Help diagnosing dim VFD on Yamaha RX-V2400

[JimBanville]

I realize vacuum fluorescent displays degrade over time.
Potential "fixes" (temporary or permanent) seem to typically involve replacing capacitors powering the VFD, or the risky and temporary "fix" of high voltage across the filament to burn off contaminants.
Can someone look at these pics from the RX-V2400 service manual and give me a starting point in determining which step to take first? The display is almost too dim to even read, so if I damage it, so be it. It's borderline useless at this point. I already did a factory reset which didn't help. It looks like power for the display comes straight from the transformer. I don't see any capacitors in the circuit. What voltage should be going to the display? THANKS!

[redwire]

A really dim fluorescent display is commonly the HV power supply (capacitors). Or the display dimmer function is turned on...
Check the VFD VP is -33.3V, starting at the power supply conn CB207 (H1). Q257 regulates it. If it is low, look at C207 100uF 50V (runs at 45V wow so conservative) and C510 47uF 50V is stashed away on the Function Board 1, doing a loop dee do connector MAIN E7 to OPERATION(1) H1. Ugh.

For the VFD filament power, a bad connection is a common occurrence but what typically happens then is the VFD just goes out - not consistently dim.
You can measure the ACV at the outside ends of the tube (pin 1 FL1 and pin 65 FL2) 6.9VAC, and to GND those two (heater) are biased at -25.7VDC by zener D851, D852, so measure that as well.
VFD filament connections are usually cracked due to bad soldering of the Kovar pins. You can see there is a long, convoluted path from the power transformer, a few hops through connectors and ribbon cables to the display board. So many places where a bad connection can happen. But vibration and tapping can flush that out.

If the tube has aged- it can be due to low emission or phosphor burn or grid contamination.
VFD phosphor burn- segments that are always on will appear dim and a bit fuzzy looking around the edges- but rarely used segments will have good brightness.
If the entire display is dim, all elements- then due to aging it can be contaminants on the heater that can be burned off to rejuvenate the VFD.
Grid contamination is another aging mechanism but complicated to deal with.

[JimBanville]

A really dim fluorescent display is commonly the HV power supply (capacitors). Or the display dimmer function is turned on...
Check the VFD VP is -33.3V, starting at the power supply conn CB207 (H1). Q257 regulates it. If it is low, look at C207 100uF 50V (runs at 45V wow so conservative) and C510 47uF 50V is stashed away on the Function Board 1, doing a loop dee do connector MAIN E7 to OPERATION(1) H1. Ugh.

For the VFD filament power, a bad connection is a common occurrence but what typically happens then is the VFD just goes out - not consistently dim.
You can measure the ACV at the outside ends of the tube (pin 1 FL1 and pin 65 FL2) 6.9VAC, and to GND those two (heater) are biased at -25.7VDC by zener D851, D852, so measure that as well.
VFD filament connections are usually cracked due to bad soldering of the Kovar pins. You can see there is a long, convoluted path from the power transformer, a few hops through connectors and ribbon cables to the display board. So many places where a bad connection can happen. But vibration and tapping can flush that out.

If the tube has aged- it can be due to low emission or phosphor burn or grid contamination.
VFD phosphor burn- segments that are always on will appear dim and a bit fuzzy looking around the edges- but rarely used segments will have good brightness.
If the entire display is dim, all elements- then due to aging it can be contaminants on the heater that can be burned off to rejuvenate the VFD.
Grid contamination is another aging mechanism but complicated to deal with.

Thanks! Here are the measurements for H1... -31~0~5.3~0~4.9~5~5~5~5~4~0~0~4.8~4.6~4.8~4.8~0~0~0~0~5
Filament measures 7v AC when unit is powered on. 0v when powdered off.
I found Q257 on Main 1 PCB, but did you mean C270? I don't see C207. Unfortunately those are too deep to get to without a lot of disassembly. As far as C510, I measured 31v on one lead and 5.6v on the other lead.

[redwire]

VFD power VP is a bit low. Your -31V vs -33.3V it could be a problem if it's got a lot of ripple due to an open filter capacitor C287 (not C207 I read that wrong) on MAIN(1). Q257 is the regulator. If C287 died then C270, C271, C510 may also have gone low value due to stress. These AV receivers always have hot spots where some parts just get baked, causing them to fail. I would expect -45V in and -33V out of Q257. Or measure ACV on it to see if there's ripple.

Check the VFD cathode bias by measuring DCV to GND at each filament pin (not ACV), should be -25.7VDC at W851 or the tube ends.

Other than this, the voltages look reasonable. Maybe post a pic of the dim display so we can see if it looks aged or not. I would go after low VP for now.