Yamaha DSP-E390 display is too bright

[abhi1981]

Hello
My Yamaha DSP-E390 display is too bright, and there is no dimmer option. I'm worried that it may shorten the life of the display.
Can the circuit be modified somehow to decrease the brightness ??

[abhi1981]

Thanks for your reply edavid
No, I don't have the manual. It's not available on manualslib. Searching for some alternatives.

[abhi1981]

VFD circuit
Now, where to add the resistor??

[Uunoctium]

tl;dr
my sight of view is that VFD brightness is less determined by filament but rather through current per lightning element. And this depends on Vcc - in your case -23.6 V. Try to reduce this voltage by inserting an zener diode eg 5V6. Should be one of more power. I recommend BZX85Cxx, 1.3W
I've done this with success to re-illuminate two mechanicall broken VFD's with totally different NOS-types

[NiHaoMike]

That zener between VEE and the resistors drops the supply voltage. Measure the voltage across it when the display is on and then replace it with one of a higher voltage. You can use a potentiometer to find out what voltage works well although that won't work well as a replacement since then the voltage drop would change wildly with the number of segments lit.

[floobydust]

OP's pics I can see the filaments glowing, they are usually much dimmer... which implies mains is too high voltage. OP what does it measure?

[abhi1981]

I also suspect that the voltage may be too high (got it used, so don't know it's history)
I'll check it and let you know.

[Chris56000]

There's a Service Manual on Elektrotanya for the DSP–E800 free that you can probably use as a guide – it's got the circuit signal paths and voltage readings marked in orange to help you pick them out!

The rated filament voltage across the end pairs of pins is normally 3.15 V AC nominal for this type of vfd display and they certainly should NOT be glowing a visible red!

I would use the DSP–E800 book as a guide and check the mains transformer against the spec given on the circuit diagram – if you're somewhere where the mains is 240/250 V or even higher, (common in more remote countries!) then buy a metal clad resistor of about 220 Ω 15 W (there's probably plenty of room in the chassis to fix one!) and wire it in series with the mains from the on/off switch to the primary of the transformer.

You might have to experiment with the two series resistors marked on your circuit sketch to get the filament to 3.15 V – change them both together otherwise the voltage distribution over the ends of the display will be uneven, causing one end to look dimmer than the other!

For example, if you find a 22 Ω resistor in one lead works, then you need a 12 Ω in series with each!

The remarks made by other Members regarding the zener diode in circuit from the filament mid–tap to the 23 V  L.T.– supply line I am in agreement with – make sure it's not gone s/c !

Chris Williams

[abhi1981]

Thanks Chris for your valuable suggestions.
I found the service manual at Radiomuseum.org, but could download only one page due to download restrictions. Also, I couldn't post it because it's copyrighted with watermark. So, that's why I posted the handwritten diagram.
You are right regarding the mains voltage, it's around 240 volts. The mains voltage fluctuates a lot( it can easily go upto 270V  ) so I'm using a mains voltage stabilizer.  It maintains the voltage between  220-240V.
I suspect that someone might have fiddled with the DSP because the seller told me that he got it "serviced".
I'll check and let you know. Actually, the DSP is at the bottom of the rack and disconnecting all the wires and taking it out will be a challenge  .

[abhi1981]

Checked the display, when the mains voltage was exactly 220V, still bright. 

[abhi1981]

How do I measure the -18.2 and -23.6 volts with a DMM??
I mean where should I connect the negative and positive leads of the DMM, to measure negative voltage??
Please advise.

[Uunoctium]

The main disadvantage is the lack of circuit diagrams

So let's look at similar devices: they all used the same procedure. Overall, a separate discrete regulator for Vp was installed, consisting of 1 Transistor, 1 Z-diode and 2 resistors.

The E492 stabilizes at -23.6V; the A780 at -32.3V and the E800 at -29.9V.

The easiest way to solve your problem is to replace the Z-diode with one at a lower voltage. (With a little more experience in electronics, you can add an extra transistor and a variable resistor to fine tune the VFD voltage.)

So trace Vp (I hope they marked it) from the display board back to the power supply. I bet that's the board with the two electrolytic capacitors directly below it. Locate Tr and the Z diode.

Change e.g. 22V to 18V or 30V to 24V. I don't know the original value, so you have to figure that out.
(The series resistor should be able to handle the small additional load - correctly it should be recalculated).

[abhi1981]

Thank you Uunoctium for your help.
I found the service manual at Radiomuseum.org, but could download only a few pages due to download restrictions. Also, I couldn't post it because it's copyrighted with watermark. So, that's why I posted the handwritten diagram. I have added the -VF circuit. The zener diode is HZS242TD. What value (part no.) should I use?
Sorry for my bad drawing.  Please let me know if I am missing something.



Also, what is R439 for? I tried replacing it with 15ohm resistor, but no effect on brightness.

[Uunoctium]

R439 limits the filament current. Altering has less effect on brightness, but protects filament
 
The HZS242TD is an 24v, 0,4W  Zener Diode. I suggest to replace it with 18V

But i'm not sure, if the TPD of 2SA1015 (Regulator transistor) rated @400mW is sufficient. His max current of 150mA is enough. I bet, they inserted that 1kOhm resistor to reduce the voltage drop and therefore total power dissipation over transistor.
On the other side: with lower operational voltage the current per segment is reduced, so it may match.
In general, check (with your finger) if it runs very hot after modification.

[abhi1981]

The -38.1 is going straight to the amplifier transistors.
Will changing the diode affect this voltage??

[abhi1981]

Hooray!!!!!! It worked!!!!
Changed the zener diode to 18v and also R439 with 15ohm resistor. Significant reduction in brightness. The glare is gone now.   
Also, the transistor is not heating up.

Thanks everyone for your help.
Special thanks to Uunoctium.

[Uunoctium]

glad to see, that your work was successful.
Btw: that reduction gives the VFD tube a significant longer live. If it turns dim in future, you can easily reinstall the original components

[james_s]

I wonder if it was a mistake in the original construction? A VFD should not be blindingly bright, there is really no good reason to drive it that hard.