Electronics > Repair
Fluke 8050a True RMS U32
tango17:
Wow! Thanks, Modemhead for those pics. You must have done that before, as you posted everything I needed to interpret them, the baselines, the settings, the input signal, etc. I'd always wanted a Fluke, and while reading on the web I came across your posts about converting an 8050a to LEDs. It was that info that caused me to buy the (for parts) 8050a I found on EBay at a price I could afford.
So back to the repair. The 8050a manual says the input of the U32 board goes to an absolute value circuit, and studying those circuits from the 1980s, many use a pair of diodes (A7- which my notes label "D1" and "D2" for convenience) between a pair of op amps (almost certainly LF351N, "U1") and possibly CA3150E ("U2") with multiple precision resistors- often laser trimmed (the thick film U32 board seems perfect for this).
I set my scope to match yours, except I used 60 Hz input and 5ms/div. The same baselines, the same 1V RMS input, the same .5V/div on the upper trace and 1V/div on the lower trace . My T1, T2 and T4 are the same as yours, so I won't post them, but my T3 differs. The signal between the positive going half wave is not zero, it's got some weaker positive signal there.
I then traced to the CA3150E ("U2") and the inverting input pin 2 shows something similar - close to full wave rectification, but not symmetrical - see below with 0.2V/div on the upper trace. The output (pin 6) shows pulses at the zero crossings - see below with 2V/div on the upper trace, still DC coupled. (I've got some better traces AC coupled and showing the pulses more clearly, but you can see the basic signal below - alternating size positive going pulses from a negative DC baseline.) The offset null (pin 1) of that chip is near -5V and comes from the trimmer pot on the U32 board. The NI input (pin 3) is at ground.
Any ideas?
tango17:
Just thinking out loud here: The inverting input pin P2 of U2 looks like (T3-T2). Note that the peaks are highest when the input signal is going positive, right where your T3 is zero and my T3 has some positive signal. I suspect the full wave (absolute value) rectified signal will be on your P2. If so, then U2 (CA3140E) is not part of the absolute value circuit, since its input is already absolute value (assuming the T3 signal is working and is zero when T2 is negative going). The output P6 of the CA3140E is what I'm not sure about. Is it doing zero detection? I can't think why. Is it just an inverting amplifier and the chip is bad? And what is causing the nonzero portion of the T3 signal on mine? Hmmmm?
wiss:
Leaky diode?
3140 is not good for much, except low input bias. I would not expect it to be part of any low level AC signal processing. High noise and high offset.
tango17:
I thought about the "leaky diode" scenario also. However, that can't be my main problem. I should be getting some sort of signal out of U32, and I am not. Using the flow diagram, I suspect that the 3140 is the 2x log converter and it's bad. The 473777 chip is a dual op amp and I suspect it performs the log and antilog functions. If the 3140 is the 2x log converter and if its input is the absolute value from the 351/diodes, then the 3140 is bad. I don't know why I'm getting the leaky diode problem, but once I've got output from U32, I can go back and try to figure out why the input to the 3140 is not exactly the absolute value. Once someone confirms for me that the output pin 6 of the 3140 is roughly 2x log(absolute value ( input)), then I'll replace that chip, since I know that's not what I'm getting.
wiss:
Pin 6 of the 3140:
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