Electronics > Repair
Deciphering Fluke 732a
dlebed:
I was able to obtain a Fluke 732a without any history whatsoever (only sticker with thermistor resistance at oven setpoint ;D) and so far I have not figured out yet whether it's a bad or good luck...
The good thing is that one of my 3458a's was reading its 10V output as 0.58ppm from exactly 10V which is a good start.
The unit had some problems though due to poor soldering/repairs that was done before me, so I decided to rebuild all the boards (incl. A5).
A3 and A4 are easy, but A5 (reference board inside the oven) is famous to be very messy to deal with.
I clearly had some poor contacts, so I decided to overhaul it as well (CC resistors and also caps).
Battery
Obviously, original batteries were completely dead, but after overhauling A3 and A4 boards I decided to give a try to LiFePO4 battery.
Luckily Amazon has a single 24V LiFePO4 battery, so there is no need to put any extra balancers, etc. for multi-battery setup.
Here is the battery I put into test Talentcell 24V 6Ah LF8011 https://www.amazon.com/dp/B0CNLKKL9C?ref=ppx_yo2ov_dt_b_fed_asin_title.
The battery was able to deliver about 5.7Ah in my discharge test (22V cutoff), which is almost as specified (they specify it as deep discharge down to 18V) and is quite a bit more than standard 4Ah Lead Acid.
Obviously you need to tweak R20 and R10 pots on A3 to make sure the charge and float voltages are good for LiFePO4 chemistry.
I put everything together, but have not yet tested it with a battery as I'm waiting while A5 will be ready too.
Here is how the battery looks inside the battery compartment - there is quite a bit free space there.
You don't even need to cut any new holes as the battery sits completely below the top plate and don't touch any metal with its terminals.
dlebed:
A5 (Regulator board)
One of the challenges with A5 board is that clearly something went wrong around CR5/VR2 area as PCB is charred and there were signs of rework in that area.
Also, U3 was replaced with LF353N in some weird DIY socket.
Here is the CR5/VR2/R9 area (already cleaned, it was worse).
There is clear PCB charring/discoloration, probably due to some failure/overload happened before and fixed.
The problem is that given the quality of other fixes, I'm kind of having a bit of distrust to this repair, so I decided to order new parts and replace CR5/VR2.
Has anybody looked into this part of A5?
It seems that this CR5 + R9 is a part of current compensation circuit...
Why R9 is a part of the ref amp set along with R5 and U2?
I understand the U2 and R5, but why the R9? It is to compensate and match the current set by R5?
dlebed:
So to replace the CR5/VR2 with trusted parts I ordered:
[VR2] 1N4574A - 6.4V temp. compensated Zener with 5ppm/C tempco - an impressive part with impressive price of $27
[CR2] A set of different CRDs from Semitec: E-101, E-301, E-501, E-102, see https://www.semitec-global.com/uploads/2022/01/P22-23-CRD.pdf
Interestingly, they all have different stabilization voltage and also a different tempco:
What I found is that the CR2 current regulating diode that I have on A5 board actually stabilizes above the rough 2.2V that it should be working at (18.6V - (10V + 6.4V) = 2.2V across CR5).
Here is a sweep of CR2 I have on the A5 board, you can see that it's not yet stable at 2.2V
Here is the sweep of CR2 + R9 in parallel, you can see that together they reach exactly 1mA at 2.2V
As for the Semitec's CRDs that I bought (10 each) I ran sweeps and figured out that I could put multiple of them in parallel to achieve lower tempco and very good regulation at 2.2V.
E-101
E-301
E-501
E-102
So the main question I'm currently battling is what is the actual role of the R9 and how to actually tweak the CR5 (I assume it will mostly affect the tempco of the U2 based on what I read about Fluke 731 tuning).
I'm thinking on putting 3 x E-301 in parallel, that will result in much better tempco and much better regulation at 2.2V.
The question is whether this R9 is in any way responsible for CR5 tempco compensation (which I doubt) and should CR5 be tuned to be exactly 1mA?
Will having a better regulation and tempco for CR5 in isolation actually improve things or make them worse (if R9 supposedly compensating for CR5 in some way)?
dlebed:
I was able to acquire the original Fluke CRDs (Fluke 334839) and run sweeps, interestingly they're quite different from the Semitec's CRDs.
BTW, if someone will ever need that rare Fluke 334839, shoot me a message, I might have some leftovers. I don't think you'll be able to easily find them on the market.
Semitec 1mA CRD has negative tempco, while Fluke 334839 has positive.
Also, for 1mA CRD, Fluke's has better regulation at ~2V than Semitec's one.
Fluke 334839 CRD sweep
Here is zoomed in version around 2-2.5V for Fluke 334839 vs Semitec E-102.
Each sweep was bidirectional, so you see two lines with the same color.
The bigger the gap between the lines the bigger the tempco - the gap is due to self-heating and tempco effect.
You can see a better regulation of Fluke's CRD and lower tempco.
I feel the same result could be achieved with 3xE-301 in parallel.
bdunham7:
I also have a 732A I'm going to be going back into at some point. I'll probably be interested in 1.0mA CRDs at some point. Just a few thoughts and a question for now.
IDK why R9 is part of the ref amp set because it, along with CR5, sets the zener current through VR2--and CR5/VR2 are not part of the ref amp set. IDK why Fluke chose to set the voltage here in this way instead of a buffer and divider deriving it from the output voltage. I suppose a 5ppm/C zener stacked on the 10V reference is pretty good and might actually be hard to match the other way. It's all in the oven anyway.
I find that this particular wiring diagram is harder than expected to understand and I often refer to the first interconnect diagram which is simplified, but actually shows how things are set up. For example, it is very hard to understand what is going on with the output circuit and U1 unless you attach the snippet from the interconnect diagram that I've attached.
The question is where are the discussions of 731B tuning? I've seen them referred to before and what people say about them seems to imply some very in-depth discussion of zeroing the tempco of the reference by finding just the right current. However, I've asked but have never seen those detailed discussions.
I'm not sure what to say about the rework, you can still buy an LM358H AFAIK. The browned boards seem to be a common issue and is probably just because those parts get a bit warm.
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