[Unigor 6e repair] Connection point for wire

[giosif]

Hi,

I am trying to repair a Unigor 6e meter and, upon disassembly, I noticed this black wire which, on one end, is connected to a potentiometer w/ on/off switch (the cable connects to one of the switch terminals of the potentiometer).
The other end of the wire, though, is not connected and I could not spot any solder traces on the board which might indicate the initial location this other end was connected to.
Would anyone have an idea where this cable is supposed to go?

Thanks!

[coromonadalix]

schematic ?

https://elektrotanya.com/metrawatt_unigor_6e_analog_electronic_mm_1973_sch.pdf/download.html

[giosif]

Thanks!

I tried using the complete set of schematics (the one you link is only a part of it, BTW), but I couldn't determine if that wire is supposed to be there in the first place (a bit strange the one end which is still connected has been soldered to the pot terminal this way vs. somewhere on the board) and, if so, where the other end should go.
Initially, I thought it might go to R35 on the next board, but that has its own inter-board link.

[m k]

It's black, other one is red, my guess is GND.
Shape points to bit right where seems to be a hole.

When bottom slider is C~ the N-E knob is always grounding R32.

[giosif]

It's black, other one is red, my guess is GND.

Fair comment, although the fact that they use battery + as GND in the schematics makes it all confusing (and, in that case, our cable is not connected to GND).

Shape points to bit right where seems to be a hole.

That was my initial thought as well, but the hole is to allow one of the terminals of the analog meter (or "dial"? not sure what to call it exactly - it's the "thingy" with the moving needle) to poke through when the board is seated in its intended position.
And that particular terminal of the meter/dial is not showing as connecting to GND in any of the schematics I've seen so far.
This said, the cable might still be supposed to go through that hole, but connect to something on the other side of the board, rather than the terminal.
However, given the length of the cable, there are not that many other locations on the other side of the board where it could connect and there are no visual clues that a cable has ever been soldered to any such potential locations.

When bottom slider is C~ the N-E knob is always grounding R32.

I am not sure what you mean with "N-E knob"?

[m k]

0/VA/RC control knob, its S19 part connects R32.
Assuming it's the one where black wire is connected.

[giosif]

Apologies for not seeing the potential obvious, but in what schematics does the 0/VA/RC control knob have an S19 part and where is R32 being grounded?

[Andy Watson]

The control/dial appears to be "RC-just" - R72 in the portion of the schematic that I've attached. This is ganged with switch S10. Looking at the photo of the PCB: the three terminals to the west of control I would guess are the variable resistor part (R72), the terminals at the south and (presummably) north are probably the switch (S10). Your black wire connects to one side of the switch. So the loose end connects to either the ground of the amplifier circuit (R76, R73, R72, B1, etc.) or the negative of the battery. Note, if it is the latter, you still need to pick-up a connection to C23 and C19.

What is the feature that I've circled in red? Is it just a trick of the camera or could the wire have come from that point? It looks very much like a ground-plane connection.

[m k]

Apologies for not seeing the potential obvious, but in what schematics does the 0/VA/RC control knob have an S19 part and where is R32 being grounded?

I'd say it's not exactly obvious.
It's the principle diagram page from above and right of RC-just square there.

But to be rational it should be a trimmer.

[giosif]

The control/dial appears to be "RC-just" - R72 in the portion of the schematic that I've attached. This is ganged with switch S10. Looking at the photo of the PCB: the three terminals to the west of control I would guess are the variable resistor part (R72), the terminals at the south and (presumably) north are probably the switch (S10). Your black wire connects to one side of the switch. So the loose end connects to either the ground of the amplifier circuit (R76, R73, R72, B1, etc.) or the negative of the battery. Note, if it is the latter, you still need to pick-up a connection to C23 and C19.

Thanks for the details!
The three terminals of the knob are indeed for the variable resistor R72 and the north and south terminals are for the switch S10.
The south terminal of the knob where the soldered end of the black wire is connected goes towards R76, R73, etc.
Problem is, there are a lot of components connecting to the left side of S10 in the schematic and finding which one(s) the black wire is supposed to go to is not straightforward.

What is the feature that I've circled in red? Is it just a trick of the camera or could the wire have come from that point? It looks very much like a ground-plane connection.

I checked all solder joints around the knob under the microscope and none appeared to have had a something like a wire previously connected to them.
I think it is just the way that solder connection appears in the photo.

I'd say it's not exactly obvious.
It's the principle diagram page from above and right of RC-just square there.

But to be rational it should be a trimmer.

In this case, I am guessing you meant to say S10, rather than S19.

[giosif]

An update on this: given I couldn't determine where the black wire was supposed to go, I decided to remove it altogether (I can always solder it back, if I eventually identify its purpose).

In terms of the overall repair, I started looking into the different circuit blocks, beginning with the square wave generator.
And it seems someone had previously played with pot R63, which controls the frequency of the wave generator.
I adjusted the frequency to be close to 205 Hz, as per the documentation, and the meter started showing signs of life.
On a quick check, most of the functions seem to work and are fairly accurate.
There is some odd behaviour here and there (e.g. with the + - AC slider set to +, if I set the rotary range selector to one of the k\$\Omega\$ ranges, the measuring needle goes all the way to the left; with the slider set to - or AC, it doesn't do that), but that may be just me being unfamiliar with this type of meter.

[m k]

I'd say it's not exactly obvious.
It's the principle diagram page from above and right of RC-just square there.

But to be rational it should be a trimmer.

In this case, I am guessing you meant to say S10, rather than S19.

No, principle circuit is a bit different.
There S10 is left of RC-just, between and below of other two squares, C26 is also a new thing.

R32 of circuit diagram is on low part of indication section and next left of 2nd lowest of 0/VA/RC set there.
So black wire would be uF/nF to GND, a bit right of the previous.

[giosif]

No, principle circuit is a bit different.
There S10 is left of RC-just, between and below of other two squares, C26 is also a new thing.

R32 of circuit diagram is on low part of indication section and next left of 2nd lowest of 0/VA/RC set there.
So black wire would be uF/nF to GND, a bit right of the previous.

Ah, I see what you mean now: the other part of the schematics (see attached).
Ok, so what you were saying in your first comment is that the black wire is related to the part of S19 which connects to GND in those schematics?

[m k]

Yes, but a bit upside down since it's a battery positive.

Also, can't find that C26 from the other page.

[giosif]

Yes, but a bit upside down since it's a battery positive.

May I ask how exactly you know the black wire is supposed to connect to S19?
Have you seen that connection in another Unigor 6e?

Also, can't find that C26 from the other page.

In other schematics, I have seen that capacitor marked as C9, instead of C26.

[m k]

I'm just guessing and all other PCB pictures I've seen are from those back cover removed and still stacked machines.

Principle circuit has clear connections for that knob.
Maybe you can check what is missing.

Principle circuit's S17 is clearly electronic circuit's low right.
Part of S18 could be between R73 and R76 in the middle of electronic circuit.

Electronic circuit's uF/nF near R32 is clearly the whole width of S.
So trimmer and switch on top must be something else.

[giosif]

I'm just guessing and all other PCB pictures I've seen are from those back cover removed and still stacked machines.

Ah, ok.
Yeah, I also have searched for photos of disassembled Unigor 6e meters but, like in your case, the one's I've found only showed the entire stack of PCBs, while the board we are discussing here is at the bottom of the stack. 

Principle circuit has clear connections for that knob.
Maybe you can check what is missing.

Principle circuit's S17 is clearly electronic circuit's low right.
Part of S18 could be between R73 and R76 in the middle of electronic circuit.

Electronic circuit's uF/nF near R32 is clearly the whole width of S.
So trimmer and switch on top must be something else.

Apologies, but I find this hard to follow.
In any case, per my update earlier, the meter seems to be mostly functional now, even with that wire entirely removed.
I might decide to disassemble the meter once again at some point in the future, to replace any components which might be out of spec (e.g. electrolytic caps, Allen Bradley resistors).
And if, in the meantime, I can get a confirmation as to where that cable is supposed to connect, I will solder the wire back.
Thank you for your help!

[m k]

Ok.

My guess would be only for AC capacitance something anyway, so not very crucial.