VFD display help.

[t0nito_]

Hi guys, I'm new here and I registered to this forum in hopes that you can help me figure this out.

I have a VCR that we brought from Canada (NTSC) many years ago, it's a 1987 GE 9-7500 manufactured by panasonic. I've looked everywhere for a schematic with no success. I've googled the main board model number and nothing shows up. All I know is that the electronics and transport are the same as the Panasonic PV-4720, as a Sylvania "3 head" vcr and a JC Penny, don't know the model number, but I've seen them on youtube.

Anyway the problem is, when I was yound and stupid I was curious and opened the VCR to see how it worked, then I don't know why but I thought it was fun to short the pins a SIP resistor over the VFD to have it display "random" segments when suddenly there was a spark and the VFD never worked since, despite that, the VCR fortunately was and still is working perfectly.

Now, 20 years later I'm finally determined to fix it. So here's what I know so far.

I get no voltage across the filament pins.
I have about 3.3V if I measure between each of the filament pins to ground.
I have 7.5 Ohm resistance across both filament pins.
I followed the traces of both filament pins and they go directly to the power supply.

If I short one of the filament pins to ground the VFD works but with a dimmer display.

So I think there's something sorted on the power supply (perhaps it's supposed to deliver 6.6V?) Any ideas?

For what it's worth the VFD is a NEC FIP90M8 (Panasonic part VSZS0076) Driven by a NEC D75206CW microprocessor.

Since some of you are familiar with VFDs I'd like to hear from you.

Thanks.

[German Dalek]

A big hello to Portugal and welcome on board!
The only thing I can do for you is to send you this link to ask for the schematic,
just because it is from northern America:

http://www.videokarma.org/

Good luck!

German Dalek

[Hunts smoothing bomb]

Hello and welcome!

By what you have said it sounds like you have shorted the filament supply all those years ago and blown something, surely there can't be much to it as you say, the VFD works when you short one of the filament pins to ground indicating there is voltage present, I wonder if it works from a split or floating rail regulated power supply and my thinking is that the regulator or a fuse has blown but there is current getting back through somehow because the other rail is above chassis potential but not the full amount which would be responsible for the the dim display. I have no schematics, It's just a hunch !

Good luck and post back your findings.

Cheers

[t0nito_]

Thank you for your replies.

I'm going to remove the boards later to access the power supply and I'm going to keep tracing those two lines to see where it goes.

Yesterday I took a peek at the supply and I couldn't visually see anything burned out. It was already late to test any component.

I'll keep you posted, thank you very much!

[Maarten]

Given the 1987 model year, it might be similar to European models from the NV-L series.

[t0nito_]

Bad news, I was checking voltages with my voltmeter e must have shorted something else. No the power supply is dead. I only hear a small tick when plugging the cord then it's completely dead.

Diodes look ok, there is a bigger diode though that if I plug and unplug the unit if I tet it it seems shorted the instant I touch with the voltmeter but the resistant starts to climb rapidly the the diode read good again. Any Ideas?

Tested the transistor and it seems ok too.

Power supply is a VEPS0167



Damn me for being so clumsy

EDIT: Just figured out my VCR is just the same as a Panasonic PV-2700.

[t0nito_]

Turned out it was just a tiny fuse, the VCR is working again though the VFD is still not working properly.

[davegsm82]

Your filament supply should be floating at some voltage BELOW ground, i.e. the filament will have a voltage across it, 3.3V sounds good, AND The whole filament should be maybe 30V or more volts negative with respect to ground though. Perhaps you shorted the part of the supply which provides this voltage?

The voltage across the filament won't necessarily be DC, it could be AC straight from the PSU (more likely if it's a Linear supply rather than SMPSU though).

You could even connect a supply from the filament to the chassis and see if the display wakes up, just be careful not to put any excessive voltage ACROSS the filament or it will go 'ping'.

Good luck,

Dave.

[John123]

You should be able to see the filaments glowing red a little if you put your hand over the VFD itself (or with the lights off).

[t0nito_]

Thank you for your replies, the supply is in fact a SMPS and it should be feeding 4 VAC to the filament pins, but turns out that the voltage is 0.05VAC when off and 0.2VAC when on, can't find anything wrong with it though, I measured nearly every component and them all seem to check out fine. The problem must be on the primary side because these low voltages are this low mesuring directly on the small transformer.

I tried supplying an external supply with around 3VDC with positive on one pin and negative on the other pin but nothing happened. Can someone draw a schematic on how to properly supply an external supply? I read that filaments don't really care that the voltage is AC or DC.

[kalee20]

The filaments don't care, but if you try and use DC you will find that the characters at one end of the display are brighter than the other, because the charatcer's anodes are are a different potential relative to that part of the filaments.

Running from AC solves this because the brightness varies above and below the average, and our eyes smooth out the flicker.

[t0nito_]

But I'd rather have a gradient on the display then no display, could you please tell me how to connect the filaments to DC power? I tried using an external power source with 3.5V but the display did not light up at all.

Funny thing is that if I "short" one of the filament pins to ground the display will light up but rather dim and some elements light up that weren't supposed to but the display is still readable, the problem is that maybe the little AC power there is left introduces ripple to ground because the video picture had a rainbow pattern on the lower part of the screen so I had to remove it again.

[Maarten]

Chances are, not only the filament voltage is missing but also the 20 to 30V bias.

[t0nito_]

Well according to another forum the pinout for the Panasonic VEPS0167 SMPS the values are:

Pin 1 AC 4V
Pin 2 AC 4V
Pin 3 UNSW +5V
Pin 4 UNSW +5V
Pin 5 UNSW -33V
Pin 6 GND
Pin 7 UNSW +14V
Pin 8 UNSW +48V
Pin 9 POWER STOP (L)

My values are:

Pin 1 AC 0.2V
Pin 2 AC 0.2V
Pin 3 UNSW +5.1V
Pin 4 UNSW +5.1V
Pin 5 UNSW -21V
Pin 6 GND
Pin 7 UNSW +18.5V
Pin 8 UNSW +48V
Pin 9 +5.1

I suppose the -21V are low when it should be -33V, I assume that 20V to 30V you mention are from that line, measuring every pin of the ribbon cable to the VFD/button PCB to ground I get no such voltage, so I assume it must be some component(s) between the power supply and the pcb.

I really wish there was a schematic of this vcr out there, preferabily one that isn't more expensive than wha the vcr is worth...

Searching more thoroughly I found out that the GE 9-7500 is exactly the same as a Panasonic PV-2700 except for the front bezel/panel.

Here are photos of both my GE-9750 and the Panasonic PV-2700:

My GE:

[t0nito_]

More photos:

Now the panasonic:

http://images.quebarato.com.ar/T440x...__5C8D74_2.jpg

http://images.quebarato.com.ar/T440x...__5C8D74_7.jpg

http://images.quebarato.com.ar/T440x...__5C8D74_5.jpg

As you can see they're pratically the same VCR. I wonder these VCRs look familiar to any of you.

[t0nito_]

Sorry to be a pain in the neck, I'm getting somewhere now, I traced the -30V line from the power supply and I found that between the VFD board and the power supply there is a single resistor but I think it's open, there's -30V on one side of the resistor and around -2V to the VFD board.

I tested it with my "universal" tester and it says "Unknown or damaged part".

Since nature decided to make me colour blind I'm having a hard time figuring what value is this resistor.

Can you please tell me what's the value of this resistor?

Edit: I just finished reading that the additional white band means it a fusible resistor, right?

[Clydeuk]

Brown, Red, Brown = 120 ohms.

Do you have a multimeter to test it?

I suspect it may be O/C as otherwise I'd expect it to get very hot with 30 volts dropped across it.

[Freya]

I think you might find it is an inductor !

[t0nito_]

Originally Posted by Clydeuk

I suspect it may be O/C as otherwise I'd expect it to get very hot with 30 volts dropped across it.

I tested it and it's open. I momentarily put a 2k ohm resistor on the VCR and the display finally came to life after all this time. I didn't leave it there though knowing it wasn't the right resistor, so I went ahead and put in a 450 Ohm which was the one I had closer to 120 cause I guess a little voltage drop wouldn't do any harm and:

Thanks for all your help guys! Good to see this thing working after all those years!

[Nickthedentist]

Ah, well done! And nice and bright too.