SA-9800 Dim Display: Options for Fix
- Thread starter AlwaysAnalog
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- dim display sa-9800 vfd
pdm4606
Super Member
I had a dim display on one of my VHS players last year.
I checked the main circuit board and found one small resistor looking burned. I could get a reading but could not find a schematic with what it is supposed to be. Finally got a few low resistance resistors and settled on a 10 ohm. The burned res. was at around 18+ ohms. Color coding fried so no use. I installed the 10 ohm and working well. Also went up to a 1/2 watt. Old one was very small. Maybe 1/8 watt.
Working fine since then.
I checked the main circuit board and found one small resistor looking burned. I could get a reading but could not find a schematic with what it is supposed to be. Finally got a few low resistance resistors and settled on a 10 ohm. The burned res. was at around 18+ ohms. Color coding fried so no use. I installed the 10 ohm and working well. Also went up to a 1/2 watt. Old one was very small. Maybe 1/8 watt.
Working fine since then.
QSilver
Super Member
From what I've read no, the SX-D5000 runs on a different voltage but it's the SA-8800 I'm worried about. Some of the dimming has gone away with some use but I want to try to reverse or at least prevent the rest.
About the VHS player, maybe your's needed more current now than it did when new - hence the resistor doing the trick.
1TUFSS
Thread Killer
As mentioned before, the SA-8800 and SA-9800 are the only Pioneer units I've ever seen which suffer from dim VFDs. The 7800 seems to use a different display voltage, and the 8800 may use a more suitable (albeit not perfect) one. The 9800 is definitely the lemon of the bunch, sadly, with the most frequent and severe failures. I've only seen 8800s with slight dimming or uneven brightness, only the 9800s go the whole hog and severely dim.
I have been reading a bit more on how to rejuniveate a vacuum tube. A good read can be found here. As I read through the article I have a theory on why the displays may go bad to begin with. Please feel free to shoot holes and or prove it right or wrong. 
The filaments in the VFD are probably the thoriated type. Thorium boils off electrons from the filament wire in equilibrium as long as the filament voltage and corresponding anode and grid voltage is correct. The thoriated wire keeps driving new thorium to the surface to be boiled off as electrons. If too much or too little voltage is applied or if the corresponding anode and grid is not biased properly then the filament can begin to build an oxide, decreasing the emissions from the filament, causing the display to dim. The voltages on the 9800 may not be optimized and eventually the filament goes out of equilibrium as an oxide builds on the surface. This may be due to the dimming circuit, IDK....
In any case it may be possible to make the display brighter in a couple of ways.
1.Remove the grid and anode voltage and leave the filament on for 30 - 90 min, possibly longer. This boils new thorium to the surface of the filament and removes the oxide. I believe that this is what's happening when reports are made of displays getting brighter with use. Removing the anode and grid voltage just allows the filament to get back into equilibrium faster.
2. Place a voltage on the filament that's 50% more than normal or until you can see the filament lines very lightly glow for a few seconds, repeat this a couple times. This rapidly removes the oxide and brings the thorium to the surface of the filament.
The filaments in the VFD are probably the thoriated type. Thorium boils off electrons from the filament wire in equilibrium as long as the filament voltage and corresponding anode and grid voltage is correct. The thoriated wire keeps driving new thorium to the surface to be boiled off as electrons. If too much or too little voltage is applied or if the corresponding anode and grid is not biased properly then the filament can begin to build an oxide, decreasing the emissions from the filament, causing the display to dim. The voltages on the 9800 may not be optimized and eventually the filament goes out of equilibrium as an oxide builds on the surface. This may be due to the dimming circuit, IDK....
In any case it may be possible to make the display brighter in a couple of ways.
1.Remove the grid and anode voltage and leave the filament on for 30 - 90 min, possibly longer. This boils new thorium to the surface of the filament and removes the oxide. I believe that this is what's happening when reports are made of displays getting brighter with use. Removing the anode and grid voltage just allows the filament to get back into equilibrium faster.
2. Place a voltage on the filament that's 50% more than normal or until you can see the filament lines very lightly glow for a few seconds, repeat this a couple times. This rapidly removes the oxide and brings the thorium to the surface of the filament.
That SA-9800 with the dim display is STILL sitting on the bench, patiently waiting for me to get back to it. I have less qualms about the time waiting to rejuvenate it now that it is mine.
This is merely a bump. And this is a search term : markthefixersa9800dimdisplay for me to find this again.
This is merely a bump. And this is a search term : markthefixersa9800dimdisplay for me to find this again.
BUMP thread to top. I was able to rejuvenate the VFD on my 40 year old Heathkit clock by using a 9V battery on the VFD filament for a few seconds. Just let it glow orange for a few seconds. I didn't have a power supply to try so I used a 9V battery. This method works!