Author Topic: Fluke 8050 Battery Power Supply Repair Advice  (Read 16621 times)

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Offline ModemHead

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #25 on: September 12, 2016, 10:44:50 pm »
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?
 

Offline tango17Topic starter

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #26 on: September 13, 2016, 07:17:38 pm »
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.
 

Offline ModemHead

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #27 on: September 14, 2016, 03:29:46 pm »
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.
 

Offline tango17Topic starter

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #28 on: September 15, 2016, 04:29:20 pm »
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?
 

Offline tango17Topic starter

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #29 on: September 15, 2016, 05:23:28 pm »
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....? 
« Last Edit: September 15, 2016, 05:35:09 pm by tango17 »
 

Offline ModemHead

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #30 on: September 15, 2016, 06:08:15 pm »
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.
 

Offline tango17Topic starter

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #31 on: September 15, 2016, 08:49:17 pm »
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. 

 

Offline ModemHead

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #32 on: September 15, 2016, 11:45:14 pm »
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.
« Last Edit: September 15, 2016, 11:50:18 pm by ModemHead »
 

Offline tango17Topic starter

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #33 on: September 16, 2016, 05:03:47 pm »
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.

 

Offline retiredcaps

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #34 on: September 16, 2016, 05:14:39 pm »
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

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.
 

Offline ModemHead

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #35 on: September 16, 2016, 05:34:32 pm »
Capacitors.  Sometimes they fail low resistance.  Gotta rule them in or out.
 

Offline tango17Topic starter

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #36 on: September 16, 2016, 05:35:47 pm »
Caps it is.... Will let you know.
 

Offline tango17Topic starter

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #37 on: September 16, 2016, 06:29:56 pm »
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.
 

Offline ModemHead

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #38 on: September 16, 2016, 07:56:56 pm »
...  but we're hot on the trail.
You're a real funny guy.  ;D
 

Offline tango17Topic starter

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #39 on: September 16, 2016, 09:04:34 pm »
...  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.
 

Offline ModemHead

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #40 on: September 16, 2016, 10:53:21 pm »
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.
 

Offline tango17Topic starter

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #41 on: September 17, 2016, 01:40:16 am »
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...
 

Offline ModemHead

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #42 on: September 17, 2016, 02:45:30 am »
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?
 

Offline tango17Topic starter

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #43 on: September 17, 2016, 03:14:17 am »
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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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #44 on: September 17, 2016, 03:54:36 am »
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...
 

Offline tango17Topic starter

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #45 on: September 17, 2016, 03:54:04 pm »
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.

 

Offline tango17Topic starter

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #46 on: September 17, 2016, 04:43:48 pm »
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.
 

Offline ModemHead

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #47 on: September 17, 2016, 05:01:02 pm »
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.
 

Offline ModemHead

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #48 on: September 17, 2016, 05:09:50 pm »
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.
« Last Edit: September 17, 2016, 05:20:59 pm by ModemHead »
 

Offline tango17Topic starter

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Re: Fluke 8050 Battery Power Supply Repair Advice
« Reply #49 on: September 17, 2016, 05:33:35 pm »
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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