EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: staze on June 10, 2013, 08:04:00 pm
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So, looking at buying a ~$100 bench multimeter (used).
At this point, it's pretty much 3 options I can see based on really 1 need: 5.5 digit (or better).
I was at first thinking an Fluke 8810a, but they only do volts and ohms, no current. =/ Guess that isn't THAT big a deal, but figured I'd try for the "big three" measurements.
So at this point, it's pretty much the:
Fluke 8840a/8842a
HP 3468a/3478a
Keithley 199
The Fluke and the Keithley are both Made in the US (a nicety, but not mandatory), all three do the measurements I'm after. And for this instance, let's assume all 3 do TRMS, as well as have GPIB (though I realize the 3468a does not).
Seems the biggest difference is the displays. The Fluke is a VFD, the HP is a non-backlit LCD, and the Keithley is a LED. At this point I'm leaning heavily toward the Keithley. The HP display seems like it just sucks unless you hack it for a backlight. The Fluke and the Keithley are both highly legible in bad/no lighting.
Can anyone enlighten as to the repairability of any of these? I think I've found Service Manuals for them all (the Keithley and Fluke both have their service manuals included in the instruction manual, I'm not sure about the HP). The Keithley is actually the most recent of the 3 as both the Fluke and HP seem to have been mid-eighties to early nineties models. And at least the Keithley provides front panel (or remote) calibration. I can't recall if this is the case with the Fluke and/or HP.
Are there others to consider? Are there any showstoppers? I've heard/read that some of the more modern Keithley's are a bit, dodgy. I also don't overly like the whole battery backed calibration data on the HPs. I get the impression the Fluke and Keithley both store this info in some type of non-volatile way?
Thoughts? Help?
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Doh, ~$100 for one of these bench multimeters? Are you serious? In what condition are they or do you expect them to be for that price?
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OP might just squeezze this one home
*Bay 221237809510
And that seems to be an "only" in that pricerange , and has the "just sucks" display , "battery backed calibration" and no GPIB.
Or the fluke (maybe with all the blessings of a VFD)
330935612705
This is in the (or better) category and currently $100 , but i suppose you'd be lucky to get it within your ~100$ budget.
281117141789
Edit: Watch out for the 281117141789 , pict looks fake , and seller has 0 in feedback ... PayPal though
/Bingo
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Two things to watch out for with the Fluke 8840a: AC and GPIB were optional (and options are sometimes removed without changing the label), and VFDs get dim after many years of 24/7 service. The advantage of the Keithley and HP is that they had all important features as standard (the 3468a obviously lacks GPIB).
Note that current ranges are somewhat limited: the Fluke is limited to one 2 A range, the HP 3468a only has one 3 A range, the HP 3478A has a 300 mA and a 3 A range, and the Keithley 199 has the oddball combination of 30 mA and 3 A (best current resolution of these models).
The HPs and Keithleys are limited to 300 V, the Fluke goes to 1000 V. The specs of the Keithley and Fluke are extremely close, I would probably rate a Fluke 8840a with AC/GPIB in good condition slightly higher than the Keithley 199, and the HP 3468a/3478a inferior due to the lousy LCD and lack of relative function. On the other hand, an 8840a in similar condition is usually a bit more expensive than the less popular Keithley 199. The huge LED display on the Keithley is very nice in my opinion.
The rebadged Keithley 2100 was quite terrible, but all designs up to the 20xx series (eg. the 2015 that Dave recently tore down) are well regarded. The Keithley 199 uses non-volatile storage for calibration constants (it's the newest of these models). Not sure about the Fluke, should be in the service manual. The Keithley 199 is late eighties (introduced in 1989), the 3468a in 1983 and the 3478a a few years later. Not sure about the Fluke.
I'm quite sure all meters in this list support front panel calibration. I know it for a fact on the Keithley 199 and HP 3468a, since I actually adjusted these.
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Two things to watch out for with the Fluke 8840a: AC and GPIB were optional (and options are sometimes removed without changing the label), and VFDs get dim after many years of 24/7 service. The advantage of the Keithley and HP is that they had all important features as standard (the 3468a obviously lacks GPIB).
Note that current ranges are somewhat limited: the Fluke is limited to one 2 A range, the HP 3468a only has one 3 A range, the HP 3478A has a 300 mA and a 3 A range, and the Keithley 199 has the oddball combination of 30 mA and 3 A (best current resolution of these models).
The HPs and Keithleys are limited to 300 V, the Fluke goes to 1000 V. The specs of the Keithley and Fluke are extremely close, I would probably rate a Fluke 8840a with AC/GPIB in good condition slightly higher than the Keithley 199, and the HP 3468a/3478a inferior due to the lousy LCD and lack of relative function. On the other hand, an 8840a in similar condition is usually a bit more expensive than the less popular Keithley 199. The huge LED display on the Keithley is very nice in my opinion.
Hey, alm, I was hoping you'd be the one that would respond. =) since it was largely your reviews that pointed me at the Keithley in the first place.
The 300V limit isn't that big a deal since I have a Fluke 189 handheld meter (which will do 1000V), so no worries there. And yeah, the VFD does seem a bit weird, but they are highly visible when new(ish).
I do like that Keithley is owned by Tek (a local (for me) company)... but then, Wikipedia makes that seem pretty confusing. Fluke is pretty local as well (Pacific Northwest). This actually reminds me a bit of the Nikon/Canon debate, and I'm over in the corner yelling "Pentax!" =)
But anyway, thanks! I'm assuming the Keithley 199's you have are still quite stable/good/etc? I still can't say I understand the "Scanner" capability.... in my mind, "Scanner" means either a Receiving only radio, or something for digitizing images/paper, etc.
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I have no complaints about the Keithleys. One of them has a scanner option, I've never used it. It's a relay card that allows the meter to measure 10 (?) channels in quick succession. Something often used for system applications. I imagine you could use it for some monitoring applications, but I just tend to throw more meters at the problem ;).
One minor annoyance is that AC/DC and auto ranging are per function, and default to manual ranging. I think the other meter defaults to auto ranging, so it may be something you can change.
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you can buy as well Yokogawa 7552, there are currently two on ebay (with RS232 instead of GPIB, which is why i didn't bought them). They area as well 5.5digit meters with 0.003% accuracy, 20A / 1000V / 4wire.
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makes sense... just weird they call it a "scanner". so you'd have a bunch of alligators at a test station that you'd hook to test points, hit "go", and it would run through all the test points. makes sense.
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Huh, the Yokogawa's seem interesting. They take a memory card!
The RS232 would be preferable... but they're a bit higher than my price point unless I could get them to come down a LOT on a "best offer" =P
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Someone either "got taken" , or made a super buy
http://www.ebay.com/itm/281117141789 (http://www.ebay.com/itm/281117141789)
/Bingo
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Doh, ~$100 for one of these bench multimeters? Are you serious? In what condition are they or do you expect them to be for that price?
Yes, he's serious. That's what they go for in the US, in good working condition (although the 8840A/8842A has been going up lately).
How about the Keithley 196?
There's also the Fluke 8502A/8505A/8506A if you have the space, have to watch out for the options though.
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Someone either "got taken" , or made a super buy
http://www.ebay.com/itm/281117141789 (http://www.ebay.com/itm/281117141789)
/Bingo
Wow, that's a hell of a deal. And yeah, either a "fluke" (not to be confused with Fluke), or it's counterfeit.
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Someone either "got taken" , or made a super buy
http://www.ebay.com/itm/281117141789 (http://www.ebay.com/itm/281117141789)
/Bingo
Wow, that's a hell of a deal. And yeah, either a "fluke" (not to be confused with Fluke), or it's counterfeit.
Looks more like a simple scam.
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Sad if that's the case... at least eBay will help out in that case.
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I do like that Keithley is owned by Tek (a local (for me) company)...
Careful. Keithley is owned by Tek, but they are both owned by the evil Danaher Corp: Destroyer of engineering departments!
Scanners are awesome! It's pretty much just a relay card which allows you to hook up multiple probes and have the meter switch between them. It's really only useful for automated measurements but when you need that it's SUPER useful. If you can, get the scanner card.
I've been rocking the Keithley 199 for a while now. I got mine for about $120 from the ebay in really good condition.
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That pretty much explains Tek recently. At least they're trying to refocus on what they're good at... and sold off all the stuff they were not so good at (interested in), like the Phaser printers.
My only problem with the older Keithley's is the color scheme. Brown? Really? The 70's called, and they want their color schemes back.... not that beige is much better, but at least it's "common".
Anyway, I purchased a meter. Once I have it, and know it works, I'll post back with details. =)
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Since the Keithley 199 was mentioned in this thread.. I have a question..
I have a chance to get a broken 199 shipped to me for $100.
The seller says that the Ohms and Volts display is always 0.0000 but the current works.
Seems like an easy fix ?? I had a look at the schematic, there is a whole lot of FETS being used on the input circuit, some as low leakage protection diodes, others as switches... I just wonder if these FETS are (a) selected for their characteristics and (b) unobtanium now
Thanks for any guidance.
Cheers!
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Since the Keithley 199 was mentioned in this thread.. I have a question..
I have a chance to get a broken 199 shipped to me for $100.
The seller says that the Ohms and Volts display is always 0.0000 but the current works.
Seems like an easy fix ?? I had a look at the schematic, there is a whole lot of FETS being used on the input circuit, some as low leakage protection diodes, others as switches... I just wonder if these FETS are (a) selected for their characteristics and (b) unobtanium now
Thanks for any guidance.
Cheers!
So this is actually one of the ones I bought... got it for less than $100 (the seller has "make offer"). I'm actually guessing the issue is a configuration one (either it's set to single shot, or control/output over GPIB, etc). Once I get one, I'll post back and let ya'll know. =)
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Could also be a calibration issue. I had one meter that always read 0 V on one ACV range. Turned out the cal constant was just zero for that particular range, probably due to someone using the same voltage for the first and second point. A calibration with a known AC voltage fixed it. This would explain the hard 0.0000 V without any noise. My other suspicion would be a short in the input circuit, since an open would result in noisy readings on the lower ranges.
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It just seems like it could be an easy fix no matter what..
Yes, the seller has more than 1 available, hard to believe they are all in the same condition though, i.e. all not working in the same way??
I think the seller has tested a few and doesn't have a whole lot of time to test them all, so maybe it's just a hit and miss...
however, the input circuit seems rather straightforward and easy to follow the signal path, the schematic is readily available, everything is soldered through hole and DIP packaging... could be an inexpensive Keithley 199 if I could get it working again.
@staze... yes, thanks... I'd like to hear your results when you get it.
Thanks!
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It just seems like it could be an easy fix no matter what..
Yes, the seller has more than 1 available, hard to believe they are all in the same condition though, i.e. all not working in the same way??
I think the seller has tested a few and doesn't have a whole lot of time to test them all, so maybe it's just a hit and miss...
however, the input circuit seems rather straightforward and easy to follow the signal path, the schematic is readily available, everything is soldered through hole and DIP packaging... could be an inexpensive Keithley 199 if I could get it working again.
@staze... yes, thanks... I'd like to hear your results when you get it.
Thanks!
No problem. I should get mine Friday.
I'm actually guessing he/she didn't have time to really test them at all. If you look at their seller page, they have 9500 listings! So they get stuff, probably briefly test it, then throw it up either as is, or working. First thing I'm going to do is reset to factory defaults.
One thing I did notice is that the one pictured didn't have a power button. The seller said he/she would make sure to send me one that did. So, we'll see.
I will definitely report back.
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Great thanks! If it's just a calibration issue as alm says, or possibly just some settings...then it's a bargain for sure.
Since the seller says volt/ohms shows 0.000 and current works I assumed it was something shorted or open in the volt/ohms path (which is clearly seen on the schematic as distinct from the current path ) .. so I was thinking maybe one of the many DG211 analog switches was fried and stuck open, or one of those FETS that are used directly in the signal path (but probably not the FETs used as protection diodes, since those go to + and - supplies, so the display wouldn't show 0.000 in that case, it would have a voltage offset)
I'm still thinking to take a risk myself and buy one anyways... really can't go wrong for less than $100 for this decent 5-1/2 digit meter which seems highly repairable :)
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okay, got the meter. good and bad.
Good: powers up fine, and displays. Board/Unit revision is A4.
Bad: none of the measurements work. Volts (DC or AC), Ohms, or Current. Did a reset, no change. Occasionally the volts/amps show "overflow" and the ohms fluctuates a bit, but that's with nothing hooked up. So I'm guessing it's something with references, transformer output, or something along those lines. I did try a calibration for DC voltage using my PSU, but no luck. no change in readings.
Sadly, none of the components are labeled on the board... so finding test points is a bit of a pain.
The manual: http://www.ko4bb.com/Manuals/09_Misc_Test_Equipment/Keithley/Keithley_199man.pdf (http://www.ko4bb.com/Manuals/09_Misc_Test_Equipment/Keithley/Keithley_199man.pdf)
Any comments, guidance would be greatly appreciated. I'm guessing once we get one of these squared away, they'll all be the same issue.
Thanks!
P.S. Question for Alm: Do you ever get any relays that audibly click... when switching between AC and DC, for example? I don't get anything. Not with the scanner either.
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The relays in the 199 click, but they are very quiet: a quite high pitched click which is barely audible over the sound of the button click. Nothing like the chatter of some other meters. You only hear them when opening, not closing. I hear them: at power on/off, when switching from AC to DC (not the other way around), when switching from ohms to volts/amps, when switching from the 3 V DC/AC range to 30 V, when switching from the 3 Mohm range to 30 Mohm, and when pressing the scanner button after selecting one of the channels other than 0.
Yes, often no silkscreen on equipment from this era. You get used to identifying parts from the parts location diagram. So no current measurement either. Not what the seller described. Then my suspicion it would be somewhere in the multiplexer/ADC circuit. I would first go through a visual inspection and the power supply checks from the manual (obviously), and after that use the troubleshooting mode on page 6-18/6-19 (just pick the 3 VDC range) to test the ADC, mux and reference/zero circuits.
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The relays in the 199 click, but they are very quiet: a quite high pitched click which is barely audible over the sound of the button click. Nothing like the chatter of some other meters. You only hear them when opening, not closing. I hear them: at power on/off, when switching from AC to DC (not the other way around), when switching from ohms to volts/amps, when switching from the 3 V DC/AC range to 30 V, when switching from the 3 Mohm range to 30 Mohm, and when pressing the scanner button after selecting one of the channels other than 0.
Yes, often no silkscreen on equipment from this era. You get used to identifying parts from the parts location diagram. So no current measurement either. Not what the seller described. Then my suspicion it would be somewhere in the multiplexer/ADC circuit. I would first go through a visual inspection and the power supply checks from the manual (obviously), and after that use the troubleshooting mode on page 6-18/6-19 (just pick the 3 VDC range) to test the ADC, mux and reference/zero circuits.
Hmmm... I'm not hearing ANY relays at all. Which obviously makes me suspicious. Visual inspection looks good so far, but I need to pull the digital card, and get under the shield over the bulk of the analog board. With the power supply checks, I'm curious, they reference checking certain points (which is fine) but they say the voltages are referenced to digital or analog common. Where are these points?
I'm guessing the +5V digital is fine (since the display works, etc), but I wouldn't be surprised if the +5V or +15V analog are not working (which would explain the relays not being heard), which they say to check pin 3 on U51 or U52/U50 (respectively), and that these are referenced to analog common (which I'm unsure of where that is.
I will also say I hooked up the "meter complete" BNC output to my scope, and the output looks pretty darn noisy. It's not just a nice TTL output.
Thanks!
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U50-53 are just three terminal linear regulators. The corresponding ground is on pin 2, except for the 79xx, then it's on pin 1.
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U50-53 are just three terminal linear regulators. The corresponding ground is on pin 2, except for the 79xx, then it's on pin 1.
I don't see a 79xx... I see a 7805, and two 78M15's. though it could be what's attached to the heatsink that they don't actually SAY what it is in the parts list. Just "heat sink, ass'y"
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Are you claiming that U50 is a 78M15? That's unlikely to work. What are the voltages on its pins?
U53 is the +5V digital regulator, an LM2940 according to the schematic.
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I like the keithley 199. I got a GPIB card recently so I will find out how it works with the computa. I should start buying all the 199's so I can destroy them and be left with the last one.
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Okay, so, V51 reads 5.007V (good) V50 reads -15.046V (good), V52 reads 15.123V (good). All less than 1% out.
On my board, V51 is a 7805CT (gnd pin 2, output pin 3), V50 is a LM79M (gnd pin 1, output pin 3). V52 is a LM340T15 (gnd pin 2, output pin 3).
V53 is a LM2940CT and outputs 4.9805V (gnd pin 2, output pin 3).
I'm not seeing any ripple (a few mV) on any of the regulator output, so that's probably not it.
So the power supply would seem to check out (I don't have an ESR meter, yet), but capacitors check out as C59 1.49mF, C63 720uF (should be 680uF), and C65 as 722uF (should be 680uF). First time I measured C65, it rose to about 780uF or so. Tested again, was the previous number, 3rd time, it was 1000uF. It is in circuit when testing, but this seems rather inconsistent. C63 and C65 are seemingly connected in series, but testing between C63 and C65 results in 345uF (which I think makes sense). So I have NO clue there.
I guess the next step is go through the 3V diag, or give it a simple 1Vpp sine into the input, and trace where that goes. If there are any other suggestions, I'd be very appreciative. Boy I was just hoping it was something obvious...
Oh, and alm, I do think I hear some of the relays on power up... VERY quiet, and tinny)... guess I just missed them before. That said, I don't think I hear any when switching between DC and AC (but that might just be me).
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If there is no significant ripple, than I wouldn't worry about electrolytic caps. This is a linear power supply, and it's not like you're trying to track down a noise problem. I would start with the diagnostic. Tracing the signal to the ADC is kind of confusing, because the multiplexer continuously switches the ADC input between zero/ref/zero/signal. The diagnostic mode lets you just choose one of those. I suggested 3 V DC because DC involves the least extra circuitry, and the 3 V range does not involve amplification or an attenuator.
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If there is no significant ripple, than I wouldn't worry about electrolytic caps. This is a linear power supply, and it's not like you're trying to track down a noise problem. I would start with the diagnostic. Tracing the signal to the ADC is kind of confusing, because the multiplexer continuously switches the ADC input between zero/ref/zero/signal. The diagnostic mode lets you just choose one of those. I suggested 3 V DC because DC involves the least extra circuitry, and the 3 V range does not involve amplification or an attenuator.
Okay, though I'm not sure I understand what it's telling me to look at/for.
There's 4 modes. And yeah, it's zero, zero, cal (guessing that's reference), and signal. So I should run like a 1V DC signal into the Hi/Lo inputs, put it in debug 3V DC, then in N01, look....? It tells me which relays/switches should be closed, but I'm not sure that helps me much. =/ Sorry, again, pretty new to this. Software and computer hardware I can diagnose, this stuff is still a bit new.
Thanks!
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Read chapter 5 of the manual (principles of operation), especially the part about the mux and the ADC. The ADC will measure the zero, reference and signal in quick succession to calculate a reading. Measuring the three signals (relative to analog ground) after the mux will tell you whether the problem is before or after the mux. TE1/TE2 looks like a convenient place to measure it. If all signals are correct, then the problem has to be in the ADC or digital logic.
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okay, but still not sure I get the process.
I'm assuming I have to input a signal of some type. TE1 and TE2 look like posts that several transistors are hooked to.
So, I hook the digital board back up (leaving the analog shield off), plug it in, and power up. I put it into debug mode for 3V DC. I set up a ? signal on my FG, and run it into the hi and lo inputs on the front, and then with either the scope, or my DMM, check TE1/TE2 (going by chapter 5, it looks like TE1, as the 3V input is routed through K1 (which I can't get to it's contacts easily), to Q30 (which hooks to TE1) for said signal relative to the lo input? Is that right?
As a side note, I note that the board isn't actually grounded, seemingly. It's all isolated from the chassis/mains ground. I'm assuming this is normal, but thought I would check. I also note some of the ceramic disk caps are looking a little worse for wear, but I haven't checked their capacitance yet, so no idea if they're actually bad, they just look a little flakey (the yellow ceramic seems to have chipped off in places).
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So, going by what I posited above, I set my FG to output a 5khz, 1Vpp sine wave, and put that into the hi-lo inputs on the front of the unit. I then put the unit into DC volts, on the 3V range, and with my scope probe ground hooked to the "lo" input, probed TE1 and TE2. Both show the 5khz wave shifted up 1V, and slightly reduced in Vpp (about 780-800mVpp). So that would seem to indicate... it's the ADC or Digital side?
Where would be best to look at this point?
Sadly, I think I've also hosed the calibration data for the 300mV and 3V DC ranges... =/ not quite sure how... but now both read "overflow" even when there is no input. Guessing I must be doing something wrong in the calibration attempt. I've giving it -300mV (or -3V), then +300mV (or +3V), then 0 volts (well, I'm turning my PSU output off for that one). Maybe this is why you need a real calibrator... =/ But until I can get a successful calibration, I can't be sure it's not just bad calibration data that's making these things not work. It seems really friggin' unlikely there'd be 6 of these things for sale, by the same seller, that all died the exact same way unless it was something other than random failure.
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I would see what the reference and cal/signal zero look like (N=1,2,3) at TE1. I would use a DC, not AC, signal to test the VDC function. Then I would check the ADC counts output (while not in the diagnostic mode) on both sides of the opto-coupler (see theory of operation, table 6-13 and schematic). I would change the input voltage and observe how the pulse chain changes if you change (eg. invert, or change by a factor 2) the input signal.
Attempting calibration if you suspect the ADC circuit is broken is a bad idea. The power supply was probably not the problem (although switching it off is not a good way to get 0 V, you should short the inputs), but the ADC was feeding the MPU garbage information. After repair, you will need to send it out for calibration or have access to accurate 300mV and 3V sources to bring it within spec. I would probably switch to the 30 VDC range for testing, although it involves more components and is therefore less ideal for testing.
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If you don't have any need for the scanner card, I need a replacement one for my 199. Send me a message and we can work something out.
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I would see what the reference and cal/signal zero look like (N=1,2,3) at TE1. I would use a DC, not AC, signal to test the VDC function. Then I would check the ADC counts output (while not in the diagnostic mode) on both sides of the opto-coupler (see theory of operation, table 6-13 and schematic). I would change the input voltage and observe how the pulse chain changes if you change (eg. invert, or change by a factor 2) the input signal.
Attempting calibration if you suspect the ADC circuit is broken is a bad idea. The power supply was probably not the problem (although switching it off is not a good way to get 0 V, you should short the inputs), but the ADC was feeding the MPU garbage information. After repair, you will need to send it out for calibration or have access to accurate 300mV and 3V sources to bring it within spec. I would probably switch to the 30 VDC range for testing, although it involves more components and is therefore less ideal for testing.
Just getting back to this.
So, putting in a 2.50V DC signal from my FG. Put the unit in Diag for 300V (3V, and 30V both give overflow) DC. Checking TE1 at N 1,2,3,4 all give nothing. zero voltage (this is with my scope lead ground hooked to the Lo input, and probing TE1). TE2 gives the same. I'd almost wonder if a relay is stuck open/closed... or a bad FET, or something.
Interestingly, I AM seeing that signal on K3 and K4 (on both sides). According to the diagnostics table, these should NOT be closed.
Checking 6-13, I'm having a bit of trouble on two points.
1. Where are all these referenced to (guessing digital ground), which should be the shared ground but I'm having a hard time finding a good grounding location. It would appear the pin closest to the 5V digital regulator that goes from the main board, to the digital board, is ground for the digital board, as it's connected to Vss (pin 1) on U8.
2. I can't find U3B, which is supposedly where to measure the A/D counts. I see U3, but U3B is no where to be found (though, I'll fully understand if it's right infront of my face).
Anyway, with all that, here's what I'm seeing:
U8-pin2: 1ms (good)
U8-pin3: 11ms (should be 15ms)
U8-pin6,7,8: 5V, pulse when button pressed.
U8-pin9: 0V (no pulse when buttons pressed)
U8-pin19: lots of pulses from 0 to +5V. Seemed to have some "runts" in there as well (much shorter duration than "normal" pulses).
U14 pin 4: 22ms 5V to 0V pulses. No variation.
U3B: Unknown (can't find it). U3 (incase it's a typo) pin 13 gives a 3.84197Mhz 0V to 5V square.
U8 pin 12: 1.92098Mhz 5V clock.
U8 pin 13: 960.491Khz 5V clock
U8 pin 14: 480.246Khz 5V clock
U8 pin 15: 240.123Khz 5V clock
U8 pin 16: 240.123Khz 5V clock
U10 pins 34 and 35: 2.0032Mhz 5V clock
U10 pin 37: 5V
U5 pin 9: 0V to 5V pulses, 1ms apart. Getting some definite noise, pre-pulses, runts, etc in there.
U5 pin 19: 5V
U5 pin 18: 2Mhz square 0V to 5V.
U5 pin 28: 3.72V
As a note, I had to re-close the "scanner" jumper (since the scanner wasn't attached) to get the A/D counts to work, as well as the 2Mhz square out of U5 to be steady.
I'm not exactly sure the U5 oddity is anything significant since I believe that's just the GPIB driver. But it behaving oddly might be a symptom. Or, maybe what it's doing is normal.
And that's it. not exactly sure what to test next. Nice to get some readings, but a bit frustrated that nothing is jumping out here.
As for measuring on each side of the optocouplers... they're UNDER the digital board... a bit hard to measure. =/
Any thoughts? Thanks for all the help!
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Correction. The first time I did my measurements of TE1 in Diag, I had not shorted the scanner jumper.
Now in 300V or 30V DC, in diag, I get 2.5V DC (my input signal) on all N settings. So, about the same as before, just that I'm actually seeing a signal now. So it still seems something isn't switching correctly. =/
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So, interestingly, Pin 18 on U8, which is tied to Pin 5 on the header between the analog and digital board, is labeled as "clock". Checking this with the scope gives a pretty bad looking "clock" signal.
Also of note, Pin 37 on U10 is not a +5V ±5%, it's seemingly either clock, or TTL of some sort. It's just a square between 0V and 5V, at about 510Khz. And pins 38 or 39 on U10, which according to the schematic should be an 8Mhz clock, are no where near that. No idea if that's just the impedance of the probe stopping the clock, or what.
The whole thing seems very inconsistent. Sometimes it does one thing, sometimes another. Seems like it could still be something funky with the power rails.
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As luck would have it, Dave covers multi-slope integration measurement in the TDT this week... which I THINK is how the 199 does it's measurements.
Still doesn't help much in what's wrong with this meter... but.
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I'm trying to wrap my head around everything you've tried so far :)
yes, the table 6-13 is the digital side, so all the voltages in that table are referenced to the digital ground, which would be pin2 of the 7805 marked U53. Or any ground pin on an IC would work too, in a pinch. It shouldn't be that hard to find a digital ground. What I often do when I am beginning a troubleshooting session is to solder a short black wire to a digital ground plane near the regulators, and a short BROWN wire to the ground plane near the analog regulators.. this is my DGND and AGND references, and I clip everyground of my scope or DMM or FG there :) If I ever need to solder an earth reference I will use a GREEN or GRN/YEL wire for that. This is just my convention, perhaps a best practice, to save time in the future clipping grounds all the time.
U3 is a dual 4-bit counter, so there is two parts inside. If you have found the IC labeled U3, then you have found both U3A and U3B.
U10 is the MC6809 processor and Pin 37 is a /RESET line, so you should see it simply pulled high during normal operation, you should not see a clock there. Make sure you are probing at x10. I don't like the loading of a x1 probe, I almost always use x10.
you won't be able to measure the clock by probing pins 38 or 39 across the crystal. But I don't think your processor is a problem, it's clearly working, as evidenced by the fact that the display is working, the front panel operations seem to basically work, and you can enter the various diagnostic N modes. So don't chase down anything in the digital section yet, you should stick with the analog side and verify the analog voltages, the reference voltages, and the signal paths as explained in table 6-11 for the various ranges. Stick with the 3VDC range first, and verify that path through the relays and muxes and that it reaches the input buffer amplifier U46. In the 3VDC range, the buffer amplifier U46 should be configured for x1 gain. Verify this. The microprocessor switches the input buffer amplifier between x1 and x10 gain using U45A. Table 5-1 show the buffer amplifier configuration for each range. Verify this buffer amplifier is at the proper gain setting for each range. Verify the signal is appearing at the output of the buffer amplifier. This is the first point before the integrator. Verify the signal from your generator is appearing at the integrator input on the gate of Q1. Verify the 3.84Mhz clock as used by the counters on page 4 of the schematic. print off the schematic and mark of signal paths that you have tested and known to be good.. use a highlighter, it can help here. You'll get a clear picture of where you are and where you need to go.
The integrator is made up of Q1/U19/C32 on page 4 of the schematic. Figure 5-6 shows a simplified schematic. Put a scope on the output of the integrator, does it look like its "integrating" :) looking at Figure 5-6 you can see the operation of the integrator , comparator and how the clock is gated out to the ADC counts. Mr. alm already said, you should check the ADC counts at both sides of the opt-isolator and verify that it is changing pulse train with varying inputs.
All this needs to work. This looks like a very serviceable machine, actually. I haven't bought one yet, but I am still considering it. After studying the schematic, they have written the service manual with a fully explained operation guide, detailed diagnostic procedures, built in diagnostic modes, and of course a complete schematic. And it's all discrete parts -- definitely a very serviceable machine.
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codeboy2k,
thanks! I'll have to give this a all a shot... and yeah, I never use 1X (though they are dual probes).
Once I'm home and can take a look, I'll probably have questions, because just looking at the schematics and parts list, I can't see where the gate actually is on Q1, or for that matter, where's the output of the integrator. As serviceable as this thing seems through the manual, it's actually a bit annoying since the digital board sits above a good chunk of the analog board, so probing is a bit troublesome.
When I'm searching for my input signal, should I just clip my probe ground to the input Lo, or should I clip to the analog ground? Guessing the answer is, both, in that when looking for signal, clip to "low", when looking at rails, clip to analog ground. Do you see anything wrong with inputting just a straight 2.5V DC signal for my test?
Also, looking at the schematic, I see say, "VOLTS OHMS HI" on page 1, it heads through the switch, then off to R13 and Q6, then over to Q30, then down and around. So I should be checking, say, the source of all those FETs to see my signal, maybe the C30 cap as well. As alm said, TE1 and TE2 look like they're in that signal path. And I have checked that, and the signal appears. So, the signal is making it to those points. So I think we're good to that point.
So then we head to U46, and check pin 6 (are they clockwise or counter clockwise from pin 1) to see if it's the same signal (or x10 if I'm at 300mV), and I'd assume also check pins 4 and 7 to see if I'm getting the ±15V rails.
Then there's the integrator. So, I look for the signal on the drain (or source) of Q4 (if I can reach it), then on G2 (if I can find it) of Q1 (maybe check the -15V on the source for Q1, and the +15V off R56/R55), then on Pin 2 of U19, and ideally, then a sloped line on pin 6. Can also check the rails on 4 and 7. Gotta say, the integrator is not what I'd normally look at and say "oh yeah, that's an integrator".
K... all that seem reasonable? I'll give results in a few hours (once I'm home).
So, all that down, how do I check the 3.84Mhz clock? I know I can't probe it directly because it loads it down too much and stops it... so...?
Still have the nagging "there are 6 of these that all died in the same way". So either it's a design flaw (unlikely), or user error (what would blow if you pushed 500V into these things? Maybe a FET or an opamp?
Will get back to you with the results.
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To answer my own question, the Volts/ohms Lo input is tied to analog ground. So, I just clipped to that.
So, starting out, I powered the unit up, and on TE1 and TE2, with a 2.5V input, they read 4.5V. Hmm. Power cycled the unit, and they now read 2.5V. So SOMETHING isn't working reliably. I also note that when I powered it up the first time, the LED on the front (guessing it signifies trigger) pulsed about 2 or so times a second. Now, it pulses about 4 or more times a second. Power cycled again, and now TE1 and TE2 read 0V.
Power cycled again, got the expected reading (2.5V) on TE1 and TE2.
So... thoughts?
What would cause that?
Anyway... the below is done outside diagnostic mode.
U46:
The rails are +15V, and -14V. The non-inverting input is a bit higher than my input (about 2.9V). The inverting input is the same. And checking, it would appear the resistance between pins 2 and 3 (inverting and non-inverting) is basically 0R. Can't say if this is normal or not.
At first, I saw seeing 10.6V out of U46 at the 3V range, but after letting the unit sit a bit, I am seeing 3.4V at the 3V range, 10.6V at the 300mV range, and 0V at the 30V and 300V range. So that would seem relays aren't closing/opening.
On Q1, Gate 2 (which I THINK is shown here http://www.datasheetarchive.com/indexer.php?file=DSA00411750.pdf&dir=Datasheet-023&keywords=MP841&database=user-highscore (http://www.datasheetarchive.com/indexer.php?file=DSA00411750.pdf&dir=Datasheet-023&keywords=MP841&database=user-highscore) as pin 7) shows 1.2V. But, checking it's "rail" I don't see a -15V on any of the pins. So I'm really quite stuck on that.
U19's output MAY be integrating... it's kind of hard to tell. I do see a +15V rail on pin 7, and a -14V rail on pin 4. The inverting and non-inverting inputs appear identical (I think this is correct). The output from pin 6 looks like about a 2.8V "noise" then a slow drop down to -80mV, then a pretty quick jump back up to 2.8V. See attached photo. Also interestingly, in the 300V range, in the 4.5 digit mode, I actually get a "useful" reading. It quickly jumps between 1.76V and 2.12V, and occasionally -95V or something. If I turn on the input, the reading drops down to .70V and .35V, still occasionally jumping to -95V. But in 5.5 digit mode, I get nothing useful. Also interestingly, the trigger pulse is consistent in the 5.5 digit mode, where in 4.5 digit mode, it pauses occasionally.
So... I got nothing.
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first things first... recheck all your voltage regulators and reference voltages too, there is a -6.4V reference, a -10V reference, a -2.8V reference and a -4.3V reference. make sure those are all ok.
For diagnostics, stick with the 3VDC range for now, try to get just that one working..
> and the signal appears [ed. at TE1 and TE2] . So, the signal is making it to those points. So I think we're good to that point
Yes if you're seeing your signal at TE1 and TE2, that is just before the input buffer, so your good there. But you say power cycling you get different values at TE1 and TE2, and I presume you are still putting 2.5VDC on the input? Just make sure every time you power cycle it still is starting up in the same mode, 3VDC range. If you power cycle it and the display says you are in the 3VDC range, and you are supplying 2.5VDC, but are seeing 4.5 volts at TE1,TE2, then don't just turn it off again, check where is that coming from?? That bus that is the input buffer (TE1,TE2) is fed from the sources of just 5 FETS. If you have 4.5 volts on the TE1,TE2 bus, then just check the drains of each of those 5 FETS, Q30,Q32,Q33,Q34,Q35 for that 4.5V. also, use a scope, not a DMM, is it 4.5VDC or is it oscillating and your DMM is just showing some average DC voltage? These are diagnostic questions you should be asking.
Having -14V on the -15V rail on U46 seems off. Is the power supply bad? -14V is not terrible, but since the -15V rail is a 7915, I know it should do better than this. So when you see -14V there, check the 7915 voltage regulator U50.
> The non-inverting input is a bit higher than my input (about 2.9V).
Thats far too much higher, I would think.. They should be equal, that's the opamp way... more importantly is the output of that opamp...
The output of the input buffer amplifier should be + or - 3.03V max. Since you are giving it 2.5VDC at the input terminals, you should see 2.5VDC there at the U46 buffer amplifier output. however, it's not that simple, because ....
> At first, I saw seeing 10.6V out of U46 at the 3V range, but after letting the unit sit a bit, I am seeing 3.4V at the 3V range,
> 10.6V at the 300mV range, and 0V at the 30V and 300V range. So that would seem relays aren't closing/opening.
Why do you say the opamp output being too high is an indicator of a relay problem? on the 3VDC range there is only 1 relay, K1 in the signal path. oh I see, you're talking about the 0 VDC on the other ranges... stick to 1 range, get it working first... understand how it works and the path it takes, don't jump around. Try to get the 3VDC range actually reading correctly.
Q1 is an MP841, that spec sheet seems correct, they show an LS841 equivalent and yes, G2 is pin7 viewed from the bottom in that drawing K1.
During a non-diagnostic mode, ie. during the normal ADC cycle, the input at G2 will be switching between AGND and the SIGNAL. This switching is done by U45B/Q4/Q17.
You can probably try to select a specific N mode of diagnostics to hold that signal at either one or the other for a check.
Note that the signal at the G2 gate is NOT the signal you input. Because the input signal can be bipolar +3.03V to -3.03V, there is an offset added to the signal before it hits the integrator, this offset will level shift the bipolar +- 3V input to a negative input only. This is done by the voltage divider formed by R58/R61/R62 and the switch U42C. I haven't been able to figure out the exact voltage shifts... maybe 0-3V becomes -4.6 to -3 and -3 to 0 input becomes -6 to -7.2... I think.. just not sure of the switch configuration when the input is positive or negative. If I am correct then your 2.5VDC input should be at the top of R58 on Sheet4-D1. The bottom of R58 should be about -3.3V ... all referenced to analog ground. This -3.3V should appear at the gate G2 in a pulse as it cycles through the ADC capture..check with a scope there.
> U19's output MAY be integrating ...
nope. It looks like it's oscillating. you should see a steady series of ramps up and down. That area is a big problem, it needs to be focused on and fixed. but also check the other signal areas I mentioned above.
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If you want to check the digital clock on that analog section, then check : ( i hope I got these right )
U43 pin 4 should be 3.84 MHz
U17 pin 3 should be 1.92 MHz
U17 pin 6 should be 768 KHz
U17 pin 7 should be 384 KHz
U18 pin 4 should be 96KHz
U18 pin 5 should be 48KHz
U18 pin 6 should be 24KHz
U18 pin 8 should be 1.5KHz
U8 pin 3 should be either 48Khz or 1.5Khz or depending on fast or slow mode (4.5 or 5.5digits)
maybe some of those will help you verify the clocks...
But generally just focus on getting the DC 3V signal correct at the front end.. what you are seeing there just doesn't seem like its right to me.
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codeboy2k,
Thanks very much, this is good info. I'm swamped today and this weekend, so I'll be getting back to this Sunday night, or Monday evening, which sucks. I really wanted to stay in the groove of figuring this out.
I am using a scope for pretty much everything except continuity/resistance testing. I'll stick with the 3V range from now on... I have noticed while working on all of this, the relays seem to be working better, or at least, I can hear them now when switching between ranges/measurements, but that's neither here nor there.
I'll see if I can check any of this today. I'll recheck my VR's, but here https://www.eevblog.com/forum/chat/bench-multimeter-(another)/msg247316/#msg247316 (https://www.eevblog.com/forum/chat/bench-multimeter-(another)/msg247316/#msg247316) is what I got last time around.
The annoying thing about the reference voltages is that they produce one thing, then divide it down, so finding where that division end point is... anyway, will see what I can find, I think last time I checked, the -2.8V was bang on, and therefore so was the -6.4V.
Will see what I can get to before I have to do, you know, work. =)
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So before other stuff takes me away, I did check the voltages on the transformer output. I can't see which pin is which, but there appear to be 3 taps off the secondary side. One of these gives 9.35V between what I'll call pins 5-6. One gives 19.100V off pins 3-4. The last gives 15.15V off pins 1 and 2. All are RMS AC. Also appears there's a 4th tap on the other side that gives 8.1V RMS. The others I must be measuring wrong, as they don't match up with what's going into the regulators. The ±15V regulators are getting 23V or so... oh.... filter caps. duh.
The 5V analog regulator is still giving 5.0065V DC. The -15V regulator is giving -15.043V. The +15V is giving 15.125V. So I think those are still good. Those are all with the meter as I didn't have time to fire up the scope.
Digital 5V gives 4.9794V. And now that I'm looking, that 4th tap seems to be for the digital regulator.
Anyway, more to come after the weekend. Thanks for all the help. If you think of anything else, let me know. I guess, hypothetically, what's the likelihood this is all caused by one broken component vs multiple failures?
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U17
Pin 1: 3.84Mhz.
Pin 3: ~1.62Mhz (fluctuates a bit)
Pin 5: ~650Khz
Pin 6: ~325Khz
Interestingly, I've found the Rigol DS1052e isn't a great multimeter. Where my Fluke says the -15V regulator is putting out a nice clean -15V, the scope at first said -14.4, but then as you zoom in, it gets to be a nice clean -15V. So it could be that the integrator and A/D buffer is getting that -15V as well. But, I'm not sure.
That said, something would seem to be up with my voltage rails, as I would think with NO input at all, U46's (input buffer) output should hover around 0V... instead, it's around -.1V. With my 2.5V input signal, I'm getting out 3.06V. The input (non-inverting) from the input buffer (read on TE1/TE2, and pin 3) read 2.56V (which is what the scope seems coming from my FG directly at the inputs to the DUT).
U46's rails read as +15V (on pin 7), and -15V on pin 4 (and the case, as it should be). Pin 2 of U46 reads as that same 2.54V. With no signal, Pin 3 (input) reads 0V (basically), and Pin 2 reads that -.1V previously indicated (so that's where it's coming from).
-10V reference checks (-10.058V). -6.4V reference checks out at -6.440V. The -2.8V checks as -2.8729V. There's a +5VR marked on the analog board, and that checks as +5.0835V. The -4.3V reference, though, is a bit hard to find. It's on CR16, which is right in the middle of some resistors on the bottom, it takes the -6.440V reference, and takes it down to a theoretical -4.3V, but in my case, it's -4.1746V (which seems a bit low).
Anyway, that's all for tonight. More tomorrow once I have some more time. At least I'm getting some use out of my test gear... my wife doesn't think it's quite so silly now. =)
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Interesting tidbit... on a whim I was fiddling with the unit listening to when relays would flip, etc... and I switched it over to ohms, and clicked auto, and it went to overflow on the Megaohms scale. Okay... so I shorted the inputs, and low and behold, it's reading 1ohm or less... though it occasionally flips to like -49R for one reading then back again. So something is still wonkey there (since, you know, negative ohms?)
Un-shorting the inputs makes it go back to overflow on the Megaohms scale. But interestingly, testing with a 4K7 resistor resulted in it clicking between K and M ranges, and never reading anything. So yeah, still doesn't work. Ignore this. =)
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Having -14V on the -15V rail on U46 seems off. Is the power supply bad? -14V is not terrible, but since the -15V rail is a 7915, I know it should do better than this. So when you see -14V there, check the 7915 voltage regulator U50.
> The non-inverting input is a bit higher than my input (about 2.9V).
Thats far too much higher, I would think.. They should be equal, that's the opamp way... more importantly is the output of that opamp...
I think this is due to what I mentioned the other day... I was just looking at too low a resolution, so the -14V on the negative rail was because that's what it said at the -5V/div setting. Zooming in showed it was actually -15V.
I'll check these again, but I'm guessing they'll be on the money. The interesting thing that may just be coincidence, is the more I play with this thing, and test various aspects, the better it behaves. Almost makes me wonder if I just left it on for a couple days if it would just suddenly work. =P
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I'm going to check the clock stuff again tonight, because looking at what I wrote, and what you said, and also given what the datasheets say for these devices:
U17: http://www.nxp.com/documents/data_sheet/74HC_HCT390_CNV.pdf (http://www.nxp.com/documents/data_sheet/74HC_HCT390_CNV.pdf)
U18: http://www.nxp.com/documents/data_sheet/74HC_HCT393.pdf (http://www.nxp.com/documents/data_sheet/74HC_HCT393.pdf)
Means that U17 should be getting, as you said, the full 3.84Mhz, then dividing it down by 2 on pin 3, then by 2 again on 5, then by 2 again on 6, and again on 7, yes?
So shouldn't it be:
1: 3.84Mhz
3: 1.92Mhz
5: 960Khz
6: 480Khz
7: 240Khz
Or am I completely crazy?
Anything I've provided help?
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Okay, finally getting back to this.
Now that I'm understanding this all a bit better (after reading the manual/schematics, another book, and these posts a few times), I understand the input buffer feeds is what's flipping between zero, ref, and signal (I think I said earlier this looked like zero, zero, ref, sig, but I can't find where I got this from). Anyway, I was measuring this on TE1 and TE2, which is BEFORE the input buffer, not after. So, measuring pin 6 of the input buffer, with a 2.5V input signal on the 3V range, I get the following:
N1: ~0V
N2: ~0V
N3: -3.3V (I think this should be -2.8V)
N4: 2.97V (shouldn't this be 2.5V)
Double checking with my Fluke, rather than scope, these check out.
Checking Q32, which according to the diagram should be what switches on the reference -2.8V, I do indeed get -2.8V on its drain (it's a JFET). Also checked on TE1 and TE2, and yes, getting the correct 0V, 0V, -2.8V, and 2.5V on each of the N stages. So I guess that was the correct place to test before.
So, why is the input buffer adding voltage to these? I do see the exact same numbers on U20 pins 5 and 6. So the LF442 seems to be working (though I can't really say what it does in the grand scheme of things other than act as a buffer...)
Checking U45 pin 14, I get 0V, 0V, -2.8V, and 2.5V perfectly. So it's working.
That said, measuring the A/D side of R58, I get 0V, 0V, -2.54V and 2.6V on N1, N2, N3, and N4 respectively. So that seems to be working as well. Where I get lost is after that...
Looking at U19, though, I'm a bit confused. Pin 6, the integrator output, shows 14.4V on N1, N2, and N3, and then shows integration on N4. I would think it should show integration on N3 as well... but I'm not sure. Alm? Codeboy2k? I will say it's not a very stable integration. See photo.
One thing I have learned... the accuracy of a scope is relatively poor to a DMM when it comes to voltage. Off a decimal or two doesn't seem to be that big a deal).
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On a side note, and maybe this is my issue... I switched my scope over to AC coupling, and measured the filter caps on the supply side of the voltage regulators. On C59 and C65 I'm getting about 300mV of ripple (see picture) on the positive side (furthest from the stripe). But oddly, I get no ripple on the negative side of C59 and C65, but I do see the same amount on the negative side of C63. So what the hell? I don't see any ripple on the outputs from the LM7805 (5V Analog), LM7915 (-15V analog), or LM340T15 (+15V analog). Not sure if it's interesting that the +15V is a LM340T15, when all the diagrams say it should be a LM7815... guessing they're basically the same.
I also see about 550mV of ripple on the negative side of C72, and about 750mV of ripple on the positive side. I thought I was seeing ripple on the +5V digital regulator too, but then I realized I was referencing analog ground... referencing to digital ground, there's no real ripple (about 120mVpp of noise).
Should I just recap and see where I am at that point? I can't see caps being an issue though since the VR's all seem noise free.
Thoughts?
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Another side note:
I decided, since it had been working better, to try another calibration (since I'd already screwed the 3V and 30V range).
And oddly, it SEEMED to work.
I've now done the 3V and 30V ranges. But it still doesn't work quite right. While it registers 30V, and 3V correctly, anything in between is pretty badly off. And I have no real way to generate anything over 30V, so... something still isn't right. =P
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Wow small world. I purchased two of these from the same seller. I figured if I could piece together one working one with the parts I'd be in great shape as even the at the price I paid for two I would come out way head.
As luck would have it, one works great! Its a tad off in calibration but otherwise seems to work just fine. I think its the one he shows in the image actually. It has a broken power button but it still works, the button it self is still there and working, the red plastic push part is missing. Physically its in not too bad condition.
Now for the other one. It's a mess, its dirty, there appears to be a bit of rust on the heatsink, its beaten to hell etc. and none of the functions read anything but 000.000. All of the front panel buttons seems to work. In spite of the rust on the heatsink, surprisingly the +/-15 and the 5V ranges work just fine. I'm not quite intelligent enough yet to get to the level of what you guys have done but I did find one of the two polystyrene capacitors reads as around 1K ohm resistance (my working one reads OL) and three of the transistors are reading around 10K ohm between collector and emitter (insteadof the 10-20Mohms I read on my working one).
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Interesting. Which caps/transistors are you getting that off?
The manual has a full schematic and layout in it, so figuring out what is what isn't too bad.
What's really interesting to me is I figured all 6 of the ones this guy had we're probably bad in the same way, but this would seem to be incorrect, since you say one of them works just fine.
What test equipment do you have access to? I'd be curious to compare some numbers. But I'll certainly check what I get off the same parts you've listed if you let me know their designation. :)
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Interesting. Which caps/transistors are you getting that off?
The manual has a full schematic and layout in it, so figuring out what is what isn't too bad.
What's really interesting to me is I figured all 6 of the ones this guy had we're probably bad in the same way, but this would seem to be incorrect, since you say one of them works just fine.
What test equipment do you have access to? I'd be curious to compare some numbers. But I'll certainly check what I get off the same parts you've listed if you let me know their designation. :)
I'm gonna work on it a bit again tomorrow. I'll let you know the exact designation.
Yeah I printed out the schematics because i was tired of jumping back and forth in the pdf.
Test Equipment I have:
A UT61e, the other working Keithley 199, an old Tenma 30Mhz analog scope with two probes, a Circuit Specialists 3645a power supply (0-37V, 0-3A), an old HP 6114A Precision Power Supply plus a few other odds and ends like a coaxial ratio transformer and a few dekapots
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Cool. I also just purchased one of the Blue ESR kits, so I plan to build that this weekend and confirm the filter caps are good/bad.
Anyway, hope there's some more suggestions as far as my troubleshooting is concerned. I'm pretty stuck at this point, since things look kind of OK, but I'm not sure the integration looks the way it should. I would think, on N4, the integration should be extremely uniform, where right now, it's a bit, off.
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I'm swamped with other stuff right now, but I'll try to take a look at your results next weekend and maybe take some measurements on a working 199.
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I'm swamped with other stuff right now, but I'll try to take a look at your results next weekend and maybe take some measurements on a working 199.
alm,
No problem at all. I'm just happy you're willing to help. I'm going to poke around a bit more this week... maybe something will jump out as obvious.
Thanks again!
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So I was just checking the clocks again, using both what's in the manual for Digital Circuitry Checks, and what Codeboy2k said.
All the digital circuitry tests check out except U8 pin 3, which shows pulses every 11ms, not 15ms. Not sure if this is an issue or not.
Checking what Codeboy2k said:
U43 pin 4 should be 3.84 MHz
U17 pin 3 should be 1.92 MHz
U17 pin 6 should be 768 KHz
U17 pin 7 should be 384 KHz
U18 pin 4 should be 96KHz
U18 pin 5 should be 48KHz
U18 pin 6 should be 24KHz
U18 pin 8 should be 1.5KHz
U43 pin 4 looks good, solid 3.84Mhz clock.
U17 pin 3, 6, and 7 all give a clock, but there are big chunks missing. There's a block of clock signal, then nothing, then more clock, then nothing. So something is clearly not quite right there (or we're mistaken on what U17 should be giving). See picture.
U18 pins 4, 5, 6, and 8 all give half of what they should, with the same missing chunks. I'm guessing that's why U17 is off as well... half the signal is missing. U8 pin 3 shows the same thing (missing chunks of clock) on both 5.5 and 4.5 resolution... about half of what you said it should be.
U17 pin one shows the same, good, 3.84Mhz input as well. So it seems unlikely U17 and U18 are bad, so much as some shared resource? I'm looking through the schematics now... trying to find what might be a likely cause. Not seeing a whole lot yet...
Looks like the reset pin on U17 and U18 is driven by U30 pin 5. Which I'm guessing is driven by the clock signal generated by the digital board... which now I see the opto-couplers in there. And U49, which should have a clock signal on pin 10. Any ideas what that clock rate should be? Obviously should be the same as what's on Pin 5 of the digital board header. Which looks pretty friggin irregular (random +5V TTL pulses). U8 pin 18 generates the clock, which looks just as irregular. Pin 23 on U8 is where IT gets it's clock, which comes from U1, Pin 16. After that, I got nothing. Pin 1,2 of U1 are getting a nice 2Mhz clock. Everything else just looks like pulse trains of some sort or other.
I would THINK that clock signal, that resets the U17 and U18 counters, should be regular... and that might be why there are chunks of data missing from those counters. But, got me.
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Good to see you perservering with this meter !
A few thoughts on your notes and observations
U17 pin one shows the same, good, 3.84Mhz input as well. So it seems unlikely U17 and U18 are bad, so much as some shared resource? I'm looking through the schematics now... trying to find what might be a likely cause. Not seeing a whole lot yet...
Your included scope trace is labelled "Clock on U17 and U18" - not having used a Rigol I cannot tell whether your trace is showing high frequency oscillations. Can you speed up the timebase or grab a 'single shot' of the clock at a higher time base to confirm you have a relatively 'clean' clock signal from U43B pin 4 ?
If the clock is oscillating it would not be good since the clocking of the other chips downstream will be wrong. You could isolate the clock output from U43B by cutting the input pins on the chips that it feeds. eg cut U17A pin 1, U8B pin 11 and U43D pin 11. This will tell you whether the clock is being loaded by a faulty chip.
Note, when I say cut I mean snip the pin off the IC from the top, close to the package. You can then bend the pin away and then just tack it back onto the chip after checking, but don't cut too close to the IC package, leave a bit for resoldering. BEFORE doing this check that you can still buy the IC's. I would not be afraid to do this with common TTL/CMOS glue logic like U17, U8 and U43 since you can always go back and replace them later on.
Looks like the reset pin on U17 and U18 is driven by U30 pin 5. Which I'm guessing is driven by the clock signal generated by the digital board... which now I see the opto-couplers in there. And U49, which should have a clock signal on pin 10. Any ideas what that clock rate should be? Obviously should be the same as what's on Pin 5 of the digital board header. Which looks pretty friggin irregular (random +5V TTL pulses). U8 pin 18 generates the clock, which looks just as irregular. Pin 23 on U8 is where IT gets it's clock, which comes from U1, Pin 16. After that, I got nothing. Pin 1,2 of U1 are getting a nice 2Mhz clock. Everything else just looks like pulse trains of some sort or other.
I would THINK that clock signal, that resets the U17 and U18 counters, should be regular... and that might be why there are chunks of data missing from those counters. But, got me.
Regarding U1 - PAL10L8 is a programmable array logic chip and looks like it is used to map the RAM, ROM, display and U5 (IEEE controller) subsystems to specific memory address ranges. The 6809 CPU is 'clocking' U1 and U1's outputs D0 - D7 are used as chip select lines for the abovementioned subsystems. Be careful with U1 since it has been programmed and if it is faulty you are in trouble. Even if you have a good U1 it most likely cannot be read without great effort since they have protection fuses which when set makes the chip virtually unreadable.
However I would assume U1 to be ok and concentrate on the clock from U43.
You also mentioned the SYNC input to U17 and U18 - if you are not sure it is ok, isolate it and tie the CLR inputs of U17 and U18 to Vcc (pin 15) via a 10K resistor to clear the counters and to gnd via 10K resistor to enable the counters. Then you can check the outputs assuming the clock input is ok. You could also again isolate the clock and apply an external clock using a generator or a simple 555 oscillator at any frequency under a few MHz.
So the general idea is to isolate small sections of the circuit (for example U43, U17, U18) to confirm they are ok then move to the next small section. Don't try to fix everything at once !
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Your included scope trace is labelled "Clock on U17 and U18" - not having used a Rigol I cannot tell whether your trace is showing high frequency oscillations. Can you speed up the timebase or grab a 'single shot' of the clock at a higher time base to confirm you have a relatively 'clean' clock signal from U43B pin 4 ?
See attached picture. It's from U43B pin 4.
If the clock is oscillating it would not be good since the clocking of the other chips downstream will be wrong. You could isolate the clock output from U43B by cutting the input pins on the chips that it feeds. eg cut U17A pin 1, U8B pin 11 and U43D pin 11. This will tell you whether the clock is being loaded by a faulty chip.
On U17A pin 1, I see the clock from U43B pin 4, no problem. On U8B pin 11 I see the same clock. On U43D pin 11, I see the same clock.
I zoomed in on the waveform for pins 3, 6, 7 on U17, and while the hardware frequency counter was off (since it was seeing the gaps), the software one showed the correct frequencies listed by Codeboy2k (well, pins 6 and 7 both give 384Khz). So the reset signal for the counters (or SYNC) was causing an issue with my previous attempt to just use the hardware freq counter. =/ Learning experience! So I'm not sure there's anything wrong with the clock signal at this point. =/
Back at the start, alm said to look at the a/d counts on each side of the opto-couplers. I just did this, and I'm a bit confused. Pin 3 on AT2 should just be +5V, yes? Because it's not. it's a big mess of + and - pulses all around a +5V signal. Pin 2 is a nice TTL pulse train. R86 has a nice +5V on the side oposite AT2, but obviously on the AT2 side, it's the same as pin 3. But maybe I'm misunderstanding the opto-coupler.
I just feel like I'm barking up trees now. I think I'll have to wait until I can see some info from a working one... Because at this point, everytime I think I've found something, I measure it a different way, and it turns out it's fine. =(
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There are people who shouldn't be allowed to touch electronics. It's times like these that I think I'm one of them. :-[ So I go back to check things out and realize that the -2.8V reference is reading -2.2V. Great maybe this is part of the problem. So I check the zener (VR2) and its reading a beautiful 6.395V then the associated op-amp(U13). Unfortunately while measuring the op-amp my hand shakes (I'm only 32 but get random hand tremors from time to time) and I bridge the In+ and V- pins (pins 3&4). I hear the relays click and now the zener reads 1.2-1.4V |O Looks like I just damaged a $10-20 zener (It is a special one with a 0.005%/oC temp co, which works out to 0.03mV/oC). Who knows what else I damaged with that foolishness.
As for the -2.2V reading, I think ti is indicative of another problem because the resistors themselves check out as a perfect match (variation only in the last digit) to my working system. Which to me indicates somewhere else the voltage is drifting off spec.
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There are people who shouldn't be allowed to touch electronics. It's times like these that I think I'm one of them. :-[ So I go back to check things out and realize that the -2.8V reference is reading -2.2V. Great maybe this is part of the problem. So I check the zener (VR2) and its reading a beautiful 6.395V then the associated op-amp(U13). Unfortunately while measuring the op-amp my hand shakes (I'm only 32 but get random hand tremors from time to time) and I bridge the In+ and V- pins (pins 3&4). I hear the relays click and now the zener reads 1.2-1.4V |O Looks like I just damaged a $10-20 zener (It is a special one with a 0.005%/oC temp co, which works out to 0.03mV/oC). Who knows what else I damaged with that foolishness.
I'm assuming power cycling the unit didn't resolve this? I can't say I've ever permanently damaged something by a mistake like this, but I've probably just been lucky. =/ I'm hoping the one you blew was on the malfunctioning 199, and not the one that worked?
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okay, hopefully this is nothing, but I'm curious if it's normal to see "noise" on the ground plane? My scope probe is "grounded" to the lo input on the meter, and touching the probe to any of the analog ground plane produces noise on the scope. I'm guessing this is just because the probe/PCB is acting like a big ol' antenna, but I thought I would check.
Anyway, any other tips for testing things? my Blue ESR meter kit arrived today, so I hope to build that tomorrow and check the filter caps, just incase.
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There are people who shouldn't be allowed to touch electronics. It's times like these that I think I'm one of them. :-[ So I go back to check things out and realize that the -2.8V reference is reading -2.2V. Great maybe this is part of the problem. So I check the zener (VR2) and its reading a beautiful 6.395V then the associated op-amp(U13). Unfortunately while measuring the op-amp my hand shakes (I'm only 32 but get random hand tremors from time to time) and I bridge the In+ and V- pins (pins 3&4). I hear the relays click and now the zener reads 1.2-1.4V |O Looks like I just damaged a $10-20 zener (It is a special one with a 0.005%/oC temp co, which works out to 0.03mV/oC). Who knows what else I damaged with that foolishness.
I'm assuming power cycling the unit didn't resolve this? I can't say I've ever permanently damaged something by a mistake like this, but I've probably just been lucky. =/ I'm hoping the one you blew was on the malfunctioning 199, and not the one that worked?
Yes luckily it was on the malfunctioning one. ;D I'm still determined to fix this one and learn a lot in the process.
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okay, hopefully this is nothing, but I'm curious if it's normal to see "noise" on the ground plane? My scope probe is "grounded" to the lo input on the meter, and touching the probe to any of the analog ground plane produces noise on the scope. I'm guessing this is just because the probe/PCB is acting like a big ol' antenna, but I thought I would check.
The loop from your ground lead around through the ground plane on the PCB to the probe tip makes a very nice antenna. I use it intentionally to check for EMI emissions...
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ah, makes sense, and that's kind of what I figured.
thanks!
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Yes luckily it was on the malfunctioning one. ;D I'm still determined to fix this one and learn a lot in the process.
You and me both. I've already learned a lot... just wish I could figure out what the heck is wrong.
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I zoomed in on the waveform for pins 3, 6, 7 on U17, and while the hardware frequency counter was off (since it was seeing the gaps), the software one showed the correct frequencies listed by Codeboy2k (well, pins 6 and 7 both give 384Khz).
This has been bugging me, since pins 6 and 7 SHOULDN'T give the same frequency.
Looking at the datasheet, and the schematic, I think it should be:
1: 3.84Mhz
3: 1.92Mhz
4: 1.92Mhz
5: NC (though I think it should give 1.92Mhz, maybe)
6: 786Khz
7: 384Khz
Which is exactly what Codeboy2k said it should be. So why are pins 6 and 7 the same?! Would a 74HC390 fail in that way? Seems kind of... odd.
Anyone know if I can breadboard this easily to check a spare one I have, to see what it does. I've never played with one of these before... If I just feed it a signal, will it just divide (assuming I give it a supply voltage, etc), or does it need a reset signal?
Thanks!
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So, I just checked those numbers, and Pin 5 actually is 960Khz, but it doesn't really matter since it's not connected.
What's interesting, though, is pin's 6 and 7.
Pin 6 is 384Khz, with a pulse width of ~1.020us. Pin 7 is also 384Khz, but the pulse width is half that, at ~520ns. I'm not really sure what that means, but maybe someone else will?
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So, I just checked those numbers, and Pin 5 actually is 960Khz, but it doesn't really matter since it's not connected.
What's interesting, though, is pin's 6 and 7.
Pin 6 is 384Khz, with a pulse width of ~1.020us. Pin 7 is also 384Khz, but the pulse width is half that, at ~520ns. I'm not really sure what that means, but maybe someone else will?
Based on my reading the logic table in the datasheet what you describe is correct. Pin 7 should be the same frequency as pin 6 but at half the duty cycle.
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okay, then I'm just baffled.
Because looking further up, it says that chunk of the counter ÷5, but I can't make heads or tails of the part you sent since it seems to contradict the previous page's IEC logic symbol (unless I'm completely misreading that).
On your busted one, what do you get out of U17?
I'm going to pull the filter caps tomorrow and do ESR tests. Guessing their fine, but who knows.
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Sad if that's the case... at least eBay will help out in that case.
Not necessarily. I got ripped off to the tune of US$1000. E-bay/Paypal told me to get lost basically, even though I could prove I was shipped a different item from the one pictured in the auction. Tried the 'appeal' process, which was a total joke, as it went to the same staff member for review! Be warned. They protected a major seller, at my expense.
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okay, then I'm just baffled.
Because looking further up, it says that chunk of the counter ÷5, but I can't make heads or tails of the part you sent since it seems to contradict the previous page's IEC logic symbol (unless I'm completely misreading that).
On your busted one, what do you get out of U17?
I'm going to pull the filter caps tomorrow and do ESR tests. Guessing their fine, but who knows.
Well on my working one, I think my multimeter has a hard time reading the frequency or something. It read the input fine 3.842Mhz for both, but the others gave weird results. I'll bust out my scope tomorrow and see what I get. That beign said pins 6 and 7 showed the same frequency on my multimeter (just the number was ~1/2 what it should be) but their duty cycles were different.
On my broken one only pin 1 showed ANYTHING. The rest all showed zero volts output and zero frequency. So on my device it appears this IC is dead! Not sure what killed it but I need to start tracing back from here. At least SOMETHING is dead (not at my own hands).
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Well on my working one, I think my multimeter has a hard time reading the frequency or something. It read the input fine 3.842Mhz for both, but the others gave weird results. I'll bust out my scope tomorrow and see what I get. That beign said pins 6 and 7 showed the same frequency on my multimeter (just the number was ~1/2 what it should be) but their duty cycles were different.
On my broken one only pin 1 showed ANYTHING. The rest all showed zero volts output and zero frequency. So on my device it appears this IC is dead! Not sure what killed it but I need to start tracing back from here. At least SOMETHING is dead (not at my own hands).
cool, thanks. certainly let me know.
I pulled the main filter caps today, and they all check out ESR/Capacitance wise. So that's not it (didn't think it was). So, still searching. Keep looking at the top and bottom of the board hoping something will jump out at me. =/
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Since I've gotten really no-where on this, I think I'm actually going to start from scratch. I'm still assuming it's something rather obvious, and probably a single point of failure, but at this point, I'm just jumping all over the place hoping to find a smoking gun (or at least a bullet hole).
I'll admit some of the signal pathway is a bit confusing (like, what the hell is the point of Q5 and Q6), but it must be that way for a reason. =P I'll report back if I find anything odd.
Any further input from alm or codeboy2k, or PedroDaGr8 would be much appreciated.
Thanks!
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Since I've gotten really no-where on this, I think I'm actually going to start from scratch. I'm still assuming it's something rather obvious, and probably a single point of failure, but at this point, I'm just jumping all over the place hoping to find a smoking gun (or at least a bullet hole).
I'll admit some of the signal pathway is a bit confusing (like, what the hell is the point of Q5 and Q6), but it must be that way for a reason. =P I'll report back if I find anything odd.
Any further input from alm or codeboy2k, or PedroDaGr8 would be much appreciated.
Thanks!
A few observations.
So yesterday I picked up a LeCroy 9410A with two Tek 10x probes for $125 locally, the thing is huge lol. Anyways, that I started tracing the clock signal from the GPIB board (which actually also controls the display). So the signal looks good going from U8 through AT1 (the optoisolator on the mainboard) but at or after that the clock signal starts looking REALLY messy. I don't know if its my fault but the signal goes from being a clean 0-5V to 2V-5V and with a lot of high frequency noise superimposed on top of it. I am not sure if U49 does it or if it is injected after U49. I'll try and figure it out tonight if I can. Also, after running the meter for a while I noticed the filter caps on the Vreg get quite warm.
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Anyways, that I started tracing the clock signal from the GPIB board (which actually also controls the display). So the signal looks good going from U8 through AT1 (the optoisolator on the mainboard) but at or after that the clock signal starts looking REALLY messy. I don't know if its my fault but the signal goes from being a clean 0-5V to 2V-5V and with a lot of high frequency noise superimposed on top of it. I am not sure if U49 does it or if it is injected after U49. I'll try and figure it out tonight if I can. Also, after running the meter for a while I noticed the filter caps on the Vreg get quite warm.
Interesting! I started with the analog board last night, and managed to trace everything successfully to U46. What's interesting is at the output of U46, there's an offset added by U45, which I'm guessing is due to what amounts to a resistor divider where U45's and U46's output come together. So where I get a nice 2.5VDC into U46 (which is my input signal), the output pin shows about 2.9V out. U45 is where the multiplier is done for some of the ranges...
Also, what's VERY interesting, is C38 is missing. Looking at the board, it was never there (the solder pads/holes look to have the factory "fill"). PedroDaGr8: could you look and see if C38 is there? It's not listed on the diagram as being purposefully omitted. Also, what revision does your digital board say it is (mine says A4). Doubt it matters that much, but it would be curious to figure that out.
As for the optoisolator... the offset MIGHT make sense (looks like R83 coming from the +5V rail may be the offset) (page 5 of the schematic), but the noise SHOULD be shunted to ground through C61. I guess my first thought would be, did you move your ground lead to the analog side when you measured AT1's output (I say this because I forgot to do this, and things looked wonky (60Hz hum) until I realized it, and moved the ground lead). But I need to recheck all that once I confirm my signal path is clean.
All that said, I do wonder if the issue is clock related.
Just some thoughts. =)
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sorry I meant after the op-amp U49 following the optoisolator. I'm going to double check tonight.
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ah. Well, wouldn't that be because U49 sits halfway between its +5V and Gnd rails... ? So zero input would get shifted up to 2.5V? I'm guessing here, so please, call me wrong if that's just complete BS.
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ah. Well, wouldn't that be because U49 sits halfway between its +5V and Gnd rails... ? So zero input would get shifted up to 2.5V? I'm guessing here, so please, call me wrong if that's just complete BS.
Actually, looking at the schematic again. U49 isn't an opamp, its a hex inverter. So it should be just fine with 0-5V output. The thing is, since the noise follows the hex inverter, I'm not sure if this noise comes from the hex inverter or is being injected backwards from somewhere else.
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ah. Well, wouldn't that be because U49 sits halfway between its +5V and Gnd rails... ? So zero input would get shifted up to 2.5V? I'm guessing here, so please, call me wrong if that's just complete BS.
Actually, looking at the schematic again. U49 isn't an opamp, its a hex inverter. So it should be just fine with 0-5V output. The thing is, since the noise follows the hex inverter, I'm not sure if this noise comes from the hex inverter or is being injected backwards from somewhere else.
Ah, okay. Sure looks like an Op-amp from the drawings. =P
hmm, I'll have to check that out, see if mine sees the same thing. What's the noise look like?
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Anyways, that I started tracing the clock signal from the GPIB board (which actually also controls the display). So the signal looks good going from U8 through AT1 (the optoisolator on the mainboard) but at or after that the clock signal starts looking REALLY messy. I don't know if its my fault but the signal goes from being a clean 0-5V to 2V-5V and with a lot of high frequency noise superimposed on top of it. I am not sure if U49 does it or if it is injected after U49. I'll try and figure it out tonight if I can. Also, after running the meter for a while I noticed the filter caps on the Vreg get quite warm.
Interesting! I started with the analog board last night, and managed to trace everything successfully to U46. What's interesting is at the output of U46, there's an offset added by U45, which I'm guessing is due to what amounts to a resistor divider where U45's and U46's output come together. So where I get a nice 2.5VDC into U46 (which is my input signal), the output pin shows about 2.9V out. U45 is where the multiplier is done for some of the ranges...
Also, what's VERY interesting, is C38 is missing. Looking at the board, it was never there (the solder pads/holes look to have the factory "fill"). PedroDaGr8: could you look and see if C38 is there? It's not listed on the diagram as being purposefully omitted. Also, what revision does your digital board say it is (mine says A4). Doubt it matters that much, but it would be curious to figure that out.
As for the optoisolator... the offset MIGHT make sense (looks like R83 coming from the +5V rail may be the offset) (page 5 of the schematic), but the noise SHOULD be shunted to ground through C61. I guess my first thought would be, did you move your ground lead to the analog side when you measured AT1's output (I say this because I forgot to do this, and things looked wonky (60Hz hum) until I realized it, and moved the ground lead). But I need to recheck all that once I confirm my signal path is clean.
All that said, I do wonder if the issue is clock related.
Just some thoughts. =)
Also, yes C38 is filled on BOTH of my devices. It is one of the ones that I thought was defective actually. Going back and measuring again I can't get the same results oddly enough.
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Also, yes C38 is filled on BOTH of my devices. It is one of the ones that I thought was defective actually. Going back and measuring again I can't get the same results oddly enough.
Well crap. I'll check again, but it's not in the space it shows on the drawing. Is it in a different space for you? Shows it as being right below R17 and R13, behind Q7.
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Also, yes C38 is filled on BOTH of my devices. It is one of the ones that I thought was defective actually. Going back and measuring again I can't get the same results oddly enough.
Well crap. I'll check again, but it's not in the space it shows on the drawing. Is it in a different space for you? Shows it as being right below R17 and R13, behind Q7.
yep it looks identical to C53 except its larger. I'll take a pic when I get home. I'm not entirely sure it is necessary
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yep it looks identical to C53 except its larger. I'll take a pic when I get home. I'm not entirely sure it is necessary
Here's hoping it's not. Guessing it must not be if the factory left it off. If it IS, hopefully a Mica cap will be a suitable replacement, as finding a 500V polystyrene 680pF cap looks to be rather troublesome.
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Okay, just checked pin 10 of U49, and I'm guessing this is what you mean by "noisy". Mind you, that's just me stopping the scope. Maybe things like this don't matter with a clock signal, but it certainly doesn't seem clean. I've confirmed this signal remains crappy looking on the CLK inputs for U29, U30, U31, and U32. Note that my channel 2 is my source signal. And while the optocoupler output is a positive going clock signal, U49's is negative going. Looks like it's a base 5V signal, that clocks at 1V. Second picture is the large gaps in the clock signal.
Indeed, C38 is missing. Guessing it must not be necessary.
The output of the AT1 for the clock signal, btw, has gaps. guessing whatever is generating the clock is being reset every X cycles. So it's not a steady lock signal, but goes for a bit, has a gap, goes a bit more, gap, etc. That what you see? =/
Also interestingly, the clock on U49 pin 10 (and AT1 input and output) shuts down in diagnostic mode (any N level). So it must be driving the MUX. Which I guess makes sense.
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Actually, i think mine may have been a grounding issue because on the other side of the inverter, I am reading non-stop 5V solid. So iether U49 is blown or one of the other ones is blown. I may start lifting ICs to see if I can isolate it.
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Actually, i think mine may have been a grounding issue because on the other side of the inverter, I am reading non-stop 5V solid. So iether U49 is blown or one of the other ones is blown. I may start lifting ICs to see if I can isolate it.
This is on the broken one I would assume.
Do you get a clock between AT1 and U49? I just get a solid 5V when I'm in diagnostic mode as well... the clock seems to only run in normal operating mode.
I've pretty much traced my signal as far as I can... since once I get to Q17, which no matter the N mode, the gate is at -10V, and my signal disappears. There's about 30R of resistance between the Source and Drain (with the device off). Does this mean that FET is toast? If so, anyone know what model FET it is? The parts list just says "Trans, N-CHANNEL FET" and "TG-128". If it's supposed to dump my signal to ground, then how the heck does the integrator work?! I see that Q4 will switch on and off... and I can see some noise (740mVpp 48khz triangle wave centered on -220mV or so in N1 and N2, then it shifts down to -800mV for N3, and I get nothing on N4). But that's certainly not my signal. My signal seems to die on the Q17 side of R58, and there it sits. I did lift R58 and confirm it is a working 15.8kR resistor. Though checking the other "TG-128"'s on the board, they seem the same... about 30R between drain and source with no power applied. =/ But I would think with a -10V bias on the gate, it should be completely closed. =/
So, in short, I'm stuck at this point. The only thing I can think is if I can lift the drain leg on Q17 and see if that gets my signal further down the line... but I'm not sure if that's safe to do. I'm not even sure what the point of Q17 is, other than for some other measurement where you don't need voltage on this signal path.
Thoughts?
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I guess there could also be a short in Q4 between the drain and gate as well... but it would seem SOMETHING is just dumping my signal to ground around that point.
Would this seem like a reasonable failure due to abuse (over-volting the meter)? Can I just lift a leg on either of these and see what happens?
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I didn't pull the jfet (yet) but checking it with the diode test of my fluke, I get about .75V DC between gate-source/drain one direction, and about 2.2V the other direction. My understanding is you shouldn't get ANYTHING in one direction, is that not true, or is it because I didn't pull it to test?
Thanks!
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I didn't pull the jfet (yet) but checking it with the diode test of my fluke, I get about .75V DC between gate-source/drain one direction, and about 2.2V the other direction. My understanding is you shouldn't get ANYTHING in one direction, is that not true, or is it because I didn't pull it to test?
Thanks!
http://www.allaboutcircuits.com/vol_3/chpt_5/3.html (http://www.allaboutcircuits.com/vol_3/chpt_5/3.html)
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I didn't pull the jfet (yet) but checking it with the diode test of my fluke, I get about .75V DC between gate-source/drain one direction, and about 2.2V the other direction. My understanding is you shouldn't get ANYTHING in one direction, is that not true, or is it because I didn't pull it to test?
Thanks!
http://www.allaboutcircuits.com/vol_3/chpt_5/3.html (http://www.allaboutcircuits.com/vol_3/chpt_5/3.html)
This also explains why I got shorts across the JFETs sometimes but not others.
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well, then I have no clue. somewhere around that area my signal is completely disappearing... so something is dumping to ground. It's there on the mux side of R58, but missing on the A/D side... just a nice 0V. Will have to check again tomorrow evening... but right now, I'm guessing this is the general area of fault. Everything LOOKS fine... but something has seemingly shorted/failed.
Any suggestions? =/
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Okay, on this again. Signal still disappears after R58, but looking at the mux circuit, I notice some overshoot on it.
I've found that it's coming from the output of U45A. My understanding is the mux is switching rapidly from 0, 0, -2.8V, and my input signal (2.5V). But there's a blip now and then, which would seem to be coming from U45A (unless it's coming from somewhere else further up the chain, but it's not on TE1/TE2. I don't see any noise issues on the rails to anything, so that's not it.
Any thoughts? For all I know, it's the same cause as the weird runt pulses on the hex inverter as well...
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Taking another approach, with the unit off, I get 30R of resistance from ground to the A/D side of R58. PedroDaGr8, or alm, can you please check this on yours? wait, nevermind. With Q17 not getting a -10V on it's gate, it just goes to ground. Crap. How the hell do I figure out where my signal is being grounded out then? Lift Q17 (or at least, the source/drain pin) and test then?
With the unit on, in diagnostic mode, with any N, I confirmed it's NOT Q17, as I get no conductivity between source and drain (because the gate is at -10V). So it's somewhere else things are going to hell.
Help!
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Looking at the integrator in diag-mode, I see pin 2 at 9.928, 9.928, 9.915, and 10.205V on N's 1-4 respectively. Pin 3 is at 10.527, 10.527, 10.583, 10.208V on N's 1-4. So Q1 must be taking input from SOMEWHERE, and creating an offset from something, and using that to drive the integrator?
And at least in N4, it's doing some type of integration. so something is getting through. maybe there's something I'm missing from the schematics with regards to where the signal goes and what drives the integrator. Maybe this is all down to the hex inverter runts or the overshoot on the mux... but unless I start taking some peyote, I'm not sure I'm going to have any epiphanies. Anyone got anything? Bueller?
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I just realized you asked me what the model of the JFETs are. On my non-working one they are ALL 2N part numbers (the working one has the weird custom parts) if you give me a few minutes I'll list the part numbres of all of the TO-92 devices.
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I just realized you asked me what the model of the JFETs are. On my non-working one they are ALL 2N part numbers (the working one has the weird custom parts) if you give me a few minutes I'll list the part numbres of all of the TO-92 devices.
Here we are.
Q17 - Harris ITE 2n4392
Q4 - 2n2771
Q9 - NOT POPULATED
Q16 - 2n5818
Q13 - 2n4392
Q3 - 2n2771
Q11 - 2n4392
Q33 - 2n2607
Q34 - 2n4393
Q32 - 2n4392
Q30 - 2n2607
Q35 - 2n2607
Q8 - 2n4392
Q12 - 2n4392
Q7 - 2n4392
Q5 - 2n4392
Q6 - 2n4392
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Well I pulled U49 and finally got around to testing it. No more constant 5V on the output to clock. So the hex inverter was clearly dead. Time to order some replacements.
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Well I pulled U49 and finally got around to testing it. No more constant 5V on the output to clock. So the hex inverter was clearly dead. Time to order some replacements.
Fascinating... because yeah, my U49 appears to be failed as well. It's clearly partially working, but not well. I have a replacement one arriving on Monday (NTE74HCT04). For now, I'm just going to put in a socket. Here's hoping.
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Just posted some pictures of what the IC (U49) does out of circuit... it ain't pretty.
https://www.eevblog.com/forum/chat/logic-inverter-'noise'/msg263583/#msg263583 (https://www.eevblog.com/forum/chat/logic-inverter-'noise'/msg263583/#msg263583)
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So... it's fixed.
I pulled U49 and it seemed to be busted when I put it on a breadboard... but then I was corrected about being wired backward. doh! So I tested with it wired right, and it was dead. fast forward a couple days, and I test it again on the board. Now it works. huh.
So, after removing U49 from the board, I put in a socket since I don't overly care of soldering ICs directly. I drop U49 into the socket, and viola, my Keithley 199 works flawlessly (after recalibrating the 3V and 30V range). Now, I think I'm still going to replace that chip with the new one that's getting here next week, since I can't be sure if the chip is just playing nice for now, or if, as someone suggested, the gnd or 5V pin was a bad solder. =/
Huh. Stupid logic. Anyway... PedroDaGr8... start with that logic chip. Bet you'll get somewhere. =)
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One other question, if anyone knows... the selector switch for changing between front and rear inputs, is rather sticky (doesn't return to "front" well when pushed). Think I can clean it somehow, or should I try to buy one somehow?
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I do want to congratulate alm. Saying to check both sides of the opto-coupler for the A/D counts was almost exactly right. The signal on the optocoupler was fine on both sides, it was one step further at the inverter were things broke down. =P
But still... damn accurate.
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great news! I think I need to get that blown diode replaced first.
As for theinverter, ut's strange IIRC they invert the signal, push it through the connector, through the optoisolator then invert it back. not sure why they did that
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yeah, wonder that too... but looking online, it seems it's due to CMOS TTL logic not being able to drive the optocoupler directly... so they use it to sink the pulses instead.
http://sound.westhost.com/project145.htm (http://sound.westhost.com/project145.htm)
Seems like this should be something built into optocouplers.
On the plus side of all of this... I think it's justified my test equipment purchases in my wife's eyes. Pretty sure I couldn't have found the problem without a scope at the least...
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Glad you were able to fix it, even without me having time to provide measurements from a know-good unit! Yes, a scope is pretty much essential for this. Now to convince your wife that spending money on the scope was good because it enables you to spend more money on electronics junk to fix ;).
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Glad you were able to fix it, even without me having time to provide measurements from a know-good unit! Yes, a scope is pretty much essential for this. Now to convince your wife that spending money on the scope was good because it enables you to spend more money on electronics junk to fix ;).
Oh yeah, that's no problem. I'm just not sure what I want/need next. with the scope, FG, PSU, and now bench and handheld meter...
I wouldn't mind sending out the bench meter for a calibration... but it seems hard to justify the $100+ for it. I got close with my PSU and FG, and calibrated based on my Fluke 189 (which I have no idea how well it's calibrated, and it's a digit lower resolution).
For fixing stuff, one of the blue ring testers would be nice, but that's not something I buy broken.
Any suggestions for something else to buy that would be "cheap" in a busted state, but could be reasonably fixed and provide a good learning experience? =)
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I would say stuff that it not any newer from this, around the late eighties custom ASICs and poorly documented circuits become common. Stuff like analog function/pulse gens and linear power supplies tends to be fairly doable. In general, I find that used prices for broken units on eBay have gotten way too high. Unless you already know the problem in advance, it's always a gamble.
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I would say stuff that it not any newer from this, around the late eighties custom ASICs and poorly documented circuits become common. Stuff like analog function/pulse gens and linear power supplies tends to be fairly doable. In general, I find that used prices for broken units on eBay have gotten way too high. Unless you already know the problem in advance, it's always a gamble.
Makes sense. Well, I'll keep an eye out.
Any ideas on the input selector switch?
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The power switch (which I think is a similar kind of switch) is quite sticky in one of the units I own. I rarely use the input selector switch, and I don't have them in front of me now, but I think it's a fairly common issue. I would try cleaning them if it bothers you. To clean switches, I usually squirt some IPA in, exercise the switch a few times, let it dry, and then optionally add lubrication. I think there are also purpose-made products (eg. the various Caig products) for this. Don't use WD-40 or contact cleaners, they tend to be aggressive and/or leave a sticky mess that attracts dust.
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The power switch (which I think is a similar kind of switch) is quite sticky in one of the units I own. I rarely use the input selector switch, and I don't have them in front of me now, but I think it's a fairly common issue. I would try cleaning them if it bothers you. To clean switches, I usually squirt some IPA in, exercise the switch a few times, let it dry, and then optionally add lubrication. I think there are also purpose-made products (eg. the various Caig products) for this. Don't use WD-40 or contact cleaners, they tend to be aggressive and/or leave a sticky mess that attracts dust.
Well, I don't have any IPA, but I have a nice Hefeweizen or some nice chocolaty stouts... but I would think this would leave a huge mess. =D
Seriously though, I'll try some isopropanol. And yeah, I've seen a lot of suggestions about using Caig DeoxIt Gold (and I think Red) for stuff like this... but like you, I don't plan to use the switch much, and dropping $25 on a bottle for this one switch is a bit much. =/
Thanks for all the help!
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Success....kind of.
So the other night before you tried your repair I tried inserting the replacement 74HCT04. No result the system just made a whining noise and I couldn't get a trace to pull up. Tired I gave up for the time being. Seeing yours I decided to go back and revisit it. Sure enough the IC was in backwards :-DD and I could have sworn I triple checked it the other night. Anyways flipped it around, BOOM, I have numbers moving on my display. Unfortuantely, because of the blown zener they make no sense. |O Even stranger, I tried the old "bad" HCT04, it works TOO! :wtf: The only thing I can figure is that there is a bad connection inside the 74HCT04 and the heat of desoldering it reflows that connection. It is the only thing I can think of that makes sense.
Now its time to order that zener and get it installed, found a place selling them for $10. Anyone have some advice to make sure I don't kill another zener?
Lastly, maybe change the name of this to Keithley 199 instead of Bench Multimeter? it will help people trying to repair theirs as this might be a common mode of failure (and one likely unexpected by most people).
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alm: IPA worked a treat. Just squired a bunch on there, and worked the switch, and it freed up pretty nicely. Guessing I'll pick up some Caig stuff in the future to lubricate, but for now, it works.
PedraDaGr8: LOL! maybe subconsciously we're thinking "it's an inverter, it should be wired backward". =) You're probably right... but hell if I'm not going to replace the inverter anyway. I do wonder if the heat is part of it, because I'll admit, to get that sucker out, I really had to heat the crap out of it. Really wish ICs were in sockets by default, especially since there's room.
So, with the zener, I'd assume you've checked the voltage on it (or at TE1/TE2 in N3 mode) and it doesn't read right? $10 isn't bad for that part... I would assume just solder it quickly, in the right direction, and you shouldn't have any issue... but I'm not an expert.
As for title... will do. I've thought about that. =)
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alm: IPA worked a treat. Just squired a bunch on there, and worked the switch, and it freed up pretty nicely. Guessing I'll pick up some Caig stuff in the future to lubricate, but for now, it works.
PedraDaGr8: LOL! maybe subconsciously we're thinking "it's an inverter, it should be wired backward". =) You're probably right... but hell if I'm not going to replace the inverter anyway. I do wonder if the heat is part of it, because I'll admit, to get that sucker out, I really had to heat the crap out of it. Really wish ICs were in sockets by default, especially since there's room.
So, with the zener, I'd assume you've checked the voltage on it (or at TE1/TE2 in N3 mode) and it doesn't read right? $10 isn't bad for that part... I would assume just solder it quickly, in the right direction, and you shouldn't have any issue... but I'm not an expert.
As for title... will do. I've thought about that. =)
Haha yes its an inverted inverter; that's my story and I'm sticking to it. O0
As for the zener: Yeah it reads 1.4V across it in all modes. It's definitely shot. I was looking at these zeners and its no wonder they are expensive. It has a very nice temp coefficient (0.0005%/oC). Anyways I purchased the zener plus a couple replacement 74HCT04's. Can't wait to get it up and fully functioning. Then maybe off to calibration (a local place does full ISO and NIST calibration with all data for $75, waiting to hear back if they will give a discount if I dont mind waiting up to a month and do both Keithleys and my 6114a at the same time).
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As for the zener: Yeah it reads 1.4V across it in all modes. It's definitely shot. I was looking at these zeners and its no wonder they are expensive. It has a very nice temp coefficient (0.0005%/oC). Anyways I purchased the zener plus a couple replacement 74HCT04's. Can't wait to get it up and fully functioning. Then maybe off to calibration (a local place does full ISO and NIST calibration with all data for $75, waiting to hear back if they will give a discount if I dont mind waiting up to a month and do both Keithleys and my 6114a at the same time).
Nice. I need to make friends with someone that has at least a calibrated 5.5digit meter (6.5 or 8.5 would be better). The Keithley's are really easy to calibrate... it's just waiting the 2 hours for them to fully heat up (at least, that's what the manual claims to wait before calibration).
Good luck, and let us know how the replacements go. Where did you source the zener from? While they aren't LM399 good (0.0001%/°C), that's still pretty damn stable.
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Hi all,
Good to see a few of the Keithley 199s being resurrected.
I recently retrofitted my 199 with an LM399 reference. On my unit the original 1N4579 appears to have drifted significantly, so I decided to replace it. Fairly easy to do, as the Zener voltages are pretty close. I added a 107 kOhm resistor in parallel to R69 to bring the reference voltage back to -2.8V. -10VR and +5VR were adjusted by replacing the following resistors:
R67: 25500 Ohm
R70: 11500 Ohm
R77: 10000 Ohm
R76: 13700 Ohm
The LM399 heater was hooked up to the +15V and -15V regulated supply rails.
While I had the unit open I replaced the NE5534 Opamps with LT1001s (DIP8).
The LM399 was burned in for 4 weeks and the unit then calibrated in a cal lab. I continue to occasionally check the unit against my Geller 10V reference. So far I'm happy with the results.
Appreciate any feedback regarding my approach...
Regards,
Martin
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Really wish ICs were in sockets by default, especially since there's room.
Careful what you wish for. Some old Tek equipment used sockets for everything, even transistors. Over time, these sockets become intermittent. Multiply this with a few dozen sockets: lots of fun to troubleshoot. Solder connections tend to be much more reliable.
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Hi all,
Good to see a few of the Keithley 199s being resurrected.
I recently retrofitted my 199 with an LM399 reference. On my unit the original 1N4579 appears to have drifted significantly, so I decided to replace it. Fairly easy to do, as the Zener voltages are pretty close. I added a 107 kOhm resistor in parallel to R69 to bring the reference voltage back to -2.8V. -10VR and +5VR were adjusted by replacing the following resistors:
R67: 25500 Ohm
R70: 11500 Ohm
R77: 10000 Ohm
R76: 13700 Ohm
The LM399 heater was hooked up to the +15V and -15V regulated supply rails.
While I had the unit open I replaced the NE5534 Opamps with LT1001s (DIP8).
The LM399 was burned in for 4 weeks and the unit then calibrated in a cal lab. I continue to occasionally check the unit against my Geller 10V reference. So far I'm happy with the results.
Okay, that's pretty awesome actually. I'll definitely keep this in mind should my ref ever start drifting. =)
For now, I just found a Keithley 197 off ebay for a song that I'll be trying to fix as well. =)
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Careful what you wish for. Some old Tek equipment used sockets for everything, even transistors. Over time, these sockets become intermittent. Multiply this with a few dozen sockets: lots of fun to troubleshoot. Solder connections tend to be much more reliable.
touché. I guess I just need more experience unsoldering ICs.
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Cutting the pins and desoldering each pin individually is usually the best strategy for jellybean parts, since saving the board is far more important than saving the part, and you're not going to reinstall a suspect $0.10 part anyway.
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Cutting the pins and desoldering each pin individually is usually the best strategy for jellybean parts, since saving the board is far more important than saving the part, and you're not going to reinstall a suspect $0.10 part anyway.
I figured that would be the easiest... though it makes it difficult to test out of circuit. =P
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It's easy to test out of circuit: install a replacement, see if it works. Hey, the original part is technically out of circuit, right? :P
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It's easy to test out of circuit: install a replacement, see if it works. Hey, the original part is technically out of circuit, right? :P
lol! I guess that's just as good... =)
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alm, or PedroDaGr8,
So, something still isn't quite right. I did replace the inverter, and that seems to be working now. But...
On the 30V and 300V range, with no voltage applied to the inputs, or the inputs not shorted, the voltage is pretty stable at 00.0000 or whatever. But on the 3V or 300mV range, if there's no voltage applied, or the inputs aren't shorted, the reading creeps up (down) toward -1.5V or more. On the 300mV range, this relatively quickly leads to an overflow, but on the 3V range, it just gradually creeps. If I hook anything up to the inputs, like my FG, even with the output off, the impedance is low enough for the creep to not happen or be much slower.
Sounds like some type of capacitance on the inputs?
Do either of your working meters do anything like this? I know some drift is common from it picking up transient voltages... but to pick up over a volt...
I will say that it reads accurately when I give it a voltage, and it zeros properly when I short the leads together...
Thoughts? And thanks!
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This is normal. The input impedance for (good) bench meters in the low ranges is in the Gohm (for all intents and purposes it is completely open) so builds up charge from the environment in short order. If you short the inputs with a megohm or ten as a handheld meter has, you should see the charge buildup is basically gone.
This is a desirable feature, and a good reason to own a bench meter, as you can measure high impedance circuits without having an impact on them.
Good job on the repair :D
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As ve7xen stated, this is the expected behavior. Many bench meters will actually range up (because the voltage exceeds the maximum of the range) until they reach a range with a low 10 Mohm input impedance. This will discharge the parasitic capacitance, and then they will downrange again. This is repeated indefinitely, and can be quite annoying and eventually wear out relays.
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This is normal. The input impedance for (good) bench meters in the low ranges is in the Gohm (for all intents and purposes it is completely open) so builds up charge from the environment in short order. If you short the inputs with a megohm or ten as a handheld meter has, you should see the charge buildup is basically gone.
This is a desirable feature, and a good reason to own a bench meter, as you can measure high impedance circuits without having an impact on them.
Good job on the repair :D
As ve7xen stated, this is the expected behavior. Many bench meters will actually range up (because the voltage exceeds the maximum of the range) until they reach a range with a low 10 Mohm input impedance. This will discharge the parasitic capacitance, and then they will downrange again. This is repeated indefinitely, and can be quite annoying and eventually wear out relays.
Huh. Okay... for some reason I thought the input impedance of the 199 was only like 1M, so I was a bit confused. So if I'm going to leave the unit on for long periods with no input (not planning on this, but even so), I should probably set it to a specific range and not leave it on auto.
Got it.
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Yep, I usually set it to 30 V (where is has an input impedance of 10 Mohm instead of the Gohms it has on the lower ranges) when not in use. If only to silence the annoying sound the relay produces. The same happens on the HP 3478A, for example, although with a lower frequency.
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It would be nice if these meters could sense this (either sensing the behavior after a couple cycles, or even a mechanical/optical sensor on the jacks) and automatically switch to a low-impedance range...
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Yep, I usually set it to 30 V (where is has an input impedance of 10 Mohm instead of the Gohms it has on the lower ranges) when not in use. If only to silence the annoying sound the relay produces. The same happens on the HP 3478A, for example, although with a lower frequency.
Perfect. I'll set that and save it so it comes up in that config. For interest sake, is it possible to measure the input impedance of the various ranges? Just set it to DCV in a given range and hook another meter up to the inputs measuring ohms?
Thanks!
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You're not going to measure the high-impedance ranges without something like a picoammeter. Of course, if you just want to know what is "high impedance" and what is the usual 10M, then yeah, just stick another DMM on it, it'll read infinity for the high impedance ranges.
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It would be nice if these meters could sense this (either sensing the behavior after a couple cycles, or even a mechanical/optical sensor on the jacks) and automatically switch to a low-impedance range...
Yeah... does seem like a weird thing to not address. The sensor on the jacks would solve it for no leads, but if you just left the leads plugged in and dangling...
Programmatically, it wouldn't be that hard to address this, like you say, keep a counter of the cycling, and how long it takes to do the cycle, and if the cycle takes in the order of tens of seconds, and if the charge disappears once you hit the 30V or 300V range, then you're likely in a loop... so just stop and disable "auto". =/
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You're not going to measure the high-impedance ranges without something like a picoammeter. Of course, if you just want to know what is "high impedance" and what is the usual 10M, then yeah, just stick another DMM on it, it'll read infinity for the high impedance ranges.
Oh, that works. =)
I would have thought the manual would cover the impedance of the ranges, but I couldn't find it... ah well.
Thanks all!
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I'd actually just be happy with not autoranging into the hi-Z ranges at all. Most of the meters with a high impedance range have more than enough resolution to be useful for almost everything up in the 20V/30V/whatever range - I always set my 3468A to the 30V range to shut it up, and that range has 100uV resolution! I can manually switch if I want gigohm impedance.
Of course, the real bastards are the ones with nonstandard impedances, like my B&K Precision 2712, which actually has 2.3M! :wtf: It took me a while to figure out what the hell was wrong with my circuit (I was probing a high impedance spot and calculating out the effects of the meter from an assumed 10M....) As long as HP and Keithley don't start doing that shit, I'll let them do whatever they want with the hi-Z behavior......
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I think some of the recent Agilient handheld DMMs had an odd input impedance (something like 2 or 5 MOhm) in some modes. Good fun when using accessories like high voltage probes, indeed.
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I would have thought the manual would cover the impedance of the ranges, but I couldn't find it... ah well.
In the specs section under DC volts, you'll see 'input resistance' noted as > 1Gohm for the 300mV and 3V range. Not sure if they mention it, or the consequences, anywhere else.
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I think some of the recent Agilient handheld DMMs had an odd input impedance (something like 2 or 5 MOhm) in some modes.
Do they? Hmm... I've been considering getting one, and I didn't notice that! (Not that it'll make me not want one, just surprised I didn't catch it while reading datasheets.)
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The Agilent U125x series is the one I had in mind:
(https://www.eevblog.com/forum/chat/bench-multimeter-(another)/?action=dlattach;attach=55735)
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Is it? That looks normal to me for a dual display meter - double the load for double the display. It's definitely a nice-looking meter, though I can't afford it right now...
I do love the dual-display setup. My Fluke 45 has that and it's quite handy. I'd much rather dual AC/DC than a single AC+DC, for instance, or AC+freq. V/I is also useful, though it annoyingly flutters the relays to do that... I'll have to probe my Fluke and see if the impedance halves with two displays active.
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Finally found a spec sheet that shows the input impedance for the ranges. I just figured 3V would be 10M like the others, but it's >1G, same as 300mV range.
Spec sheet is attached, since it took a bit of work to find (found it on one of the calibration/repair sites). Just incase anyone wants it. =)
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KO4BB has archived the full user manual, which includes this spec as well, just assuming you didn't find it. Manuals for your equipment are always good to have handy, sometimes you need to RTFM just to get a sensible reading from the instrument!
Keithley kindly even includes schematics in this one (their newer equipment they are very stingy about technical information). The manual is actually available on the Keithley site directly, but it's behind a registration wall, so this is better ;)
http://www.ko4bb.com/Manuals/09)_Misc_Test_Equipment/Keithley/Keithley_199man.pdf (http://www.ko4bb.com/Manuals/09)_Misc_Test_Equipment/Keithley/Keithley_199man.pdf)
Edit: fixed URL
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KO4BB has archived the full user manual, which includes this spec as well, just assuming you didn't find it. Manuals for your equipment are always good to have handy, sometimes you need to RTFM just to get a sensible reading from the instrument!
Keithley kindly even includes schematics in this one (their newer equipment they are very stingy about technical information). The manual is actually available on the Keithley site directly, but it's behind a registration wall, so this is better ;)
http://www.ko4bb.com/Manuals/09)_Misc_Test_Equipment/Keithley/Keithley_199man.pdf (http://www.ko4bb.com/Manuals/09)_Misc_Test_Equipment/Keithley/Keithley_199man.pdf)
Edit: fixed URL
Oh, I have the manual, that's how I fixed the thing (with the schematic)... but for the life of me, I can't see the specs in that manual.
Or... I'm completely blind... I just found it. Fair enough... well, thing I attached should show up in google if someone's lookin'. =)
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PedroDaGr8,
Any luck fixing yours?