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Old Fluke Multimeters

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bdunham7:

--- Quote from: yo0 on October 12, 2022, 10:10:33 pm ---I have 3 fluke dmms 8010a, 8012a and 8050a, all working good in all functions except diodes check, all show higher than normal readings, any idea? All with 1n4148 under test.

--- End quote ---

What do you get if you use the 200K range instead of the 2K range for the diode check?

mqsaharan:
I also have an 8060A with Rev H board. About 5 years ago, during recapping I forgot the orientation of C19 and got confused if it was installed backward or not so I installed it as per schematic/silk screen. Today, after seeing another message regarding C19 I remembered that I never actually checked it.
During checking this thread for 8060A pictures for C19 orientation, I found Mr. Taylor's post #871 saying that there could be small reverse voltage possible (https://www.eevblog.com/forum/testgear/old-fluke-multimeters/msg4045864/#msg4045864). So, I finally hooked it up to my 1054Z and got the following readings:

Please take the following readings with a grain of salt. And do your own research before changing the orientation of the capacitor.

Voltage at C19 with +ve terminal pointing towards the display (installed as per schematic/silk screen)
Voltage shown are measured using Min function on Vertical menu. Just to be clear the following figures are the lowest peak of the waveform. The rest of it was above 0. I forgot to add that the scope channel was DC coupled with probe set to 1x and bandwidth limit was off. Timebase was 20ms/div. I was so hung up on keeping the probe on the board steady with one hand and not letting it slipped, I never carefully noted the peak to peak voltage of the waveform at C19 terminals. Neither I noted the V/div scale. I think it was 20mV/div but I am not sure. And peak to peak voltage were about 40mV or there about. It was fresh in my mind then but not anymore.

AC function OFF = +1.6 mVDC (mostly scope's own noise, I guess)
AC function ON = +1.6 mVDC (same as above)
   with cables attached = -36mVDC (no input signal)
   with 100V ac applied from an isolation transformer. The waveform at the capacitor terminals matches the input waveform (because I got a clean 50Hz signal there).
750V range = -1.2mV
200V range = -1.8mV
20V range = -23.2mV (OL)   (Waveform amplitude decreased)
2V range = +368mV (OL)   (I think at 2V range, waveform became triangular, instead of sine. Meter was in overload condition as in 20V range.)
200mV range = +4.00VDC (OL)   (A steady DC 4.00V)

The results were almost repeatable with +/- 1.0mv (approx).

After seeing the waveform on scope, I am glad I installed the capacitor as per schematic/silk screen.

As per the advertising material shared by Mr. Taylor in post 49 (https://www.eevblog.com/forum/testgear/old-fluke-multimeters/msg323088/#msg323088), this capacitor (C19) is installed between inverting terminals of two input opamps.

Edit: Added/updated a few details.

BillyO:
Perhaps replacing it with a non-polar capacitor would take the confusion away.

yo0:

--- Quote from: bdunham7 on October 13, 2022, 04:09:33 am ---
--- Quote from: yo0 on October 12, 2022, 10:10:33 pm ---I have 3 fluke dmms 8010a, 8012a and 8050a, all working good in all functions except diodes check, all show higher than normal readings, any idea? All with 1n4148 under test.

--- End quote ---

What do you get if you use the 200K range instead of the 2K range for the diode check?

--- End quote ---

16.93 kohm

Best regards

Pio

Zoli:

--- Quote from: BillyO on October 13, 2022, 04:54:39 pm ---Perhaps replacing it with a non-polar capacitor would take the confusion away.

--- End quote ---
The caps which I would use are 47µF@25V X7R 2220 TDK transformed in THD, which have practically no capacity loss between 0-5V; best of all, it fits.
Alternative is the 2X22µF/16V film 2220 Rubycon(still fits), four times the price of the TDK, which is five times the price of the Al-poly(~$1).
So yes, is possible to replace C19 with with proper non-polar caps; the question is if it's worth it financially.

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