I probably paid too much for this Protek 506, but it took my fancy.

[rcrook]

Generally it appears that the talk on the forum classes this as a mid range DMM. I haven't had a chance to verify everything is ok with it but am interested in if you think I paid to much for it? I paid $125 on a local auction site.

When I got it home I found the original receipt in the manual. It was purchased on 13-09-2001 from the Fyshwick Jaycar in ACT for $299. Still alive after 18+ years not too bad in my books.

The RS232 and Sig Gen were the attraction. Plus its age.

[AVGresponding]

Nice looking display.

Be aware the RS232 is NOT isolated...  https://www.eevblog.com/forum/suggestions/protek-506-review/

[masterx81]

I have one... not too bad. It's not that fast, and neither too accurate, but for me has done it's job for a lot of years, and it's still there.

[exe]

I haven't had a chance to verify everything is ok with it but am interested in if you think I paid to much for it? I paid $125 on a local auction site.

It only makes sense to ask if you can return it, or willing to sell .

I've paid ~$125 for my Power Designs power supply, of which $25 was the price, the rest was shipping plus taxes . It's also 110V (or thereabout), and is bigger than I'd like. So, it's almost useless for me, but it was an emotional purchase, and I know that.

As of your DMM, it sounds a bit expensive, but hey, it at least works, unlike my power supply which collects dust . Anyway, if it makes you happy, then why bother about price? And if it's Australian dollars, then I wouldn't worry at all. Currently T&M secondhand market is high, so getting bargains is difficult

[AVGresponding]

I haven't had a chance to verify everything is ok with it but am interested in if you think I paid to much for it? I paid $125 on a local auction site.

It only makes sense to ask if you can return it, or willing to sell .

I've paid ~$125 for my Power Designs power supply, of which $25 was the price, the rest was shipping plus taxes . It's also 110V (or thereabout), and is bigger than I'd like. So, it's almost useless for me, but it was an emotional purchase, and I know that.

As of your DMM, it sounds a bit expensive, but hey, it at least works, unlike my power supply which collects dust . Anyway, if it makes you happy, then why bother about price? And if it's Australian dollars, then I wouldn't worry at all. Currently T&M secondhand market is high, so getting bargains is difficult

Emotional purchases will kick the shit out of your wallet every time! We've all been there...   

Really though, it's a nice dual-reading display, and for most general work you don't need more than 3.5 digits. So maybe it's a bit expensive, but I bet it'll give good service for many more years.

[shakalnokturn]

I have the same model with same accessories, except user manual. I paid less though.
There are some discussions throughout internet on genuine Vs. fake models, I don't know if this is founded.

The display and function range are nice, overall build quality very average, the mA fuse holder is a disgrace.

The function switch was much to stiff to my liking so I replaced or squashed the springs.

I'll have to check but seem to remember there being optocouplers somewhere on the RS232.

[masterx81]

I like the on off button. So much less stress on the selector knob.
With common nimh battery it not last much, but with li ion it last a lot. I use bonai 600mah.
I attach a picture of mine, boughti think 10/ 15 years ago. I think that is genuine.
The serial interface is optocoupled, as seen in photo.


Would be nice to know the calibration procedure. Mine have not lost it's calibration (always the same results with same references) but would be nice to try to increase a bit it's accuracy. I see some pots, but i have a bit of fear touching them 

[EEVblog]

Wow, that brings back memories, yeah 2001 at least.
IIRC it had the stiffest range switch I'd ever felt.

[AVGresponding]

I have the same model with same accessories, except user manual. I paid less though.
There are some discussions throughout internet on genuine Vs. fake models, I don't know if this is founded.

The display and function range are nice, overall build quality very average, the mA fuse holder is a disgrace.

The function switch was much to stiff to my liking so I replaced or squashed the springs.

I'll have to check but seem to remember there being optocouplers somewhere on the RS232.

Yeah my bad, I skimmed through the thread I linked instead of reading it fully   

The creepage distances are pretty laughable though, but if you're just measuring general electronics stuff and not venturing into hundreds of volts territory it shouldn't be an issue.

Another one to add to my ebay watch lists!   

[shakalnokturn]

@ masterx81
Calibration should be easy enough, all the trimmers are labelled.
Start with the MAX134 datasheet and look for Vref, adjust that first using another DMM.
Check / adjust all the others feeding known values close to but less than "full scale".

[masterx81]

After the post i've already tried without touching the vref. I've compared the readings with a good know dmm, and only temperature and dc were out, all the other well in tollerances.. I've fixed temperature, but dcv wasn't possible to set accurate because setting on the low side made worse on high values. So it remains in that way

[NoisyBoy]

What you observed is very common with low end DMMs or older DMMs, where they calibrate using pots.  Due to the non-linear nature of their A/D converter, and with the limitation to only one calibration point with the pot, when the low end is right, the top end might be off, and vice-versa. 

Often, if there's a service manual, the calibration voltage is a single digit voltage for these meters, and not necessarily near the top of a range.  In a lab grade DMM where there are multiple adjustment points, there are multiple zero and gain adjustment for each range.  For example, in DC alone, the Keysight 34401A has six separate gain adjustment across the voltage ranges, and 5 different zero adjustments for DC, for a total of 11. 

It used to drive me crazy trying to optimize the accuracy of a handheld DMM using the service manual, as the high end is always way off spec.  Now I am down to one of two ways:
* You will need a voltage calibrator for the first approach, you first would map out the measured voltage on a graph across the entire measurement range.  Then pick a calibration voltage which will allow both the high and low end to fall within spec.
* Determine the voltage that you care about, pick the point you use most often, then calibrate against that particular point.  So if 3V is what you measure the most, set the calibration to 3V and adjust the pot to give you a 3V reading.  Often we care about <20V DC, you will likely find high end to be off spec, but at least the voltage that matters to you is accurate.

[CDaniel]

What you observed is very common with low end DMMs or older DMMs, where they calibrate using pots.  Due to the non-linear nature of their A/D converter, and with the limitation to only one calibration point with the pot, when the low end is right, the top end might be off, and vice-versa. 

The cause is much more likely due to input voltage dividers using cheap resistors lets say 1% and not using expensive laser trimmed resistors on ceramic substrate ...

[NoisyBoy]

The cause is much more likely due to input voltage dividers using cheap resistors lets say 1% and not using expensive laser trimmed resistors on ceramic substrate ...

Great point!  Thanks.

[Krisby]

If you are interested in selling, I seem to be developing a Protek 506 collection, so I am interested in buying.  I bought one from the USA on ebay, a hardly if ever used one, and recently I've taken delivery of a new one from a firm in Kolkata, India.  I had bought one originally from Jaycar or Altronics, and it was my great leap forward from the little yellow $10 DMM, which seemed like astonishing technology at the time.  Two of my Protek 506s came in a cardboard box which was inscribed with the words 'The "Next Generation" Digital Multimeter'