EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: tango17 on September 08, 2016, 06:41:34 pm
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This is the power supply for a Fluke 8050 with the battery - 01 option. The batteries are new, and it seems to work, except the readings are all about 40% too high. One of the battery leads broke off, the manual says don't run it without batteries, and that's about when the problem started. Tracking it down, the VR1 reference is too low, causing the deintegrate to take longer, causing the high readings. Further checking finds that all the power supply voltages are low. I've marked the voltages in red that I'm seeing now.
Advice?
The zener VR2 has about 3.6v across it, but out of circuit it's ok at 5.6v.
Am I correct assuming Q5 and Q6 must be ok, otherwise I'd get no output voltages?
My problem is I'm busted and can only sit or stand for,, 5 minutes at a time. I run down, make a measurement, and then consider it while lying on my back, so I need to plan carefully for each test.
Thanks for suggestions.
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The +6V and -5V is definitely a problem. I have several of these meters and I've seen quite a wide range on the +13V and -10V supplies on working meters, basically anything 10V or more (absolute value) seems to be OK. Your -10V is high because it is relative to the ailing -5V, the difference being the battery voltage, 4.8V in this case. The pack must be flat?
Keep in mind the -5V powers all the CMOS logic, relative to ground.
I tend to agree with you about Q5 nd Q6, but the amplitude may still be down for some reason, maybe something on the rail is loading the +6 or -5 down. Hard to say.
Away from home right now, but in 2 or 3 days I can get you some scope traces on the primary if it will help.
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The pack has some gairly new NiCd in it, although they are a bit lightweight in mAh. The battery warning isn't on, and it reads pretty much the same with and without AC power on. Just to be sure, I did run a power supply across the batteries at 5.4v, but it didn't help. I was trying to decide if it was a loading problem or something in the supply itself. I haven't finished checking all the caps with the ESR meter. The waveforms would help, I've got a scope. Any suggestions on what to pull to see if it's a loading problem downstream of the supply?
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The only reason I questioned the battery status is that a healthy pack will usually rise to 1.4V per cell under trickle charge.
My memory isn't good enough to advise on break points, I need to look at one of my units first. I'm pretty gonzo about those things, I'd just start cutting tracks. ;D But do continue to check the caps and also the diodes in the area, esp. CR4.
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Actually, CR4 was next on my list. I'm glad we're on the same wwavelength. Then I'll finish the cap testing. Under load, the NiCds should be about 1.2, per cell so the 4.8 differential looks ok to me. Then I'll cut some traces to see if it's a loading problem. However, my first thought was that a loading issue wouldn't affect both the positive and negative supplies together. That's why I posted the supply circuit... suspecting it was probably in there somewhere. Plus, it might have been damaged when it ran without the batteries connected.
Thanks for the help!
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Ah, you had the batteries uner load, I was assuming we were on mains, with current being pumped into the pack by the LM317.
This power supply is an interesting animal. It's a basic inverter circuit for the +13 and +6 rails from the secondary. But it also uses the primary for a boost to get 5V above the battery, thru CR4, VR2, and C22. Since its all inter-related, I figure a problem on one rail would affect the others. Given your measurements, my intuition tells me to be most concerned about the -5. Could easily be wrong...
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Testing in circuit with the esr meter, C20 seems to be open. That could be it. CR4 tests good. When I can stand/sit again, I'll swap it out and report back.
Nope, it wasn't C20. On closer inspection, I'd measured the wrong terminals. C20 is ok.
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So where would you go next? Cut the -5 line after C22 to see if it's a loading problem there?
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I have one of these and even when working properly its a simple truckle charge circuit with no shutoff so it kills the batteries every few years. I finally removed the batteries and mounted a regulator so it works like the non battery option.
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Yes, I know about the battery removal conversion and the constant charge current design of this model. I thought about converting it, but I was buying some NiCds for another project, and it only cost $6 to get a set for the 8050, too. It's occasionally useful to be able to run it on batteries. If the display goes, I'll do the led conversion and that will probably take more power, making the battery life shorter, so I'll remove the batteries then. That's assuming I figure out what is wrong with it now.
CR4 reads about 3.8v reverse biased on the meter. I think its time to connect the scope and get more info.
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So where would you go next? Cut the -5 line after C22 to see if it's a loading problem there?
Well it is a quick way to answer the loading question, at least on that rail. Being that I'm both gonzo and also cautious, I'd probably cut the whole supply loose, to avoid any possible damage from energizing inputs on unpowered devices.
Just saw your latest... I should be able to get you some traces Sunday afternoon.
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I have one of these and even when working properly its a simple truckle charge circuit with no shutoff so it kills the batteries every few years. I finally removed the batteries and mounted a regulator so it works like the non battery option.
I have one unit that I converted to AC-only by modifying the LM317 circuit from a current regulator to a voltage regulator. Works OK.
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My problems was it only runs from batteries. If they had designed it to run while charging I might have kept the battery option.
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CR4 is switching at about 5-6 kHz. For 75 uS it's forward biased. It then goes about 8v reverse biased and drops to about 6v over the next 100 uS. Looks to me like it's a loading issue. I'll start removing loads.
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... If they had designed it to run while charging I might have kept the battery option.
It was designed to be run while charging*:
(https://www.eevblog.com/forum/repair/fluke-8050-battery-power-supply-repair-advice/?action=dlattach;attach=254413;image)
When the power button is pressed, half of the switch connects R25 in parallel with R26 (re: diagram in opening post). Thereby, increasing the regulator (U26) output current by 100mA, which should be sufficient to power the meter. Initially, my secondhand Fluke 8050A wouldn't operate while charging, but replacing R25 (which had gone high) cured the problem.
*The charge current while running the meter is sufficient to maintain the battery pack charge level, but not to increase it.
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The+13v rail has steady dc, if a bit low. The others all have the same 60/40 duty cycle at 6 kHz as CR4.
Following photos are all 200mv/div AC coupled 50uS/div. CR4 is forward biased for the 40% period of about75 uS.
Photos are +6v, then - 5v,then - 10v.
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The ripple looks excessive, esp. on the -10V. Have you considered just replacing the caps? They're old enough and cheap enough to do that as a matter of course.
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Well, I've checked esr on most of the pwr supply caps and they seem fine. In theory, the -10 is 4.8 offset from the -5 rail, so why the big ripple difference? Also, that ripple is on a DC average that's way off the nominal +6 and -5. Something's going on.
It makes no difference if iit's running on batteries or batteries plus AC. As expected, there's lots of ripple on the battery pack.
Notice how the -10v levels out. TThat's happening at about 8v plus.
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Following photos are all 200mv/div AC coupled 50uS/div.
The attached photographs are a repeat of your test point checks, on my Fluke 8050A, using the same V/div and Time/div settings.
NB My meter's power converter operates at a frequency of 25 kHz (measurement shown in last image).
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Thanks for the photos. Removing loads has been tricky, but so far I've seen no improvement. Your frequency is 4 times mine. I know C20 is good. Do you think I could have a problem with Q5/Q6?
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Had some difficulty getting non-blurry shots off of an analog scope. The frequency is modulating a tiny bit at about 2Hz. The trace looks like it has a heartbeat. I assume this is the variation in loading as the ADC is doing it's thing.
Running from freshly charged battery pack:
TP2: 14.4V
TP3: 6.1V
TP4: -5.3V
TP5:-10.7V
Oscillator frequency here is about 33kHz.
For the following shots, note that I was using 100mV per division instead of 200mV. The ripple on the +6V was too low to be interesting, so no shot of that. The last shot is the collector of Q6 (the tab), which is essentially the voltage across CR4.
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That last shot of CR4 looks a lot like mine, except the frequency of yours is higher and mine sags down the voltage more. I've removed the display board, with no effect, and I'm cutting traces, but I'm having trouble finding the right spot for -5. I guess its a loading issue, but I admit I don't fully understand how that affects frequency.
Thanks to both of you for the help. It's odd that my only symptom I saw was high readings. I later saw the display start to fade out after a few minutes of operation, but I figured that was a typical bad LCD display issue. While testing, I ran down the batteries, so I connected 5.3v to replace them, and saw 500 mA draw, which seems high to run the meter, so I'm back to a loading problem.
I'm only able to sit/stand for short periods, so it will be slow going before I can report back, but I will. Again, thanks!
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While testing, I ran down the batteries, so I connected 5.3v to replace them, and saw 500 mA draw
500mA? :o
This one is averaging 143mA off the battery pack, which is at 5.07V at the moment.
Sorry to hear of your sitting/standing difficulties, I know that must be frustrating.
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Quick experiment, I added 100mA extra load on the -5V and the frequency dropped to 16.7kHz.
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I'd previously heard that 150mA was about the draw to be expected, so, given the 500mA I saw, it's looking more like a loading problem. The frequency drop you saw under heavier load also points that way.
We had a power outage all day, so nothing yet, but I'll get the power supply isolated eventually, and verify it's working, then track down the load.
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With 350mA extra current draw at 5V, that's 1.75W of heat. Assuming a single fault in a limited area is responsible for that, something should be getting warm, no?
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Yes, something should be hot, but I didn't see/feel it. I also realized later that much of that current was going into charging the batteries. I need to pull them out next time I test, but I've already removed the display board and cut some traces, so I can't do that test yet. I'm getting there, but finding the right trace to cut is slow going.
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FYI, while buttoning up this unit that I got the scope traces from, I noted a small blue-green fuzzy place near C23. Removed the cap and found electrolyte leakage underneath. While replacing it, also found that the plating on the negative hole had eaten through, necessitating a patch wire. Interestingly, the cap still measures with nominal capacitance and 0.04 \$\Omega\$ ESR. Probably would have failed soon though.
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My C23 looks OK, AFAICT. I just can'tfind a good place to cut the - 5v trace. C22, the regulator U26, and pin2 of T3 are first, but the batteries are on the other side. and R25, R26 aren't connected until after that trace touches R15. BTW, what is the - 5F rail? R15 connects between -5v and -5F, but I can'tfind its source, unless it comes from the RMS module?
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Ok, advice requested:
Option 1- replace all caps in supply, C21, C24, C22, C34, even if they seem ok.
Option 2- isolate supply, disconnecting cathodes of CR5, CR6 connecting to new caps, pull cathodes of VR2, CR4, emitter of Q6, pin2 of T3 and batteries to test supply and hopefully find a short in the load
Option3- keep cutting traces
Option 4- reconnect all, remove batteries, check current draw from batterirs or supply (avoiding battery charge current) , if still high, one by one check current of each supply. The +13 and +6 should be easy.
Other?
I guess I lean towards 2, but....?
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The "-5F" rail is the "filtered" 5V rail, where the filter consists of R15 and C38 (shown just left of the controller crystal on the schematic.) That does mean that R15 is a good place to observe the current for a good portion of the -5V load.
Option 1: I would go here first personally, just as a matter of course with old electronics. I have seen electrolytics fail short before.
Option 2: Maybe, but it doesnt seem like you have a hard short that can be found with the power supply disabled.
Option 3: It just may not be physically possible to cleanly cut the supply loose. You may just have to make an assumption on whether the power supply or the load is faulty and try to go from there.
Option 4: I was confused by your operating conditions back up the thread, on battery, on AC, or on a DC power supply. I do think you need a good handle on the current measurements under a controlled condition. The data is a little haphazard at the moment, at least to me.
I'm happy to provide reference measurements, as long as I'm at home. I do travel frequently.
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I'll replace the caps. They probably need it.
Option 2 was to separate the supply enough to test it. Once separated, perhaps I can detect which rail has the excess load. I didn't mean to say I thought there was a dead short anywhere.
I agree, I need to check the current load. I've got the positive rails isolated, but the negatives are giving trouble. Thanks for the comments.
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Check/replace caps first, good. :-+
You got me thinking about this, so I came up with a way to measure the -5V supply current, without cutting tracks.
Looking at the schematic, it appears that all the current responsible for the -5V supply comes from the "boost" current from T3 primary that is dumped through CR4 into C22 while Q6 is off. So isolating the junction of CR4 cathode and C22 positive terminal, and then measuring current from there back to ground will be the -5V current. [Upon further reflection, this probably includes the -10V current too.] I am ignoring the relatively small and steady-state bias current for VR2.
I lifted CR4's cathode and removed C22 completely. Attached C22 negative terminal back to the board (white wire). Attached CR4 cathode to C22 positive (blue wire). Yellow wire goes back to the board where C22 positive was (actually ground). Measured the current between C22 positive and the yellow wire. It is about 58 mA. The meter was operating normally during the test, in 20VDC mode.
As you can see from the picture, my -5V supply actually measures -5.2V. As a further experiment, I tested how much current I could draw from this supply until it fell to -5.0V. It took 235mA additional load to get down to -5.00V. The frequency dropped to 9.7kHz.
Conclusions: The -5V supply is not the least bit wimpy or marginal. Given the voltage drop and frequency drop you're seeing, any fault on the -5V rail would have to be substantial, and easily noticeable in the battery current draw. So if you restore your meter to it's original state and measure the current draw at it's battery terminals, it might be revealing.
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I restored to original state, and tested battery draw with good batteries, no AC. It's slightly in excess of 500 mA! Would you check draw on the - 5 rail next, swap the caps or look for something hot? The only thing I've seen hot is the regulator U26 when charging.
I hesitate to let it run for long now that I know how high the current is, but it has previously run for many minutes without harm. I have noticed that the display will eventually fade out if it runs for more than a few minutes, but it's not a problem on the display board since disconnecting that board doesn't help.
I supose I lean towards checking current on the rails, but the caps need replacing anyway. Thanks for the test info and suggestions.
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I don't have a 8050A and certainly modemhead is providing you excellent suggestions. My only contribution is to say that in this thread below
https://www.eevblog.com/forum/testgear/fluke-8060a-refurbish-questions/?all (https://www.eevblog.com/forum/testgear/fluke-8060a-refurbish-questions/?all)
the OP noted a 25mA draw on the 8060A. After replacing the caps, which were obviously bad at the bottom bung, the current draw went back to the normal 2.5mA.
I'm not saying this is your root cause, but it is possible.
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Capacitors. Sometimes they fail low resistance. Gotta rule them in or out.
(https://www.eevblog.com/forum/repair/fluke-8050-battery-power-supply-repair-advice/?action=dlattach;attach=256267;image)
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Caps it is.... Will let you know.
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I replaced a few caps, and had to quit, but decided to get a more accurate current draw measurement than my old Simpson would give. The new Fluke said 930 mA! With that much, something had to be getting hot. It was U26. It will be tomorrow before I can sit/stand again, but we're hot on the trail. I'll check the U30 bridge, then replace C23 and the U26 regulator.
If I recover early, perhaps I'll just remove U26 tonight to see if that's it.
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... but we're hot on the trail.
You're a real funny guy. ;D
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... but we're hot on the trail.
You're a real funny guy. ;D
A poet, but don't know it. :)
Can't stand yet, but friend dropped by to see the invalid and he pulled C23 and regulator for me. The cap tested good. With both out of circuit, it's still drawing near an amp. Q6 is hot. I can understand Q6 being hot if it's driving 4x-5x the normal current, but why did U26 heat? I'll check it later. C22 and C34 are still to be replaced (across - 5v and - 10v rails), so I go there next. Then I'll check current draw on the rails. Shouldn't be too hard to figure out which rail.
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Assuming no AC power, I can't see a way for U26 to heat up without a simultaneous fault in C23 or the bridge rectifier, and then it would conduct even with the switch off.
With all the trouble-shooting activity, it might be a good idea to do a close visual inspection for solder bridges, bits of wire, etc.
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The U30 bridge is good, as is C23. The regulator U26 is bad, with no regulation (tested out of circuit) and Vi connected to Vo with a low resistance /diode path. I'm pretty sure pins 2 and 3 of the regulator are mismarked on the circuit diagram.
There is a fuse wire between the two battery packs soldered in and located just above CR4. Presumably it acts to prevent battery shorts. That fuse wire just burned out. Oddly, neither C23, nor U26 were installed when it blew, unless it let go earlier and I didn't notice it. In fact, I don't think it was even turned on when it let go, but I can't be sure
I'm pretty sure I smelled it let go at a time when one pack was not even connected. I found no shorts, yet...
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I'm pretty sure pins 2 and 3 of the regulator are mismarked on the circuit diagram.
Yes they're mismarked. I meant to mention that and I forgot. My mnemonic is 1-2-3 = AOI "Age Of Information".
It seems more probable to me that the regulator failed when the meter was on AC power, or some kind of "oops" moment when attaching a DC power supply to sub for the batteries.
Please just go ahead and replace C22 and C34, if you haven't already done so. And don't try to power the unit without a known good C22 in place.
Do you think the unit is in the same condition as when you started, or have things gotten worse?
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Hard to say if it has gotten worse. I'll check C22. I'll also have to repair that wire fuse.
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You know I had never noticed those fusible links before. There are two of them, one on each side.
Since NiCds are obviously capable of destructive current, consider doing all further testing by disconnecting the packs and powering with a 5V current-limited supply at the battery terminals. If your DC power supply has variable current limiting great, otherwise maybe a 12V light bulb or somesuch in series.
Retiring for the evening...
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C34 and C22 both are replaced. That makes C20-C24 and C34, all of which are replaced and all of which tested good (original and replacement) for capacitance, ESR and resistance for good measure.
I was in the process of designing and building a current limited dual supply, when I got access to a calibrated Fluke and thought I'd calibrate my two 8050s, and this started. Half of the current controlled supply is working, albeit not yet in a case, so I'll set it back up and then wire in the fusible with a 1A fuse.
Before powering anything, I want to do more investigation of how the link could have blown, and how U26 could have gotten hot. There must be something there.
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I got some help this morning. It's drawing 1.06 amps at 4.8v. I can actually hear the oscillator running... Eeeeeeeeeeeee. There don't seem to be any shorts near U26. DC resistance the batteries see is about 580ohms.
I think I'll monitor current flow through CR4 next. U26 and C23 are still removed, and to prevent damage to the ribbon cable I've removed the display board and installed a temporary power switch.
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C34 and C22 both are replaced. That makes C20-C24 and C34, all of which are replaced and all of which tested good (original and replacement) for capacitance, ESR and resistance for good measure.
Ok, I feel better now. Those caps are obviously right in best location to cause this kind of fault.
The reason I asked about the problem getting worse was because we started with low p.s. voltages and high ripple, and now it seems we have a 1 amp near-short. I wondered if that has existed the whole time. This, and the mysterious heating U26, is the reason I suggested a good physical inspection.
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I think I'll monitor current flow through CR4 next.
Remember it is a square wave at the oscillator frequency. Current dumps across CR4 to C22 during the time Q6 is off. If I understand the circuit correctly, Q5 regulates Q6's charging current to keep C22 5V above the positive battery.
Added: According to my previous measurements, it should average about 60mA.
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I should have said I'll lift one end of C22 and CR4, and measure current as you did.
I checked DC resistance across C21 C24 and C22 while in circuit, which correspond to the two positive rails and-5. There was nothing that would account for an amp of current draw. I wonder if I could have a short in a T3 winding?
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I should have said I'll lift one end of C22 and CR4, and measure current as you did.
I checked DC resistance across C21 C24 and C22 while in circuit, which correspond to the two positive rails and-5. There was nothing that would account for an amp of current draw. I wonder if I could have a short in a T3 winding?
I'm relatively sure you can also just lift CR4 and measure the average DC current through the break and get the about the same number.
You said the resistance on the battery terminals is 580 ohms. You're right, this does not account for the current draw. That suggests to me the fault is on the downstream side of a silicon junction, which isn't turned on by a low-power ohm-meter.
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Well, we'll learn more once I get some measurements of current on the rails.
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In a few I have to go out for the rest of the day, so the peanut gallery goes silent.
Regarding T3: This looks like a blocking type oscillator, not too much unlike your basic joule thief. If a T3 winding were shorted, would the oscillator run at all?
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My grateful thanks to the peanut gallery. CR4 has 298mA read by a calibrated Fluke 87V. Freq is 5.5 kHz.
Hmmmm.
After some time flat on my back, I'll grab some more current measurements through CR5 and CR6.
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Recalling that I have removed the display board, these are the currents measured:
Total battery current - 1022 mA
CR4 - 290 mA
CR5 - 5 mA
CR6 - 254 mA
Presumably the rest of the current flows from the -5 rail back to the -10 rail or via Q5/6.
Where to go from here....
1 Hunt for the missing current.
2 I haven't tried changing the selector switches to see if it affects current.
3 Try again to isolate the supply from the load to make sure it works.
4 Measure resistance between various rails, like between +6 and -5
5 Try the heat test again and see what's getting hot from that 1+ amp
6 Remove socketed DIPs. Many are powered +6 to -5.
I'm flat on my back for a few more hours, so I'll keep on contemplating.
I realize I can't just add up currents, since it's being transformed to different voltages, but it still looks like there's some more there somewhere. The +6 flow indicates to me that there is a heavy load on it somewhere.
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I made reference DC mA measurements from my working 8050A for you. Operating normally in 20VDC mode, display board connected. Changing ranges doesn't seem to affect the readings much.
Used AVG reading from a Fluke 87V in DC mA range, with MIN/MAX engaged. Here the current is a 31kHz waveform, and it does vary periodically with the integration/de-integration cycling of the meter, which is why I used the AVG number.
Battery - 151.8mA @ 5.1V
CR4 - 58.3mA
CR5 - 5.2mA
CR6 - 13.8mA
Looks like you have a serious problem on the +13V and possibly both negative rails.
Yes, to get the currents to add up, you would really have to count watts, and presumably a good chunk of it is being dissipated as heat.
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Success!
It was the LM358 on the U32 hybrid RMS converter board. I cut its power leads and the battery current dropped to 192 mA.
The rails now measure +14.6, +6.0, -5.2 and -10.2.
I don't have much luck with the U32 hybrid RMS converter board. My other 8050a had the transistor array fail. It's fixed, but not calibrated, and that was one of the goals when this started, to see if this unit matched the Fluke 87V and to get the repaired unit AC calibrated.
That U32 board is supposed to be replaced as a unit, and it's hard for me to calibrate AC as I don't have a decent transfer reference. Does a computer soundcard work for low frequencies?
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Awesome! :phew:
Anything special that led you in the direction of the LM358?
I've had really good luck with the 8050A, except for LCDs and the usual trouble with dead NiCd cells screwing things up. In fact, the reason I have several units is because I bought "non-working" ones on eBay to get good LCDs, but they were all easy to get running. I ended up converting two to 7-segment LEDs, in my usual gonzo sort of way.
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Both of mine were non working, too. This one just needed batteries. The other was sold as working, but wasn't, and after complaining, I got a full refund, and fixed the U32. Its display is dying, but I've got LEDs to repair it (I've read your post on that repair).
As to what lead me there, I figured if it's not capacitors, it's usually an active component. I marked all the semiconductor components driven by +6 and figured it would be getting hot. I felt them all. None were warm, then I remembered that the components on hybrid U32 are not shown on the schematic, and checked them. As usual, in hindsight, I should have been smarter earlier.
Now to put it back together, replace the LM358 and regulator and do some calibration.
Thanks for the help. It's easier when you can bounce ideas off someone else.
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You're welcome. I've never had to really trouble-shoot this power supply before, so this exercise caused me to stare at the schematic enough that I understand it better now.
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Update 1: All the traces have been repaired, the diodes resoldered, the display board reconnected, the regulator replaced and all caps replaced (which was probably needed anyway). The LM358 is disconnected, but not yet replaced. I'll have to check my supplies or order a new one. The DC is low, but that's because I had the calibration turned way down to compensate for the high readings Iwwas seeing. I'll do the DC calibration next. All voltages are good and it's charging and running off AC correctly. I don't know what blew the regulator. Probably an oops or a piece of metal on the bench shorting something.
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What do you do when you need to replace the batteries on one of these? I have one with the batteries missing (but have the leads) and a bad LCD. I'm assuming your rolling your own battery packs but I do not have an existing pack to replicate. I know it was taken out of service because of dead batteries and the LCD was swapped into another unit with a bad LCD. I might try an LED conversion on it if I can solve the battery issue (which I'm guessing is 3-4 NiCad batteries in parallel on each side as I seem to recall seeing something about 1.2V on the silkscreen.
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Sounds like great progress. It's fortunate you can fix the RMS module with a jellybean part.
I don't know what blew the regulator. Probably an oops or a piece of metal on the bench shorting something.
If the LM358 was the first fault to develop, then the batteries were actually providing all the extra current. When the batteries became disconnected, that huge load was shifted entirely to the LM317, which must have given up quickly. Just guessing.
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What do you do when you need to replace the batteries on one of these?
The original battery was two packs of two C-size NiCds, wired in series for 4.8V nominal. The power supply is designed to deliver a constant trickle charge of 140mAh with the switch off, which is a reasonable overnight C/10 charge rate for cells in the 1200 to 1500mAh capacity range of that time. With the switch on, the charge rate is reduced to about 100mA.
I have one battery-powered unit I converted to AC-only by changing the regulator from constant-current to voltage regulation at 5.5V (https://www.eevblog.com/forum/repair/fluke-8050a-repair/msg79394/#msg79394). For others, I just used four generic eBay NiCd sub-C cells. The little plastic boxes are over-sized for sub-C, so I stuffed some foam in to fill up the space.
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I use ebay and buy 4 sub-C NiCds with tabs of at least 1200mAh each. The last set I bought was 1300 mAh, but you can go as high as will fit. The inner dimensions of the plastic box are 53 mm by 50mm by 27mm. All sub c NiCds are 23mm in diameter, so worry only about length and keep it under 53 mm. You could probably fit 5/4ths sub cs into that space.
When the Option 01 8050a unit is off, but plugged in, it puts 1.2v across a 9.1 ohm resistor as a constant current source to charge the battery pack. That's130mA. For my batteries, that's C/10, which is high for always on charging, and will likely kill the batteries in a few weeks/months. You'd likely be better off with 2400 mAh batteries, plus they last longer if you really use it without AC. I solve that by plugging it into to a power strip that I turn on only when I'm working. It's kept them alive.
Update 2: I had added all the U5 jumpers in the original calibration attempt with low voltages. Now that it's repaired, I followed the steps to measure voltage and cut the U5 jumpers to get the R11 and R12 trimpots into calibration range. With the correct voltages, lo and behold, the jumpers it told me to cut were the same as those I'd added. It was still in the same range (out of 16). Further, after calibrating with R11 at the 1.9000V range, the R12 190.00 mV calibration (which I hadn't touched) was at 190.01mV. Not bad.
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. With the switch on, the charge rate is reduced to about 100mA.
The power on switch adds 12 ohms in parallel with the 9.1 ohm, so output current from the regulator goes up to about 230mA by my calculation. My unit was drawing 190mA without the display board, so charge current into the batteries might be even less than 40 mA on mine. If the batteries are charged, and you're going to leave it plugged in, the batteries might last longer if you keep tho 8050 power switch on rather than off and constantly pumping 130-140 mA into the batteries for months on end.
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Thanks for the tips on which batteries to obtain and the link to bypassing them. I'm still undecided what to do with this meter but converting it to LED sounds fun :)
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If the batteries are charged, and you're going to leave it plugged in, the batteries might last longer if you keep tho 8050 power switch on rather than off and constantly pumping 130-140 mA into the batteries for months on end.
The unit I was measuring upthread seems to be on the low end of current consumption. Possibly different board revisions. With the switch on, whatever current is left over will charge the batteries obviously.
The unit I use all the time is an LED-converted one that averages 200mA consumption with a 40mA charge rate with the switch on. I leave it powered on 24/7 unless I'm away from home, in which case it is unplugged. The batteries have lasted for years this way. It is sort of handy in a power outage, it will be sitting there glowing red, ready to check batteries in flashlights.
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Thanks for the tips on which batteries to obtain...... converting it to LED sounds fun :)
You can get 2200 mAh on eBay for under $2 each, and 2400mAh for a bit more. Mine only cost $1.50, but are smaller (I needed them for another project.) I was pretty pleased with my 8050 purchases, two units for $36 plus $6 for a ser of batteries. One LCD display is dying, so I'm also going to do the LED conversion soon.
Doed anyone have any suggestions for a cheap way to do the AC calibration? I need a stable 200Hz AC source at 2 volts. It also calls for 10kHz 19 volts and 100 volts, but I can live without calibration of those ranges.
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Doed anyone have any suggestions for a cheap way to do the AC calibration? I need a stable 200Hz AC source at 2 volts. It also calls for 10kHz 19 volts and 100 volts, but I can live without calibration of those ranges.
For tweaking the low-end AC, I found my cheap GwInstek DDS function generator becomes fairly amplitude-stable if I let it warm up a couple hours. Also I did some experiments and found that a computer sound card playing a sine-wave tone was quite stable. I have one of those function-generator apps on my Android tablet, but I haven't really checked it out yet.
Mine were reasonably well-calibrated when I got them running, so I just left the adjustments alone for the high ranges and the frequency compensation. Indeed I found that simultaneous monitoring of mains voltage agrees with my better meters, so I'm happy enough with that.
I seem to recall there have been threads on this forum before about generating stable AC for calibration. There are some volt-nuts hanging around here. I am only a volt-kumquat.
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I've got a sound card, so I could use that, although I'd need to amplify it to get 1.9 volts it recommends for calibration. I also remember I've got an old crystal controlled oscilloscope calibrator that runs from 10 Hz to 100 kHz in steps of 1-2-5 and with outputs from 10mV to 10V. It's square wave, but I suppose I could try it, or build a filter to get closer to sine wave.
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The sound card idea works great. I found an android app that simulates a 2 channel signal generator called Function Generator that does what I need.
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Thanks for the feedback and updates, it's good to document what the outcomes are in these threads, for future readers.
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Well, I haven't really updated what happened, because I didn't have an LM358A. I only had LM358s, so I need to place an order, and I hate to pay $6 shipping for a $0.42 part. I'm putting together a larger order for some other projects and will post on the final results..... eventually. However, here's an interesting tidbit from the manual errata page I'd never seen.
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Never noticed that either. Bizarre little bug. I'm away from home again, but when I get back I'm going to check that out.