Fluke 97 Scopemeter - resistance not working

[jzkmath]

Hello,
I just recently bought a Fluke 97 Scopemeter from a Hamfest. After playing around with it, I have figured out that everything works on the meter, except for the backlight, and the resistance measurement (and diode mode).
As shown in the attached picture, even when the probes are connected together, I still get the reading for Open Loop.
I know that the banana connector used for measuring resistance is also used as a signal generator, and I know that works.

Does anyone know what might be causing it to not work?
Thanks!

[jeroen79]

Elektrotanya.com has a servicemanual.

Is the meter putting out a small current in diode or resistance mode?

[jzkmath]

I am not getting any current when I use my other multimeter to test, though I might be testing it wrong. (I put my multimeter in amp mode and connected the probes together so that the multimeter was in series with the scopemeter)

I looked on that site and did not see any service manual for the 97 scopemeter. I have a pdf of the service manual though from another site, but it only how to repair using fluke's specialized equipment.

[jeroen79]

I searched for PM97 and found it on the second page:
https://elektrotanya.com/philips_pm93_95_97_fluke_93_95_97_sm.pdf/download.html

One thing I would check is the relays switching the ohms circuit to the outputs.

[jzkmath]

I have just been looking at the circuit and honestly I don't even think the schematic is correct. The component location diagrams and actual PCB do not match and the relays appear to toggle differently then what is diagrammed in the manual.
According to the manual, relay pins 4 and 13 are common, 6 and 11 are normally closed, 8 and 9 are normally open, 1 and 16 are the control,
yet the states appear to be reversed when you turn on ohm mode.

Other than that, the relays are behaving like they should.

[joeqsmith]

On the plus side, they seem to be fairly well protected for basic faults and could very well be a simple fix.   Then again, it's easy to underestimate the human factor. 

Using a DMM  w/10M  input, what voltage is between the BNC shield and the Black banana, with ohms selected and nothing attached?   Same test but between the red and black?  Switch the meter to ond of the voltage modes, again measure between the BNC shield and black.

In all cases I am guessing you read something under 100mV.   

[jzkmath]

Between BNC Shield and COM is 3.46V, and changes by 10mV when I change the range on the scopemeter;
(300 = 3.81V, 3k = 3.58V, 30k = 3.45, 300k = 3.35V, 3M = 23.26V, 30M = 3.26V)
Getting no voltage between red and COM

With the scopemeter in voltage mode, there is 0V between BNC shield and COM, and 5mV between red and COM

[Samogon]

I have just been looking at the circuit and honestly I don't even think the schematic is correct. The component location diagrams and actual PCB do not match and the relays appear to toggle differently then what is diagrammed in the manual.
According to the manual, relay pins 4 and 13 are common, 6 and 11 are normally closed, 8 and 9 are normally open, 1 and 16 are the control,
yet the states appear to be reversed when you turn on ohm mode.

Other than that, the relays are behaving like they should.

Fluke was redesigning this scopes multiple times, i have seen all sorts of builds and all were far from schematics available

[jzkmath]

I doubt the PTC is dead but I would check from that back at least to the clamps.   I would just do a quick check of the all those diodes and transistors in-circuit.

When you say PTC are you referring to the temperature resistor (R2750)? If so, I already tested that and it appears to work.
Not sure how I would test the diodes and transistors but I will look that up.

[jzkmath]

I doubt the PTC is dead but I would check from that back at least to the clamps.   I would just do a quick check of the all those diodes and transistors in-circuit.

I just checked and it appears that the protection circuitry is working fine. Don't have a clue as to what value that PTC should be, as the service manual only gives the ordering code, not the value.

[jzkmath]

Somethings that I found:
- diode mode works if I connect the black banana to the probe ground. Even so, it reads either 108k or 0 in resistance mode. Removal of the scope probe makes it go back to 'OL'.
- the IC chip responsible for selecting 'Rr' works as intended.
- the two tansistors for the protection circuit are not on the board. There are the pads for them, but it seems they were put on the board.
Anyone have any ideas as to what else could be causing it to not work?

[targit]

My old 99 had a problem similar to what you are describing, it was one of the latching relays not making a good connection, as far as I can remember it had been subject to over current between one of the BNC screen wires and the black banana socket. It was a long time ago, no internet and no service manuals. It don't think those relays had a high current capacity so maybe worth taking out and thoroughly testing.