Fluke 87-I repair

[retiredcaps]

Can you retest the input impedance on the 87 again?  Please report the measurement for DC mV, 4VDC, 40VDC, 400VDC, and 1000V (go into manual mode and cycle through the ranges).

Can you also get the 87 to measure a 12V DC power supply?  Please report the measurement for each range.  For example, if it is working properly, it should be

4VDC = 0L
40VDC = 12.00
400VDC = 12.0
1000VDC = 12

[staze]

Well get these tonight.

A quick measurement of input impedance with no battery connected was 13.6M on the DCV range, and I think 4.3M on the DCmV range.

Will confirm tonight, and get the other measurements, with a battery installed. That said, since there's still voltage on the input jacks in DCV, I'm sure the input impedance measurements are going to be WAY off.

Hopefully I can get the LCD to cooperate as well... it started out perfectly, and now it's pretty bad. =/

[retiredcaps]

That said, since there's still voltage on the input jacks in DCV, I'm sure the input impedance measurements are going to be WAY off.

I think you will be correct, but I just wanted to see if there is a change depending on the range.

The zebra strips probably just need an IPA cleaning.

[staze]

That said, since there's still voltage on the input jacks in DCV, I'm sure the input impedance measurements are going to be WAY off.

I think you will be correct, but I just wanted to see if there is a change depending on the range.

The zebra strips probably just need an IPA cleaning.

Yup. And yeah, will get those tonight. Also see if I can figure out where that voltage is coming from...

[retiredcaps]

So... where would voltage be leaking in from? My thinking would be in section 2-4 of the manual... the voltage clamping network of Q1, Q2, Q6, CR7, CR8, and R58. Maybe one of those transistors shorted from an overvolt?

I can't read a schematic to save my life, but I wonder if removing R59 (510ohms) would allow the meter to function while eliminating the above as possible source of leakage?  R59 is connected to S1 rear contact 2 (section 2-7).

edit: added more detail.

[staze]

So... where would voltage be leaking in from? My thinking would be in section 2-4 of the manual... the voltage clamping network of Q1, Q2, Q6, CR7, CR8, and R58. Maybe one of those transistors shorted from an overvolt?

I can't read a schematic to save my life, but I wonder if removing R59 (510ohms) would allow the meter to function while eliminating the above as possible source of leakage?  R59 is connected to S1 rear contact 2 (section 2-7).

edit: added more detail.

I'll give that a shot... should also be easy enough to see if the voltage is higher or lower on each side of that resistor. Yay 5.5 and 6.5 digit Multimeters. =)

[retiredcaps]

R58 is connected to VSS which might also make another interesting measurement point?

[staze]

okay, had my hopes up that I found the issue when I saw some solder-like material between the guard and input traces on U4, but seem to have been nothing. =/

Anyway, the info you asked for.

Input Impedance:
DC mV: 9.9845M
4VDC: -1.45M-1.74M (all over the place)
40VDC: 6.6M (all over the place as well)
400VDC: 9.990M (rock solid) (87 even sees 1.0V coming from my 189's resistance measurement)
1000V: 9.980M (ditto).

So the issue would seem to be the 4VDC and 40VDC. Hmmm...

Measurements of 12V DC PSU... set it to 1mA so I didn't risk blowing out my wimpy 1/4W input resistors I hacked in there). PSU always read 0mA. Actual measured draw on each range is listed in parenthesis below.

4VDC = OL (1.275uA)
40VDC = -25.55V (all over the place) (1.223uA)
400VDC = 12.0 (1.195uA)
1000VDC = 12 (1.196uA)

Calculating from that, the input impedance for each range, 12VDC input, with the listed currents, would be 9.411M, 9.811M, 10.042M, and 10.033M, respectively.

The 40VDC range is interesting since with no input, the 87 reads -33.45VDC. One would think with 12V input, it should read -21.45VDC. Huh.

R58 and R59, however, both check out as far as resistance. There's a 1.58V drop across R58, and a 0V drop across R59 (which, I'm not sure I understand, but like you, I have a difficult time with schematics... especially since Fluke didn't actually label the parts on their schematics) in DCV mode. Reading through the service manual section on Voltage measurements... I'm either thinking U4 is bad, or they've way oversimplified things... Because somehow, Q1, Q2, Q6, Q13, and Q14 enter into the equation.

One thing that keeps me wondering... did you ever manage to measure C20 (capacitance, maybe ECR/Resistance as well if you can)? I do see a bit of voltage on it in DCV mode...

Alright, time for bed.

[ModemHead]

I'm sorry I haven't been able to participate in this thread due to circumstances from which I have a temporary reprieve today.  So I'll toss out a few quick thoughts.

The current draw when turned off is a red herring.  The 1M pull-up resistor R54 is directly across the battery terminals when the unit is off, which accounts for the draw.  In addition the Vdd/Vss pins for U6 is also across the battery permanently, but the resistor accounts for most all the current.

Looks like an interesting case of serious over-volting as you know.  I'm just a hobbyist so I can't say I've seen everything, but over-volting seems to mostly affect the fusible resistor (R1), the MOVs, the thermistor, and (unfortunately) the main chip in cases I've seen personally or read about, assuming you clean up the carbonized mess sometimes left.  Of course there's a first time for everything.  Wouldn't hurt to check all the clamping transistors. (the ones with the base and collector shorted)

In addition to PCB contamination, failures in the main chip will lead to current being sourced or sunk where it shouldn't be, wacky resistance readings, and voltage appearing at the input jacks.  Do not hesitate to isolate the lines into the main chip by finding a convenient place to cut a PCB trace.  It will save you tons of time scratching your head, and as long as you're careful, it's easy to re-connect with a solder bridge or bit of 30ga wire.  I have an 83 here that reads 100 ohms from its ohms sense input (P93) to ground, with the lead isolated from everything else and the power turned off.  It must be toasted!

Pay real close attention to the APV0, APV1, and APV2 lines.  You might break one at a time and see if the voltage on the meter input jacks disappears.  Maybe start with APV2, since it seems that the 40V range is the screwiest?

[retiredcaps]

okay, had my hopes up that I found the issue when I saw some solder-like material between the guard and input traces on U4, but seem to have been nothing. =/

Yes, sometimes it is not obvious at first, second or even third glance where contamination lies.  I put tough to solve problems away for a couple of days and then look at it again with "fresh" eyes.

So the issue would seem to be the 4VDC and 40VDC. Hmmm...

By measuring the input impedance of each range, it gives us more clues as to what is wrong.

One thing that keeps me wondering... did you ever manage to measure C20 (capacitance, maybe ECR/Resistance as well if you can)?

I did measure C20 back in post #25.  Unless I misunderstood?

[retiredcaps]

I have an 83 here that reads 100 ohms from its ohms sense input (P93) to ground, with the lead isolated from everything else and the power turned off. It must be toasted!

Is it sheer coincidence that staze's Fluke 87 and modemheads's recent 83 both have bad main ICs from the same ebay seller?  Maybe the seller has some quick method of testing the IC to know it is bad?

I'm not saying the seller is dodgy as both were clearly listed as "parts/as is" and the problems with each meter clearly disclosed.

[staze]

Wouldn't hurt to check all the clamping transistors. (the ones with the base and collector shorted)

Will check other stuff later, but looking at the clamping transistors, Q1 and Q2 both test 0.61V one way, and 2.4V the other, which I believe is normal. They're also directly connected to each other. Q6, however, reads 0.61V one way, and Open (OL) the other. I think that's what it should be... not sure why Q1 and Q2 read how they do. =/ CR7 and CR8 both read 1.23V in one direction, and 1.83V in the other. And I previously checked R58, and it's fine. That's the whole of the clamping network.

One question is, Modemhead, do you have a full understanding of which pins on Z1 to test against? In a previous post I listed resistances, but the manual makes a complete mess out of simply describing how it should work (it's like a sentence was left out in editing).

Also, am I right in thinking there are guard traces that run along either side of the APCC and APV0 pair? there are two traces next to U4 that are not connected (they look like guard traces since they run down to Z1 and around).

Thanks!

[staze]

Yes, sometimes it is not obvious at first, second or even third glance where contamination lies.  I put tough to solve problems away for a couple of days and then look at it again with "fresh" eyes.

One thing that keeps me wondering... did you ever manage to measure C20 (capacitance, maybe ECR/Resistance as well if you can)?

I did measure C20 back in post #25.  Unless I misunderstood?
[/quote]

Yes, I think I might take a break here after tonight, unless the suggestions keep coming!

And yes, I totally missed that when you posted it. Sounds pretty similar.

Thanks!

[staze]

<edit>

Nevermind, I'm an idiot. Of course there's voltage across there with the fuses out, that removes the "short" to COM... so, ignore the below. Though it is weird there's ALWAYS a voltage across the shunt rather than switching it on/off with the rotary.

</edit>

The plot thickens? In DCV mode, with the fuses removed, there is a whole 1.4931V between the 10A jack, and COM. Between the mA jack, and COM, there's 0.7804V (basically, half). All the while, there's still the -0.1840V (AVG) across the Volts and COM jack. But, maybe that's normal, since it looks like those jacks are tied to VDD through a voltage divider... Is that for the shunt?

[ModemHead]

The diode arrays should read two diode drops in the forward direction.  The clamp transistors should read one diode drop in the forward direction.  In reverse, they should be open-circuit, so that any reading is the just the result of other parallel impedances reacting to the stimulus current. Somewhat unpredictable.

Fluke left off the pin numbers for Z1 on the schematic, so measuring it would be somewhat trial and error.  It's actually a very sturdy component, so it wouldn't be my first suspect anyway, as long as there's no physical damage.

Yes there are guard traces around APV0, this is the very high-impedance analog-to-digital converter input.  Not really sure what the APCC input is for, but if it's inside the guard traces, it must be a high-impedance input also.

[staze]

Always really hesitant to cut traces. Any chance I would see voltage on that pin when there is no input?

What is the op-amp in the unit used for if pretty much everything is in U4? I thought at first it was the AC buffer, but it seems that's in U4 as well. :/

Out of curiosity, what resistance do you get between black "pads" on the PTF when the switch is "shorting" them? From what I gather, the CPU just sees a "bit" when the switch is moved, and changes function from that.

[staze]

so, reason(s) I ask.

1. There's a "hole" in the top shield right above that op-amp. Doesn't look melted... but could be (op-amp doesn't get hot, it seems). Though, looking at modemhead's pictures, I see the same hole on his... must be a molding defect. =/

2. Voltages on the op-amp seem... off. Pin 4 and 8 both are right for the rails, but given the inputs on pins 2 and 3 (few hundred milliamps), pin 1 is slammed to the negative rail (-3.2V), and weirder, pins 5 and 6 are the same as pins 2 and 3, yet pin 7 fluctuates pretty wildly. Looking at the schematic, pin 1 being slammed at the negative rail makes some sense since the diode on it's output would mean it's not getting any feedback... so maybe I don't quite understand how this isn't the expected behavior of this thing...

[staze]

On another note... CR5 (up by the power regulator circuitry in the upper right corner), reads 0.61V in both directions... Which, doesn't seem right. But I'm not sure how, if it was bad, it would cause any of this... since it seems to just be involved in the power rail generation, which as I said before, all check out.

That said, I do have a dual tracking PSU coming, incase I want to bypass the battery input completely... =/

[retiredcaps]

Out of curiosity, what resistance do you get between black "pads" on the PTF when the switch is "shorting" them?

I'm not sure what you are asking here?  Perhaps a photo?

[retiredcaps]

On another note... CR5 (up by the power regulator circuitry in the upper right corner), reads 0.61V in both directions...

I get the exact same reading of 0.61V.

[staze]

weird.

And yeah, I'll post some pictures. I am going to break out the good camera and Macro lens and take some real pictures.

[staze]

Alright, didn't get pictures up. But I ordered some REAL IPA (99.9% anhydrous). Going to clean the whole thing again... since I'm really hoping it's not the CPU (Fluke said they stopped making a replacement in 2006).

I go see a voltage between the APV2 pin and the V? connection. =/

[PedroDaGr8]

Alright, didn't get pictures up. But I ordered some REAL IPA (99.9% anhydrous). Going to clean the whole thing again... since I'm really hoping it's not the CPU (Fluke said they stopped making a replacement in 2006).

I go see a voltage between the APV2 pin and the V? connection. =/

Thats overkill mate, there was no need for anhydrous. Unless you plan on storing the IPA under molecular sieves and using a needle containing N2 to pull out the alcohol it will rapidly rehydrate by absorbing moisture from the air (the molecular sieves scavenge any excess water that finds its way in). That's the "beauty" of azeotropes, they will rapidly form at will. I know this from experience being a nanomaterials chemist. I use anhydrous stuff regularly and its a pain to work with if you need to KEEP it anhydrous.

Otherwise, good luck mate.

[staze]

Alright, didn't get pictures up. But I ordered some REAL IPA (99.9% anhydrous). Going to clean the whole thing again... since I'm really hoping it's not the CPU (Fluke said they stopped making a replacement in 2006).

I go see a voltage between the APV2 pin and the V? connection. =/

There was no need for anhydrous. Unless you plan on storing the IPA under molecular sieves and using a needle containing N2 to pull out the alcohol it will rapidly rehydrate by absorbing moisture from the air (the molecular sieves scavenge any excess water that finds its way in). That's the "beauty" of azeotropes, they will rapidly form at will.

Otherwise, good luck mate.

True. I guess I meant more than the drug store 99% stuff has god-knows-what in it to keep people from drinking it, as well as getting the water out. I bought actual MGChem 99.9% anhydrous just because that, theoretically, doesn't have benzene, etc in it. =) Now, I doubt that'll make any difference, but it'll at least not make me question if my IPA is leaving behind some residue. Of course, I probably could have gone over to chem-stores on campus and gotten the same 99.9% IPA... but I don't like to abuse that (they gave me enough looks when I asked for straight Ethylene Glycol, and some nice Tygon 5/8th-inch tubing  (for a computer water cooling system)). =) Though, storing something under something just makes me recall dealing with solid K or Li stored in/under kerosene.

[JoeS76]

Hi , how are yous - enjoying the forum- can i ask- have you checked the bridge rectifier- i've found that sometimes with readings like yours- the bridge is blown and needs replacement which is a simple enough fix