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Electronics => Repair => Topic started by: tango17 on March 06, 2015, 08:36:02 pm

Title: Fluke 8050a True RMS U32
Post by: tango17 on March 06, 2015, 08:36:02 pm
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.
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 07, 2015, 02:50:47 pm
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?
Title: Re: Fluke 8050a True RMS U32
Post by: wiss on March 07, 2015, 04:14:04 pm
I have 2, can check tomorrow.
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 07, 2015, 08:58:36 pm
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.
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 07, 2015, 09:13:19 pm
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.
Title: Re: Fluke 8050a True RMS U32
Post by: wiss on March 08, 2015, 05:54:50 pm
On the two pads towards the edge of the board there are nice looking halfwave rectified voltages, one positive and one negative, on the pad facing the LF351 there are a sine plus a diode-drop in the direction of the sine.

"if (sin(x) > 0) {
  reutrn sin(x) + 0.6
} else {
  return sin(x) - 0.6
}"

Or, more like 0.4 V, and it is not symmetrical.
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 08, 2015, 06:43:28 pm
Hmmm. I've labeled the four pads symetrically positioned around the A7- diode pair in my photo above.  At T2 I see a weak half wave negative going rectified signal. It's inverted from the positive half. At T3 I see strong positive going half wave (inverted from the input negative half cycle).  If I look closely at T3, Ican tell that between the strong positive  peaks, there are weak positive peaks that match the negative going peaks on T2. The strong peaks are roughly four times the amplitude of the weak peaks, so on some scales and inputs it looks like half wave rectification.

On T1 and T4, which are connected, I see a symmetrical near square wave.  The tops and bottoms of the signal bulge out, up at the top, down below.

Do you see full wave rectification anywhere? I sort of expect it (absolute value) at a second op amp, and the logical suspect would be the output (pin 6) of CA3140E.

I'm seeing that pseudo full wave signal (strong and weak peaks) at the inverting input (pin 2) and sharp narrow pulses at the output (pin 6) of CA3140E.
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 08, 2015, 08:46:48 pm
"if (sin(x) > 0) {
  reutrn sin(x) + 0.6
} else {
  return sin(x) - 0.6
}"

Or, more like 0.4 V, and it is not symmetrical.
That sounds like what I see at T1, T4, and the other pins seem close to the positive and negative half wave rectification I see, except for scale.  I was beginning to wonder if I should be looking at the 473777 chip. There is another report of it failing, and I'm not seeing any output from it. It's grounded at the output (pin1) on the op amp connected to the output of the U32 TRMS, and both the inputs on that output op amp (pins 2, 3) are also always grounded.
Title: Re: Fluke 8050a True RMS U32
Post by: ModemHead on March 08, 2015, 09:17:27 pm
Here's what I see...

Meter input is 1.00Vrms sine at 200Hz, shown as bottom trace (2V/div) on all shots.

Top traces are T1, T2, T3, T4 (0.5V/div)

Sweep at 1ms/div.
Title: Re: Fluke 8050a True RMS U32
Post by: wiss on March 08, 2015, 09:20:46 pm
That look simular to what I saw on the diode.
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 09, 2015, 04:34:43 pm
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?
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 09, 2015, 05:01:49 pm
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?
Title: Re: Fluke 8050a True RMS U32
Post by: wiss on March 09, 2015, 06:15:23 pm
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.
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 09, 2015, 06:39:58 pm
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.
Title: Re: Fluke 8050a True RMS U32
Post by: wiss on March 09, 2015, 07:04:25 pm
Pin 6 of the 3140:
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 09, 2015, 07:14:51 pm
Yes! That's what the log of the absolute value of a sine wave looks like. I'll order a CA3140E, if I can find one. I'll grab some diodes, too, if I dont have any with similar specs lying around.  I wonder if I can do this without desoldering the 29 pins of U32?
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 09, 2015, 07:40:32 pm
Before I do,  however,  here is a typical log circuit using a BJT. Its in the feedback loop. If it was blown, I might be seeing something like I see now where the op amp goes to the negative rail. The CA3046 chip has 5 BJTs, and the circuit has 3 more log or antilog functions to perform. They probably use those transistors.  Some more checking is warranted.

Thanks, wiss and Modemhead.
Title: Re: Fluke 8050a True RMS U32
Post by: wiss on March 09, 2015, 08:05:45 pm
In a normal one quadrant square circuit you'd use a log-anti-log circuit, the I0 tempco will be cancelled out by other junctions and the Vbe tempco cancelled automatically in the log-anti-log conversion, well, beats me how they do it here :)
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 09, 2015, 08:46:29 pm
beats me how they do it here :)
It's really tricky to work on this. There's no circuit diagram in the manual, like there is for the rrst of the 8050a. There's no visible resistors on the thick film substrate, and there aren't any visible traces to see the interconnections.  I have to buzz out every possible interconnect, and if there's a resistor, I get no buzz signal.
Title: Re: Fluke 8050a True RMS U32
Post by: ModemHead on March 09, 2015, 08:48:10 pm
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.

I could not find a signal on the inverting or non-inverting inputs of the CA3140E (U2P2, P3).  Both are basically 0Vdc, plus maybe 20mV of noise.  (The meter is really working, the display shows "1.0037".)  U2 output, and its strobe pin 8, are shown here with the same scope setup, except the input reference trace moved to the top.

I really don't have a clue how this circuit works.  :-\  It sounds like you're getting it narrowed down though.  :-+  And I don't mind getting a little practice for making "old-style" scope shots.
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 09, 2015, 09:17:54 pm
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.)
Title: Re: Fluke 8050a True RMS U32
Post by: wiss on March 09, 2015, 09:19:00 pm
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... ?
Title: Re: Fluke 8050a True RMS U32
Post by: ModemHead on March 09, 2015, 09:37:26 pm
Not getting a signal on pin2 (inverting input) is very confusing to me.
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.
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 10, 2015, 02:49:17 pm
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?
Title: Re: Fluke 8050a True RMS U32
Post by: krivx on March 10, 2015, 03:30:33 pm
Is the CA3140E hard to source? I think I have some old RCA parts in DIP8.
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 10, 2015, 03:45:34 pm
I found some TI LM3046s that should work for the CA3046. The CA3140 is available from Digikey.
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 10, 2015, 05:26:12 pm
I will admit that one of the reasons I started with the 3046 was it was easy to access. Easier than any of the other chips. I didn't have to remove the u32 circuit board from the main PCB in order to desolder the 3046. It's sort of like the drunk who searched for his lost wallet under the streetlamp even though the wallet was lost elsewhere simply because the light was better under the streetlamp. None the less the 3046 has now been removed and I'm pleased to report that two of the five transistors are definitely bad.

The bad ones were the transistor closest to the diode pair near the input and the transistor closest to the output.
Now I need to put together an order from digikey for all of the parts I need. I hate paying shipping cost for a 50 cent part. I might as well order the other parts as well just in case they need to be replaced also.
Title: Re: Fluke 8050a True RMS U32
Post by: ModemHead on March 10, 2015, 06:49:09 pm
None the less the 3046 has now been removed and I'm pleased to report that two of the five transistors are definitely bad.
OK I'm a little late with this, but I'll post it for completeness. :)

Meter input remains at 1.00Vrms, at 200Hz.  The 8050A display reads 1.0007.  Scope settings remain the same.  I found my non-battery option 8050A and switched to it so the probing is easier.

Pin 1 of the 473777 is the module output at 1.069 Vdc.  The output of the other opamp on this chip is -1.805 Vdc.  All the inputs are very near 0V, with no AC signals present.  The -5V supply is -4.98V and the +6V supply is at 6.24V.

The transistor array pins have interesting signals which I've captured.  I'll call it U3.  Pins 6, 8, 9, and 11 are at 0 Vdc.  Pins 2 and 9 are both at -0.657 Vdc.  Pins 4, 5, and 7 are all the same signal.

I think it's safe to say the circuit does not follow the block diagram exactly. The input goes into the LF351 where it looks like the log(Vi) is done.  The CA3140E apparently then does the absolute value.  There's still an antilog and summation happening somewhere, but I don't know where exactly.

Hopefully the dead transistors will be your only problem...
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 10, 2015, 08:05:58 pm
Success! Just so I'd know if I needed to buy more chips, I cut off the legs of the two bad transistors, hot melt glued a couple of spare NPN transistors with similar specs on top of the 3046 and bent the legs to match the SOIC pins I'd cut off.  I soldered in the whole contraption and I'm now getting output that's looking pretty good.   I need to check calibration and make sure that it works on all ranges, but it was displaying values close to what I expected with the few tests I had time to make.

Thanks for the help, everyone, particularly the "beyond the call" effort from Modemhead with all that recent work. The information that there was no signal at the input of the 3140 helped me decide the problem was probably in the feedback  loop.

I still want to check signals around the "leaky diode" problem area to see if that's been fixed, and I'll compare my signals to those Modemhead just posted.

For the record, the bad transistors were on pins 6-8 and 12-14. I'm going to replace the chip anyway.
Title: Re: Fluke 8050a True RMS U32
Post by: ModemHead on March 10, 2015, 09:27:55 pm
Excellent!  Great job on the diagnosis and repair.  Pleased to contribute a bit, sometime in the future someone will google "8050A RMS converter" and find some good info here.

Regarding the silver solder, the only thing I know about that is the ceramic terminal strips in old Tek scopes are supposed to be soldered with silver-bearing solder, because regular 60/40 can weaken the bonds between the metal contacts and the ceramic material.  Not sure if that applies here.
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 11, 2015, 01:29:14 am
My report of success was somewhat premature. I had the scope on the 2 volt/div scale when I thought it was on the 1V/div scale. Everything looked more or less correct, but in reality, the repaired 8050a was reading about 50% low on all AC scales.  By chance, the numbers seemed to match up.  I'll be using Modemhead's numbers to track down what's going on.  At least I've got something other than zero on the display. :)
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 11, 2015, 01:50:17 am
I took some readings from 1 volt to 200 VAC on another meter and the 8050a, then X-Y plotted them in Excel. They lie accurately on a line, so at least I know the error is just a linear scaling error.
Title: Re: Fluke 8050a True RMS U32
Post by: wiss on March 11, 2015, 08:14:56 am
You must get really bad matching-errors from the two hacked-in transistors!
These circuits are annoyingly temperature dependant. That's why no one try to build one with discreet transistors.
Even when you get a 3046-replacement I would guess that adjustments will be needed.
Title: Re: Fluke 8050a True RMS U32
Post by: ModemHead on March 11, 2015, 01:25:37 pm
Even when you get a 3046-replacement I would guess that adjustments will be needed.
For those who haven't seen the schematic, there is a configurable voltage divider and trim pot on the output of this module. Hopefully when the matched array is installed the output will fall within the expected output range.
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 11, 2015, 09:05:57 pm
Yes, there are 4 resistors for the DC calibration, and two more resistors and two trim caps for AC calibration, plus one adjustable resistor on the U32 TRMS board itself.  I've been looking at the calibration procedures, but I decided not to touch anything until the transistor array is installed. I ordered three of them today. With luck, one of them will bring it within calibration range.

Again, I'd like to express my thanks for the help.  I'll let you know how it goes.
Title: Re: Fluke 8050a True RMS U32
Post by: tango17 on March 18, 2015, 04:52:02 pm
The 3046 came in yesterday and I removed the hacked transistors and bad CA3046.  The new chip was an LM3046 in an SOIC-14 package with 0.05" pitch  The old chip was a surface mount DIP package with 0.1" pitch, so the first step was to solder on 14 lead extensions and reform the SOIC to DIP shape. There's probably an adapter sold that would make this job easier, but I couldnt find it.

This morning I was ready to install. After some tricky soldering, with the U32 hybrid board still installed, I turned it on and ran ACV tests. The results were all within 1 percent of my best meter from 2 millivolts to 146 volts at 60 Hz. That meter (3.5 digits and purchased in the 1980s) only has 1.2 percent accuracy plus or minus 10 counts over the test range, so that looked pretty good to me. I'm not going to touch any of the 8050a AC calibration until I get access to a better meter and test equipment.

I guess that constitutes success!

Thanks for the help, guys!
Title: Re: Fluke 8050a True RMS U32
Post by: ModemHead on March 18, 2015, 05:42:48 pm
Nice thorough job on the repairs, I hope you get years of service from your "new" 8050A.  Mine stays on 24/7, it's always ready for a quick measure or for monitoring something for hours with no battery-saver timeouts.
Title: Re: Fluke 8050a True RMS U32
Post by: wiss on March 18, 2015, 06:12:12 pm
 :-+

I would be surprised if the cal was dependant on the parameters of the 3046, that would mean lots of thermal drift.