If the transistor has failed then all the 5V supply is pulled through the 7805 regulator, and can be more than it's rated for.

See Dave's page on this here:
http://primrosebank.net/computers/mtx/r ... _power.htm

Thanks for replying Martin. I'll check it out. When you say hot, you mean hot enough to (really) burn my fingers? i.e should I use a thermometer or can I touch it?

You can touch it, then . . . . . either :-

1. Rush to the sink and cool your throbbing finger under the cold tap, or

2. It’s slightly warm, so likely OK

Seriously, if it is u/s, it will probably be VERY hot, but be careful and a fleeting touch will tell you that it’s not well

Regards
Dave

Or you could just test the relevant supply voltages, like the +5V, +12V and the -5V.

If any are out of specification, then the power supply circuitry likely has a problem.

Mark

You can test all three voltages on the 4116 video DRAM chips:



A copy of the full data sheet is here

Pin 1 is VBB, which is -5V (-5.5V to -4.5V).
Pin 8 is VDD, which is +12V (+10.8V to +13.2V)
Pin 9 is VCC, which is +5V (+4.75V to +5.25V)
Pin 16 is VSS, which is 0V (ground/GND)

Mark

Well, I looked at that page on the R62 and R60 resistors. R62 seemed ok, the voltage measured at 0.67V.

R60 was 11.07V which suggests we have an issue!

Are you sure it wasn’t -11?

http://www.primrosebank.net/computers/ ... mtxpsu.htm

Checking on the vram, all is good except the 12v, which reads around 8v.

What do you measure across capacitor C51?

Mark