Fluke 87-I repair

[staze]

All,

Got a Fluke 87 off fleabay for $20 as broken. Indeed, on firing it up, it behaves rather strangely.

So, knowing MrModemhead did some work with these, I immediately tested the 1K fusible resistor. Indeed, it's open. Great, so wired a 1K 1% 1/4w resistor in across it (with some leads), and now the ohms and diode read correctly. But, the ACV and DCV both read all over the place. ACV reads random mv's, and DCV reads about -34V. DCmV reads correctly. Pretty sure current measurements are correct too, but haven't checked. Range switch test checks out as well.

Interestingly, ACV reads random on all ranges (about -80mV to -100mV on 400mV range, about the same on the 4V range, .33V on 40V range, slowly decreasing to 0.5V on the 400V range, and 4V on the 4000V range (last couple seem to just slowly be decreasing to 0V)), and does nothing when you short the leads together. DCV reads -34.xx V on the 40V range, 0V on the 400V and 4000V range. OL on the 4V range. Makes me wonder if C1 might be bad (though, that shouldn't have anything to do with DCV since it's shorted out for those readings). RT1 seems to be good, as it's at 1.48K. I do only get 850K out of R2 (that's in circuit).

I'm really hoping there isn't something blown with the resistor network. I'll fully admit, I haven't tested the power supply(s) yet, but that's mainly because I haven't mapped out where the test points are yet (the service manual for these is great, albeit, a bit difficult to jump back and forth on).

Was mainly hoping someone had an idea to point at. Obviously if that fusible resistor is bad, then someone shocked this thing. It reminds me a bit of high input impedance, but it would zero out if the leads were shorted, and that isn't happening. Guess it also reminds me a bit of a digital issue (bad logic signals), but haven't gotten that far.

So, anyone seen anything like this? got any ideas? I thought at first the spark gap (E1) (guessing that's what it is) was bad since it had a big split in it, but that seems to be normal from pictures I've seen. =P Big hope is I don't have to order a replacement R1 until I figure out the issue, and can order whatever else at the same time. =)

Thanks!

[staze]

I just noticed R29 looks blown. There's some black residue around it, and the numbers are partially gone. It's reading 176K in circuit. Also looks like there might be some scorching around the solder blob between R1 and RT1. Looks like it arched toward the trace going to pin 1 (I think) of Z1, which is probably what blew out R29. I can see R29 in the schematics, and it's definitely in the AC and DC volts path, but it seems to also be in the millivolts, so I'm not sure why it wouldn't matter there (other than maybe 100K and 170K look pretty similar when you're only dealing with 400mV).

Thoughts?

[retiredcaps]

Got a Fluke 87 off fleabay for $20 as broken. Indeed, on firing it up, it behaves rather strangely.

I was going to bid on that and then it disappeared.  At least I know who now. 

I'm really hoping there isn't something blown with the resistor network.

That should be easy to measure to verify its values.

I thought at first the spark gap (E1) (guessing that's what it is) was bad since it had a big split in it, but that seems to be normal from pictures I've seen.

Yes, there should be a split in the spark gap.

Did you check the varistors (RV1 and RV2)?  They should measure infinite resistance with another multimeter.  You do the measurement in circuit.  A bad varistor will measure shorted.

[retiredcaps]

Thoughts?

A clear focused picture will help.

Black residue might be carbon which conducts.  From modemhead's site

http://mrmodemhead.com/blog/ideal-carbon-footprint/

[retiredcaps]

Another thing to check.

Put the 87 into DCV.  With another multimeter, measure the input impedance.  You should get around 11.11Mohm.  The input impedance for the mV range should be around 10.00Mohm.

[staze]

Did you check the varistors (RV1 and RV2)?  They should measure infinite resistance with another multimeter.  You do the measurement in circuit.  A bad varistor will measure shorted.

RV1 in circuit shows infinite, RV2 (one that bridges the cutout) shows 100meg, decreasing. Actually, reversing the leads results in infinite... testing RV1 with reversed leads results in a non-infinite reading as well. Cap in series?

Pictures of R29 attached, as well as the arch marking on the back where RT1 and R1 connect.

[staze]

Another thing to check.

Put the 87 into DCV.  With another multimeter, measure the input impedance.  You should get around 11.11Mohm.  The input impedance for the mV range should be around 10.00Mohm.

DCV I get 8.74MR ±4KR (fluctuating). DCmV I get 9.945MR ± 1KR.

[retiredcaps]

I just noticed R29 looks blown.  It's reading 176K in circuit.

My Fluke 87 R29 reads 99.5kohm in circuit.

[retiredcaps]

Pictures of R29 attached, as well as the arch marking on the back where RT1 and R1 connect.

I would clean up those areas with some IPA and scrub away all that black residue.

[retiredcaps]

Another thing to check.

Put the 87 into DCV.  With another multimeter, measure the input impedance.  You should get around 11.11Mohm.  The input impedance for the mV range should be around 10.00Mohm.

DCV I get 8.74MR ±4KR (fluctuating). DCmV I get 9.945MR ± 1KR.

I get a rock solid 11.11M and 10.00M ohm.

[staze]

Hmm... so resistor network, or those varistors, or.... Could you check those, and see if you get infinite in both directions?

Waiting for the end of the daily show before I clean up that soot. But I would think if it were conducting, I'd get lower rather than higher resistance.

[staze]

yup, no difference.

Okay, so, RV1, RV2, R29, or the resistor network. I'm guessing Pin 1 of the network is the one closest to the input jacks (all by itself). So, which pins should I be measuring between for what values? I'm not sure the service manual is very clear on this (it says pin 1, but not pin 1 to what).

Though, guessing RV1 and RV2 are fine if they're in the 100+ megaohm range. I don't have a SMT replacement for R29, but I could probably colder on a replacement axial one. The part I don't get is the changing value... open resistor acting as a capacitor?

Oh, and what does R2 measure in circuit for you?

[staze]

Pictures of R29 attached, as well as the arch marking on the back where RT1 and R1 connect.

I would clean up those areas with some IPA and scrub away all that black residue.

Done, no change. There's actually a physical divot in the PCB on the backside where that arc occurred. =/ 

[staze]

okay, something is clearly wacko... measuring R2 one way, I get 850K, the other way (leads reversed), I get 1.6M. This is with the unit off. Something is clearly acting as a cap, or I'm being dumb about this...

[staze]

So, on a whim, I set my second meter (right now, a Fluke 189) to voltage, and interestingly, I'm getting a voltage out of the 87 when it's set to DCV or DCmV proportional to the reading on the 87. In the case of the DCV, it's about -170mV when the 87 shows about -34V. And -7mV when the 87 shows -7mV. So maybe those resistors are all fine, I'm just getting voltage leaking into the circuit from somewhere and throwing off my resistance readings. Also explains the polarity issue.

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? (someone really blew the ass outta this thing)... but the MOVs didn't blow... RT1 did... so... what the hell?

[retiredcaps]

Hmm... so resistor network, or those varistors, or.... Could you check those, and see if you get infinite in both directions?

As you probably guessed, after I took the photo, I went to sleep.

I have to run a bunch of errands today, but I took some quick measurements using a Fluke 187 (500Mohm capacity).  I can take more measurements later tonight and read your replies more fully.

For now (all readings in circuit - both directions),

R2 = 909k
E1 = infinite
RV1 = infinite
RV2 = infinite
R29 = 99.5k
R1 = 0.9971k

Z1 = to come later, but a quick measurement shows they all match values shown in Figure 5-3 (I didn't measure the 9.996M as it wasn't "obvious" to me which pins to measure)

Since your RV1 and RV2 are reading in the 100Mohm area, I suggest you remove (desolder) RV1 and RV2 and see if that fixes the problem.  If you are correct about the 87 getting blown out, it is likely the varistors are defective even though they don't measure shorted.

[staze]

Cool. I'll remove those MOVs and try that. I did clean everything up, and not seeing anything indicating the damage to the PCB is causing issues. R29 still looks physically damaged.

One other question... could you take a picture of the top and bottom rotary switch in the "off" position? I'm not 100% sure I didn't screw something up there. =P

Those MOV's shouldn't be leading to voltage on the input side of things, would they? Anyway, at this point, guess I'm looking at at least replacing R1, and R29. But also figure it's RV1 and RV2.

[retiredcaps]

R29 still looks physically damaged.

Since my R29 measures close to the 100kohm in the schematic, yours is definitely bad.

Those MOV's shouldn't be leading to voltage on the input side of things, would they? Anyway, at this point, guess I'm looking at at least replacing R1, and R29. But also figure it's RV1 and RV2.

Here is a thread where bad varistors was causing Robomeds to have incorrect voltage readings.

https://www.eevblog.com/forum/testgear/fluke-87-v-mystery-low-impedance-input/

PS. Did you measure E1?  If you did, I do not see the measured value?

[retiredcaps]

Z1 = to come later,

Pin Z1 is closest to input jacks.  Measured with Fluke, unknown calibration status, 187 and in both directions. Figures in the brackets are from Figure 5-3 of service manual.

Z1 to Z2 = 10.012M (9.996)
Z3 to Z7 = 1.1117M (1.1111)
Z4 to Z7 = 101.11k (101.01)
Z5 to Z7 = 10.014k (10.010)
Z6 to Z7 = 1.0008k (1.0001)

edit: typos

[staze]

Was this with the 87 on or off?

[retiredcaps]

could you take a picture of the top and bottom rotary switch in the "off" position?

The sun is gone here so I can't use natural light and get a clear focused shot. This is the best I can do with a flash.

The top and bottom pic is shot at angle to get rid of the flash.

[retiredcaps]

Was this with the 87 on or off?

All readings I have taken is with the 87 off with no battery installed.  Rotary position is in the off position.

[staze]

Perfect! I'll check my switch, and those values when I get home. Maybe also remove those MOVs and check with them out.

Also printed up the schematics today on 11x17 (A3) so I can take notes on values. Will post back later tonight. Thanks!

[staze]

Okay, checking Z1:

Z1 to Z2: 9.984M.
Z3 to Z7: 1.1091M
Z4 to Z7: 100.87K
Z5 to Z7: 9.994K
Z6 to Z7: 0.9986K

This is with, as you, an unknown cal 189 with switch off, and no battery. So, Z1 seems to be okay, if a little lower than it should be.

Also with the unit off, and no battery, the two MOVs register as OL both directions (getting somewhere).

E1 also open both ways. R2 still shows as 0.8517M (which seems wrong), and R29 definitely has physical damage... just going to be a bitch to replace with Z1 in the way. C1 checks out (just incase it was shorted). C20, on the other hand, in circuit appears to be bad, and I might spy some cracking around a leg (which may or may not be normal for a ceramic cap of this age). Could you test C20 in circuit and let me know what you get? I get about 0.136nF. I'll lift if I need to, but curious what it shows for you.

At least I'm fairly confident Z1 is okay at this point. =) So, one thing down. Leakage could be coming from one of the clamping transistors (Q1, Q2, Q6)...

Interestingly, it is drawing a very small amount of current when turned off... about 9.80uA. Not sure if that's abnormal or not, but from what I read on Mr Modemhead's site, it should draw nothing when off. So... leakage somewhere?

[lowimpedance]

Interestingly, it is drawing a very small amount of current when turned off... about 9.80uA. Not sure if that's abnormal or not, but from what I read on Mr Modemhead's site, it should draw nothing when off. So... leakage somewhere?

Give the PCB another good clean with IPA and check again.