Display Problem (vertical lines)

I saw your thread and the pictures. I just thoght you resolved the problem since the thread died. It happened to mine on a syncing procedure. Emplod balked, so i restarted them both, and when i did, lines. I tried re-flashing... etc, but no luck. Do you have any idea what on the board is doing it? Is it possibly a cable failure?
Thanks
Hans

Thanks for the warning... this sounds like an offshoot of the low voltage at the display problem that I found on mine awhile back. I've been dealing with mine. I am at the 45-48V range (which is low) and I have one area which is a line of solid pixels vertically. Only one area. I thought I'd screwed up the display but was waiting to change it. Now I know that won't solve the problem.

What I also found was that sometimes when on AC power, when I plug in the Empeg, the screen is almost not visible as the votage dropped so low. But an unplug and plug in makes it back to full (well full for 48V) brightness. It appears to be either capacitor or voltage regulator dependent but I've given up on solving it.

Does anyone have an idea what capacitor could fail that would cause this sort of issue? And could it possibly be that little coil thing near the top that got messed up?

Yours sounds like a different problem.

On mine, when I swapped display boards, the problem stayed with the board, but when I let the good one stay on the original empeg for a while, it started having the problem, too.

Also, ISTR testing the voltage and finding that it was fine.

Does the problem I describe seem to be related to coil issues you've seen?

There is a 100nF capacitor in the small high-voltage power supply on the display board. If it fails, the 60V line fails to regulate properly and either you get low, or unstable voltage. I am not honestly sure which component this is, or where.

Alternatively, if there is an internal failure in the VFD, then you can also see low voltage at the test point TP1 on the display.

If the coil breaks a contact, the circuit fails to work completely.

If one of the two electrolytic capacitors on the back of the display fails, then you can also get low voltage.

How would you recomend testing for these failures. Is it your opinion that the display could still be good if given the propper supply?
Thanks
Hans

It's not guaranteed: I always use this face when I see a complaint of 46V plus banding in the display, since it's a bit chicken and egg. The display may be pulling the voltage down (it's duff - expensive repair) or the supply might have gone bad, to which the display responds by doing tricks like this (cheap repair).

Generally, if the display goes dimmer without bands when the voltage drops, then the glass is still good. As soon as I hear "bands", I reach for the soldering iron and wire clippers.....

What happens if you turn down the brightness with the dimmer? Does it stop banding as the brightness is reduced?

There's no reason why low voltage would show banding unless one bank of pixel drivers - which are embedded in the glass - is dead or dying. In that case, as RobS says, turning down the brightness can sometimes restore a full picture, as the anode drivers can cope with the lower voltage in their dying state.

The bands are because the pixel drivers built into the glass do alternate column pairs (column 0 & column 127 are paired, then 1&2, 3&4... 125&126). Each pair of columns are driven by a set of 64 anode drivers, one for each pixel in that set. Both anode drivers get the same data shifted into them for every line, but the blanking signals turn the set you're not refreshing off - this then reverses for the next pair of columns.

All the pixels are driven by the same pixel data line from the main board, so if you have any display then the data is ok. By the same token, the frame sync line must be ok as must the line sync - otherwide you'd see gibberish.

The only thing I can think of - if it isn't the driver in the glass that has died - is that the blanking select pins aren't being driven correctly. This comes out of the display PIC (pin 6, IC4), goes through the PEEL (coming out as two non-overlapping signals, BK1 and BK2 on pins 12 & 13 of IC1) on the display board, and then into the display on pins 4 & 23.

These signals should look about the same (on a 1 ch scope) and like two signals which are never low at the same time if you look at them together. If one of them is high all the time, then that's why you're getting stripes. I think. ISTR that Stu at Eutronix saw something like this but the signals looked ok.

Hugo

I don't think turning the display brightness down will prevent any actual failures of the display components.

What turning the brightness down WILL do is reduce the amount of phosphor burn, if that's a concern to you.

This is an update to the problem of zebra striping described in this thread. We also had a display doing exactly this. It turned out to be the display PIC on the display board that caused this. Rob S. replaced the display PIC and all was well. We have plenty of the display PICs now so we can repair these ourselves.

Thanks Hugo and Rob S!
Stu

FAQed.

Thanks so much for being such a great source of repair information and services. And thanks Hugo and Rob S for continuing to help us out with our favorite toy.

Do you think the pic could be the cause of my display problem as well? One bright stripe down the right side that comes and goes as well as the whole display dimmed. It will change if I lower the voltage via hijack.....I would hate to buy a VFD and not solve the problem.

It's certainly a possibility, I'd say. I highly recommend you send it to Stu or Rob, so that way they can trace it down to either the PIC or the VFD and replace the correct component.