Toneohm 850A short finder by Polar Instruments - a look under the hood.

[PA4TIM]

Those X-rays are really cool. Together with some thermal image cameras a very usable tool for repairing (but a bit out of budget)

For the readers who like to build a shortfinder
http://www.pa4tim.nl/?p=5339  (a Bob Pease design and a Bob Smith design)
I made two shortfinders but I also have a DE5000 and some 6,5 and 7,5 digit meters. The problem in short-finding is that you need to look at the meter and the pcb at te same time. That is why the tone is helpfull.

But when you have a multilayer board a multimeter or other contact-making probe is not handy. An other problem is that some use a rather high current. The inductive pickup like in the Smith model does not have those limitations. The problem with this inductive pickup is that it needs some practice and a careful placement/direction of the probe. But by making serveral probes it can work over a huge current range (I start at 6mA) To overcome the inductive "problems" I made a probe from an old cassette tape player. And that works even better and a lot more easy as the inductive probes.  It is very directional.

An other way could be to use a Hall sensor probe. I tried this (not as a probe for this tracers) but my sensor is not sensitive enough for this purpose.

[Shock]

That is the HP current tracer probe that I have 
An ex MoD new set of HP current probe, logic pulser, logic probe and logic clip cost me £60, but they sell for around £100 these days.

I have two, one for each hand.

[lukier]

Thanks to Fraser , equipped with all the information he provided here, I decided to go the DIY way and make the probes.

For the miliohm probes I bought these cheapies, fairly sharp:

http://www.ebay.co.uk/itm/221771743126

The cable is pretty much just held by strain relief plastic, so I cut cable there, drilled through the strain relief and by pulling the tip I could get remaining wire out. Then cut remaining cable close to the back side of the tip and I've used RG174, poor-man Kelvin soldered there. Coax diameter roughly matches the original cable. After assembling everything I put a drop of super glue in the strain relief and between needle plastic and handle tube. On the other side I used NYS322 connector from Farnell.

Drive probe is just MAS 50 connector from Farnell, fine silicone insulated wires terminated with pin header connectors. This way I will be able to change between small micrograbbers (from logic analyzer), normal hooks and pin headers.

Current sense probe was a bit of long shot. NYS322 connector as before, 0603 1 Ohm resistor there inside, RG174 coax and LQH32CN221K23L inductor from Farnell. It is very small (1210) and I checked it with DE5000 and got similar readings to what Fraser measured.

I still need to read the manual to make sense of the noise this box makes, but the current sense probe test was I think most intuitive and it seems to work well. Now I need to design & 3D print appropriate case for this inductor.

[Fraser]

Hi Lukier,

Nice work 

I will replicate it to make probe sets for my two units.

Thanks for sharing your work

Fraser

[PA4TIM]

Nice done. That is funny, I have done almost the same thing http://www.pa4tim.nl/?p=4207 (but then for use with a multimeter.

I also used RG179 but in my case for a pair of Kelvin clips.
However, for the multimeter style probes I used a pair of old probes from Hirschmann if I remember well. You can scew the points out. I soldered the (lemo multistrand) wire as close to the tips as possible. This works very well. For the "mechanically challenged" person that could be an alternative option.

This kinda semi kelvin probes work fine for most cases and are often much more easy to use as kelvin clips.

Robertz has a video about making real multimeter style kelvin probes. I am rather handy in mechanical things and I can make a lot of things but after watching Robertz making his probes I feel like I'm a mechanical retard

[lukier]

Nice done. That is funny, I have done almost the same thing http://www.pa4tim.nl/?p=4207 (but then for use with a multimeter.

I also used RG179 but in my case for a pair of Kelvin clips.
However, for the multimeter style probes I used a pair of old probes from Hirschmann if I remember well. You can scew the points out. I soldered the (lemo multistrand) wire as close to the tips as possible. This works very well. For the "mechanically challenged" person that could be an alternative option.

Thanks. In my case I wanted to replicate the original Polar probes as close as possible and continue being cheapskate at the same time

Nice write up. I like the splitter box in particular. I want to build something similar for my DE-5000. I have crocodile clips and tweezers adapters, but I also want proper kelvin clips so recently I've ordered these cheap BNC to Kelvin Clips from China and I want to 3D print some kind of adapter from DE-5000 to 4 BNCs (as seen on benchtop HP LCR meters).

Robertz has a video about making real multimeter style kelvin probes. I am rather handy in mechanical things and I can make a lot of things but after watching Robertz making his probes I feel like I'm a mechanical retard

Me too I pretty much just have Proxxon Micromot kit, few hand tools and CNC 3020T that is still "work in progress" (working on my own controller) so every time I read Robrenz posts I'm amazed at the spectrum of the tools he has at hand. And then I've read his post on restoring Tek plugins and there he uses old school font engraving pantograph - mind blown

[Fraser]

With regard to inductive probe casings,..... In the past I have needed nicely shaped probe handles so took a look at the pens sold in the supermarket. There are some very nice looking pens that can be adapted into prone handles. I am not talking about cheap BIC pens, but nicer offerings from the likes of Papermate etc. Some are even shaped to fit comfortably in the hand.

Fraser

[Fraser]

Lukier,

I just measured the dimensions of the inductive probe tip and I must say your choice of Murata SMD inductor looks spot on.

The tip measures 3.3mm x 3.7mm at the point where the inductor is located.

Those dimensions match the Murata inductor well. The original probe appears to have had the probe tip moulded In resin around the inductor. I will likely place the inductor inside a thin wall ABS tube used in modelling. Such is perfect for the task and may be filled with glue or silicone before being mounted in a pen shell.

Fraser

[lukier]

Thanks!

Now the only problem is that I bought the last 3 inductors from Farnell  Even had 2 on back order (it was min qty 5).

The replacement they are suggesting looks like a shielded inductor (full ferrite), not very similar to what I deducted from your x-ray photos. I don't know if this matters much.

On the website they suggest alternative and there is some lead time date, however I got an email from them saying:

Code: [Select]

Thank you for placing an order with us recently. Unfortunately, we are unable
to supply you with the item(s) listed below, and have had to delete them from
your order(s). Where there is a suitable Alternative we have listed this below.
Please accept our apologies for any inconvenience this may cause. If this does
not affect you directly, please forward to the user of this product(s).

Original Product
Product Notification Reason          NO LONGER STOCKED
Farnell Order Code                   9522239
Product Description                  INDUCTOR, 1210 CASE, 220UH
Manufacturer/Brand Name              MURATA
Manufacturer Part No.                LQH32CN221K23L
Purchase Order Number                14/09/15 14.16
Date Ordered                         14/09/2015
Qty Outstanding                              2

There is no suitable alternative for this product.

I think Mouser still has them.

[Fraser]

Thanks, I saw that Farnell no longer have stock 

I ordered 5 from an eBay seller but I am expecting him to refund as he likely just buys Farnell parts and on forwards them.  There is a Murata 1208 part that may be available. I will check. If all else fails, I can find something similar that is nor screened.  I don't see a major issue with using a slightly different inductor value either. This is a non resonant, non critical role for the inductor.

Fraser

[Fraser]

Interesting development..... The eBay seller of the Murata SMD inductor had despatched 5 to me today.

Either he has a different supplier or was holding stock.

I just ordered another 10.

Seller is Petelox. I have bought from him before and he has been reliable

Fraser

[Fraser]

The inductors arrived from Peter Lox. They are the correct bobbin size and spec but have come from Radio Spares. I now have some to experiment with.

Fraser

[caius]

Sorry for bumping this thread.
I bought a Polar Toneohm 850 , I still have to receive it but it will come witouth any probes so I'll probably build them myself rather than buying the original ones from Polar.Is there any pinout of the various connectors in order to know which are the pins to use on the cables?Thanks.

[lukier]

I bought a Polar Toneohm 850 , I still have to receive it but it will come witouth any probes so I'll probably build them myself rather than buying the original ones from Polar.Is there any pinout of the various connectors in order to know which are the pins to use on the cables?Thanks.

Check Fraser's post for photos of the plugs and pinouts:
https://www.eevblog.com/forum/testgear/toneohm-850a-short-finder-by-polar-instruments-a-look-under-the-hood/msg747123/#msg747123

and my post for part numbers:
https://www.eevblog.com/forum/testgear/toneohm-850a-short-finder-by-polar-instruments-a-look-under-the-hood/msg755627/#msg755627

also service manual / schematic. Both probe connectors can accept either Kelvin or Current Tracer probe, only Drive Source is a separate one.

[caius]

OK, thanks, I got it.Looking at schematics of Toneohm 850A (whose PCB should br the same of 850) I figured out pinout for building my cables.I guess the black clip of the drive source must be connected to the pin of J3 which goes to the inverting input of the LM339 OP-AMP while the red one to the pin that goes to the not-inverting input.Obviously this matters when you track the voltage injecting direct current to the circuit and not when you use the non-contact current tracing and the inductive probe since you are injecting alternate current.
I attach part of schematics for the three connector J1-J2-J3.

[mmagin]

3. Signal tracer probe is not a DIY option until its internal parts are identified and understood. The probe is likely a coil on a ferrite core and it is very directional. I may be able to X Ray my probes to see what can be revealed. I bought new probes from POLAR industries for £40 as I wanted good performance and the time to make the probes would have exceeded the new parts purchase price. If I can reveal more about the probes contents, I shall post it in this thread shortly.

Ooh!  This sounds very similar to that 1970s hp current tracing probe that was introduced along with the pulser and logic probe.  (Which goes for too much on eBay)

[caius]

I received my Polar Toneohm 850, sadly the case came damaged and cracked.Anyway the PCB is intact.I'm struggling to find the correct voltage jumper setting for my country (I'm in Europe so 220V), now the unit is set to 120V.I removed the two 120V  jumpers (B----D and A----C)  and close the 240V one (B----C) but unit doesn't power up at all.I can't find the service manual of model 850 but only the one of 850A which has different PCB.

[caius]

OK, the unit is faulty (good bargain..), I don't know if this depends from the damage occured during shipping but it seems strange.PCB inside is intact, no sign of cracks.From my first check I  could ascertain that no voltages come out from main transformer 'T1' both in 120V and 220V line selected.
Are schematcis available for this model? 850A ones are different and 'T1' transformer changes accordingly the serial number.

[Fraser]

The 850 has a combination of metal and fibreglass (?) case. Sorry to hear yours got smashed. It should be possible to repair the resin though. Then tidy up with a respray with car paint.

Please PM me your email address and I will see if I can supply the 850 service manual. I think I have it in my archives 🙂

Fraser

[Fraser]

Yep, I have the standard 850 (non A) service manual and schematics.

I will send once I have your email

Fraser

[caius]

The 850 has a combination of metal and fibreglass (?) case. Sorry to hear yours got smashed. It should be possible to repair the resin though. Then tidy up with a respray with car paint.
Please PM me your email address and I will see if I can supply the 850 service manual. I think I have it in my archives 🙂

Fraser

Hi and thanks for replying.I already put pieces together with a superglue, damage (pictures attached) was not wide.But this has few importance, the main problem is that the unit doesn't power on while seller told me it did and sent me also a picture of the display ON.I can measure only 1VAC on the two secondary  of the main 'T1' transformer, I think this is the culprit.One of the output goes, after being rectified by a bridge, to the 7805 regulator whch has on his input only 0.5VDC obvioulsy, that's clearly too few for regulation Therefore all the logic inside is not powered.

[caius]

Yep, I have the standard 850 (non A) service manual and schematics.

I will send once I have your email

Fraser

I PMed you my e-mail.Thanks again for the help.

[Fraser]

Caius,

I cannot remember whether the transformer is PCB mounted. If so, it is not uncommon for the weight of the transformer to snap a winding connection where it attaches to the PCB pin. I have repaired such failures with a bridging wire between the pin and remaining end of the wire. The primary winding is the most fragile and so most likely to snap.

Thankfully the transformer is not too unusual if it has to be replaced. It has three secondary windings that feed three 5V regulators. You might need two replacement transformers to get the three isolated windings though. I suspect a 9V-0-9V and a 0-9V would be a good match.

Fraser

[Shock]

If you do not have a PolarCare contract there is an admin fee of GBP 50 for electronic copies of operator manuals and GBP 495 for service manuals.

Did you read that on their website, what a joke.

[Fraser]

Caius,

I have sent you the 850 user manual as well.

Fraser