Test Equipment Anonymous (TEA) group therapy thread

[med6753]

Yes butchered it has been. coils got cut when I cut the yellow "jacket", sadly.

Chinese Vref well yes I guess I could add a pot to it and use that as an external ref for the meter, why not...

Well, my point was why reinvent the wheel. Make a circuit that works using the Cheap & Cheerful China-Direct™ Vref, then once you have your meter working, or at least functional enuf that you know you're gonna stick with it til it does, then you can start shopping some AD584s and design your PCB based on the VRef, which is doubtless directly based on the dev example circuits in the AD584 datasheet.

Have fun!

mnem
 

I just handed him a good starting point on a silver platter. 

[mnementh]

Yeah, sure... no argument there. But as Vince stated... all he needs for this purpose is a 10K/25 turn pot across the 10V output and filtering as needed for the existing 15V (IIRC) power source.

When he has the rest of it working, then he can design something as a permanent replacement.

mnem
"It's all fun & games until someone loses a eye! Vref!

Then you get the fun of playing Let's find the Eyeball Let's Design a Vref !"

[mnementh]

argl.
Trip to Reichelt 2 days ago, pulled a portable Scopemeter, but alas it is not booting.
did take a bloody zbox PC along, should not have done it, but, ...

and did have a water leak in the house with the flat that I just sold.
I'm still responsible for that, as I still carry the risk until transfer of ownership is complete.

FML ...
 

   

You get one of the VR GO backpack PCs...? 

mnem
 

[mnementh]

What do you mean I have a problem? I don’t have a problem. You’re the one with a problem.

I only had the MD78 in December. This is just the Fluke meters I own. Doesn’t include some other equipment.

I’ve held off buying vintage stuff. Only because I’m in the process of moving. Once I’m settled in though I’m going old school analog Fluke equipment.

(Attachment Link)

Around these parts that's considered a "starter" set. I have 12 Flukes. 3 are frequency counters, the balance DMM's. 25 pieces of Tektronix gear and that doesn't include plug-in units. I'm a little light on hp gear. Only 4. I need to step up my game here.

So yes, you don't have a problem.   

Hold my beer, let me do a stocktake of my gear. 

So... you say "hold muh beer..." because there's no place to put it down for all the gear you need to count...? 

mnem
"Oh look! 1000 square millimeters of empty bench!!!"

[TERRA Operative]

What do you mean I have a problem? I don’t have a problem. You’re the one with a problem.

I only had the MD78 in December. This is just the Fluke meters I own. Doesn’t include some other equipment.

I’ve held off buying vintage stuff. Only because I’m in the process of moving. Once I’m settled in though I’m going old school analog Fluke equipment.

(Attachment Link)

Around these parts that's considered a "starter" set. I have 12 Flukes. 3 are frequency counters, the balance DMM's. 25 pieces of Tektronix gear and that doesn't include plug-in units. I'm a little light on hp gear. Only 4. I need to step up my game here.

So yes, you don't have a problem.   

Hold my beer, let me do a stocktake of my gear. 

So... you say "hold muh beer..." because there's no place to put it down for all the gear you need to count...? 

mnem
"Oh look! 1000 square millimeters of empty bench!!!"

Horizontal space is a precious and scarce resource here.

[ch_scr]

...
Horizontal space is a precious and scarce resource here.

I guess that's why you like the big-box Tek stuff...
Once you've made horizontal space for one, you can stack to the ceiling, no fear of crushing!
An such variety of instruments in the same case...

[mansaxel]

"Oh look! 1000 square millimeters of empty bench!!!"

I looked at my as of yet unrepaired 5440 scope and comparing it to the cluttered bench the other night, as I was patching up my services after a power cut.

Code: [Select]


while !benchmalloc(sizeof(scope)) {
       printf "repair queue not shortened\n";
       };
printf "scope repair intiated\n";
while (!trace && !trigged) {
       printf ".";
       sleep 1000;
       };
printf "scope repair complete\n";
exit 0;


[mnementh]

   (Thanks, Randall)   https://xkcd.com/208/

Go ahead, make fun of the No Kung-Fu dwagon!!!  I'll make fun right back!   

mnem
I know just enough to be dangerous... or to guess how little I really know.

[TERRA Operative]

...
Horizontal space is a precious and scarce resource here.

I guess that's why you like the big-box Tek stuff...
Once you've made horizontal space for one, you can stack to the ceiling, no fear of crushing!
An such variety of instruments in the same case...

You found my secret! The HP gear is great for the same reason, stack them up like big Lego bricks. haha.
I have found a cheap 19" server rack that is handy for stacking gear in (and on) too.

[Vince]

Nixie DMM update

Weather is crap today, no way I am working outside on the garage construction. So I am spending all day on that DMM...
Just looked at the forecast, decent weather coming back only on Saturday. So, today and the next couple days, I will stay inside, working on the DMM... hopefully I will make some progress !

So I am starting with the basics of course. Power rails and filter caps...

The power rails and many signals are readily available at the back of the instrument, on the track side of the board that holds all the sockets for the card edge connectors, as you noooo doubt saw in my presentation pics. No doubt...

PSU schematic below.

So we have 200V for the Nixie tubes and the Neon bulb indicators.
Then +/- 6V, and +/- 12V.

Full wave rectification, and Zener diodes. Two in series, so that if you tap one only, you get 6V, and if you tap the two of them, you get 12V.

The 200V has lots of ripple, 12Vpp or so, so the cap could do with a replacement but... the Nixie tubes couldn't care less, they work fine regardless. So since it can't cause my problems, I will leave that cap alone for now.

Then the two positive rails... they are good, zero ripple.
However the two negative rails had a lot of ripple. 400mVpp for the -6V, and one volt or so for the -12V rail.

I removed all the Nixie boards so I can get to the PSU board which is hiding in the middle of the instrument, sitting atop of the transformer.
It has 17 terminals and none of them are identified. Fun. They are numbered on the schematic but what is the pinout on the board ? Good luck. The filter caps are vertical cans bolted to the chassis. They are not identified either....

So I spent a bit of time probing things and drawing notes... then I could start troubleshooting the thing.
Tested all diodes, looking good. Tested all the beefy 82R 1W series resistors upstream of the Zener, good too. Good enough at least.
Then scoped the filter caps (3 of them, in different places) of the negative supply. See the screen capture of the main cap, right at the output of the bridge rectifier.
It does not filter anything whatsoever, ie, it's open circuit...
Power off, disconnect one of its wires, test it with the Chinese component tester : "No, unknown or damaged part ". OK.

So I replaced it. It's a 1000uF 50V one. Looked in my stock of crusty old salvaged caps. Found a 1000uF 63V "Jamicon", and it tested good on the Chinese meter. ESR at 10R looked a bit high I guess, but still good enough for a quick test. Better than open circuit anyway...
I soldered it quick an dirty underneath the chassis, hence the DMM laying on its side no choice.
Power up. A few seconds later that cap goes POP !!!!! 
Pulled the power plug quick. Noticed the cap was BURNING HOT ! 
I thought oh, maybe the ESR was too high then....

So I searched my stock / left over of brand new Nichicon low ESR caps. No direct replacement sadly. Best I could come up with was a couple 470uF 63V in //. Good enough.
I tested them first.. and their ESR is 5R ?! Not much better than the one that popped ! 
So I thought well, the old cap must have been defective, who knows.
So I power up. I kept my fingers on the caps to feel if they would heat up like the previous cap.
THEY DID !! It was getting freaking hot freaking fast ! 
So pulled the plug again, didn't want my nice Nichicon to go pop, for sure.

Then started scratching my head.. what the fuck is going on here....

After a few seconds, light bulb moment : it's the NEGATIVE rail, that black wire is NOT supposed to go to the negative terminals of the caps !!!   

Oh dear.....

I let the caps cool down, soldered them the right way around and scoped the result. Looking much better for sure !

Put all the Nixie boards back in place so that the power rails are loaded as they should and I can do a realistic ripple measurement.
Ripple now ? Couple volts on the first cap I just replaced, and zero at the final output.
Voltage levels are closer to nominal as well. -12V used to be -11,4V, now -11.86V
-6V was -5.8V, now -6.0V.

So.. making progress, hopes are high... but no, bloody instrument is not grateful at all, symptoms still 100% the same.

OK maybe it's because of the missing resistor hence missing Vref, on the Vref board.
So, let me cobble some external Vref quick, see if that improves things.

Anyway, testing and fixing the power rails was a pre-requisite, it needed to be done. So I still call that progress, and I can now rule the PSU out.

Stay tuned...

[capt bullshot]

I guess we'll have to invite this guy to the TEA forum now:

[med6753]

Nixie DMM update



Then started scratching my head.. what the fuck is going on here....

After a few seconds, light bulb moment : it's the NEGATIVE rail, that black wire is NOT supposed to go to the negative terminals of the caps !!!   

Oh dear.....

I let the caps cool down, soldered them the right way around and scoped the result. Looking much better for sure !

Trust me. You are not the only one to make that mistake.  Just like having a scope on AC coupling and wondering why the square is distorted we have ALL soldered caps in backwards or not observed polarity. If someone sez they never did they be lying.

[Cubdriver]

Nixie DMM update



Then started scratching my head.. what the fuck is going on here....

After a few seconds, light bulb moment : it's the NEGATIVE rail, that black wire is NOT supposed to go to the negative terminals of the caps !!!   

Oh dear.....

I let the caps cool down, soldered them the right way around and scoped the result. Looking much better for sure !

Trust me. You are not the only one to make that mistake.  Just like having a scope on AC coupling and wondering why the square is distorted we have ALL soldered in caps in backwards or not observed polarity. If someone sez they never did they be lying.

...or they haven't been doing this for very long.  Like taildragger pilots and ground loops, there's them that have and them that will...

-Pat

[med6753]

Your calibrations are only as good as your calibrator source. For scope calibrations, mainly vertical compensation and time base adjustments, go to tool is this Heath IG-4244 Scope Calibrator. So how accurate is it? It's high speed square wave is spec'ed at 1nS rise time. Does it meet that spec? Let's find out. 7904 with 500MHz 7A19 vertical. I had to use a 7B80 timebase on X10 expansion because the 7B92 timebase isn't working. So that X10 expansion will induce some error.



Damn close if not right on. Looks like about 1.2 – 1.3nS. The X10 expansion might play a role here but I'm happy with that.



What about speed accuracy? The manual doesn't spec it but typically it's within +/- 100Hz although sometimes can be out as much as +/- 200Hz @ 10MHz. I did replace the fixed 15pf cap on the crystal oscillator with a variable trimmer cap to bring it within +/- 50Hz if needed. Most scope timebases are spec'ed at +/- 3% so this is more than adequate. Yes, a time mark generator would be a better tool here but currently don't have one.



Some have criticized the IG-4244 for it's characteristic ringing after the trailing edge. I actually regard it as a valuable tool for assessing the high frequency response of scopes with B/W's of 200MHz or greater. As you see the 500MHz 7904 rings like hell. Since I don't have the tools to verify it accurately the ringing gives me a certain level of confidence. Want some proof? Second picture is the 500MHz 7A19 vertical plugged into a 7603 with 100MHz B/W. Very minimal ringing since the mainframe doesn't have the band pass to display it.     

[med6753]

Nixie DMM update



Then started scratching my head.. what the fuck is going on here....

After a few seconds, light bulb moment : it's the NEGATIVE rail, that black wire is NOT supposed to go to the negative terminals of the caps !!!   

Oh dear.....

I let the caps cool down, soldered them the right way around and scoped the result. Looking much better for sure !

Trust me. You are not the only one to make that mistake.  Just like having a scope on AC coupling and wondering why the square is distorted we have ALL soldered in caps in backwards or not observed polarity. If someone sez they never did they be lying.

...or they haven't been doing this for very long.  Like taildragger pilots and ground loops, there's them that have and them that will...

-Pat

Bingo. 

[Saskia]

argl.
Trip to Reichelt 2 days ago, pulled a portable Scopemeter, but alas it is not booting.
did take a bloody zbox PC along, should not have done it, but, ...

and did have a water leak in the house with the flat that I just sold.
I'm still responsible for that, as I still carry the risk until transfer of ownership is complete.

FML ...
 

ok, Reichelt refunded in full for the UTD 1050DL, no discussion whatsoever ...

[Cubdriver]

Nixie DMM update



Then started scratching my head.. what the fuck is going on here....

After a few seconds, light bulb moment : it's the NEGATIVE rail, that black wire is NOT supposed to go to the negative terminals of the caps !!!   

Oh dear.....

I let the caps cool down, soldered them the right way around and scoped the result. Looking much better for sure !

Trust me. You are not the only one to make that mistake.  Just like having a scope on AC coupling and wondering why the square is distorted we have ALL soldered in caps in backwards or not observed polarity. If someone sez they never did they be lying.

...or they haven't been doing this for very long.  Like taildragger pilots and ground loops, there's them that have and them that will...

-Pat

Bingo. 

Ouch!!  Though it could have been a lot worse.  Looked like he was landing in a bit of a crosswind, and put the downwind main on the ground first.  That tends to be a recipe for disaster.

-Pat

[factory]

...snip...

- Components : on the underside, see close up, there are a couple yellow axial components, that are leaky. Not even sure at this point if they are caps or.. maybe batteries/cells (for the voltage reference maybe... will need to look that up in the schematics).

Those crusty axial components look to be wire-wound resistors for the reference circuit, it uses a 6.2V Zener as the reference.


David

A resistor that leaks green stuff ?! 

But... resistors they are indeed !
Looking at the schematic, and the wiring on the board... it adds up. They correspond to the two low tempco resistors.
I pulled one leg on each, to measure them... schematic says they must be 3k and 2.1k ... I measured 1.25k and.. open circuit. That green stuff must have been bad...
Then one one of them, the lead I had slightly lifted... broke off, flush with yellow buddy. So it was definitely the end for it.
I sacrificed it in the name of science. Indeed it's a wire wound resistor in there. The green stuff looks like it's corrosion of the metal caps at each end, not the resistor per se.
I guess the meter must have gathered too much moisture for too long sitting on its arse on the ground of a damp garage... moisture got inside the resistor, yellow rubber not air tight, and then it was the end for poor little resistor.

So I must replace them. Tempco wise, should not be a problem, as schematic reads 1,050 ppm... which is... atrociously crap ?!

1,000ppm when any cheap metal film is 50ppm ? I guess in th '60s 1,000ppm was good, and the Allen Bradley carbon composition were even worse ?...
So the real problem is... to know the value of these resistors because.... I can't trust the schematic 100% seeing as my board has 2 extra resistors that aren't depicted in the schematic. So... who knows if those yellow resistors in my instrument still have the same value that the schematic pretends hmmmm.... 

I will compare with the other meter.

Other problem : looks like the service manual is based on the 'B' suffix model, not the 'D' suffix I have to fix here ! 
Pfff..... that doesn't help. The first 4 nixie boards are the same I am sure, as is the power supply too I would guess but... what about the other board and wiring....

This repair is looking like it's gonna be a lot of fun...

EDIT : I measured the Zener in circuit, powered up. Reads 6.34V. That's 2.2% off. Good enough I guess, the calibration trimmers should have enough range to adjust for that.

Good night....   

You need to slow down a bit, take a bit more time & be very careful next time. 
Could you have disconnected the wires instead? i.e. the yellow one at the top right (in picture below) and the red to the pot. Then measured the each resistor without disturbing the fragile parts with green spot corrosion.


Is there any possibility to find the end of the damaged resistor & reattach it?
Another problem now I've had more time to look at the circuit I found last night, is that is differs a bit from what you have, also the reference feeds two ways, one via the "4000" switch to the attenuator and the other to the comparator board. Really could do with the correct diagram for this variant.


P.S. the capacitor & diodes that supply the +200V for the tubes looks to be a voltage doubler circuit.

P.P.S. the last caps I installed backwards were four Nichicon BT series, I was very disappointed that they survived a 4 hour load test, only getting domed at the top. 

David

[mnementh]

Nixie DMM update



Then started scratching my head.. what the fuck is going on here....

After a few seconds, light bulb moment : it's the NEGATIVE rail, that black wire is NOT supposed to go to the negative terminals of the caps !!!   

Oh dear.....

I let the caps cool down, soldered them the right way around and scoped the result. Looking much better for sure !

Trust me. You are not the only one to make that mistake.  Just like having a scope on AC coupling and wondering why the square is distorted we have ALL soldered in caps in backwards or not observed polarity. If someone sez they never did they be lying.

...or they haven't been doing this for very long.  Like taildragger pilots and ground loops, there's them that have and them that will...

-Pat

Just a few minutes ago did one bass-ackwards of a pair of BFF caps on the output of that old Apple SMPS brick for my 32in Cinema display... and you know the drill: It's a low-profile 1200uF/35V cap which the closest I can get is 470uF/63V in a package that'll fit.

So of course, now I'm pawing through my bin of scrap PCBs hoping I get lucky...

mnem
Been a few years since I did a ground loop, tho. But... I was flying my Cub.

[Vince]

You need to slow down a bit, take a bit more time & be very careful next time. 
Could you have disconnected the wires instead? i.e. the yellow one at the top right (in picture below) and the red to the pot. Then measured the each resistor without disturbing the fragile parts with green spot corrosion.

Sorry mam ! 
Thing is, from the get go I expected to replace these things just based on how crusty they looked, and the green stuff leaking from the terminals... so I was not even trying to rescue these yellow buggers ! 
To me it was like looking at an old cap or battery...if you find one that leaks and is so crusty, it does not even cross your mind to even try to reuse them 

Now obviously rewriting history is easy and had I known what I would find inside these yellow jackets.. yes maybe I would have tried to open them up much more carefully..... next time ?     I will take that as a learning experience... maybe one day I will get another piece of TE that might make use of this experience...

P.S. the capacitor & diodes that supply the +200V for the tubes looks to be a voltage doubler circuit.

Wow, I don't only get moral support on TEA... I also get ACTUAL technical support as well !  Thanks for taking the time look at this thing, much appreciated !  :-+0

Good spot !    I had been trying to make sense of this part of the PSU... and of the fact that I measured only 100V or so at the input, half the expected voltage ! Makes perfect sense now !  Boy it didn't even cross my mind that they could be doubling the voltage...never seen it done before in such circumstances. Transformer is always custom made, so the secondaries are just wired to output whatever it is that you need usually ! 
No idea why they didn't make that winding produce 200V to begin with ! Save on copper ?!... at the detriment of adding more components and cost ?! 
I will keep that at the back of my mind next time I look at an antique power supply... I won't get fooled twice !

Another problem now I've had more time to look at the circuit I found last night, is that is differs a bit from what you have, also the reference feeds two ways, one via the "4000" switch to the attenuator and the other to the comparator board. Really could do with the correct diagram for this variant.

Yes it's been bugging me all day as well... I got carried away when I said I would make a replacement board... I can't do that until I figure out what this second output does, how it interacts with where it's going... 

My board has 2 extra resistors...
So I cracked the other metrer opened, since it is the same model that the manual is based on.
It's Vref board, see below, is 100% identical to the schematic. The resistors on it look a bit more "modern". The two "special" / yellow resistors have been replaced by small plastic cylindrical ones. They are clearly marked. Their value is exactly what the schematic says. The 1,050 tempco is also printed on them.
It's.. perfect.

So I put the two meters/boards side by side and compared them carefully.
I eventually figured the older board out.

To help with understanding, I will call "lower branch" the vertical path that start at the Zener diode anode, and goes downward. That's the branch that produces the 4V ref and goes to the attenuator. Then there is the "upper branch", starting rom the Zener anode also, and that buggers off to the comparator board. That's the mysterious / tricky branch to me 

So. basically, the two boards are 100% identical in topology, wiring, same interface to the outside world, they are interchangeable for all I can see.
All that changes, is the value f the resistors, slightly, and also how what they are made out of (single resistor or a // or series combination).

Both the upper and lower branches are affected :

- Upper branch:  schematic shows 6,7k total resistance, about 50/50 split between that 1% 3.7k resistor, and the 1050 tempco 3k one.
In the older board, the total resistance is still the same, but the split is different : 5,7k for the 1% resistance, and only 1k for the yellow one. Well I think at one point I did manage to measure it and it was about 1k IIRC... and since it would add up here, so let's say it was indeed 1k ! 

- Lower branch : The fixed  1% 3,9k at the very bottom, has been replaced by what you spotted, a // combination of 56k and 3,75k, which measures at 3,5k.
So still ballpark the same as the newer board.
Then the other resistance, the 1050 tempco one, has been replaced with a combination of two R in series : the yellow one in series with a 1,33k resistor. Which means I guess that yellow one which I could never measure, must have been in the order of 1,000 ohms or something (like the other one then), if we want everything to add up...


So.... I feel a bit less confused and scared than I did this morning.. things are clearer now. The mods are minor. They are basically the same boards, interchangeable, same pinout same everything really... just minors mods to how the resistors are "implemented".

What I think :

1) it's clear the lower branch is what generates the -4V ref, so if I make my own Vref circuitry, I need to connect it to this output.

2) The upper branch I still have no idea what it does in this comparator board, nor why it too needs a 1050 tempco resistor. However, what I think at the least, is that the Zenr diode provides a low output impedance 6,2V source to BOTH the upper and lower branches. Hence, to me, the upper branch is not concerned with what's going on in the lower branch, so I could separate the two : disconnect the lower branch and replace it with my own Vref, and leave the upper branch as it is, so it can keep doing whatever it's doing...

3) The lower branch, the ref, I can just replace the dead yellow resistor with a 1k resistor, or trimmer if I get fancy, to restore basic operation. Of course out the window with the 1050 special R, but for now, it should be enough to make the board operate again. It's quick and easy and free to do, so I think that's what I am going to do right now, it's only 10PM I still have a bit of juice and time left before I go snoring.

[Vince]

EDIT : just took some measurements in the newer board, to help me konw what I am doing when replacng the yellow resistors on the older board.

I measured the voltage across the 200R trim pot in the lower branch, that fine tunes the 4V ref.

I have exactly  +/- 0.1V of adjustment range : the upper connection of the pot sees -3,9V and the lower connection sees -4.1
The cursor/wiper does as expected show spot on 4,000V.

Well, 4,001 on my Fluke 11 

So I shall be targeting that...

The 6.2V Zener diode, 55 years old... is at 6,23V   

[Vince]

WARNING - Light bulb moment !!!

I think I got it !!! 

I sussed all my problems out I think ! 

That mysterious upper branch in the Vref board.... it's not a mystery anymore ! It's simply the Vref for the comparator that does the voltage to time conversion, against the ramping voltage !
The 4V ref of the lower branch feeds only the front panel to apply 4V to the input jack when you press the " Test" button on the front panel, to calibrate the full scale of the display with the pot available on the front panel as well. This 4V ref feeds ONLY the input jack, nothing else !

Yes, they use TWO Vref not just one !!! WHY ?! 

To confuse the hell out of me of course...

So it's all clear now.

1) The upper branch is not a mystery any more, it's just ANOTHER Vref, to feed the comparator, the brain of the DMM.
2) Lower branch is 100% independent of the upper one, and feeds 4V only to the front jack.

This has several nice consequences :

1) It explains what these 1050 tempco resistors are for.
2) They are just there most likely to counteract the temperature drift of the Zener, since temperature is the biggest factor that makes any PN junction drift.
3) These two resistors have NOTHING to do with one another. They each do their business in there own respective branch, no more no less. I can treat them independently.

4) and #4..... this means I can easily solve all my problems : now that we know what these 2 branches do, and what these two tempco R are for exactly.... I can simply GET RID OF THEM !!!
I don't need a 4V Vref ! The one and only ref here is the 6.2V Zener, that's all, and hence why the schematic says it's hand picked. The two tempco are there to compensate the drift of the Zener in each branch. So, all I need to do is make a replacement board with a stable 6.2V Vref using a modern chip, which will replace the entire " Zener + tempco resistors "combo, and I can then, therefore... replace these tempco resistors with regular precision stable resistors, available off the shelf ! 

PROBLEMS SOLVED !!!!   

[TaylorD93]

Repairing a Sefelec Electrical Safety Tester tonight at work, i had a slight brain fart.

calibration requires a 300mOhm Load to be tested @ 30A AC.

Left my 2x TO-220 50mOhm load resistors connected (in series) on a tiny bit of scrap copper heatsink. It didnt even complete the 5s Ramp time before i smelt them getting hot, now i have two nearly 300 Ohm TO-220 resistors 


Time for a cup of tea me thinks!

[Vince]

Wow, that's pure genius, how to make do with what you have, fine tune your own 300mohms resistors, well done !! 

[mnementh]

Wow, that's pure genius, how to make do with you have, fine tune your own 300mohms resistors, well done !! 

*launches a volley of soggy ol' boots in the general direction of Château Picard*   

mnem