Adjustment question - Fluke 8600A (fixed) - Page 1
Forum section: electronics / testgear

author: c4757p
author_rank: Super Contributor
timestamp: May 27, 2013, 05:20:13 pm
content: Yep, another adjustment question. I grabbed a couple of Fluke 8600A DMMs broken, dirt cheap (wouldn't power on). Fixed the power supplies, and the calibration is out quite a bit, so I'm adjusting them. Both of them have this problem. Any ideas?The first step in the DC V adjustment is to set two trim capacitors attached to a JFET in what appears to be the ADC:1. Adjust C24 with a 1M resistor across the input to get a reading of -0.01mV to +0.01mV.2. Apply +190mV; note the reading.3. Apply -190mV; adjust C30 to get a reading within 1 digit of the +190mV reading.4. Repeat until no adjustment is necessary.The problem is that with every step I'm backing the screws out of the capacitors more and more, and it never quite reaches proper calibration before the screws are falling out! I've cleaned the crap out of the PCB in case there was gunk adding stray capacitance, with no effect. Should I be looking at replacing that JFET? And if so - what can I use? They list it as an F2691 from Solitron, which seems remarkably hard to get. Would any old switching JFET work, or is this a special part?

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 27, 2013, 06:47:45 pm
content: Just to state the obvious here: in step 3 you are aiming for thenegativevalue you got in step 2 are you?

author: c4757p
author_rank: Super Contributor
timestamp: May 27, 2013, 06:59:21 pm
content: I am aiming for equal magnitude - matching +194.04 to -194.04.

author: c4757p
author_rank: Super Contributor
timestamp: May 27, 2013, 07:11:01 pm
content: And there it is. When I repaired the power supply, I had to replace the transformer - it was faulty. I used a small SMPS module that I had in the junk box to replace the entire line->DC system. Unfortunately I didn't notice that the SMPS module was also "slightly faulty" - at least, it was putting out just a bit too much ripple to be sensible (almost 500mV p-p). I added some filtering and now it's just fine.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 27, 2013, 07:15:41 pm
content: Which in turn made me read the circuit description of the meter and I learned a few things again today

author: c4757p
author_rank: Super Contributor
timestamp: May 27, 2013, 08:47:15 pm
content: Stupid SMPS had such a finicky feedback loop - if the filter had any substantial inductance at all (down to 22uH) it would develop a nice 1Vpp 1kHz-ish oscillation. And it puts out exactly the required voltage, so I couldn't use an RC filter and drop more voltage. I considered changing a feedback resistor to get 7V out instead of 5V, then using a linear regulator after that (the ripple was relatively low frequency), but eventually I tried just putting a small ferrite bead on the power input and that fixed it. There must have been bigger, shorter spikes I wasn't seeing on the scope screen. The low frequency ripple is still there, but it doesn't seem to be causing any problems, even on the AC modes (the +/-15V analog rails developed by a separate inverter are perfectly clean).

author: Unknown
author_rank: Unknown
timestamp: May 27, 2013, 09:52:12 pm
content: I wonder why mains powered DMMs almost all use a linear PSU... I can't actually think of a single counterexample of the top of my head, though I wouldn't be surprised if one of the cheaper manufacturers did it to cut costs. The fact that they only draw a small amount of power probably make the savings negligible anyhow.

author: don.r
author_rank: Frequent Contributor
timestamp: May 27, 2013, 11:41:47 pm
content: Adds weight and volume to the box so you think you're getting something more than you are. Most 4.5 bench dmms, at least these days, are handhelds with a xformer, rectifier and cap. Older models like he 8600A, of course, can be forgiven.

author: c4757p
author_rank: Super Contributor
timestamp: May 27, 2013, 11:56:25 pm
content: Quote from: alm on May 27, 2013, 09:52:12 pmThis one's a semi-counterexample, but just because of the battery option. The main PSU puts out +5V, and the +/-15V comes from an inverter. (Though their idea of 'regulation' was to put 19V into an LM7815...) I'm pretty sure the non-battery version is fully linear, though.Actually, the power supply in this one is pretty odd. Weird non-standard transformer connected to the mainsthrough a 5uF capacitor(yes, in series). It puts out 20V RMS unloaded, but obviously sags significantly under load, because that 20VAC is rectifieddirectly into the 5V rail, which is shunt-regulated by the batteries, and if I'm deciphering the schematic correctly, by a Zener-controlled TRIAC on the AC secondary in lieu of the batteries.

author: SeanB
author_rank: Super Contributor
timestamp: May 28, 2013, 04:57:35 am
content: Adding a linear PSU reduces the noise injected into the common line of the meter from the power supply, unless you ground the input common. A transformer can have a very low interwinding capacitance and good filtering reduces noise to below 1 digit of flicker.As to using the transformer with a series capacitor, that increases output impedance, and gives a close to constant current source quite cheaply which works well to charge NiCd battery packs, though there are problems as you saw when the pack dies, you need to have a beefy shunt regulator or zener diode or other regulation method otherwise you cook the circuit.

author: Postal666
author_rank: Regular Contributor
timestamp: April 04, 2014, 01:21:30 am
content: Sorry for resurrecting an old thread, but does anyone have an idea on what would make an 8600a (w/ NiCads) show ALL decimal points lit up ALL the time? The meter itself seems to work fine other than that, although it was last calibrated 8/87. Auto range DOES work, and you can see the display cycle through the ranges as it should, but with every decimal point lit, "auto" is useless. The meter is still useable by selecting the range manually, since you know where the decimal point is, but it would be nice to find a fix.

author: Postal666
author_rank: Regular Contributor
timestamp: April 04, 2014, 11:10:42 pm
content: Got it. Per suggestion on another forum, I pulled pin 9 of U8 to ground through a 560 ohm resistor. Damn thing works like a charm. Thumbs up for bodge repairs.

author: casinada
author_rank: Frequent Contributor
timestamp: April 05, 2014, 12:20:20 am
content: Postal666,Don't feel shy to post the full info.Which other forum?

author: Postal666
author_rank: Regular Contributor
timestamp: April 05, 2014, 04:05:01 pm
content: http://www.vintage-radio.net/forum/showthread.php?t=84157http://www.vintage-radio.net/forum/The original suggestion was 470 ohm, but closest I had without going to the garage was 560. Unfortunately, when I fired it back up today, NO dp's will come on.....unless it's running off the batteries, then it works fine. Plugged into the mains the dp slowly fades until gone. Weird. Probably time to dig a little deeper.

author: Postal666
author_rank: Regular Contributor
timestamp: April 06, 2014, 12:35:34 am
content: Ok, so....facepalm time! Apparently I was chasing a "fault" that was actually a feature?Here's the complete story:I scored this 8600a from ebay a few days ago. When I fired it up, all the decimal points stayed lit, so I figured there was a problem. Anyway, after not finding any solutions on the 'net, I tried the aforementioned "bodge" which seemed to work; problem solved.So, today when I turned it back on, the dp worked like it should have, UNTIL I plugged it back into the mains.It worked fine on batteries though.Well, after digging a little further into the schematics, it turns out that there is a "low batt" feature that illuminates all the dp's when the batteries are low (I think). Before the "bodge" I was reading ~700mV on the base of Q39, so I figured pin9 U8 was stuck and the 560 ohm bodge pulled it down enough to work, and it did, but here's why: it was still letting enough of the smaller voltage from pin 9 of U8 through, but effectively killing the "low batt" feature voltage. Now, as for why the dp dissapeared when running off the mains: You guessed it, the 560 ohm (without the low batt voltage) was completely killing the U8 dpl signal.So I've removed the bodge resistor, and on mains it works fine now. When on battery power, the dp's all light up again, but after plugging it back in they slowly flicker and drop off leaving the correct dp lit. Duh, it does this because it's at the "low batt" threshold, and flickers until charged back above it? Can somebody confirm that this assumption is correct? I messed with this thing for a couple hours the other day, but never caught this behavior. After reading the manual, I know why...it states that although it will charge while powered on, it can take up to 43 hours to reach full charge.Throw on top of that some pretty crusty looking leaky NiCads, and there you have another case of RTFM!Even though it was last cal'd in 1987, it still reads real close to my HP 3468A, which means nothing since it was last cal'd in 2011, but it's good enough for me. I'm going to order some new batteries for it if someone can confirm that the above behavior is normal, regarding the "low batt" thing.

author: edavid
author_rank: Super Contributor
timestamp: April 06, 2014, 04:21:49 am
content: Quote from: Postal666 on April 06, 2014, 12:35:34 amYes, it's normal, but I'd suggest not buying new batteries for it, or at least not new NiCds. The charger circuit is so primitive, it cooks them quickly, so you're just generating toxic waste for no reason. Also, I don't think there are any NiMH cells that can tolerate the trickle current. Unless you want to add a real charger, keep the 8600A on the bench, and use a nice LCD meter for your on-the-go measurements.

author: Postal666
author_rank: Regular Contributor
timestamp: April 06, 2014, 04:02:10 pm
content: Fair enough. Honestly the battery option isn't a big feature for me (wasn't even listed in auction), so losing it makes no difference. This will just be a secondary bench meter anyhow. My concern now is what to do with it. In your opinion(s), should I:1. go through the trouble of designing a new charging circuit2. try to source the parts to convert back to line supply only3. just keep using it until the old batts go bad and take out something elseI've attached the ps schematic for reference. Would it just be better/easier to modify the existing supply to provide the +5V for SW13? Many thanks!edit: I read that Q43 & CR27 may be for regulation when the batteries are not there, but I can tell you (at least in my case) simply removing the batteries does not work. Seems like I should be able to just remove the batteries and feed the regulated +5V into SW13. What am I missing? (go easy, I'm still a noob)

author: edavid
author_rank: Super Contributor
timestamp: April 06, 2014, 04:23:53 pm
content: Quote from: Postal666 on April 06, 2014, 04:02:10 pmOption 3 is good... I have seen many 8600As (and 8000As) where the batteries have failed and/or leaked, and it's never damaged anything else.

author: SeanB
author_rank: Super Contributor
timestamp: April 06, 2014, 04:34:01 pm
content: Simpler to just make a shunt regulator using a TL431 and an external transistor mounted on a heat sink to emulate the battery, and bypass it with a 10000uF 16V capacitor. Modify S13F to disconnect the red lead to the transformer, and remove CR21 and CR23 to depower the circuit totally when power is off. Disconnect Q43 and CR27 as well to remove the battery regulation system. That has the advantage of not doing any non reversible modifications, as you just remove the battery pack and disconnect 3 leads, and install a small number of components in the place that was previously occupied by the battery.If the battery fails then you lose some regulation on the 5V rail, and then you rely only on CR27 and Q43 working still as they do rough voltage regulation on the battery derived 5V supply. If they fail you run the risk of cooking all the unobtanium TTL in the meter.The circuit for the power shunt is available in the TL431 datasheet.http://www.ti.com/lit/ds/symlink/tl431.pdfpage 22That will all then be easy to reverse if you want the battery option again.I have the opposite, with a 1900 counter where the battery option would have been good to have. Just means it will remain mains powered.

author: edavid
author_rank: Super Contributor
timestamp: April 06, 2014, 05:14:50 pm
content: Quote from: SeanB on April 06, 2014, 04:34:01 pmThis doesn't happen. Fluke designed it not to blow anything up if you try to run it without batteries.

author: SeanB
author_rank: Super Contributor
timestamp: April 06, 2014, 05:43:58 pm
content: SCR's do fail open or short, and if this fails open you are just relying on the original device processing to keep it working as the supply goes above "Absolute Maximum" values. Many TTL devices will work at 8V but you might get the unlucky ones that cook themselves. I have seen this before. Custom ASIC devices typically have poor voltage tolerance, ask those who used Ferranti ASICS that died if they were too cold.

author: edavid
author_rank: Super Contributor
timestamp: April 06, 2014, 05:51:44 pm
content: Quote from: SeanB on April 06, 2014, 05:43:58 pmThose are some good reasons why there could hypothetically be a problem, but in actual reality, there is no problem. You can operate a Fluke 8600A with no batteries and it won't cook anything.

author: SeanB
author_rank: Super Contributor
timestamp: April 06, 2014, 06:20:15 pm
content: YMMV, some survive, some do not.crDSC01484bySeanB_ZA, on FlickrHere is a board that had a little overvoltage event, and most aside from a PM5403 survived with no issues.

author: Postal666
author_rank: Regular Contributor
timestamp: April 06, 2014, 08:47:42 pm
content: Quote from: edavid on April 06, 2014, 05:14:50 pmI disonnected one of the batteries and it didn't hurt anything. Maybe I got lucky? (see below)Quote from: edavid on April 06, 2014, 05:51:44 pmI can only confirm from this particular unit that: it will NOT work without the batteries. I got a garbled display, and a high pitch chirping/whirr noise (imagine a sparrow in a blender) coming from the area with the vertical AC/ohms boards. After reconnecting the battery and saying a prayer to the electron gods, it fired back up with no apparent damage.So, either it won't work without them, or the Q43 +5V regulation is already toast?For now I guess I'll just use it as is while exploring forthcoming suggestions such as SeanB's TL431.

author: edavid
author_rank: Super Contributor
timestamp: April 06, 2014, 09:09:29 pm
content: Quote from: Postal666 on April 06, 2014, 08:47:42 pmTo clarify: It won't work without the batteries, but it won't damage anything to try to turn it on with the batteries disconnected.

author: Postal666
author_rank: Regular Contributor
timestamp: April 06, 2014, 11:36:53 pm
content: Quote from: edavid on April 06, 2014, 09:09:29 pmGotcha. Glad I'm finally on the same page wih everyone else. haha

author: wiss
author_rank: Frequent Contributor
timestamp: April 08, 2014, 09:12:47 pm
content: Some flukes (8050) might get damaged (the lcd) by running without NiCds, they are the only regulation of the primary supply.

author: edavid
author_rank: Super Contributor
timestamp: April 08, 2014, 10:22:01 pm
content: Quote from: wiss on April 08, 2014, 09:12:47 pmNo! Fluke's designers were not that stupid.

author: wiss
author_rank: Frequent Contributor
timestamp: April 10, 2014, 02:43:57 pm
content: Quote from: edavid on April 08, 2014, 10:22:01 pmAre you sure? The supply goes up from 5 to 7 V (or so) when the NiCds are removed on the 8050, the 8000 seems to be ok without them.

author: edavid
author_rank: Super Contributor
timestamp: April 10, 2014, 03:03:56 pm
content: Quote from: wiss on April 10, 2014, 02:43:57 pmYes, I am sure. All of the Fluke meters that use NiCds, use them as shunt regulators. None of them will be damaged if you try to power them up without the NiCds.

author: pipe
author_rank: Contributor
timestamp: May 27, 2014, 04:27:29 pm
content: Resurrecting a somewhat old thread here, but I ripped all the cells and charging crap out of my 8600A and replaced it with the 5V regulation from the schematics. The 8600A also comes in a non-battery version, and that version is using a standard transformer, rectifier and a 7805. I kept their primitive switched +/-15V card. If you rip the batteries out, there's a footprint on the PCB for adding the normal linear regulator.

author: Postal666
author_rank: Regular Contributor
timestamp: November 10, 2014, 07:06:19 am
content: Hey pipe! Gonna follow your lead and dig this thread back up...haha. So, I've been using mine as is, but two of the batteries are totally shot now, thus the thing won't even turn on. Let me ask you just so I'm clear: You used the existing xfmr and built the +5V regulation of CR102, U102, C104 (from the line supply version) and fed that +5V into the original +/-15V board?

author: edavid
author_rank: Super Contributor
timestamp: November 10, 2014, 04:11:25 pm
content: Quote from: Postal666 on November 10, 2014, 07:06:19 amI found that the DC-DC converter needs 5.5V input to produce an output of 15V, and it is also quieter at that voltage than 5.0V. So, I suggest using an LM317 and a trimpot so you can tweak the output voltage where you want it.Also, the power switch is in the wrong place in the circuit. If you care about that, there is an unused set of contacts on it that you can use to switch the AC input.