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Fluke 732A + Fluke 752A
Posted by
Theboel
on 11 Aug, 2017 00:15
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#1 Reply
Posted by
TiN
on 11 Aug, 2017 00:41
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Bad idea
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If there's anything wrong inside the resistance cans of the 752, there isn't much to do about it. OTOH, it could have a bad switch or something. Those have internal jumpers to trim them- mine needed to be changed to work properly. It sounds like the 732 has a bad battery. It could be a great deal if you can fix them, or a horrible deal if you can't. If the money tree is in bloom and it's not a huge amount to you, maybe go for it. If it seems like a lot of money, don't do it.
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#3 Reply
Posted by
jh15
on 11 Aug, 2017 04:11
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why would a battery be a bad problem?
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The eBay 732a's almost always have a dead battery pack. Usually people replace them with new batteries don't realize that you have to turn the high-charge current cutout voltage down from 32V to a more reasonable 28~28.5V for the newer battery chem setups. Look at at your newer SLA batteries and stay under the maximum charge voltage - these are not quite the same as the earlier SLA's of 70's / 80's where max batt voltage was on the order of 8V per batt.
If you don't adjust that charge cutout voltage, you just cook the battery pack at every charge and they don't last long. I'll do a how-to later if time allows.
Other problems on 732a are the power supply caps (always) and charge pass transistor, and the batt pack on-off switch goes flakey. If the fuse is popped you might have bigger problems.
If there is something wrong in the ovenized section, just forget it - there are so many hand-selected parts in there it's not very repairable. Good luck even removing that oven section from the case if the insulating foam has gotten hard. SOMETIMES you get lucky and the oven box slips out - but usually the whole assembly just disintegrates into a white powdery mess, especially if you have a unit from the 70's.
Trust me - if that oven is stuck in there, it's probably not coming out until the magical insulating foam falls apart around it.
A 752 with a dead output? Yes it could be a dead switch or if somebody over-stressed it to the point of failure. The switches in those things are not very repairable either if it's a busted wiper. You'd almost have to get a parts donor 752, but that is rare. Usually those work OK - there is not a lot to go wrong if they aren't over-stressed and the switches + cal pots stay working.
If that 752 is really dead, well you just bought yourself a pile of really special resistors.
Conrad's and TiN's advice is good - absolutely steer clear unless it has no effect on your budget.
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#5 Reply
Posted by
alm
on 11 Aug, 2017 12:39
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It is an auction, so expect the price to go up substantially. Especially if it is linked to at a metrology forum full of volt nuts
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#6 Reply
Posted by
saturnin
on 11 Aug, 2017 16:55
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Exactly, there is just the calm before the storm in the auction. If you have ever been interested in the offered items, it was not very tactical to publish the auction here.
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#7 Reply
Posted by
CalMachine
on 11 Aug, 2017 18:42
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The eBay 732a's almost always have a dead battery pack. Usually people replace them with new batteries don't realize that you have to turn the high-charge current cutout voltage down from 32V to a more reasonable 28~28.5V for the newer battery chem setups. Look at at your newer SLA batteries and stay under the maximum charge voltage - these are not quite the same as the earlier SLA's of 70's / 80's where max batt voltage was on the order of 8V per batt.
I just picked up a 732A and the 7003 battery charger and am needing to change out the batteries. Is this something that is detailed in the manual?
R10 on the A3 board?
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There are two manuals available for download. One is the older 732A version and there is a slightly newer 732A/AN manual.
Both have the charger adjustment procedure but one or two set points are different. I didn't find an explanation as to the difference but IIRC the values were different by 2-3vdc. I tend to use the newer setting but I try and see how it is currently set, and if it is close to one of the procedures, leave it as is.
I like to charge each battery separately before installing especially if they are not balanced.
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Yep, R10. Forget the stuff in the manual about adding the pot / Variac. Just clip on your voltmeter to the battery board and watch the battery pack voltage while it charges and it should switch from high rate charge to float charge at around 28 ~28.5 volts on the pack. Your batts are probably rated for no more than 7.25 VDC, so 4 in series will mean they get balanced at around 28~29VDC anyway for sure, and probably less. DON'T set for 32VDC as it tells you in the original manual, or you just destroy the batts. A lot of times newer batts won't ever get to 32VDC, they just start getting warm and toasty (and ruined). You don't want that, and you find out in a hurry what "sealed" means for an overheated battery.
You'll also find out that modern batteries are slightly bigger and you probably don't need those spacer cards that were in with the old batteries.
R10 gets turned down to minimum (you hear the wiper clutch click) and up from there a couple turns - start about there and see about where it is. When I do these that's a good starting point. That's a multiturn pot in there. You have the top covers off.
The Batt charge light goes out when it switches to float mode, plus you'll see batt voltage drop to around 27~27.2 VDC or in there during float. That's correct. Don't over-do it.
To test you just let batts charge up until you know they are at full capacity, and then run on batt power for say 60~90 seconds, then power up on AC again and watch the system switch the batts on to high current cycle mode, then drop back to float when high-rate mode hits somewhere around 28 ~ 28.5 VDC.
You might have to readjust R10 as the unit gets warm, so set the covers on loose and let everything run a few hours and then recheck R10.
You test your unit at any time by dropping the AC power, let it run on battery power maybe 5 minutes or so, then re-apply AC. You should see the Batt Charge light come on for a while then go out when batts are charged up again. For instance if you run on battery power for 5 minutes, and then re-power up on AC: The Batt light comes on for about 30 to 45 seconds or so for a good battery pack.
A battery light that flickers or stays on always, or your unit doesn't run on battery power for at least 12 hrs means your pack or charger board is broken.
Once you get new batts in, replace 3 main power supply / regulator 'lytic caps and adjust the battery charger, button it up for another 5 or 7 years. Don't take any measures until she's all dressed up again and all screws in place, and wait a week or two after any cold power cycle. You can also check the other pot settings, but usually those are OK anyway. That's about all you do.
On older units they almost always will have drifted "up". Either leave it be and know the current voltage when you send it in for cal (comes in handy with a KVD when you have it slightly high, that means you can hit 10V on the output of KVD) or make an adjustment at cal time. Wait at least several days or so in between touching that cal pot to see where she settles in. We never worry about touching the 1.018 or 1V outputs - that's what your 752 is for. You can do those extra 732 outputs for general entertainment but a 752 on that 10V output is very, very accurate & stable for a 1V source.
Once you get it to where you want, hit that reset hole with a wire to LO to light In Cal lamp SCR circuit. That just serves to tell you if the unit went cold - out of power. Tag the front of the unit with exact output voltage / date and she's ready for service.
If you really have to there is the coarse setting board on top of the oven box, but if you have to tweak that there might be something else wrong. We've had units from the 70's that never have needed the coarse setting adjusted - from the factory they usually have the 40ppm option jumper-ed, but sometimes some other combo. The cal pot on the front should give you about 8~10ppm adjust range, more or less.
Now you wait several months and see if you have a drifty unit or not. It takes quite a while to know the full story, be patient. If you got the unit off eBay and it sat cold for a long time - it can be months (or never) before it stops wandering around. Sometimes they get good again though - you just have to wait and see.
The worst would be if a 732a sat in a cold (and hot) warehouse for years, so be very careful of "equipment dealers" on eBay.
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#10 Reply
Posted by
martinr33
on 11 Aug, 2017 23:48
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I just replaced the batteries in an eBay 732a, ex Australian Air Force.
I used two 12V Yuasa-branded batteries Yuasa NP4-12. Amazon didn't have them as prime, but the eBay seller got them to me in 3 days.
I measure the charge voltage at 26.2V. I got this from the external DC connector, which connects to the battery string. I'll check it again, see how it rises.
The wires to this connector, and one of the wires to the front panel switch, were badly fatigued with most of the strands broken. Other wiring was fine. It is all in PTFE insulation.
I needed to add two spacers, which I cut from .236 acrylic - gave me a perfect fit. I had to notch out the aluminum top plate to accommodate two of the terminals.
The unit seems to be settling in at about 10V + 50uV. I'll let it cook off for a few weeks, and see how it is stabilizing.
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You don't need to hack into your 732a to fit new batteries - just use four each of the correct size 6V 4.5Ah battery (these are commonly used for exit signs, alarms etc.). We get these by the case when they go on sale, have lots of uses:
https://www.atbatt.com/power-sonic-ps-640-6v-45ah-sealed-lead-acid-battery-w-f1-terminal.aspWe've used those for decades, no problem. If you keep the charger adjusted, those will last 5~7 years or so - depending on how many times you full cycle them.
You do have to take apart the battery cage assembly pretty far to get the batteries to slip in, but they fit fine. You might need to remove a cardboard spacer that was tucked in with the batteries if you had the old 1981 era batteries installed.
Handy tip: Keep the battery terminal covers in place when you slip the new batteries in, otherwise you'll find out how to weld the battery terminals to the top aluminum battery plate - and smoke a new battery.
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Also: When you get that 732a running, check the thermistor sensor for the oven. Probably somewhere around 4k ohms (3k to 5k is common) but that should stabilize as the unit warms up and settles in.
You keep an eye on that to see how your oven is doing - that should stay relatively stable even if room temp changes a few degrees.
It will be interesting to see how that Aussie unit works out. Seller wasn't sure of history but the price wasn't too bad - he had it for a set price earlier and then changed it to auction where it sold.
Have fun!
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#13 Reply
Posted by
martinr33
on 12 Aug, 2017 03:50
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Thanks for those tips!
I replaced the four 6V batteries with two 12V batteries (the batteries actually cost the same per unit, $20 each, for 6V or 12V), but it took a bit of metal hacking.
You are so right on the terminal covers, I left them on and there were many times I would have had a battery short otherwise.
I did have to bend the terminals up, because the terminals interfere with the wiring board.
That oven point is really important - I'll start monitoring that once the unit has stabilized.
Rather bizarrely - I have two 8.5 digit Advantest meters watching it. They come pretty close to agreement some of the time (microvolt), but can drift maybe 7uV apart as the temperature shifts, even with an autocal. So short-term stability is looking decent.
I figured that the AAF would have kept these things in spec over a couple of decades. In other words, they aren't being surplused because of bad behavior, they are just being replaced because they are old.
If the batteries were replaced in 2008, the would have needed servicing a few years ago, so this system may have been sitting. As with all these things, a bit of a flier but we'll see how it plays out over a few months.
At +50uV, it is a bit on the high side. That's too much for the easy adjustment, and I do not want to crack the oven open. So I will live with it.
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A) If the AAF got rid of it simply because it's "Old" then they are probably mistaken. Older units that are stable, working and with a long drift history are much more valuable than a new unknown 732b. BUT these 732a are more expensive to ship for cal than a 732b - and probably somebody in accounting got worried. This is just like the US Navy dumping good expensive metal filing cabinets because they had reached a 10yr "expiration date" which is asinine. 732a's will probably be OK until about 2025~ 2030 when we expect the first units to start experiencing real failure of the zener junction on that crystal lattice, but that is a guess projection (estimated 50 ~ 70 years on that junction lifetime, powered on 24/7). We have to just wait and see - we could be coming up on other oven or transistor amp failures, etc. Live in the moment and enjoy it!
B) Hopefully you replaced those electrolytic caps also. They last around 7 / 10 years and I bet the AAF didn't know those need to be replaced. These work for a living 24/7 and don't last forever. Use a good quality, long lifetime cap replacement. When you replace the caps and batteries you tag the back of the unit with a date so you know when it needs to be done again. Otherwise you'll see destructive leaks and a bit more noise on the output if you're using it while on mains power. Normally you switch over to batt power when making quality measures though. That's the whole point.
C) 50uV is 5ppm high - should be very easily adjustable by that cal pot depending on where it's sitting now, unless it's already been cranked down to the stops (Keep the cal pot between 15% and 85%, don't run it at the stops - if you see that you have to use the coarse adjust board). PLUS unless your other meters are in cal you're at an unknown state for absolute value - even if they seem to agree that doesn't mean anything, really. Probably fine to leave it be. If you touch the pot you wait a while (a week or so) to see where it settles in, and usually you do that when this unit is sitting next to one or two or more other 732's for reference. Normally you NEVER adjust a 732 to a meter (no meter is really good enough), only adjust to another freshly cal'd 732 via null meter. If you make an adjustment you always record the old setting, the new setting, the total delta adjustment and the date. That becomes part of the drift history.
That is very normal - most units we've seen from late 70's early 80's will be around 4~8ppm high, give or take. If they haven't been cal'd / adjusted in a long time (99.9% eBay / dealer stuff).
Once you see it working it is highly recommended to send it in for cal - that will start an accurate drift history record and make the unit much more valuable later. The 732 will be a more accurate absolute reference than your meters. Otherwise you just have an unknown quiet power supply the size of a boat anchor, approximately somewhere in the ballpark vicinity of 10V...maybe.
Now you need a few more cal'd 732's to build up your Vref stability and lower uncertainty...
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#15 Reply
Posted by
martinr33
on 13 Aug, 2017 03:18
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A) That's the bad news. No history. so I have to start collecting. I am going to let it warm up for a couple of weeks. Then, I think I will be able to check it against a couple of known units. I am lucky to have a couple of well -equipped friends in the area. (and they know who they are, and I am very grateful for their engagement).
B) Two of the caps were newer, and two looked original. But the ones most at risk are the caps on the AC power module. I need to get spares first. Modern caps with higher voltage rating will have a much, much longer life. I may also up the capacitance, which also helps. (capacitor life is directly related to temperature and linked to physical size, whether there is current flowing or not... I know this from working on electronic products in the '80s, when the QA team was able to run extensive hot soak experiments). I'll figure out what will fit. Fluke already used 90V capacitors, which is very smart. The higher voltage really helps with longer life. (for those of you following along, electrolytics are typically rated for 1000 hour life - this is why we use higher voltage and capacitance than necessary! There are plenty of recent examples where the designer either did not know this, or somebody cut production costs).
C) Yes, I slipped a digit. I'll have to check the adjustment, see where it sits. Although at 5ppm off, there's a case to leave it alone.
Thanks again for your insights. I'm working through all of this.
And by the time those diode references fail, someone will have built an LTZ1000 version that patches in and uses the Fluke calibration resistors.
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I measure the charge voltage at 26.2V. I got this from the external DC connector, which connects to the battery string. I'll check it again, see how it rises.
Check your unit - unless it was modified at some point, you can't measure the battery pack voltage at the rear connector accurately because of protection diode CR1 in series to that connector. For the instructions I gave on adjusting battery charger, you have to measure the voltage across the battery pack proper. If you measure at the rear DC power connector you might be reading about 0.8V low. Just be aware.
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Spare parts for 732a's - these will keep your unit running for about 7-10 yrs before replacement. You always replace the caps when changing batteries anyway.
For caps -only- go with minimum 2000hr or more rated at 85° or 105°C, 100V rating. Anything less won't last long term for 24/7 operation. These will fit just fine - Your unit might have slightly different values depending on build date but these are the only ones we've ever needed:
1. Nichicon or Vishay 220uF X 100V axial - 2000 hr life minimum 85°C, usually need 2 per unit depending on rev. For instance Vishay / BC # MAL212019221E3
2. Nichicon 330uF X 100V axial, 2000 hr life minimum 85°C - for instance part number TVX2A331MCD. Usually need one (or two) per unit depending on rev. Very early units might have two so pick up an extra.
Transistors - for when you forget you unplugged the unit and left it on batteries all weekend and the batteries went 100% flat, and about the 3rd time you do that on old batteries you wind up with a shorted battery that takes out the charger transistor... it's handy to have these in stock (but check your unit since they might have changed part numbers) these are most common used by Fluke in 732a for power transistors - we've never had to replace the signal transistors (some of which are probably like 2N3904 / 06):
3. MJE15028 NPN 8A 120V 50W in TO-220.
Plus the usual power supply diodes, bridge etc. Those vary depending on when the 732a was built so just grab some to match your unit, we like to replace with the exact part number if possible. They probably won't be exotic numbers. Replacing these parts is more rare but "sh!t happens" so you have a box of spares.
If you plan on adjusting the 10V cal pot on your unit (ONLY if you have another cal'd 732a for reference) you're going to find out that by design, that cal pot shaft is just about impossible to reach with any standard length screwdriver. You can get (or build) a very long reach small flat blade plastic screwdriver to fit in that hole, and then it'll take about 20 or 30 minutes to get it to slip into the slotted shaft (Or you get really lucky and you find the shaft on first try), because you can only go by what you feel several inches away from the front panel. Fluke has a special tubular adjust tool that originally shipped with these units or your can make your own - we make them out of small brass / plastic tubing with a small piece of shim stick mounted cross-wise near the end of the tube to engage the pot slot; the tube acts as a short guide to slip over the pot adjust shaft, then you turn the tube until you feel the shim stick slip into the shaft slot. It still takes practice to feel when you've got the pot shaft engaged. You will want the brass tube / shim stock at the end for the pot and then transition that to a plastic rod that's going to come in contact with the housing.
You can also take an existing pot adjuster tool and add a long plastic extension rod to it, but the idea is you want that insulated so you don't connect the pot shaft to the outer housing during adjustment, or else you'll see a slight VDC offset appear. Sometimes you can find an extra-long pot adjust tool that will fit but it does take some searching, that's why we just made our own version.
You'll know what you need when you examine your unit up close. Better yet is to get the real Fluke special adjust tool if you can get one - Last I checked Fluke didn't have any left for 732a's.
Once you touch that 10V cal pot and get it to where you think you want it you put down all your tools and back away from the unit - and come back a few days or a week later and see where it settled at for real. Always cross check to other cal'd 732a's (at least two) to determine where your unit was before and after adjustment. The best technique is to send it to Fluke for a real cal and maybe adjustment. If you're sort of close already, like within a few PPM: when you send it to Fluke for cal and they will leave it be and measure the voltage traceable to NIST, but not do any adjustment. As long as you know exactly what your Vref voltage is, you're good - that's the whole idea. Generally if Fluke spots excess noise or if it's way out of whack for some reason they will tag it as "Non-Functional - Do Not Use" and return it to you. They do not repair 732a's at this point - so that's why you want to make sure your 732a is really working before sending it in for cal.
If you get or build yourself an extra long adjust tool like that it comes in handy for adjusting R10 and lots of other equipment - much easier than trying to use a regular or plastic screwdriver. Radio hams and tube amp guys will probably have access to all sort of strange, really long pot adjuster tools.
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#18 Reply
Posted by
martinr33
on 14 Aug, 2017 06:26
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Correct, but that's an optimistic Vishay datasheet so we always take it with a grain of salt. 27,500 hrs is only ~3 years powered on 24/7 but that's at the rated voltage and temperature of the cap. Stay well under the peak voltage / temperature / ripple current specs and you'll certainly get good lifetime, but we -always- replace about 7 ~ 10 yrs as part of the preventative maintenance on these 732a (and other equipment in the lab) to prevent unscheduled downtime. We've also learned from experience how far off some of the MTBF calcs and datasheets really are.
Regardless of lifetime rating these better quality caps are still electrolytic, and are allways prone to the "battle of the bulge" including increased current leakage and reduced capacitance - in our experience you can start having subsequent electrolyte leakage as early as around 12 ~ 15 years, maybe 18~ 20yrs. And that's a pain to clean up and can cause other damage as well. Even if the cap doesn't leak you'll have a real drop-off in performance over time, and the excess 120Hz noise in the power supply can reach that precious stabilized Zener and cause excess current ripple. Don't want that.
From a business point of view: Even when you count the labor time for replacing the batteries and caps, the cost of the actual parts is trivial compared to surprise downtime and then waiting for the unit to recover / stabilize from a cold start and then of course a calibration cycle. That's why we have a very strict replacement schedule for any parts subject to limited lifetime for ALL lab equipment in use. A 732a wants to run for very long stretches of time with very minimal power cycles - that's what it was born to do.
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#20 Reply
Posted by
Dr. Frank
on 14 Aug, 2017 18:03
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Whoever has won this set, congrats
, will get a perfectly working 752A.
That 10V range obviously works fine, which the seller didn't understand.
In the 10V range, the 10:1 / 100:1 divider is de-coupled from reference input and DUT input, so the divider output will always give 0V.
It's a differential instrument, so the differential output will be active, in any range.
Frank
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Wow, to bad, I was not home. It was sold for a good price, I had expected twice as much.
Was it bought by anyone here on the forum?
May be we get a nice repair thread.
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#22 Reply
Posted by
Bill158
on 14 Aug, 2017 21:00
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If you plan on adjusting the 10V cal pot on your unit (ONLY if you have another cal'd 732a for reference) you're going to find out that by design, that cal pot shaft is just about impossible to reach with any standard length screwdriver.
I have used a General Cement GC 8276 to adjust the three pots for the 10V 1.018 and 1V adjustments. The pots are about 3.5 inches from the front panel and require a LOT of patience to get the blade to engage the pot slot. Sometimes you get lucky. I usually try to look into the hole and determine about where ( i.e. 12 to 6 oclock ) the slot is and then put the adjustment tool in about the same position and then slowly twist CW and CCW. Another problem is to get the adjustment tool through the hole in the oven assembly and then engage the pot.
The company is now called GC Electronics. I see that Newark has them in stock.
http://www.newark.com/gc-electronics/8276/alignment-tool/dp/35F616?ost=8276&scope=partnumberlookahead&exaMfpn=true&searchref=searchlookahead&searchView=table&iscrfnonsku=false&ddkey=http%3Aen-US%2FElement14_US%2Fw%2FsearchMost suppliers don't seem to have them. This may have been the tool that FLUKE supplied but I cannot be sure.
Bill
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I am surprised no mention was made of replacing the power input filter. I have replaced them on all of my 732A plus several other older gear. It's a quick job and doesn't cost a lot. I think it should be common practice on gear plugged in 24/7.
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Yeup, that's kind of like the brass tube + plastic rod tool we made (similar to the real Fluke version we've seen) except at the business end the blade would be recessed back into the tube a bit, maybe an 1/16~1/8" or so. I've never seen a really long one exactly like that outside of Fluke and an older Heathkit radio alignment toolkit (very handy!).
That way when you poke the tool in it will slip into the inner housing hole opening for the pot and then it "usually" slips right over the pot adjust shaft acting as a guide (if you're gentle). Once you feel that then very gently rotate until you feel the blade fall into the pot adjust shaft slot and then you're good - start making small adjustments and watch your null meter.
The advantage to a recessed-blade-in-tube style is that easily "stays put" without your hand on it - you can leave the tool in place while you fiddle with the null meter or whatever and then come back to where you left off.
The usual technique on that pot rotation is to get to where you want, go slightly past, then back up and rock it in that final bit - let the pot wiper clean the new resting position. Once you do it a few times you know.
Even with the proper tool it takes a bit of practice but much easier. If you try to do it with any regular screwdriver you'll be there a while. It's loads of fun to give the job to the "smarty-pants" rookie grad who thinks it'll be a no-brainer simple job, and he's still at it 30 minutes later hopelessly fishing around in the hole trying to find the pot. Great Theory vs Reality learning experience!
What throws most people: with a flashlight you can look in there and see the pot shaft...but it's devilishly hard to actually get in with a tool to adjust it. By design.
OH - By the way - that is a very high quality Bourns PWW pot in there, and the cal pot shaft should be not very hard to rotate. Some guys get a metal screwdriver wedged in alongside the pot shaft, wedged between the shaft and the inner housing hole, and then torque hard. That's a great way to really mess up the cal pot and oven assembly. We've seen more than one used unit with that type of damage.