VDP output monochrome and strobing

[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!

Not as long as the key is in position - that’s why it’s there

I just thought that I should mention the, albeit ultra-low probability, risk that it could happen.

[gunrock]

Capacitor C51, 9.65V DC which is too low? So I'm assuming either C51 has deteriorated (possibly exacerbated by my small push to make room for the CFX-II) and/or one or more VRAM chips are not in their "happy state"?

[1024MAK]

The official minimum input voltage for a 7812 regulator is 14.5V. This is the supply that C51 is supposed to be smoothing. C51 is connected to the input pin of the 7812. Normally, I would expect between 16V to 18V here.

I’ve not had a short circuit on the +12V line in a MTX, so can’t say for certain, given the unusual way that the power supply circuitry is arranged, if an short circuit on the 12V rail would drag voltage across C51 down to 9.65V DC.

The 7812 itself is rated at 1A maximum. With a maximum short circuit current of 1.2A.

So I’m not entirely convinced that a faulty 4116 DRAM chip would cause these problems. But I do know, that if a 4116 is consuming lots of current, it will get very hot. Are any of the 4116 chips getting very hot (that is, a lot hotter than normal)?

Other possible faults: it could be that diode D14 has failed (1N4004, but actually any 1N4001 to 1N4007 will do), or the electrical connection from the DIN socket J9 pin 1 back to the PSU (transformer) has been lost. Either due to a broken wire, or poor connection.

Mark

[gunrock]

Hi Mark,

Well, given that I built the PSU myself it's likely to be user error on my part. I'll check the din connector as it's easy to do and I wasn't overly pleased with the soldering on that part of the job. The PSU hasn't been particularly used much, as with no storage solution until now, I've probably spent no more than 5 or 6 hours in basic/assembly larks and panel shenanigans.

Thanks.

[gunrock]

Ok, sorry for the delay.

This is somewhat embarrassing, at some point in my testing of the CFX-II, fuse 3 of my 1024MAK PSU had blown (and I didn't notice the LED wasn't lit ). So, I have replaced the fuse and tested again....

Still have the same issues with the display. Tested the C51 capacitor and get 9.55V DC. So I tested the 12V on Pin 1 or the PSU with the black probe on Pin 4 and instead of 16V DC, I get about 10.9V DC.

Switching to AC, i get 23V!!

I tested these outputs before when I built the PSU and definitely got 16V DC. Totally confused, right now.

[Dave]

Were your voltage measurements made while you had the PSU disconnected from the MTX?

Mark will likely help you diagnose the problem with your PSU, but I would guess that the step-up DC converter has failed when you blew the fuse.

[1024MAK]

In the PSU, what DC voltage do you get on fuse 3 to 0V/GND (use any convenient point). Or test across the input terminals of the DC-DC converter (red wire, black wire in my schematic).

Then test the output of the DC-DC converter (yellow or orange wire) again to any convenient 0V/GND point. What DC voltage do you get here?

Mark

[gunrock]

From F3 to GND, I get 11.3V DC.

From yellow wire on step up transformer, 11.0V DC.

Step up transformer is dodgy?

[1024MAK]

So the DC-DC converter has died. Due to the way that they work, if the switching chip is not active or has failed, you will still get an output voltage, but it will only be a value that is slightly less than the input voltage. This is due to a diode that connects between the input and the output.

You need a new DC-DC converter.

Mark

[gunrock]

Thanks, actually that will teach me for buying cheap.

When I was sourcing parts, I was looking for an LM2577 part and came across a XL6009 which seemed to be similarly specified. However, having just done some more in-depth googling, seems the XL6009 is a poorly designed load of junk, that is potentially harmful to the kit it's attached to.

This might also explain when after I set the output to 16V and then when actually placing it in the circuit a week or so later, when checking the output it had changed to a lower value. It was stored in the house, in a box, inside my electronics crate (a plastic storage box), so no jolts or friction against other items, could have changed the potentiometer.

So, I've spent a bit more (quite a bit more, with this being the Nordics ) on a LM2587 based converter, which should hopefully be more reliable. Just hope the mounting holes line up broadly with the old unit, otherwise I'll be turning the box into swiss cheese!

Thank you Mark and thanks to all for your assistance, I'll post back once I've swapped them out.