Display smeared

Posted by: presslab

Display smeared - 16/06/2011 04:31

I picked up a used Mk2a and it has a problem. When I first powered up the unit the display looked fine, but after warming up for a half hour or so, the display is smeared side to side. And now, it is completely blank with nothing on it at all.

I checked the suspect fuse on the main board and it is fine. I even took the display board from the my other empeg and it also works fine. So it's something on the display itself.

I suspect that there is something wrong with the PIC chips. Any ideas on where I should start looking with the scope?
Posted by: tanstaafl.

Re: Display smeared - 16/06/2011 13:00

Originally Posted By: presslab
I checked the suspect fuse on the main board and it is fine. I even took the display board from the my other empeg and it also works fine. So it's something on the display itself.
If you already looked at fuses, it probably isn't this, but on the off chance...

tansaafl.
Posted by: presslab

Re: Display smeared - 16/06/2011 14:13

Thanks. Yeah I looked at that page, and the fuse tests fine and my other display board works in this empeg. The VFD looked different than that, it was smeared side to side, I guess that's how I would describe it.

I measured the TP1 voltage, which looks like it should be 68V, and I'm getting 117V... The capacitor is only rated to 100V, so who knows what's gone bad even after I fix the supply. smirk

I'm hoping someone has some test point I can probe to verify that the PIC is at least working, or the GAL, or something.
Posted by: presslab

Re: Display smeared - 17/06/2011 15:58

Well, I fixed the power supply. But it looks like the VFD is smoked. I can see the serial clock signals, the filament is ok (I see it glow), and the voltage is ok now. But nothing.
Posted by: presslab

Re: Display smeared - 20/06/2011 14:44

This looks like the same VFD glass:

http://www.noritake-itron.com/Specs/GU800/GU128X32-800B-02.pdf
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=286-1059-ND

However the vacuum nipple is in the wrong spot. Has anyone used this VFD glass? I think I can just move the VFD over a bit and it should work out okay.
Posted by: presslab

Re: Display smeared - 28/06/2011 01:48

Thanks to a member here, I procured a replacement VFD glass. It works now!

The problem seems to be the capacitor mod to fix the glitchy dimming. I put on a 0.01 uF cap (maybe the wrong value?) and it obliterated the CS signal to the pot. This in turn cranked up the voltage to 117V which fried the VFD. frown

So instead of the cap I decided to fix the source of the problem and I put a 820 ohm resistor on the one-shot circuit. This is a resistor under the VFD; it used to be a 620 ohm if I recall. No more dimmer problems and the voltage is good now. This empeg will live on!
Posted by: mlord

Re: Display smeared - 28/06/2011 10:57

The dimming cap fix requires a 10uF 10nF capacitor.

Here's the nearly 10-year old original posting from Hugo:

Originally Posted By: altman
Re: Dimmer prob on my backup RIO (22/02/2002 05:37)

I've been looking at this on the scope and *may* have found the issue.

There is no separate chip-select line to the AD8400 dimmer chip, as we ran out of pins on the PIC. So, the CS is driven by an unused monostable from the clock line of the dimmer - when there is a clock, CS goes low and when the clock is absent for a while it goes high again.

There appears to be an issue that the R/C timing circuit for this is a bit close to the edge - ie, if the PIC is running slightly slow there will be a phantom rising edge on CS halfway through the dimmer setting byte - this is only ~20ns wide, but the dimmer chip is very fast and catches it.

The solution appears to be either to change the R/C circuit (hard, as it's under the display) or to add capacitance to the CS line to hide the glitch. On *most* mk2a players, the single chip on the back of the display board is a DIP 8400 (we couldn't source SMT ones for love nor money - they were used in some mobile phones, allegedly).

Anyway, if someone with this problem, a 10nF capacitor (SMT is ideal, very neat) and electronics knowledge/a soldering iron can try fitting it between pins 2 & 3 (ground and CS) of the AD8400 - ie, the middle 2 pins of the chip closest to the top of the display board, and see if this helps them. The one unit we've had returned which would repeatedly show this problem was fixed by this capacitor addition.

We're awaiting some more confirmation that this does address the issue before we do any warranty work.

Hugo
Posted by: presslab

Re: Display smeared - 28/06/2011 14:00

Originally Posted By: mlord
The dimming cap fix requires a 10uF 10nF capacitor.


Ah, so I had it right then. 10 nF == 0.01 uF.