Help with a power supply repair?

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earthsled

Well-known member
Joined
Dec 1, 2009
Messages
405
Location
Asheville, NC USA
Hi folks!

I'm hoping someone can help me sort out this power supply. I have an old CD player I'm repairing for a friend (yes, feel free to laugh). The CD player in question is a California Audio Labs model Delta. Anyway, the unit has 90s era fluorescent display and I expect the driver IC is bad (the display appears to be dead). The display driver IC is a Technics / Panasonic MN15283PEY-1. I've ordered a replacement IC, but I haven't received it yet.

From what I understand, if the driver IC were to fail it would often damage the -33V section of the power supply (see attached schematic). Indeed, I can see that someone has attempted a repair to the components in the -33V rail.

When I look closer, I can see that D306 is not a 6V2 zener like the schematic shows. In it's place is a 17V zener. I assume this change was to adjust the output of this supply from -33V to something closer to -28V.

I'm unable to find a data sheet for the display driver IC, but I have found another service manual that uses it. In this other service manual, I can see the supply voltage for the driver IC is -28.3V. This is why I assume someone was trying to adjust the voltage from the original value.

So here are my questions:

1. I'm struggling to understand why D306 is attached to an AC voltage instead of ground. What is the function for the zener in this position?

2. I gather that the circuit is using a voltage doubler (or tripler ?). In any case, I would assume D305 is being used for regulation. If the original repair was intended to lower the voltage, wouldn't it be a better idea to change D305 to a 28V zener?

3. The voltage on this rail seems to be rising extremely slow - over the span of 60 seconds or more. Is this typical for the way the circuit was designed, or should I suspect other faulty components in the circuit?

Thanks for reading!
 

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There must be a ground connection for the display. D306 is just fixing the otherwise floating 5VAC at ~6.2V above -33V. There's probably a resistor from ground to the 5VAC supply circuitry on the display board which then drains down through D306. Using the 17V may have been an attempt to reduce the current being dumped into the -33V line reasoning that that's why it's slow to reach steady state. I don't know much about that type of display but if I'm correct, it's not a completely unreasonable thing to do. The -33 is for the backlight and presumably it doesn't really care where the 5VAC is relative to it as long as it's within a certain range.

Yes, it's a multiplier of some sort and D305 sets the voltage. Yes, a 28V zener minus the 0.6V emitter junction should make around  -27.4V.

It should not take 60 seconds. A multiplier like that will be slow but not that slow. The electrolytics could be leaky and generally too old. If you replace those diodes with fast recovery types and replace the electroytics, it might fix that. However, it might also be that the backlight is old and drawing too much current. Unfortunately replacing the display IC probably won't fix that.

All of this is wild speculation of course. It's hard to tell what's going on without more detailed info about the display.
 
The zener appears to be connected to one side of the display heater supply. This implies the display is directly heated so this connection is effectively the cathode of the display IOW its local 0V.

Cheers

Ian
 
earthsled said:
I'm unable to find a data sheet for the display driver IC, but I have found another service manual that uses it. In this other service manual, I can see the supply voltage for the driver IC is -28.3V. This is why I assume someone was trying to adjust the voltage from the original value.
This value isn't so critical.

1. I'm struggling to understand why D306 is attached to an AC voltage instead of ground. What is the function for the zener in this position?
It biases the heater positively vs. -Vdd to ensure that un-addressed segments do not lit in mux mode. 6V2 is just fine, IMO.

3. The voltage on this rail seems to be rising extremely slow - over the span of 60 seconds or more. Is this typical for the way the circuit was designed, or should I suspect other faulty components in the circuit?
Something is faulty and overloads this PS. Most probably the IC you like to replace.

http://www.noritake-itron.com/NewWeb/Glass/GlassApp.shtml

@squarewave
There is no backlight in VFDs.

Here is an example  with test values.
 

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Thank you for your comments and suggestions everyone! This discussion is massively helpful!

squarewave - You were correct about the electrolytics. After I replaced C303, the lag-time in the voltage was eliminated. Now I have a strong -33V right when I flip the power switch.  Good stuff!!!

I had already pulled the display driver IC previous to my first post here. I've ordered a socket to help with testing. 64-pins is not something that I want to rework too much. Perhaps the original IC is still good? Maybe it was C303 causing the issue all along? At least with the socket, I could potentially swap the old IC with the new one fairly easily.

I've tested the old IC for shorts from pin 36 (Vpp supply) to all other pins. I did not find any shorts or low ohms. Can I assume it's safe to test with the old driver IC in circuit?

Also, should I worry that the -33V supply voltage might be too much for the IC? Just like the schematic posted by moamps, I've seen -28.3V to be a more typical supply voltage when using this display driver. Is -33V close enough, or is there potential for over-voltage here?

Thanks again!
 
The chip seems to be rated for 72V.

The choice of 28V or 33V may be more about the price of the product, desired light output, and type of face-plate used.
"This voltage may be varied within the specified rated voltage range in order to adjust  the basic brightness of the VFD"
 
Thank you, PRR!

I was not able to find a data sheet for the MN15283PEY-1. If it's rated for 72V, then I won't worry about the difference in voltage.

I'm feeling pretty confident that the old driver IC is safe to test with.  Worst case, it takes out the same section in the power supply. Given the discovery of the faulty cap, would anyone here advise against using the old IC?

Thanks!
 
earthsled said:
Thank you, PRR!

I was not able to find a data sheet for the MN15283PEY-1. If it's rated for 72V, then I won't worry about the difference in voltage.

I'm feeling pretty confident that the old driver IC is safe to test with.  Worst case, it takes out the same section in the power supply. Given the discovery of the faulty cap, would anyone here advise against using the old IC?

Thanks!
In general when making repairs our time is worth more than a few  dollar ICs...  A failure elsewhere may take out the IC with it , or not.  YMMV

JR
 
Well, the old IC is good after all. The display is working once again! Just one faulty cap was all it took.

Thank you everyone for your time.

Cheers!
 

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earthsled said:
Well, the old IC is good after all. The display is working once again! Just one faulty cap was all it took.

Thank you everyone for your time.

Cheers!

Nice one, congrats!

Replacing old Electrolytics can be a wonderfull thing.
 
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