[FIXED] Schematic required for repair of poorly Polar ToneOhm 700

[PK_LEDMAN]

I'm wondering if anyone has a copy of the schematic for the polar tone ohm 700, I have found copies for the 550 and such but not this 700 version. The 550 is just too different in enough places and not good enough.

After wanting one of these for quite some time, I've come across one, but it needs repairing, someone has already tried by the look of it, but not tried hard enough.
Amongst other problems, this thing has got a burnt out resistor marked circuit ref C29 that I cannot tell what it was, may have been 100R...or 10R....or 1R.... it began with a one anyway....but I don't recognise the coding for a scorch mark, or big gap in the middle.

The current source transistors have also burnt out, but I've fixed that. I'm confident I' can get the thing going again, but I would rather not spend the time tracing the whole circuit out to figure out what else is likely to have blown or work out what the resistor value should be if a schematic exists 

My guess is that one upon a time the last user decided to use this to measure something that was very plugged in and turned on. either shorted out a charged cap or applied the leads to a higher voltage source than this thing can handle.

When I do get this thing going, I shall post a list of all the things that need changing in case anyone else comes across a tone ohm in this condition.

I saw previous posts where people had talked about making test leads and they suggested the waybackmachine, (which is brilliant) but I cannot find a single place to download a manual from....

[PK_LEDMAN]

Looks like I found it....Had a moment of inspiration.

When the link on polar's website on the way back machine didn't work I sort of gave up.

then another post had a link to the manuals, but these were dead, so I just decided to search for those links...probably should have thought of that sooner!

sorry guys

Polar 550 & 700 service manual

https://web.archive.org/web/20060311135708/http://www.polarinstruments.com/manuals/service/Toneohm%20700%20and%20550%20Service%20manual.pdf

[PK_LEDMAN]

So good News, for me anyhow. I did fix the damn thing. so here's what was wrong for anyone who stumbles across a similar fault.

When I got the Toneohm, the mains lead was cut off....Bad sign.
It also rattled a bit....even worse a sign!

So I took the lid off.
R29 was Completly busted, into two pieces in fact (see photo).
So after finding the schematic, I replaced this. But after realising that R29 was responsible for referencing to ground the low side of the Kelvin input - I wasn't hopeful that this was the whole story.
Indeed, there were a few more problems lurking.

After a few quick checks of the +/- 5V rails to ensure there were no shorts I decided to power the ToneOhm Up.
It sprung into life, with a few digits on the LCD.
When I put the ToneOhm into Resistance mode and shorted the input wires, instead of a small current (without looking too much at the schematic I decided I should expect somewhere around 10-50mA on the highest setting), instead I got >300mA.

I set the multimeter to voltmeter mode and discovered that this worked fine reading 1V and 10V correctly when i attached my PSU.

So at least knowing that it wasn't a total write-off, I whipped out the multimeter and found Q2 (BC212L), Q3 (BC212L), and Q4 (BC184) to be shorted.

I also noted that the 5V regulator that should have been fitted would have had a current limit of just above 500mA, but I noted that someone had tried to repair this before me and stuck in a big T0-220 LM7805.
With a current limit of over 1A I deemed this too high, in the event of a fault. so I replaced it with the original part (MC78M05).

Now a little more confident I powered the thing up again......
Well it wasn't any better. a quick scope out of the opamp pins showed that the +ve input was at a negative voltage. Since this pins voltage was supposed to be derived from a pot divider from 5V to 0V I knew this meant that the opamp was shafted.

So not having the right part to hand I replaced this with an OP07 instrumentation amp, didn't have the bandwidth, but I decided it would do.

Hey, presto...It still didn't work....Grrr

This was where I was at fault. The 10 BC212L i ordered from Ebay were not BC212L the were just plain old BC212....different pinout.....In my haste I ordered the wrong things and in my eagerness, I didn't check this when I soldered them in....

with the pads already being desoldered once, I was taking no chances. I put some fresh ones in rotating the devices and bending the pins to match the 212L order.

And it worked!. phew.

TonyOhm (for that is what he is now called) is now working....well it would be if I had a current probe.

The drive output seems to work absolutely fine, I will now need to make a probe to utilise this and hopefully the probe input circuit works......

No rest for the wicked!

[Shock]

Good stuff, you ended up getting 3 different scenarios in one go, check others work, check your own work and check the silicon.

Manuals are here by the way, if you found anything missing upload it.
http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=06_Misc_Test_Equipment/Polar_Instruments

[Mjolinor]

I realise this is late but the current probe is just an inductor. You can mess with lots of different values but whatever you use it seems to work. I just settled on a small ferrite rod about 2 mm diameter and 4 mm long and stuck about 5 turns on there. I do have a proper one but did this just to see what would work so I don't actually use a home made one.

Question now. Does yours have an R100 and does your schematic show an R100. I have had mine for 20 odd years and regularly blow it up, the BC212L being the first thing that goes and I too have stuck BC212 in without noticing.

Also. Is your R32 33 Ohms as stated in the diagram or is it 330 Ohm?

I killed it this time by connecting it to a 40,000 uf cap charged to rectified mains, it let out the magic smoke.

Got it all working again but has faults I cannot fix. 2k resistance on 20k range with the leads shorted, errors also on a couple of the voltage ranges. I reckon I have a resistor off spec but can't find it. How is yours for accuracy?