Fluke 6080a Display Problem

[Daniel787]

Of the two display chips, the Frequency read out side will not display properly in either of its two fields.
I am sure the numbers are correct, but just not readable.
Something seems to be wrong with Frequency display board (A1 display PCA), Amplitude display works fine.
If it were something in the latches or buffers, I would think that only a partial frailer of the frequency display would be seen.
As it is, the whole display is affected.
I am thinking of checking the filament voltages between the two displays?
I was wondering if anyone can help with the problem, or recommend a parts supplier.
Attached is a picture of the display, depicting the problem.
I would appreciate any help.


Added picture of the unit opened up.
I think if I can’t get a replacement board lined up, It would be nuts to dig into this.
The unit works fine – you punch in the numbers and you know what the display says any way.
At 4.7k to replace you have to judge how far you’re willing to go.

 

[kpatz]

Is the Modulation display part of the same VFD as Frequency, or is it separate?

My first thought was the G1-G9 which I assume select each digit in the display while multiplexing, but if it were that the amplitude display would be affected as well.

Do you have a scope? Try tracing the signals from U2 and U3 to the Pxx pins on the display. Also see if the clock pin on U2 and U3 have the correct signal on them.

Checking the filament voltages can't hurt, and checking the anode* (Pxx) and grid* (Gxx) pin voltages too, compare them to those on the amplitude display.

(* I could have the terms incorrect here, VFDs aren't something I've really done much with).

 

[Daniel787]

Your help is welcome

Yes the mod. Disp. part Is in the same VFD. because the problem is throughout the display seem to be a tell.
You wouldn't believe how well built the unit is. It's a major operation to get at the front of the display board.
If I do get to it, I'm not sure I can power it up.
Although you can see the complete back of board from the inside.

You gave me something to think about. I wondering about PS1 and PS2. I know it's not RS1 because it's common with amp. Dsp.
Will upload more pictures tomorrow.
Thanks

 

[kpatz]

Your pic of the schematic doesn't show where PS1 and PS2 go, and I don't know offhand what purpose they serve inside the display.

Normally VFDs have 2 filament leads (cathode), grids for each digit (Gxx), and plates/anodes for each segment (Pxx), similar to a triode vacuum tube. When both the grid and the anode is positive relative to the filament, that anode lights up. When the grid is negative relative to the filament, the digit behind the grid is turned off, and any anodes that are negative don't light either. Segments are lit via the anodes and digit selection for multiplexing is done through the grids.

Based on how they're wired in the schematic, it appears that the freq/mod display is treated as 2 displays with 9 grids instead of a single display with 18. So maybe PS1 and PS2 are secondary grids in each half of the display? Without a datasheet for the VFD there's no way to know.

Also, do all the segments light up uniformly and constantly, or do they flicker as the displayed digits "try to" change? I wonder, if you can get the entire display to show zeros, if the middle segments will go out.

I'd probably pursue PS1 and PS2 since they seem to be unique to that display and not the amplitude one. All the usual causes of this issue (grid drivers, plate drivers, and filament bias) would affect the amplitude display as well. Well, maybe not for the plate drivers, since the amplitude display has its own set. So, I'd concentrate on the Pxx lines (including PS1 and PS2) as the potential source of the problem.

 

[Daniel787]

PS1 & 2 and RS1 are generated by the 3 flip flops to the right on schematic.
Not shown is the combination of the three signals into bus shown at the bottom of the flip flops.
PS1 &2 go to the display in question.
My thinking hear is, what you are saying about (Gxx) possibly = PS1 & 2. This would fit with both display portions having trouble.
All the segments light up uniformly it activated.
When a feature is activated its lighted at low level when making a change in the field the display item goes bright.
The bright light is done by 3 extra refresh cycles for that particular item that’s being changed.

 

[Daniel787]

I had what I think is break through The input and output signals of the driver for signal PS1 do not match.
UDN6118 is a special VFD driver chip.
I still need to check PS2 & RS1.
There were no signals on the voltage VDF with what looks like a shelf in the signal.
Wave forms shown are from pins 1 and 18 respectively of the PS1 driver.
This new scope is crazy cool, been using and old analog tek.

 

[Pyramid]

Daniel-

Did you ever find a solution to this problem. I have a 6080 with exactly the same issue. I haven't investigated PS1 or PS2 inputs to the display as they seem to be for blanking the display for protection in case the micro stops functioning.

Any help would be greatly appreciated.

 

[shrtrnd]

I'm not Daniel but the problem with these Vacuum Flourescent Displays is usually the Display Driver IC's or the Displays themselves.
What exactly is your problem? Are you saying you have a display, but that it blanks when you input a signal?
That would be a different problem that maybe somebody here can figure-out.

 

[Pyramid]

Thanks for the response. I may have found the problem. My freq/mod display looks the same as Daniel's Img_0153.jpg above with all of the segments illuminated. As Daniel said, the PCB is difficult to work on live; I removed it from the front panel and removed the front panel by disconnecting the BNC connectors so that I could access the front of the PCB while it is powered. It looks like U3 (74HCT273 latch) is not working as it outputs a nearly constant high on all pins except for a very narrow negative going pulse at the refresh rate. The other latches have variable width hi/lo outputs that correspond with either the display data or the grid strobing. The UDN6118 drivers seem to be working OK. I have parts on order and should have them by the weekend.

 

[shrtrnd]

OK. Hope you found the problem.
Just for general information:
I know you found Daniel's post to ask about the problem he had, but I haven't seen him on here for a long time.
Usually, the Moderators on this forum want you to start a new post with YOUR problem, maybe referencing this
old post from Daniel for reference.
When I've done it, people sometimes jump me for 'hijacking' somebody else's post.
Glad to see you on Electronics Point. Hope to see more from you on this site.

 

[Andrew Towler]

Of the two display chips, the Frequency read out side will not display properly in either of its two fields.
I am sure the numbers are correct, but just not readable.
Something seems to be wrong with Frequency display board (A1 display PCA), Amplitude display works fine.
If it were something in the latches or buffers, I would think that only a partial frailer of the frequency display would be seen.
As it is, the whole display is affected.
I am thinking of checking the filament voltages between the two displays?
I was wondering if anyone can help with the problem, or recommend a parts supplier.
Attached is a picture of the display, depicting the problem.
I would appreciate any help.


Added picture of the unit opened up.
I think if I can’t get a replacement board lined up, It would be nuts to dig into this.
The unit works fine – you punch in the numbers and you know what the display says any way.
At 4.7k to replace you have to judge how far you’re willing to go.

Daniel,

This may be rude of me to ask, but I have a Fluke 6080A/AN as well. However, my problem seems to be with the coarse loop being unable to lock. I notice you have schematics; were you able to obtain these online? I have been unable to find any schematics online and I'm becoming increasingly frustrated attempting to repair such a complicated unit without schematics. Could you possibly share where you obtained them?

Thanks,
Andrew