Keithley 199 -- FIXED!!!!

[staze]

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

[staze]

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
U18: 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?

[staze]

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).

-------

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?

[staze]

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

[PedroDaGr8]

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).

[staze]

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.

[PedroDaGr8]

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

[staze]

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.

[alm]

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.

[staze]

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!

[staze]

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.

[tjw]

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 !

[staze]

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. =(

 

[PedroDaGr8]

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  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.

[staze]

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  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?

[staze]

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.

[PedroDaGr8]

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  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.  I'm still determined to fix this one and learn a lot in the process.

[c4757p]

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...

[staze]

ah, makes sense, and that's kind of what I figured.

thanks!

[staze]

Yes luckily it was on  the malfunctioning one.  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.

[staze]

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!

[staze]

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?

[PedroDaGr8]

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.

[staze]

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.

[Chalky]

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.