EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: num8 on August 03, 2018, 01:43:34 pm
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Hello,
so I bought a Fluke 8050A where the TRMS AC measurements are about 2.4 times to big. DC and Ohms are fine.
Here are the Display readings for the current with a shorted input with RMS.
200µA: 0.05, open input 0.13
2mA: 0.001, open input: 0.001
20mA: 0.01, 0.01
200mA: 0.1, .1
2A: 1.0, 1.0
On the RMS hybrid there are testpoints. I measured TP1 on the hybrid and TP10 on the PCB.
with an open input:
200mV: Reading: overrange, TP10: sinewave .7V P-P 50Hz, TP1: distorted sine 1.2V P-P 50Hz
2V: 0.25, Sine 100mV 50Hz, distorted sine .7V P-P 50Hz
20V: 0.37, Sine 10mV P-P 50Hz, .4V P-P 50hz
200V: 0.35, Nothing on Tp10, .2V P-P
2000V: - 1.4, Nothing on TP10, .06V P-P
shorted input rms Volts, nothing on tp10 across all ranges
200mV: 0.12, TP1: distorted sine .12V P-P 50Hz
2V: -0.0014, .06V P-P
20V: 0.013, .1V P-P
200V: 0.14, .06V P-P
2000V: -1.4, .04V P-P
with a square wave at the input I noticed some ringing on TP2. while TP3 doesn't have that ringing
Comparing that to the wave forms of a working unit shown here: https://www.eevblog.com/forum/repair/fluke-8050a-true-rms-u32/ (https://www.eevblog.com/forum/repair/fluke-8050a-true-rms-u32/) , it seems that only the TP2 signal differs.
i think that's the fault.
Sadly there are no schematics of the hybrid.
If I heat the hybrid very gently with hot air, so that it is warm to the touch, the open input voltages decrease significantly.
There are two diodes on the hybrid, the one on the left corner has two diodes in that sot23 package, the one on the right marked "-9V" appears to only have one diode in it, or is it a zener?
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I know most of the time the transistor array CA3046 ic is to blame for troubles with AC measurement, those are easy to get but I haven't had any luck with replacing them on the hybrid. Maybe I've heated it to much trying to remove the "bad" chip with hot air... I didn't have time to go back to the troubleshooting... I'm still not certain that the CA was a problem.. in my case.
Great tip with tp points on hybrid.
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you have certainly found the service manual ? :
http://www.opweb.de/en/model.php?id=2966 (http://www.opweb.de/en/model.php?id=2966)
https://madnessinthedarkness.transsys.com/_media/projects:fluke8050a:fluke-8050a.pdf (https://madnessinthedarkness.transsys.com/_media/projects:fluke8050a:fluke-8050a.pdf)
http://bama.edebris.com/manuals/fluke/8050a/ (http://bama.edebris.com/manuals/fluke/8050a/)
Maybe some leaking capacitors ??
Have you checked tp10 on main board, it would be the hybrid input pin3 thru c15 and some feed back to pin 2 thru c14, at the output pin 21 you have c27 going to pin 27
and finally you have an 100mv vac 100hz adjustment / reference, an 1 meg trimpot ... is the voltage ok ?
And finally you have the supply voltages of +6 and -5 volts and an -5F (floating ???) are they okay too ?
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Hello,
i have found the service manual.
TP10 seems fine. The power supply too. The 1meg Trimpot (R7) supply is ok. The caps seem fine , I desoldered one and checked it with a tester... it's ok. The other ones aren't short circuit either.
The CA3046 seems weird. Transistor Q4 has a resistance of about 65kOhm across the CE junction (pins 10 and 11) and the CE Junction of Q3 between pins 7 and 8 has about 7MOhm.
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CA3046 look similar to UL1111. You can remove this IC and solder single NPN transistors like BC547 and test results or measure voltage on any B-E,C-E junction. Exchanging all electrolytic capacitors not worse situation but prevent PCB damage with lacking of electrolyte.
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I know this thread is old, but I was wondering if the OP has made the repairs to the hybrid board.
I've checked the board in my 8050A and found the CA3046 IC to be bad. Haven't had time to replace it yet though.
I used a home made "octopus" component tester on the IC.
https://youtu.be/4cc6OyvuuhE (https://youtu.be/4cc6OyvuuhE)