All about resistors...

I've managed to mainly fix a Megger 1502 MFT that was so far out of calibration it got laughed out of a tester day.
I have high current loop. rcd test currents, and low resistance within spec now. (I can share more info about this process separately if anyone's interested, it's been an interesting learning curve).

IR results are just about within +-2% +-2 digits. To get them more accurate I need a 5M resistor with lower than 5% tolerance. I struggled to find anything cheap and these results were obtained using a 3M and 2M in series, they both have 5% tolerance and overall it's not quite good enough.
So the first easier question is any recommendations for better resistor accuracy, hopefully without spending a small fortune. I'm wondering if adding a trim pot is a good idea.

The 2nd question is more complex and regards the low-trip loop calibration which is the last real hurdle and may prove to be the brick wall I hit.
Part of the test passes 25A current between Live and Neutral as a basis for further maths.
Part of the calibration involves adding 10 ohms to the Neutral path and storing the result, and adding 10 ohms to the earth path and storing the result. No problem so far.
But then adding 10 ohm load to the Live path....On the face of it this would seem to need a resistor rated at 5.29kw ?! Or am I getting something basic very wrong? Obviously this is lethal enough anyway and I will admit defeat and draw stumps if I can't be sure what is required here.
Thanks for any thoughts.

Excerpt from the service manual:

timhoward said:

I've managed to mainly fix a Megger 1502 MFT that was so far out of calibration it got laughed out of a tester day.
I have high current loop. rcd test currents, and low resistance within spec now. (I can share more info about this process separately if anyone's interested, it's been an interesting learning curve).

IR results are just about within +-2% +-2 digits. To get them more accurate I need a 5M resistor with lower than 5% tolerance. I struggled to find anything cheap and these results were obtained using a 3M and 2M in series, they both have 5% tolerance and overall it's not quite good enough.
So the first easier question is any recommendations for better resistor accuracy, hopefully without spending a small fortune. I'm wondering if adding a trim pot is a good idea.

Click to expand...

timhoward said:

The 2nd question is more complex and regards the low-trip loop calibration which is the last real hurdle and may prove to be the brick wall I hit.
Part of the test passes 25A current between Live and Neutral as a basis for further maths.
Part of the calibration involves adding 10 ohms to the Neutral path and storing the result, and adding 10 ohms to the earth path and storing the result. No problem so far.
But then adding 10 ohm load to the Live path....On the face of it this would seem to need a resistor rated at 5.29kw ?! Or am I getting something basic very wrong? Obviously this is lethal enough anyway and I will admit defeat and draw stumps if I can't be sure what is required here.
Thanks for any thoughts.

Click to expand...

I would have thought that the measurement is done very fast.

mainline said:

would have thought that the measurement is done very fast.

Click to expand...

That is one of the many competing thoughts I’ve had.
At the moment the highest rated resistor I have is 300w. One thing is for sure - if I eventually go for it I’ll film it!

timhoward said:

That is one of the many competing thoughts I’ve had.
At the moment the highest rated resistor I have is 300w. One thing is for sure - if I eventually go for it I’ll film it!

Click to expand...

Look at the current rating

mainline said:

Megger 249000 CALIBRATION RESISTOR 10 Ohm | OMNICONTROLS

CALIBRATION RESISTOR 10 Ohm, current rating 0.001A

www.omnicontrols.com

Look at the current rating

Click to expand...

Interesting! Thanks.

timhoward said:

Interesting! Thanks.

Click to expand...

It's meant for ohmmeter, so probably no good for what you want, but it looks massive compared to the current rating.

mainline said:

I would have thought that the measurement is done very fast.

Click to expand...

Well, you were spot on! Nothing exciting happened at all! The resistor wasn't even warm afterwards.
Thanks also for the link to the 5M resistor - now ordered.

I'm pleased to say that I've now got the No Trip loop test working as well as it ever does:

and with 50 ohms added using the check box, fairly close:

and finally the High Current test is exactly matching the calibrated Metrel:


I believe it's now close enough to not instantly be rejected at a tester day.

I've now moved onto the next challenge - a long-abandoned 1730 that gives very low loop readings.
There are several easy ways to get a 1500 series into calibration mode. None of them work on a 17xx.
Last night I finally found a way using the service port and a PC connection. This was a cause of great celebration.
I'm starting to think a "Hacking Meggers" thread in a less public area of the forum might be of interest to the small select number of crazy people like me on here who can't bear to not know how something works...

I'm interested and I don't have a Megger MFT. I'm up for a hacking thread, not just for Meggers but all sorts of equipment.

I would be interested too. I think the last thing I hacked was to use a hex editor to remove the reprogramming lock on a two-way radio.

Same here, I don't like to be shut out from my own equipment. However, with my present health situation, it's unlikely I'll return to using an MFT in anger and the separate testers I've got are still relatively new and little used. Any hacking will therefore be for entertainment only.

It has been quite widely publicised that certain brands of electronic test equipment such as scopes can be hacked to higher spec with modest effort, because the lower models are mainly hobbled in configuration. Because my gear tends to get used on high-pressure, high stakes jobs, I'm not keen on anything with any question marks hanging over it. I tend to get the best and look after it and the quality pays for itself in the long run.

Here's a functionality upgrade hack for all you Wurlitzer home organ owners out there with lazy left hands, with late 1970s models e.g 950 and 630 that have the 'Programmed Rhythm Orchestra' using the General Instruments 141099 LSI. You'll be familiar with it producing major, minor and 7th chords in the accompaniment, but the IC itself can also generate minor 7ths when selected by a pin that is not connected on the PCB. Simply replicate the circuit used on the other three select pins, extend the key decode matrix D17-D47 to accommodate seven new inputs, and connect them to vacant lower manual keys of your choosing; then you can have minor 7ths in the accompaniment too. Of course, if you are a proper organist you'll have no interest in this function anyway, but don't say I'm not generous with creative tips.

For the loop impedance test I would expect the tester limits things to no more than 50V for safety, so 250W max, and even then probably only a low duty cycle. You can get 1% power resistors at not too stupid prices such as this:

You probably have them now, but if in doubt these are 5kV rated and 1%: