VFD Display Failure - Voltage Readings Clarification - Page 1
Forum section: electronics / beginners

author: Vonsgarage
author_rank: Contributor
timestamp: September 15, 2023, 10:59:50 pm
content: Good evening.Device (Onkyo TX-SR353 HDMI Stereo AVR) works perfectly with the exception of a 100% dead vacuum fluorescent display. Not dim; dead.Thanks to you guys and the Internet I have basic understanding that if the tube is functional you need three voltage sources to get an segment / element to illuminate.a) Filament: needs to be AC voltage at around 5 volts.b) Grid: needs to be DC voltage at around -30 volts. That is negative in relation to chassis ground.c) Segment: same voltage requirements as the grid.What would you think if a device had all these voltage present and the display was still dark? But adding that the odd thing is the grid and segment voltages are correct at -31VDC but they are all at this voltage! Each and every pin on the VFD is -31VDC. I would think that that would drive the VFD to illuminate every darn segment and element! So even if the VFD Gods dropped a new VFD in my lap, I would still have issues.Am I missing something?

author: macboy
author_rank: Super Contributor
timestamp: September 15, 2023, 11:43:14 pm
content: Look at the front glass of the VFD. In one corner there should be a black bit of stuff about 1/4 inch or 5mm diameter. This is the getter, and it's purpose is to absorb (react with) any small amounts of oxygen that may have gotten into the tube, to prevent it damaging the important bits. If this spot has turned white/grey then the seal has broken and the VFD will never work again.

author: Ian.M
author_rank: Super Contributor
timestamp: September 16, 2023, 02:01:31 am
content: You've got it backwards. The filament is the cathode, the segments are the phosphor coated anode that lights up when electrons strike it hard enough, which happens when *both* the grid over the segment and the segment are sufficientlypositivewith respect to the filament cathode. Significant grid and segment current flows in the on state. The off state (assuming the tube is good and the filament is hot) is with either the grid or the segment or both at the filament cathode DC potential, or preferably slightly negative. Negligible current flows in either in the off state.Due to the MOS IC technologies back in the day it was easier (and required less level shifting) to run the on state segment and grid electrodes at +5V, the logic supply voltage, and achieve the required voltage differential by biassing the filament/cathode around -25V or -30V, i.e. negative with respect to ground. Isolated AC filament drive was used with the negative DC bias superimposed on it.Nowadays its usual to run the filament a bit above ground and the segments and grids 30V or so positive as level shifting high side drivers with a SPI shift register interface are now cheap(ish) and readily available.N.B. 5V AC is far too much filament voltage for many VFDs. The filament should glow slightly when observed in a dark room, but the glow should be almost imperceptible under normal room lighting.If all the pins are at -31V the display is blanked or in standby. I would expect active segments/grids to be pulsing between -31V and +5V.

author: pqass
author_rank: Super Contributor
timestamp: September 16, 2023, 02:02:03 am
content: If you ignore those negative voltages in the schematic, a VFD is relatively simple to drive. Electrons flow from the filament (cathode) through a grid to the anodes. To see them glow, the anodes and one grid must be positive with respect to the filament. Your VFD requires multiplexing since the same anode of each digit is connected and each digit has its own grid; 16 anodes per digit, 12 digits+annunciators (grids). All other grids are off; brought down to the same potential (or lower) as the filament. Then the next digit (grid) is made positive with its choice of anodes, etc. See thisprimer.Best practices are to drive the filament with an AC center-tapped transformer (5VAC in your case; see connector on left: FL1,FLCT,FL2). Also, the center-tapped point is raised +7.5V above VEE which you've measured to be -31VDC from GNDU (see zener ZD701). This guarantees that any grid and anode at -31V is truly off (below filament potential).Q: Why do it this way?A: Since the LD1970 also has regular logic inputs/ouputs (CLK, STB, DIN, DOUT, LEDx), it makes for a simpler grid/anode output driver; only need a single open-drain P-FET.LD1970 datasheet page 7 "I/O Pins Schematic Diagram"Therefore, if the getter spot seems fine (dark) then check if the LD1970 is receiving serial input (CLK, STB, DIN) or if you see any activity on the OSC pin (sets internal clock frequency). And of course, check if VDD and VEE are making it to the chip.

author: Vonsgarage
author_rank: Contributor
timestamp: September 16, 2023, 08:51:03 pm
content: Thanks all!The getter is nice and dark and silverary. Tube nib looks solid.The real eye opener is the revelation that what I thought was a signal to turn all the darn thingsonis actually a signal to turn them alloff. Opps!The filament is being driven by a dedicated three lead output including center tap from the transformer just for Mr. VFD. But the filament does not seem to be glowing. I will try to check the glow this evening in the pitch dark. I bet it's going to be OK.Knowing that the VFD is probably fine I will start to see if I can figure out why the chips are not talking to the display. Total tyro here so wish me luck. If you all would be so kind as to keep an eye on my post I would be super happy.And I also have to confess that the schematic I am working from and the screenshot provided from it is not the one for my unit. It is the closest I could grab of the Net. A Panasonic version that shares many of the same PCB's as this Onkyo.I will dig in. If I have any good news I'll write back.

author: pqass
author_rank: Super Contributor
timestamp: September 18, 2023, 08:32:12 pm
content: It just occurred to me that if you want to simply confirm that the VFD works,turn on the receiver and temporarily jumper a 1K resistor between VDD and one VFD anode pin (segment) AND another 1K resistor between VDD and one VFD grid pin. This is going on the assumption that there is no comms between the LD1970 and the MCU (or it's fried) and the 56K pull-down resistors are defaulting them to all off.The two temporary 1K resistors will pull-up the anode and grid pins such that there is about +25V between filament (FLCT) and anode/grid. The 1K resistors are effectively in wire-OR with the (currently off) P-FETs inside the LD1970. That should make an anode visible and confirm that the tube is fine. Why 1K? A:It's an order of magnitude smaller than the 56K pull-downs while still limiting current in case you poke the wrong pin.

author: floobydust
author_rank: Super Contributor
timestamp: September 18, 2023, 08:42:02 pm
content: It's common to have a bad solder joint for the VFD filaments (due to the Kovar). You should be able to see a faint orange glow.The VFD outside pins at either end of the tube, or the jumpers and ribbon cables etc. all the way back to the power transformer should be eyeballed or simply resoldered. Onkyo lead-free solder is a bitch, terrible stuff.Also, Onkyo AV receivers have a display dimmer/on/off via the remote control. It's the DIMMER button.

author: Vonsgarage
author_rank: Contributor
timestamp: September 19, 2023, 10:39:01 am
content: Good morning. My reply last night never posted. Will draft in Word from now on.All this information you all have provided is exceptional.Here’s an update:I slipped while probing the data line going to the display board from the CPU board. Around pin 17 – OSD-FLT/DATA. May have hit pins 15, 16, 18, or 20. The unit shut itself off. Uh Oh; now what have I done? I repowered and the display turned on for the very first time ever and displayed gibberish! This really happened! Probably good data, just missing segments.Now I can’t remember what I did next but at some point, the display started working 100% A-OK! I then tried to pin down what caused this. I flexed the PCB’s, wiggled stuff, freeze sprayed some things, put the task lamp close to some chips to heat. Display continued to work perfect. Power cycled many times. Started right up with perfect display.I then played some music and turned the volume up. Encoder style control. No change. Turned up some more, and more, and more. Suddenly the change kicked in. I then turned the volume back down fast and the volume went up instead of down! Luckily, I had the remote control nearby to turn it down. The remote works fine. Powered down, added a drop of contact cleaner on the control, repowered. NO DISPLAY. Volume control still doing the bad thing.I am going to pull the control this morning. Perhaps it is sending such mixed signals to the CPU that the CPU shuts down the display. Maybe?? Everything else, when the display was functional worked perfect. USB indexing, all functions read out correctly. Everything but the volume control worked.

author: Vonsgarage
author_rank: Contributor
timestamp: September 20, 2023, 11:45:26 am
content: Good grief. Could this have been it all along? Bad ribbon @ CN704? I had pulled this ribbon a couple of times and was careful to be gentle. I finally took a good look at the darn foils. Peeling back at the bottom two foils – close to the volume control up/down. I flattened back the best I could and no change. I then pulled a scrap ribbon from the junk pile and plugged it in. Totally dead. Not even power up. Different AWM number. Really? That makes a difference? Back to the original ribbon which at this point is falling apart all over the place. Flattened everybody back down and tried again. SUCCESS! Display A-OK / Volume Control A-OK. Learned my lesson. These ribbons are good for maybe one pull/replace; if that!Why this unit did not work from the original get-go before I started ripping things apart, I’ll never know.And how you guys and gals work on these things amazes me. Think I’ll stick with thru-hole and point-to-point repair attempts from now on.Wish me luck trying to order a replacement ribbon. Mouser? Digikey? Just super glue the one that’s there now? I'm afraid to touch it or even put the thing back together.Remember that I would not have fixed this thing without your help.