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SA-9800 fluorescent display issue

Backlight

Backlight

Active Member
Hi all,

I have an issue with the display of my SA-9800. The brightness is gone, also when the switch is set to bright.

I started an investigation and I found the display "heater" is receiving only 3.6V AC, where is should be 18v AC (if I'm correct) on PIN 14 and 15 of the indicator PCB. These are fed by the small transformer on the PSU board PIN 25 and 27. The voltage between PIN either 25 and 26 or 26 and 27 is 1.7v AC. PIN 26 goes to the protection assembly where I have no clue what it does...

The small transformer receives about 45VAC. I don't know what this should be, but it seems a bit low. Nevertheless it results in only 3.6VAC on the transformer output side (PIN 25 and 27)

Voltages on the PSU board give good stable readings except PIN 12 which reads +45VDC (should be +50VDC), PIN 13 which reads +66VDC (should be +68VDC). PIN 15 reads -67VDC by the way. The sound of the amp is good and the protection circuit kicks in and works when overloaded (volume all up with loud music fed to the AUX input and max bass and loudness on).

I have just replaced all electrolytic caps on the PSU, Protection and Indicator board by Nichicon UPW replacements.

Might the transformer be faulty? Or is there any other thing wrong on the PSU board, like the resistor R23 or one of the diodes that are in the path to the small transformer?

Please help me out here.
 
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I don't know where you got the idea that the heaters on the vacuum florescent displays use 18v ac...

This is being covered in a current thread - and before you try to increase the heater voltage, you had better get a confirmation from someone with a working 9800 display - that much over voltage would destroy it.

I have a 3900 with dual displays, and one uses 3.2vac and the other 4.3vac. The 9800 display is slightly different because it uses a few extra labels inside that light up... other than that I don't think it's any different.

Basically below, the gist is that the center "grid" is at 68 volts or reduced for dim, the heater's near ground, with some additional ground resistance for dim, and the dark segments are near ground potential while the lit segments are near 18 volts.

the quote below from today is in the current thread: " Pioneer SA-9800 fluorescent display failure?" by quadmatrix82 thread and directly applies to your 9800 as well.

markthefixer said:
Here's the "from there":

Check the power supply DC voltages on the awr-194 power supply board (power on, be CAREFUL)

referenced from a bare metal chassis ground(black probe):
to (red probe, insulate all but the very tip of the red probe, for when you slip - so no shorts):
pin 13 +68v important for a lot of things, but this feeds the display tube's grid through the bright/dim switch
pins 3&4 +52v
pins 5&6 +32v
pin 7 +18v *** important for display, no +18, no display!! ****
pins 10&11 -32v
pins 1&2 -52v

and between pins 25(red probe) & 27(black probe)about 3 volts AC

Then pin 26 should show zero ohms (power off!!) to ground with the bright / dim switch set to bright (when dim it's 470 ohms to ground, bypassed with a 8.1v zener diode). If it isn't.... no display.....

for general information, these pins 26,26 & 27 are fed by a transformer on the power supply board, that has a 330 ohm resistor in series with it's primary - which if the 330 ohm resistor dies - no 3 volts AC and no display....

also
1. pin 13 +68v for the display goes through the bright dim switch (bright 1600 ohms, dim 1200 ohms) to pin 10 on the avw-002 indicator board.
2. Thus with power off, pin 13 of the awr-194 power supply board shhould show with the bright switch bright 1600 ohms to pin 10 on the avw-002 indicator board.
3. And also with power off, pin 13 of the awr-194 power supply board shhould show with the bright switch dim 1200 ohms to pin 10 on the avw-002 indicator board.

as an aside, all those little function indicator labels on the fluroscan tube, are grounded when dark and at +18v when lit. This is also the "signal" that is switched on and off for the bar segments in the display.
Click to expand...
 
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Thanks Mark for pointing to the other thread... missed it :o

I read about the 18.5 volts in the service manual. I misunderstood that voltage as the voltage for the heater. All voltages are correct, including the output of the small transformer.

I compared the filament voltage to the value of my SA-7800. My SA-7800 reads 43 volts DC. My SA-9800 reads 48 volts DC in bright position and 53 volts DC in dim position. I would think that the voltage should be about 43 volts in bright position. This could explain why the display is less bright than the SA-7800. The values of the resistors in the line (from the 68 volts supply) are of correct value (measured them).

So what is going on? The display was bright just a week ago. Might there be something wrong inside the display increasing the resistance of the filament which could result in the higher voltage? I hope not!

Is replacing the 1.2K 2W resistor with a higher value an option?
 
Backlight said:
Thanks Mark for pointing to the other thread... missed it :o

I read about the 18.5 volts in the service manual. I misunderstood that voltage as the voltage for the heater. All voltages are correct, including the output of the small transformer.

I compared the filament voltage to the value of my SA-7800. My SA-7800 reads 43 volts DC. My SA-9800 reads 48 volts DC in bright position and 53 volts DC in dim position. I would think that the voltage should be about 43 volts in bright position. This could explain why the display is less bright than the SA-7800. The values of the resistors in the line (from the 68 volts supply) are of correct value (measured them).

So what is going on? The display was bright just a week ago. Might there be something wrong inside the display increasing the resistance of the filament which could result in the higher voltage? I hope not!

Is replacing the 1.2K 2W resistor with a higher value an option?
Click to expand...

There's two component voltages at the heater, the low ac voltage to warm the heater, and the dc voltage that it is floating at, referenced to ground.

and between pins 25(red probe) & 27(black probe)about 3 volts AC
Click to expand...

The heater voltage is measured ACROSS the two heater terminals, and the DC bias voltage is measured at pin 26, the center tap of the transformer feeding pins 25 & 27.

Then pin 26 should show zero ohms (power off!!) to ground with the bright / dim switch set to bright (when dim it's 470 ohms to ground, bypassed with a 8.1v zener diode). If it isn't.... no display.....
Click to expand...

While examining the sa-7800 schematic shows that the center tap of the filament transformer, IN THIS MODEL IS GROUNDED. Zero volts...

Thus the statements that :
I compared the filament voltage to the value of my SA-7800. My SA-7800 reads 43 volts DC. My SA-9800 reads 48 volts DC in bright position and 53 volts DC in dim position. I would think that the voltage should be about 43 volts in bright position. This could explain why the display is less bright than the SA-7800. The values of the resistors in the line (from the 68 volts supply) are of correct value (measured them).
Click to expand...

makes absolutely no sense at all and points to some sort of instrument or technique problem. The difference between 43 and 48 volts is a blind alley.

The 7800 should show zero DC volts on the filaments, while on the 9800 should show a few volts, rising to a maximum of about 8.1v due to the presence of an 8.1volt zener diode clamp D5 on the awm-163 board.

The ONLY thing I can think of is that your meter cannot accurately measure AC voltages with a DC voltage present, and that a 0.1uf capacitor is needed in series with the probe to block the dc voltage for an accurate ac voltage reading. It is called a DC blocking capacitor and is omitted on inexpensive meters.

Another difference between the 7800 and the 9800 is that on the 9800, 18 volts is fed TO the display board from the power supply, while on the 7800, there is an onboard voltage regulator that develops 15 volts on board.

Your sudden symptoms point to a single component failure, and it is essential that you measure the AC voltage at the heater terminals 25 & 27.
 
Thanks Mark, I learn a lot here.

My measurements (not a professional meter indeed):

Across PIN 25 and 27 on the PSU board 3.6 volts AC
PIN 26 to ground: 0 volts DC in bright position and 4.6 volts DC in dim position

What is the next step?

EDIT: on my meter the AC voltage between PIN 26 and ground reads 9.5 volts (dim position). Is this caused by a possibly missing DC blocking cap in my meter?
 
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Things are looking better....

But it's going to take a while to reconcile everything, integrate the new information, and bash my way through the schematics.

So it happened a little while after a recap? Was ok, now it's dimmer, but not off... right?

to rehash:

The display is rather straightforward, cathode/heater near or at ground, with an ac voltage on the filament/heater, a higher (50-60v) on a grid inside, and 15v or so switched between ground and 18v to illuminate or darken segments...

pin 10 is that 68v from the power supply, through R8(390 ohm 1 watt), R9(1.2k ohm 2 watt ) and s15, the bright dim switch on the protection board and it goes directly into the display's grid - nothing else is connected to pin 10 on tht board. The 68v line with the resistors IS carrying a lot of current, otherwise Pioneer wouldn't have used 1 and 2 watt power resistors... also, the voltage DOES NOT have any capacitors connected to it on the indicator board - so a cap failure there is out....

(the other half of the bright dim switch is the grounding of the transformer center tap either directly(bright) or through that 470 ohm 1/2 watt resistor (R7) and clamped to 8v by the 8v zener diode D5.)

These are your two brightness elements - as long as the 18v from the power supply IS 18v.... and there are no caps on it anywhere to fail.


measure it on both pin 7 of the power supply board and pin 2 of the indicator board.

it is only switched to light up special (aux phono etc) segments on the display and fed into the display driver integrated circuits Q3 & Q4.

there's not much more to it...

we could use ohm's law with the resistors (r8, R9 & R7) and their wattage to figure out a maximum display current from their heat dissipation limits, but that's about it....


any more will be very late tonight or tomorrow - I HAVE to get back to the bench...
 
Backlight said:
Thanks Mark, I learn a lot here.

My measurements (not a professional meter indeed):

Across PIN 25 and 27 on the PSU board 3.6 volts AC
PIN 26 to ground: 0 volts DC in bright position and 4.6 volts DC in dim position

What is the next step?

EDIT: on my meter the AC voltage between PIN 26 and ground reads 9.5 volts (dim position). Is this caused by a possibly missing DC blocking cap in my meter?
Click to expand...

hmmm.......... 4.6v DC across 470 ohms and 9.5 volts AC at the same point????????

Sounds like either the +68v isn't, or the currents from the segments being multiplexed is showing up. Would need to scope it. BUT Remember, the bright position has pin 26 grounded.

Measure on the power supply the DC and AC voltages across the 220uf 80v + and -68 volt capacitors C2 & C3... maybe a rectifier has shorted, or opened up, affecting the voltage. The DC and AC voltages across the capacitors should have similar values (dc to dc and ac to ac,, not ac to dc)

this REALLY is signing off for the afternoon now....
 
Thanks Mark for all the work in the schematics. I try to learn along. Pitty we live so far apart, otherwise I would bring you a couple of beers and watch you working on all this vintage gear! :)

The voltages:

PIN 7 (PSU): 19 volts DC
PIN 2 (Ind): 19 volts DC

PIN 10 (Ind): 50 Volts DC in bright position
PIN 10 (Ind): 54 Volts DC in dim position

This PIN 10 is my concern. On the equivalent in the SA-7800 this reads 43 Volts DC

PIN 7 on the PSU board is 19.2 Volts DC

The zener diode has been replaced by a previous owner. Don't know if it's 8 volts. As quoted PIN 26 (PSU) is now 4 Volts DC in dim position, so it might not be replaced with the best option...

Could a solder crack from PIN 10 (Ind board) to the display be the cause?
 
I started some experimenting:

Since PIN 10 on the indicator board was already soldered (wirerap was cut) I unsoldered it and powered up: The display is bright (again) but the dim/bright switch has minimal effect (but noticable). The side effect is that the bar segments (music input on AUX) seem a bit "blurry" on the edges. They only light up sharp when the bar next to it also lights up. May I conclude the PIN 10 feeds voltage (43-48 volts) to solve this?

This builds my confidence that the voltage at PIN 10 is too high. Perhaps the resistors on the protection board in this path are bad, even when measured ok, when the high current flows they may short quite a bit.

I await your response Mark. But I am ready to order a new 2W 1.2Kohm and 1W 390Ohm resistor. Especially the 1.2KOhm is suspected since it is "always on".

I will also replace the "alien" zener diode with a decent 8V one and solder it on the right side of the board (although hard to get there!).

Thanks again Mark, you could say I owe you a beer (or two)... :)
 
I continued some tests.

After playing the amp for a couple of hours with PIN 10 disconnected on the indicator PCB I reconnected it and the display was bright as it should be. I quickly measured the voltage of PIN 10 to ground: 26 volts DC in bright position. In dim position about 35 volts DC.

But... within half an hour the display slowly dimmed again and the voltage on PIN 10 increased to 42 volts DC. I disconnected it again and the display was bright but with somewhat blurry edges on the bars. After about 5 minutes I reconnected PIN 10 again: dimmed again.

Something strange is going on. Something seems to have a memory. I am now playing somewhat longer with PIN 10 disconnected and see what happens if I reconnect PIN 10 again.

Couple of questions:

1. The 68 volts line that feeds the two resistors (1.2 and 390 switched) has a lot of AC voltage (on my meter): 145 volts AC. Is this ok? The rest of the amp works great. By the way, the two 220uF/100V are philips and have been installed by a previous owner. I wanted to replace them but one of the caps I ordered (Mouser) was damaged (flattened somewhat), so I decided to keep the Philips for some time, but I have no clue when they were installed. Philips sold their capacitor production to Vishay quite some time ago...

2. Where does the high current from PIN 10 go too? To ground I assume. So might there be something building resistance on the indicator PCB or inside the display?
 
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1. you need a blocking cap in series with your probe to measure ac accurately when there is dc present. I use a 0.1uf 500v ceramic disc cap. Like in my signal probe picture.

start checking all your ground wires. I think you have a broken one.
 
It's 4:30 here. That is something to check tomorrow...

I will get the cap to make proper AC readings with DC present.

I think all ground wires are ok. PIN 9 on the indicator board is shorted to ground for sure. I checked that in all measurements I mentioned above. But to be sure I will check all other ground wires to the other boards.

One question: how can a broken ground wire affect the display?
 
"sneak circuits" where current flows because it has no other alternative.

doing strange things.
 
Can't find any broken wires. They all have zero resistance to ground. But I realise I can only judge individual wires if I cut them or desolder them... How likely is it this might be the cause? Is it the logical next step to check all individual wires disconnected. All wires and connections look good by the way.

If all goes well I have the 0.1uF/500V cap this afternoon and I can do some AC readings.
 
Don't disconnect anything....

What I would check is what's happening to that 18 volts as you do your display experiments with the wires connected and disconnected.

It is a zener regulator, and if the 18v is drawing too much current through the series resistor, it WILL drop out of the regulation range it was designed for as the voltage goes lower than the zener's.

overheating could explain the changes with "memory" you see.

I have a strong suspicion that the display brightness is STRONGLY related to the 18 volts, BUT the designers would have had a very hard time controlling that many aspects of the design (changing the 18v) so instead they fool with the +68 volts on the grid (reducing it), and when that wasn't enough - or had bad side effects after "going too far" - they then RAISED the ground with that resistor, and when something BAD happened THERE, they threw in the 8v zener diode...

kludge on kludge on kludge...... sigh.....
 
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Hi, this what happened today:

I got a 0.1uF/630 volts MKP (WIMA) cap because the local store did not have ceramic caps of these values. I tried to do AC readings, but this cap also blocks 50Hz AC... So no readings at all. That's a pity.

So I decided to check the DC and AC voltage in my SA-7800 as a reference with "my" meter. PIN 4 on the SA-7800 indicator board reads 43VDC and 95VAC to ground. So on my meter reading high AC seems to be normal in these amps. By the way, the SA-7800 uses 15VDC to light the segments, at least that is what my meter reads. Different than the 18VDC (19.1 on mine) the SA-9800 uses.

Now back to the SA-9800. Pin 2 (comes from pin 7 on the PSU board) on the indicator boards reads 19.1 VDC with both pin 10 connected (the display dims slowly in about half an hour) and disconnected (display is bright but with blurry segment edges). The resistor in this path on the PSU board looks normal. The zener diode in parallel is an extremely small white one.

With pin 10 disconnected, when I run the amp for a couple of hours everything seems to have "reset" to normal operation (bright display and the switch has significant effect) when I connect pin 10 again. But the display slowly dims again while the voltage on pin 10 raises from about 28VDC to 49VDC (bright position)

I am puzzled :scratch2:
 
One more comment:

On the indicator board are 4 caps missing: C13-C16 (0.047 pin 12/13 to pin 9 and pin 3/4 to the negative side of C12). According to the SM these should be soldered on the solder side on the PCB. They are NOT present on the indicator board of my SA-9800.

I imagine these caps kill all kind of high frequencies which could affect the functioning of the circuitry. Could this be causing an issue? This does not explain however why this only happened after I recapped the PSU, indicator and protection board...
 
I had a bunch of 0.047uF ceramics caps laying here so I soldered them on the board temporarily as you can see in the attachment. This did not solve the problem.

I took a picture of the PSU board. As you can see it has quite some burn marks indicating there has been some trouble in past as well. Especially the location where the zener and resitor regulating the 19VDC to the indicator board is showing heat in the past. All electrolytic caps except the two blue 220uF/100V philips ones are new.

Would be a good plan to replace all semiconductors (diodes and transistors) and of course the two philips elco's? This might result in closer to spec voltages, since not all voltages are up to spec, but 2 or 3 volts higher or lower. The lamp recieves only 3.6 VAC which should be about 8 I suppose, to start with. The lamp is not as bright as the lamps on my SA-7800 and SA-8800.

I also added a picture of the protection board which shows the zener diode that is soldered on the wrong side. This might not the best option as mentioned earlier in this thread (it does not regulate for 8 volts, but around 4.5 on pin 26 on the PSU board).

Another option is to replace all replaceable parts on the indicator board as well: diodes, resistors and caps (except the elco's which are already new).

Please help me out here. I have no clue what to do next :worried:
 

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Backlight said:
<snip>

Now back to the SA-9800. Pin 2 (comes from pin 7 on the PSU board) on the indicator boards reads 19.1 VDC with both pin 10 connected (the display dims slowly in about half an hour) and disconnected (display is bright but with blurry segment edges). The resistor in this path on the PSU board looks normal. The zener diode in parallel is an extremely small white one.

With pin 10 disconnected, when I run the amp for a couple of hours everything seems to have "reset" to normal operation (bright display and the switch has significant effect) when I connect pin 10 again. But the display slowly dims again while the voltage on pin 10 raises from about 28VDC to 49VDC (bright position)

I am puzzled :scratch2:
Click to expand...

You are assuming that the 19.1v dc stays constant. You are NOT proving it. The 7800 has a 15v voltage regulator on the board that has a FAR greater current delivery capability than the 9800's zener / resistor regulator.

Plus there are other resistors inserted in power lines that assume certain voltage drops for certain currents - and that ALL depends upon the internal conditions INSIDE the display - and if those conditions inside the display change - the currents and the voltages CHANGE.

the 9800 is complicated by the fact that the 19.1v is one of those voltages that DEPENDS upon certain ranges of current draw.

somehow you just don't get the gist of what I have been asking.

read ALL the voltages when the display is bright at the start, then read ALL the voltages when it dims down.

something is changing and you are only giving me disconnected keyhole glances at the big picture.

read EVERY TIME, even the "hours later", with the bright switch in the bright position:
awr-194 pin 13 to metal chassis ground DC voltage
awv-002 pin 10 to metal chassis ground dc voltage

awr-194 pin 3 to metal chassis ground DC voltage
awr-194 pin 5 to metal chassis ground DC voltage
awr-194 pin 7 to metal chassis ground DC voltage

awr-194 pin 26 to metal chassis ground DC voltage
awr-194 pin 25 to pin 27 AC VOLTAGE
the voltage across R23, the 330 ohm resistor across the transformer's primary.
 
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