Electronics > Repair

Fluke 8050a True RMS U32

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I'm trying to repair a Fluke 8050a.

I've already fixed a problem with the power supply, a bad selector switch (the Voltage selector 4PDT switch, which fortunately only used 3 of the 4 poles, allowing me to reconfigure it to use the unused 4th pole), some intermittent segments on the LCD display and a bad solder joint.

Everything is now working except for AC functions. I've tracked a good AC signal to the input of the true rms board -  the manual labels that board as a single component U32. It's a ceramic thick film board mounted vertically. It has 4 surface mount ICs and a few SMD diodes and capacitors.  It's marked on the back "Fluke 53 - 3045" and has approximately 30 pins soldered to the main PCB. There is no circuit diagram for that TRMS board, but its operation is described as comprising an absolute value circuit, a 2x log converter circuit, an antilog converter circuit and a log converter circuit.

It's difficult to track the circuit on the thick film board, but the input to that board is close to an LF351N single op amp, which I suspect is the absolute value circuit. A second IC is a CA3140e, which I suspect is the 2x log converter. A third IC is marked 473777. I've seen another post in which that IC failed. That post identified it as a dual op amp. With that in mind I suspect that op amp pair performs the log and antilog functions. The last IC is marked CA3046. It includes 5 transistors. I believe it works with the other components to perform the mathematical operations.

An AC input signal gets through the LF351N and arrives at an SMD diode pair marked A7-. The output of the LF 351 n  is a distorted sine wave from the true sine wave input. It has very vertical rise times and fall times with smooth almost square wave tops. It is both negative and positive and it's certainly not an absolute value. I did see a circuit from the 1980s using this chip in a precision absolute value full wave rectifier. There is no AC signal at any of the other chips or at the output. I suspect the LF 351N (JFET) inputs is bad but it is very difficult to get access to.

any suggestions for obtaining a replacement TRMS bored or finding a replacement LF 351 n? I'm going to try inserting a full-wave rectified ac signal after the LM 351 n and see if I get any AC output indication. At the present time I get only 0. Thanks for any suggestions.

If there is anyone here that has a Fluke 8050a and a scope, it would be very helpful if they could check the signal on the A7- diode of the U32 board (with a 60 Hz sine wave input on the meter)and tell me what they see. That diode has easily acessible test points nearby and is connected to the output of the op amp in the  LF351N. If a full wave rectified output appears there, I would know that part is bad on mine. I could supply a photo of the board and TP location?

I have 2, can check tomorrow.

Thank you for the offer. I don't want to put you to any effort until I'm certain I need help. I've made some progress.  The LF351N at the input is connected in the classical way to the dual diodes for a precision absolute value circuit. The output from that op amp is close to what it should be for the absolute value circuit. There should be another op amp performing the second half of the job, and I suspect it's the CA3140E. If it's connected as required for that circuit,then it would be the culprit, as it is not producing the full wave precision rectification I should be seeing on pin6 of the CA3140E.

I'm temporarily away from the bench, but as soon as I get back, I will check the circuit around those two opamps and confirm that it is designed as a precision absolute value circuit. Then I may be able to verify that the CA 3104 E is bad or that there is a bad component between the two pants in the absolute value circuit.

I really appreciate the offer of the help.I just do not want you to spend any time until I am 100% certain I need the help and, right now, I am still making progress.

A photo of the TRMS board. The chip behind the orange cap is the CA3140E. The chip to its  left behind the transformer is the LF351N at the input.


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