Pulsing Vacuum Fluorescent Display  - Page 1
Forum section: electronics / beginners

author: Electro_Jr
author_rank: Contributor
timestamp: November 08, 2018, 08:21:28 pm
content: Hey Guys,Hope someone can help me out I have a Sony DXA-H2750 stereo that I am working on. The display has a pulse to it in the lower left corner of the display and appears dim overall.Checking it with a scope pin1 has 6V @60hz and pin 39 has 6V 0HZ. This has me stumped because pin 1 and pin 39 (Fla1, Fla2) are wired together in the transformer (PG 28 of the service manual)Having never worked with a Vacuum Display before I'm a little lost.I have attached pages from the service manual 3 pages of information for the VFD and Transformer. (Couldn't upload the full 40 pages service manual it was too large)Does pins 1,39 (FLA1 FLA2) provide the positive voltage, and the controller provide the negative, which causes the electrons to be drawn across the phosphor and fluoresce?Should pin 1 and 39 have matching voltages/frequencies? What voltages should I see at pin 1 and 39?Am I looking at the wrong spot for this pulsing?Thanks for any help provided.

author: Electro_Jr
author_rank: Contributor
timestamp: November 08, 2018, 08:25:32 pm
content: Uhg....that pdf turned out horrible. Let me see if I can't get you guys some better files.Ok. JPG it will be today.

author: stj
author_rank: Super Contributor
timestamp: November 08, 2018, 11:33:58 pm
content: i really hate japanese engineers,they treat schematics like some kind of copy-protection device!what a disgusting rats nest.from what i can see your display heater is driven by some type of booster circuit - over complex as usual.that's another thing i hate about japanese designers - they would rather use 100 discrete parts than a simple dedicated i.c. for anything!

author: james_s
author_rank: Super Contributor
timestamp: November 09, 2018, 01:47:47 am
content: I fixed an old Russian VFD clock that had a severe flicker. The problem turned out to be a bad electrolytic capacitor filtering the ~30V supply to the panel.

author: sarahMCML
author_rank: Regular Contributor
timestamp: November 09, 2018, 02:19:00 am
content: Hi,It looks to me that the VFD filament lines FLA1 and FLA2 are driven by the second from the bottom winding on the transformer board, therefore A.C., biased to a DC potential of around +6V DC by Zener D944 connected to FLA2.Regards,Sarah.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 09, 2018, 02:51:33 am
content: Quote from: stj on November 08, 2018, 11:33:58 pmShoganai-ne...Changing the way it's done is a bit difficult (sucks teeth).We'll have to send a fax to the boss so he can contact his boss to tell us to organise a committee to organise the string of day long meetings to discuss the issue and finally have the top boss make an over-ruling decision to not change anything beyond one small inconsequential detail.

author: Electro_Jr
author_rank: Contributor
timestamp: November 09, 2018, 05:22:12 pm
content: Quote from: sarahMCML on November 09, 2018, 02:19:00 amSarah, Thanks for the reply.So I am understanding this correctly we are getting an AC voltage generated in both FLA1 and FLA2 in the transformer. FLA2 is then getting converted to +6V DC by the Zener.This matches what I am measuring at the display.I will check the +30 coming in and see if it has any ripple to it. The issue may be C523 on the 30V supply line.Looking back at page 19 that I attached, The Flicker/Pulse (as steady as a heartbeat) is only affecting the bottom two rows of the equalizer display. If you are looking at the graphic of the display that would be 13G and 12G. The Flicker/Pulse lessens as you move up the display and to the right.Just for giggles can I jumper pin 39 to pin1 to test? Will that damage the display or the FLA circuit? As I think this through they are already wired together somewhat as it passes through the display.

author: stj
author_rank: Super Contributor
timestamp: November 09, 2018, 08:44:10 pm
content: the display heater is a resistive load with a limited current - DO NOT short it.you could damage the drive circuit.

author: sarahMCML
author_rank: Regular Contributor
timestamp: November 09, 2018, 08:54:33 pm
content: Hi again,Can you measure the A.C. voltage between pins 1 and 39 with a multimeter and tell us its value (it should be fairly small)?The filament is biased at a small (+6V in this case) positive voltage so that the grids of the characters can be pulled down to a more negative potential, and so turn off relevant ones as required. It is usually A.C. driven so as to give equal bias to both ends and therefore equal brightness along the line of characters. In your case FL2 is held at +6V and FL1 swings slightly above and below +6V.If your display has become worn with use and the filament isn't emiting as many electrons as it did, the difference in potential as FL1 swings more positive and then less positive MIGHT produce the pulsing effect you see.Just my guess, for what it's worth!Regards,Sarah.

author: Electro_Jr
author_rank: Contributor
timestamp: November 20, 2018, 06:27:34 pm
content: Sorry for the delay in replying. Been tied up at work.The voltage between Pins 1 and 39 is reading 3.35V AC.I am seeing +36V at the C523 capacitor fort he 30V signal feeding the display driver chip.

author: james_s
author_rank: Super Contributor
timestamp: November 20, 2018, 06:28:48 pm
content: What do you get if you check AC volts across that capacitor? If the capacitor is shot the display will flicker.

author: Electro_Jr
author_rank: Contributor
timestamp: November 20, 2018, 07:52:13 pm
content: Capicator C523 shows 0.01-0.009V measured in ACCapicator C522 I checked it for comparison and because it was right beside C523 didn't show any voltage.

author: Electro_Jr
author_rank: Contributor
timestamp: November 20, 2018, 08:03:12 pm
content: I'll add that both capacitors are 0.1uf Ceramic. If it matters.

author: james_s
author_rank: Super Contributor
timestamp: November 20, 2018, 09:10:03 pm
content: Oh, I assumed we were talking electrolytic capacitors, ceramic caps almost never fail.

author: Electro_Jr
author_rank: Contributor
timestamp: November 21, 2018, 02:34:42 pm
content: How would a bad capacitor show up?For example on the one in question, I tested it across it in AC mode with a multimeter. If it were bad what readings would I see?

author: james_s
author_rank: Super Contributor
timestamp: November 21, 2018, 05:39:57 pm
content: A bad filter capacitor for example would stop doing its job, so instead of smooth DC you would see ripple which would be indicated as an AC voltage on a meter. If you're supposed to have say 24VDC and the meter shows a few Volts of AC then you know you have a lot of ripple.

author: sarahMCML
author_rank: Regular Contributor
timestamp: November 21, 2018, 07:56:29 pm
content: Hi,What ripple voltage do you see across points FLA2, and also FL30V, with respect to 0V, using your scope when set to A.C. mode? Hopefully just relatively small values. Also, does FL30V measure about 36V, as it says on the schematic?Regards,Sarah.

author: Electro_Jr
author_rank: Contributor
timestamp: November 26, 2018, 10:44:58 pm
content: FLV30 is showing about 37 Volts. With no ripple.FLA2 is showing no ripple either.I did realize that I am reading the scope wrong FLA1 has 9.86V Vmax 5.23 Vmin -4.65V.

author: sarahMCML
author_rank: Regular Contributor
timestamp: November 27, 2018, 02:40:55 am
content: Hi again,Quote from: Electro_Jr on November 26, 2018, 10:44:58 pmIs the reading of FLA1 of 9.86V a DC or an AC voltage, and to what is it referenced, 0V or FLA2?And what are the meanings of the other two values: surely Vmin isn't really going negative of 0V?Regards,Sarah.

author: Electro_Jr
author_rank: Contributor
timestamp: November 27, 2018, 04:12:28 pm
content: Its this crazy scope or the crazy operator. With it setting on AC mode it is showing those +/- voltages. With it on Dc mode its shows +~10V to 0V.This was reading FLA1 referenced to ground.The FLV30 is showing nothing on AC mode but 37V on Dc mode referenced to ground.FLA2 is showing nothing on Ac mode but 6V on Dc mode referenced to ground.I did make an observation when measuring FLA1 setting the trigger voltage to 0 Volts setting the scope to read Ac with a ground reference. Every time scope triggers it flashes an led. The pulsing of the led matches that of the display.

author: sarahMCML
author_rank: Regular Contributor
timestamp: November 27, 2018, 06:26:56 pm
content: Hi again,I get it now, your Vmax and Vmin readings are the peak to peak values of the 3.35 Volts that you measured across pins1 and 39 with your multimeter.Does the VFDisplay module itself have a part number marked on the back that you can find? I ask because if we can find out the maximum operating voltage for the filament it might be possible to increase it a little, get rid of the pulsing effect, and get a little more brightness!Regards,Sarah.

author: Electro_Jr
author_rank: Contributor
timestamp: November 27, 2018, 07:12:28 pm
content: The service manual for the stereo calls this out as IC551 part number of....TD62C950RF Sony Part number of 8-759-075-35.It also gives a part number for FL521 1-519-735-11 Indicator Tube, Fluorescent.My readings are thisFLV30: +37V DC no ripple respect to GroundFLA2: +6V DC no ripple respect to GroundFLA1: see picsFla1 to Fla2: See Pics

author: sarahMCML
author_rank: Regular Contributor
timestamp: November 27, 2018, 09:16:54 pm
content: Hi once again,I couldn't find any data for that VFD, so the only thing to do is to try a couple of things, if you think that they are worth the risk.You can use a DC power supply, connected directly across FLA1 and FLA2, and, starting from 0 volts, slowly raise the voltage until the filament glows visibly red. Leave it like that for about 60 seconds, remove the PSU, and power the unit. With luck this MAY rejuvenate the filament. But if you put too much voltage across it you could burn it out!The other thing you could try is to put an extra turn or two on the transformer (if there is space for winding it around the outside of the existing windings, of course), checking that it only gives about an extra 10% voltage (0.35V-ish AC), and connecting it in phase, in series with the existing filament supply. Again, it may or may not work!Regards,Sarah.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 28, 2018, 09:47:34 am
content: Your display may be "gassy"---- in other words has lost it's vacuum to some extent.From memory, this can cause oscillation in vacuum tubes--- usually they just stop working, but it does occur.The vacuum displays may have the same problem.

author: macboy
author_rank: Super Contributor
timestamp: November 28, 2018, 08:22:35 pm
content: Quote from: vk6zgo on November 28, 2018, 09:47:34 amThis can be easy to test. There will be a "getter" inside the VFD glass (google it). This should look like a block spot in one or more corners of the tube. If it is white or cloudy, you have air ingress.

author: Electro_Jr
author_rank: Contributor
timestamp: December 03, 2018, 06:02:30 pm
content: I do not have a variable DC power supply. Can I just use a 9V battery?

author: sarahMCML
author_rank: Regular Contributor
timestamp: December 04, 2018, 01:41:59 am
content: Hi again,Quote from: Electro_Jr on December 03, 2018, 06:02:30 pmIf you're responding to my idea in reply of 27th November then yes, you could use a fairly beefy 9V battery (a PP3 probably won't cut it!), in series with a wire wound potentiometer of about 100 ohms. Start with the pot at maximum resistance setting and gradually reduce until you see the filament glow visibly red, as I stated before. But be very careful to check that you do start from the high resistance setting!Regards,Sarah.

author: james_s
author_rank: Super Contributor
timestamp: December 05, 2018, 07:10:43 am
content: Depending on the size of the display, a standard 9V "transistor radio" battery will probably work, although it's not likely to last long. NiMH rechargeable types can supply quite a bit more current.