Electronics > Repair

Fluke 8050a True RMS U32

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tango17:
Not getting a signal on pin2 (inverting input) is very confusing to me. Wiss didn't measure that pin,  but I was sure it was going to be the absolute value full wave rectified. My input is close to that, and it makes sense that the 3140 is doing the 2x log function of the flow chart, working with one of the 5 BJTs in the 3046. At least your output is the same as his.
(Sorry I'm not faster on my testing. I'm not able to sit or stand for longer than a few minutes at a time.)

wiss:
If +in and -in both are zero, then it has to be an inverting amp, could it be a summing op? Other than that I would say output buffer, but then it would not output a waveform... ?

ModemHead:

--- Quote from: tango17 on March 09, 2015, 09:17:54 pm ---Not getting a signal on pin2 (inverting input) is very confusing to me.

--- End quote ---
Seemed odd to me, so I double-, triple, and quadruple-checked both in+ and in- pins.  Nothing but a little fuzz.  I thought maybe I had broken something, but the meter is definitely working.  Also I have the -01 battery model, so it has a bigger power transformer and a battery pack that obscures the module, probing can be a bit difficult.

tango17:
As far as I can tell, my AC signal is having trouble at the 3140. The signal at the input should be close to ground and the signal at the output should look like the logarithm of the absolute value of the sine wave. instead I see a signal at the input that looks like the absolute value of the sine wave with a leaky diode offset and at the output I see a signal that is usually at the negative rail but occasionally spikes towards ground.

One possibility is that the 3140 is bad. However,if it was bad I would expect the output to be either at ground or at one of the rails. I don't see that.Instead I am getting something that looks more like a comparator, where the signal goes to one rail except when the input goes to zero at which point I get a spike towards ground.

 So my best guess is that there is a problem in the feedback loop around the 3140, and I think there is a transistor from the 3046 involved in that feedback loop. I'm going to pull the 3046 transistor array and test it. It seems likely to me that the transistors are used in a 2x log circuit in combination with the 3140 and that the transistor feedback loop, which should produce a log signal at the output and a virtual ground at the input is just not working correctly. The transistor array may also be involved in the leaky diode problem I'm seeing.

After pulling the chip off, I will test it to at least see if the pn junctions on the transistors are working correctly. in-circuit testing didn't tell me much.

 Does anyone think I need to use special silver containing solder for connections on the thin film ceramic circuit board? Or will regular 60 40 work?

krivx:
Is the CA3140E hard to source? I think I have some old RCA parts in DIP8.

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