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

Oops, yes -11V. I still can't read schematics, isn't that linked to the -5V?

@Mark: which capacitor is C51?

Yes, see the page I referred you too

The zener diode sets the voltage from there to -5

gunrock wrote: ↑21 Mar 2021 00:06 @Mark: which capacitor is C51?

The large 4700uF one that hangs over the right hand edge of the board. Mind, if the sleeving is well fitted, you may have difficulty getting a good connection. In which case very carefully test on the left hand leg of the 7812 voltage regulator, which is on the left of the heatsink. But be VERY careful that your meter probe does not touch any part of the heatsink.

You can use any convenient 0V/GND connection, such as the modulator case.

Mark

R60 is in series between an unregulated -V supply (typically around -11V) and the 5.1V zener diode (ZD3), and limits the current to the zener so that it can do it’s job without overheating. This design is called a ‘shunt regulator’.

If you want help with reading and understanding schematic/circuit diagrams, I’m happy to help.

Mark

Can confirm that the video file you posted is repeatable?

At power on, do you always get that flashing cursor with no “Ready” text?

(Apart from the screen colours, that is the same as you'd get if you hit the <space> bar after power on, i.e., the "Ready" would clear and the system would display the text being entered.)

If there was a voltage issue, then I think it unlikely that you'd get that flashing cursor.

Okay, thanks to you all for replying.

@Dave, the ready prompt is displayed, it's just in that gif, I had leant forward to push in the composite cable (ensuring it wasn't stupidity and something simple) and my ample frame (my fat gut), had pressed the space bar!

@Mark. As for C51, it's AC right? It reads 20.07V at AC.

Other observations: ram chips seem luke warm, CPU and CTC are cooler than that. The VDP is hot, 5 or 6 seconds touching it (after the machine has been on for 20 mins) is bearable but reminiscent of a hot dashboard of a car.

Hi Steve,

Yes, that makes sense. I think it’s more than likely “just” a video problem. (But can you double check that 12v reading, it is used by the video board, so could be a problem with the 7812.)

Have a read of this page http://www.primrosebank.net/computers/ ... _board.htm

I would suggest pinning the video board for mono to see if you can get a stable picture by eliminating the colour video components and we can take it from there

(The VDP runs pretty hot in normal use)

gunrock wrote: ↑21 Mar 2021 01:25 @Dave, the ready prompt is displayed, it's just in that gif, I had leant forward to push in the composite cable (ensuring it wasn't stupidity and something simple) and my ample frame (my fat gut), had pressed the space bar!

Gives a whole new meaning to PEBKAC

gunrock wrote: ↑21 Mar 2021 01:25@Mark. As for C51, it's AC right? It reads 20.07V at AC.

No, it’s mostly supposed to be DC, but with some AC ripple. But NOT 20.07V of AC ripple. Note that cheap (here cheap means NOT expensive) multimeters often don’t give a true AC value if there is a DC voltage present. So keep that in mind. If there is 20.07V of AC ripple present, then it’s possible that C51 has either died (gone open circuit or lost a lot of capacity), or one of the solder joints has become dry jointed.

Visit Dave’s MTX Power Supply Details page, then find the heading called “+12 Volts DC” and read on from there.

gunrock wrote: ↑21 Mar 2021 01:25 Other observations: ram chips seem luke warm, CPU and CTC are cooler than that. The VDP is hot, 5 or 6 seconds touching it (after the machine has been on for 20 mins) is bearable but reminiscent of a hot dashboard of a car.

That’s all fairly normal.

The +12V being low means that (1) the 4116 DRAM chips are no longer in their happy normal zone, and (2), the colour encoder chip on the PAL Video daughter board is very unhappy, hence the loss of colour.

The good news, is that all the main logic systems including the Z80A, the CTC, the ROMs and the main RAM are all happy, because they only need the still present +5V supply.

Mark

Steve,

Just in case you were wondering . . . .

CFX does not connect to the +12v line so won’t have caused the issue.

I suppose there may be an issue if you install CFX incorrectly, I.e., with the connector offset from its proper position, but that should not be possible with the key way inserted in position 5 of the connector. Besides, CFX wouldn’t have worked and more things would likely be broken

(FYI, the +5v, +12v and 0v lines are on adjacent fingers on the edge connector)

Hi Dave,

Thanks, I'm pretty strong and no electronics expert but not even I could force the CFX onto the edge connector the wrong way!

Though you and Mark offer something for thought, I did move C51 a little to put the CFX-II in, so perhaps I damaged its joints or stressed it, in some way.

I'll recheck the output in DC tomorrow.