Fluke 8050A Repair

[Cigarsnob]

Howdy,

I just picked up a fluke 8000A and 8050A DMM from a surplus store yesterday. The 8050A model is the battery option (Connects to mains, but it comes with recharable bat). Well I knew the 8050A DMM needed some repair and when I switch it on, the "bat chg" LED comes on, but the DMM does not work. No LCD display. Although when I switch through the ranges, I do get a flash on the LCD.

I plan on buying some new batteries to see if that does the trick, but ideally, I would like to save myself the money and remove the batteries and make the 8050A function off the mains directly. I found one thread saying to just remove the batteries and install some beefy caps with a resistor. Does anyone know if this will do the trick?

I found the manual with the schematic and plan on digging into the DMM once I get home after the holiday, but thought I would seek some advise here while I wait.

Thanks in advance for any information.

Phil

[alm]

Some circuits use batteries as crude voltage regulators, in that case a cap wouldn't do. If it's just for smoothing, a cap might work found. This post in the TekScopes Yahoo! group suggests some minor circuit modifications (eg. replacing the voltage regulator and removing some resistors) to get the 8000A/8050A working without the batteries, so just putting in a cap may not be sufficient.

[ModemHead]

The battery-operated 8050A-01 uses an LM317 in current-regulator configuration to supply both charging current for the batteries and operating current for the meter.  The batteries need to be there to regulate the voltage (roughly 5V).  I modified one of my 8050As by removing the batteries and modding the circuit board to use the LM317 as a voltage regulator.  It works fine that way.

If you need more details on how I did the mod, I can provide them but it will take a few days since I am away from home for the holidays.

[Cigarsnob]

Modemhead,

If it's not too much of an inconvinance, I would appreciate more info regarding the mod you spoke of. I don't mind the wait as I'm also away for the holidays.

I should be able to make the mod just by looking at the datasheet of the lm317 and the schematic of the 8050A, but you can't go wrong with having more information.

Thank you,
Phil

[Conrad Hoffman]

I had a similar instrument that was a constant problem because of the batteries. I didn't use it often enough to keep the expensive battery pack in good shape. I replaced the battery pack with a big aluminum electrolytic cap, shunted with some power zener diodes to imitate the clamping action of a real battery. The zeners have to be sized to take the usually minimum charging current. It probably works better now than it ever did with batteries.

[grenert]

Hey, anyone have a spare LCD from one of these?  I've got one where I replaced the batteries with modern NiCd (NiMH self-discharge to quickly), but the display was squashed at some point, and it is "blurry".

Thanks and Merry Christmas to those who celebrate it!

[ModemHead]

If it's not too much of an inconvinance, I would appreciate more info regarding the mod you spoke of. I don't mind the wait as I'm also away for the holidays.

It's now on my to-do list.   

[amspire]

Hey, anyone have a spare LCD from one of these? 

I may have a working display. I have a non working meter I keep for spares. I well power out up and see if I can get any life out of the display.

What country are you in?

Richard.

[amspire]

I decided to look at what was faulty in my spare 8050a, and unfortunately, the only thing wrong is the display.

I think the display is fine, but there is a problem with the conductive rubber contract strip.

The contact with the most problem is pin 1 - the display common.

I do have spare Fluke 120v transformers, so if anybody wants to convert from battery to Mains, it looks like all you need is the transformer and about 9 other standard components. All the locations for the Mains supply part are actually on the board under R&D that is only used for the battery version.

Richard

[amspire]

This is not a bad alternative to sourcing a replacement LCD display for a Fluke 8050A DMM.



http://lous.home.xs4all.nl/fluke/Fluke8050Asite.html

The 8050A LCD does suffer from low contrast.

Richard

[alm]

This seems like an excellent alternative providing you don't care about battery life. In my opinion LED displays are superior to LCD for basic 8 segment displays. Much better viewing angle and contrast.

[ModemHead]

Modemhead,

If it's not too much of an inconvinance, I would appreciate more info regarding the mod you spoke of. I don't mind the wait as I'm also away for the holidays.

I should be able to make the mod just by looking at the datasheet of the lm317 and the schematic of the 8050A, but you can't go wrong with having more information.

Thank you,
Phil

Originally, U26 (LM317) is configured as a current regulator. When the front-panel switch is off, a 137mA charging current (determined by R26) is provided to the 4.8V battery pack. When the meter is turned on, an additional 104mA (determined by R25) is supplied to operate the meter and continue trickle-charging the batteries. As I recall, the meter required about 150mA operating current. The voltage from this arrangement would probably vary between about 4.8V for a discharged-but-healthy battery pack and 5.6V for a charged pack. Note that this Fluke schematic is in error regarding the LM317 input and output pin numbers.



I changed the LM317 configuration to regulate voltage at 5.5V, as follows:

• Cut two traces at the points indicated.
• Replace R25 with a wire jumper.
• Replace R26 with a 150R 0.25W resistor.
• Add wire jumper from R26 to U26P1.
• Add 510R 0.25W resistor from U26P1 to C23-neg.

This shows the mods from the solder side of the PCB.  The upper cut trace is obscured by the resistor lead.



Optionally, you can remove the red "Bat Chg" LED DS1, because it will still illuminate when the power switch is off.  Note that when the power switch is off, the transformer, rectifier, and regulator are still energized, and will consume some power (but much less than if batteries were installed.)

Re-installing batteries after these mods may lead to unfortunate consequences, so you may want to remove the battery pins or somesuch to prevent that.

[ModemHead]

This is not a bad alternative to sourcing a replacement LCD display for a Fluke 8050A DMM.

<snip>

http://lous.home.xs4all.nl/fluke/Fluke8050Asite.html

The 8050A LCD does suffer from low contrast.

Richard

I've done that one too, but the back side of that LED display is a labor of love.  Eh, some people climb rocks for the heck of it, I play with old gear. 

[Cigarsnob]

ModemHead,

Great description. Thank you very much. I will do this mod soon.

But before I do, without knowing anything else, do you think the batteries is causing the DMM to not function properly? I had it plugged in for a couple of hours when I first got home and switched the thing on. It worked at that point, but the low battery indicator on the LCD was always on. Then I noticed last night that it worked for a bit and then when I switched it off and back on again, I would get the low battery indicator on the LCD again along with a "1" and then the LCD would fade out. After switching the thing on and off again several times and sweeping through different ranges and voltage/resistor setting, the same result would occur. All of this after the DMM has been plugged in for the past 48 hours.

Well thanks again and I really appreciation the tutorial you provided. Hopefully this will get my DMM running again and consistently this time.

[ModemHead]

ModemHead,

Great description. Thank you very much. I will do this mod soon.

But before I do, without knowing anything else, do you think the batteries is causing the DMM to not function properly? I had it plugged in for a couple of hours when I first got home and switched the thing on. It worked at that point, but the low battery indicator on the LCD was always on. Then I noticed last night that it worked for a bit and then when I switched it off and back on again, I would get the low battery indicator on the LCD again along with a "1" and then the LCD would fade out. After switching the thing on and off again several times and sweeping through different ranges and voltage/resistor setting, the same result would occur. All of this after the DMM has been plugged in for the past 48 hours.

Well thanks again and I really appreciation the tutorial you provided. Hopefully this will get my DMM running again and consistently this time.

You're welcome, happy modding/repairing.

Your problem sounds exactly like a dead battery pack.  Bad cells will pull the voltage on this 5V nominal supply down far enough that the meter will not operate, or won't operate for long.

An alternate, even easier fix, is just to solder in any working sub-C NiCd cells that you have. If you're not concerned about actual battery operation, four 1.2V cells of just about any size will allow the meter to work fine on AC power.  Until the cells go bad, as they all do.  Smaller cells may get warm on the 140mA constant trickle...

[Cigarsnob]

You're welcome, happy modding/repairing.

Your problem sounds exactly like a dead battery pack.  Bad cells will pull the voltage on this 5V nominal supply down far enough that the meter will not operate, or won't operate for long.

An alternate, even easier fix, is just to solder in any working sub-C NiCd cells that you have. If you're not concerned about actual battery operation, four 1.2V cells of just about any size will allow the meter to work fine on AC power.  Until the cells go bad, as they all do.  Smaller cells may get warm on the 140mA constant trickle...

I don't have any C NiCd cells so I'll just stick with the mod you provided. Thanks again for all your help. I'll post my results in a day or two.

[ModemHead]

Since the transformer and regulator are energized as soon as you plug the meter in, 5.5V or so should appear across U26P2 to C23-negative even before turning the power switch on.  You can check your work this way without possibly damaging the rest of the meter if you make any mistakes.

[amspire]

This is not a bad alternative to sourcing a replacement LCD display for a Fluke 8050A DMM.

<snip>

http://lous.home.xs4all.nl/fluke/Fluke8050Asite.html

The 8050A LCD does suffer from low contrast.

Richard

I've done that one too, but the back side of that LED display is a labor of love.  Eh, some people climb rocks for the heck of it, I play with old gear. 

Thanks for the photo's.  It turns out my spare parts 8050A has a dud LCD - the backplane connection is open circuit internally. I have 3 other working ones (got four of them for $25 each), so I am going to do the LED display mod, so I have one meter that works in poor lighting.

After seeing your photos, I will make a custom PCB and find a way (I hope) of connecting to the LCD contact row, rather then all that wiring to the display driver IC's.  I should be able to salvage some SMD resistors and transistors, and I have ordered the displays and some acrylic filter sheet. Total cost will be about $20. 

Main reason for doing the work is that in debugging the LCD display, I found out that the rest of the meter works perfectly (I swapped display boards with a working 8050A), and I don't really need a 4th working LCD display at a cost of a new display - something like $50. To get one with with a LED display makes it worth the effort.

Richard.

[amspire]

I don't have any C NiCd cells so I'll just stick with the mod you provided. Thanks again for all your help. I'll post my results in a day or two.

If it were me, I would spend the $7 for a set of 4 NiCd cells.

http://www.ebay.com.au/itm/4-Sub-C-SubC-Tab-2500mAh-NiCd-Rechargeable-Battery-B1-/130617181876

They will die eventually, but you then get another set, and you can run the meter on battery power for a day's work. It is not just the portability issue, then meter has a higher safety rating when running from batteries. Running off battery also eliminates the 40pF Common Input-to-Mains capacitance that might cause a problem while measuring switching circuits.

Richard

[Cigarsnob]

Since the transformer and regulator are energized as soon as you plug the meter in, 5.5V or so should appear across U26P2 to C23-negative even before turning the power switch on.  You can check your work this way without possibly damaging the rest of the meter if you make any mistakes.

Hey ModemHead,

I am having an issue with the mod. I performed the steps that you listed, although my PCB was slightly different than the picture provided. When cutting the trace that connects to U26.1, I had multiple traces (3) ending at U26.1 besides the battery pack. These multiple connections was the transformer, T3, C22, and C34. The cut disconnected those components from U26.1. I take it this is correct, no?

Also, after adding the 510 ohm resistor from C23 (negative pin) to U26.1 and plugged in the power cord (after all other steps were performed), I only got a voltage of 4.5V when probing U26.1 and C23 (negative pin), so I ended up upping the resistance to ~650ish ohms to get 5.5V. At this point, I turned it on and nothing appeared on the LCD.

I checked my work a few times and I'm fairly certain I did the steps correctly. When I checked the voltages at TP2, 3, 4, and 5, I don't get the voltages as described in the schematic. I was checking these voltages when having one probe placed on C23 (negative pin) and the other on the test points. The voltages observed were roughly 6V (6.5V on some). But then I thought that I may be placing my other probe, (that connects to C23 negative pin) incorrectly so I placed in on the front panel common plug and the voltages on the test points were in the mV range.

I don't know if any of what I said provides any insight as to what I did wrong, but any advise would be appreciated.

I'll have another crack at it in the morning.

Thanks

[ModemHead]

I am having an issue with the mod. I performed the steps that you listed, although my PCB was slightly different than the picture provided. When cutting the trace that connects to U26.1, I had multiple traces (3) ending at U26.1 besides the battery pack. These multiple connections was the transformer, T3, C22, and C34. The cut disconnected those components from U26.1. I take it this is correct, no?

Yes, the cut should simply disconnect U26.1, leaving the rest of the circuit intact.  I've attached a diagram which I hope makes clear exactly how things should be connected before and after, regardless of PCB layout.

Also, after adding the 510 ohm resistor from C23 (negative pin) to U26.1 and plugged in the power cord (after all other steps were performed), I only got a voltage of 4.5V when probing U26.1 and C23 (negative pin), so I ended up upping the resistance to ~650ish ohms to get 5.5V. At this point, I turned it on and nothing appeared on the LCD.

The 5.5V should be from U26 pin 2 to C23 negative pin.  That is the voltage regulator output.  The voltage from U26.1 to C23-neg should be about 1.25V lower than 5.5V, or 4.25V.  Your measurement sounds like it was OK to begin with.  There should be no need to raise the resistor value, which would lead to an output voltage that is too high.

I checked my work a few times and I'm fairly certain I did the steps correctly. When I checked the voltages at TP2, 3, 4, and 5, I don't get the voltages as described in the schematic. I was checking these voltages when having one probe placed on C23 (negative pin) and the other on the test points. The voltages observed were roughly 6V (6.5V on some). But then I thought that I may be placing my other probe, (that connects to C23 negative pin) incorrectly so I placed in on the front panel common plug and the voltages on the test points were in the mV range.

The test point voltages are referenced to the meter ground, which is TP1.  The front-panel common input is also tied to the meter ground.

[matt09]

Not meaning to drag up a dead topic but I've just spent AGES trying to get this seemingly very simple circuit to work. I had no problem with getting the correct voltage out the regulator but the other board layout is deceptive. It appears very similar so you break the tracks to find that because of different spur off layouts on the pcb things don't join up anymore as they should.

Got there in the end.. 

[eugenenine]

I did similar with mine after the second set of batteries died.  Since its a simple charger it will wear out the batteries every few years.  IIRc I desoldered their regulator and mounted my own 7805 with a jumper wire so it didn't require cutting any traces.

[matt09]

I came close to doing the same but my stubbornness prevailed. 

[eugenenine]

Mine would never work and charge the batteries at the same time.  So I'd be in the middle of something and it would die.