Fluke 8000a multimeter question (lightbulb)

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I have an old 8000a fluke multimeter. I managed to fix it up by replacing the fuse and putting new solder on the switches. However there is one fault left. Next to its mains transformer there is a incandescent light bulb and it makes an annoying buzzing noise. It is a fluke brand bulb. It lights up fairly dimly.

Why is there a light bulb inside the housing? It does not provide any light to the outside. I figured it might have been there to warm up the housing but then why not just use a resistor? Is it working as some kind of inductor? As a load?

I really wanna replace it but I don't know if its something non obvious...

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On second thought it may just be the transformer making the annoying noise. I am still curious why they decided to put a light bulb there.

[PA0PBZ]

Hmm, looks like a battery charge control

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that's pretty bootleg

I wonder how I can get rid of this bloody buzzing noise though.
If I flex the board so it is convex (so the parts are on the hill and not the valley) the hum is reduced significantly.

[SeanB]

If it is an open transformer paint it with transformer varnish. That should reduce it a lot.  Flexing the board closes up the loose plates in the core.  If you are careful paint the core only with an enamel paint, but do not get it on the windings or the bobbin, as it may remove the insulating varnish on the windings.

As well check CR13 and CR14, one could be short or open circuit.

[PA4TIM]

I restored three 8000 meters but never seen a lightbulb in there. But two were standard and one had the 20A option, no batterypack.
I'm curious for a picture.

The 8000 uses a VF converter, maybe it resonates something ? otherwise I would think the transformer . Check solderings if flexing has effect

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I checked the cr14 and cr15 diodes. They have 9V passing through them when the device is on.

Also I would like to add that the noise is the worst when the display is flashing (from a overload, e.g. no leads connected during ohm measurement). The light bulb actually gets dim as the display flashes on and off.


I put additional solder on all the transformers pin and this had no effect. I might desolder it and do the coating as suggested.

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Here is a picture:

Oh yea I soldered in those capacitors myself because the batteries that were there leaked 10 years ago lol.
It buzzed before and after the capacitors.

http://image.bayimg.com/gabofaaeg.jpg
bright spot is the lightbulb


It was doing this weird counting after I removed the dead batteries. Basically it would start at 0mV and count up to the end of the range and display overload. The capacitors seem to fix that issue but I had it pop up once or twice. It needs to be calibrated but the only other meter I have is a radioshack meter from the early 90's and an analog one. It agrees with the radioshack one up to about 3 volts where a deviation begins that goes up to 300mV at 9 volts.

Someone threw it out because the fuse fried haha

[muvideo]

Hmm, looks like a battery charge control

Were batteries NiCd?
The lightbulb acts like some sort of current limited power supply, with the
added benefit of not needing heatsink

Fabio.

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Yes the batteries were NiCad.

I found a link that people may find useful:
http://en.allexperts.com/q/Electronics-3923/2008/11/Ni-Cd-Batterry.htm

"Old NiCad charges used a neat trick - they simply put an incandescent light bulb in series with the charging current.  When the batteries were at low voltage at the beginning of the charge, the light would get bright, raising its resistance, and charging with less current.  As the batteries charged and their voltage went up, the light bulb got dim and therefore charged with more voltage, but since the battery voltage was higher, this tended to be almost constant current.  Trouble is if you shorted the battery connection the light bulb would burn out and you didn't have a charger anymore."

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Ugh now this mulitmeter is jumping from 6 to 8.
I'm about to melt this sucker down for parts!

it is incapable of displaying 6 and 7! It skips to 8!
So, 4-5-4-5-8.
The range does not matter, it just can't seem to display 7's.
WHat the hell is this magic?

it does not display 3's either!
I resoldered the pins on the flex board going to the actual display modules. Continuity test does not reveal borken traces.

I found the manual so I may go through all their troubleshooting diagrams 1 by 1.

does anyone think that the lm7447 7 segment chip could be at fault here?

[poptones]

It sounds like bit 2 is stuck. Does it display 2?

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It does not display 2. It displays 0 instead of 2.

This happens for all measurement types on all range types.

This is like playing Russian roulette with measurements! \


any advice?


just a hunch but there seems to be a hex inverter that leads into the bcd converter chip. I wonder if its stuck high? Hmm...

I'm getting the feeling that i will need a logic analyzer to fix this thing.

[poptones]

Bit two is stuck at zero logic level. You can use a vom, just measure the high and low voltage. Or make a probe from a transistor a resistor and a led.

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I am going to start working on this tonight.
Here is a schematic for anyone interested :
http://image.bayimg.com/babipaaeh.jpg

thank you for the direction poptones.

[ModemHead]

Do I understand correctly that you replaced the NiCd batteries with capacitors?  The circuit appears to rely on the batteries for voltage regulation on the 5V supply.  With capacitors in place of the batteries, the 5V line might be too high.

[oh2bab]

Hi folks.

Any news with this one? I just bought a 8000a, with same kind symptoms.
It makes a noise, and without batteries inside the numbers kept bouncing. Now i have batteries inside, and the bouncing is not so bad.

0.01
0.03
-0.01

like that it goes (not exactly, but, well, you know?)

So, what i have now is a multimeter which makes a noise, and does not show any readings (edit: when i try to measure something) on any range. Damn.

Now i´m dropping in this question.. Where should i start with this one? Fuses are ok.