EEVblog #1189 - Ebay $140 6.5 digit DMM Bargain!?

Dave bought a 6.5 digit Phillips PM2534 on Ebay for $140 delivered. Did it even survive the shipping?

#BenchDMM #Ebay #Packaging

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[EEVblog]

Got an email from a viewer without a Youtube account:
Quote:
"The reference in your < US$200.00, 6.5 digit multimeter blog has some really ‘old school’ wizardry in it. Way cool!

If you are interested, I would like to point out a few things lurking inside the LM136:

First its (surprisingly) mostly a Brokaw bandgap cell. Being National Semiconductor, one would have expected it to be a Widler bandgap cell IF it went all the way back to the birth of the bandgap reference, which happened at National.


The Brokaw part of it uses 2 transistors (versus 3 for the Widler) in Q15 & Q16, which are almost certainly laid out with an emitter area ratio of 8:1. If both Qs are operating at the same current, the voltage across resistor R8 will be kT/q * Ln(8), or about 53.44 mV. At room temperature.

This is multiplied up via R7, R8 and R10 by 23X, which is about 1.2292 Vdc. This is added to Q17 & Q18 Vbe connected diodes. Assume Vbe = 0.6354 Vdc and you get a total of 2.5000 Vdc!

The collectors of Q15 & Q16 have equal load resistors and Q7 & Q8 are the input stage of the op-amp that balances the Brokaw cell. Q1 is the ‘power’ shunt pull down regulator. Actually, this is not quite a Brokaw cell (due to patent concerns), so the tail current of the first stage is NOT derived by reflecting the collector currents back down to the current source that feels them.

Instead, a really neat hardly ever talked about ‘Cross-Coupled Cell’ is used to generate a PTAT (Proportional To Absolute Temperature) current at emitter-degenerated Q14.

The flatness of a bandgap over temperature is much better when the collector currents are PTAT than when they are NPO. (Note: the ideal TC is process dependent on the so-called ‘eta’ factor which, if close to 2.0, all is golden).

This CCC (cross-coupled cell = Q10, Q11, Q12 & Q13) has a difference in ‘bottom’ emitter voltages that is independent of the ratio of left to right collector currents! This happens because the Vbe(Ic) for both Left & right show up in the sum of Vbes on BOTH SIDES!

This is so useful, Dave.

For example, if you need a really low input or termination impedance and you don’t want to throw current and power at it, this is the cat’s meow. Zin can be Beta/2 lower than Re (emitter impedance). For example, at 1 mA, Re = 25.7 Ohms, but the CCC can have an input impedance of 0.1 to 0.25 Ohms at the same current.

Here, those bottom NPNs also have an area ratio (probably 8:1 also) and this makes a PTAT voltage across resistor R2, and that flows into a, ready for it?

A Split-Collector lateral PNP! Q9.

That produces an equal PTAT current copy that feeds the tail of the lateral PNP 1st stage transistors, Q7 & Q8. Underneath is the NPN current mirror, Q4 and Q6.

The next neat trick is that the sum of the current mirror transistors’ current is also PTAT, and that is used to feed another NPN current mirror Q3 & Q5 whose job it is to make the collector, and therefore the base currents of 2nd stage NPNs Q4 & Q6 not only equal but also PTAT.

Making everything possible run at PTAT currents keeps all sources of errors essentially constant over temperature, and is almost always exploited.

Finally, Q1 is the final stage of the balancing op-amp, and it operates in the shunt mode, diverting any current in excess of the sum of all the other transistor currents, all of which are running PTAT.

R1 is a ‘pinch-resistor, ’ which gives very high resistance in limited areas but is otherwise abysmal, with wildly varying initial value and horrible TCs. It is used as a fail safe startup circuit and the only reason you can get away with using it is because of the in

[excavatoree]

The "Dave Effect" will make it impossible to duplicate this kind of a bargain for a while.

[ChristopherJohnsons]

I recently read a story about a bike manufacturer from the Netherlands that had massive problems in the US with damaged goods due to shipping. Their solution to the problem was to print a TV on the outside of the package because TVs are expected to be more important than bikes in the US ;) Their returns due to damaged shipments dropped by 80 percent right after that. So you clearly should give any seller from the US the hint to mark the package with "TV inside" or the like to have it shipped without damage :D

[Rob]

Philips - Made in Holland
All those original Philips components... brings a tear to the eye.

[Henchman]

I sell on ebay for a living (mostly used items). There's absolutely no excuse for that packing job.

[kippie]

Dave, dont forget to re-label the back. She's 230V now!

[ramosel]

I inherited my dad's so I had to go take a peek at it... yes, your buttons on the top row are pushed in. Hopefully an easy realignment of the board behind the fascia.

[Agroulinggrwaler]

The reason the packaging was so bad was because it was shipped through the eBay Global Shipping Program where eBay supposedly open up every package to make sure it's the right stuff and repacks it in a much more fragile box... Such a wonderful way to make things easier for consumers.

[trevorvanbremen]

And suddenly, the EBAY pricing of Philips Bench DMMs goes thru the roof!

[kefler]

I think the panels are designed to be swap-able depending on where you want your jacks. If you want rear facing jacks for continuous monitoring through GPIB then you can swap the clean panel to the front and the jack panel to the rear.

[ckm-mkc]

Backlight is burnt out. It's a EL strip and is a pain to replace.... You can get a larger EL strip & cut it down, but IRC it's a little involved (very tight fit) but only the EL strip needs replacing, the HV power supply is fine. BTDT on a similar display but on a power supply. That old Philips stuff is great quality overall, except the backlights...

[jlucasound]

I am surprised that was as intact as it was. If it spent one more day in transit, it would have been destroyed! You got it just in the nick! (Did I say that right? I am in the US and no, I did not box that beautiful unit. I would have put it in a wooden crate! Yes, then shipping would have been much higher.) I am so glad it wasn't damaged badly. Thanks Dave!! Love all your videos.

[Rosscoff]

I think all those loose panels when you open it are a good thing - they make for really easy access to the internals. They also are part of the reason it's light for shipping. A couple of screws and it's all back together rigidly. Nice one.

[gregvioral]

This was our eBay store. We did not pack it like this, we added foam inside with more wrap The eBay global program repackaged this unit when it cleared customs.

[shana_dmr]

Ability to individually calibrate ranges is wonderful, since some old precision meters sometimes required something silly like 1000V DC 30 ppm and even if you never used the range you still had to put at least something around 1kV DC during the cal process.

[bgdwiepp]

It was probably re-packaged by ebays global shipping program - that is unlikely the packaging from the original seller

[BlackEpyon]

I've gotten stuff from China that was better packaged than that. Yikes!
All things considered though, not a bad score!

[tongordebeke]

Dave, You can see that made in the Netherlands stands for perfect quality and ingenious solutions.
And still works perfectly after 25 years.

[Stelios.Posantzis]

7:14: "It's not terrific!" You've got to applaud this sort of ingenuity. Saves on costs, manufacturing, shipping, assembly - I bet you are glad you don't have to unscrew 4 more screws. It should last longer too (as long as you don't plan to bash the thing) and when it eventually breaks it can be recycled and a replacement can be easily made (esp. if you own a 3d printer). One of the cases where use of plastic over metal wins.

[KenWPeek]

The resistance specs are for 4W mode, not 2W mode. Also, the standard may not be exactly nominal-- so the difference between the 4W reading and the calibrated value of the resistance standard is what you should be looking at. The resistance in the leads must be nulled out to check 2W mode.