STC Resistance Capacity Bridge

[Crackle]

Hi
This example of a RC Bridge made by STC was in rather a sorry state when I found it lurking in a pile of scrap at a boot sale a year or so ago.
It came with a Siemens grid bias battery. (which still has 1.25v on some cells)
   
The front panel markings were masked by the degree of corrosion. So I decided to have a go at cleaning it up.
   
   
I scraped all the old paint and aluminium corrosion off and sanded back the best I could to clean metal, and then resprayed it.  
   
   
   
   
This RC bridge looks as if it has had the circuit wiring adapted, possibly because some contacts in the switches were not working properly. I have got the switches working with switch cleaner and repeated operation so would like to wire back to how it should be.
But I am sorry to say that I dont know how RC bridges work or how the circuit should go.
I was wondering if some of you clever people could help me get this little RC brigge made by STC working again as it should.
I have traced out the wiring as it stands at the moment. please see attached drawing.
It would be great to know how to use it and to experiment to see how it performs when working correctly.
   
Oh, I forgot to mention the metal box the unit is fitted in is made very nicely, from solid copper plate, mitred at the corners and riveted and soldered at the seams.
Thanks
Mike

[Pat Pending]

Beautiful work Mike. In essence, the vibrator provides the bridge with a signal, (it could easily be an electronic audio oscillator), one 'leg of the bridge contains the unknown component, and the other leg a component of a known value, basically the calibrated potentiometer is swung through its travel and if the unknown component is in range the signal across the phones will null and that point is read off the scale as being the value.

The vibrator is the weak link with these instruments, does it work? If faulty, they can be replaced with a simple oscillator, should wish to use it. They were and are very useful to have in the workshop, if I get a moment I will trace your layout diagram for interest, but I would say that it looks about right.

Thanks for posting.
Andy.

[Refugee]

That is a neat little portable L C bridge.
I am tethered to a mains socket with the little bridge I already have to hand.
The big Marconi I have got on the bench now looks very impressive and can do ESR to some extent as well as inductance.
It is a TF1313 and was acquired as working I think but it is going to get a service before I fully adopt it anyway.
I will start a new thread for it over the next few days.

[Crackle]

Hi
Thanks for your replies, the problem is if you look at S3, a DSDP switch (or change over switch) intended I believe to switch from resistance measurement to capacitor measurement, you will see that the 2 central common terminals have been bridged together. There are 2 other wires, one from R3 R2 R1 common and one from the switch itself which are soldered together. I cant work out how these would have been fitted to the switch.
I would have thought that for resistance measurement the vibrator, which incidentally is working, would have been switched out of circuit as DC is all that is required. At the moment S3 seems to not do much, there is a slight change in the vibrator when it is changed over from R to C.
A similar thing had been done with S1 which was faulty, it had been rewired and the 2 common terminals had been also bridged. This was simple to rectify as it simply switches from internal battery to external battery.
Another thing which is odd is the 2 small 100pF capacitors C4 on my layout, (at first I thought they were back to back diodes as I have not seen this style of capacitor before) look like they may have been an addition and only seem to be switched into circuit when S5 is in the middle position.
S4 and S5 are on a single 2 wafer rotary switch, S41 and S42 are on the top wafer and S5 is on its own on the bottom wafer. (sorry I forgot to mark S41 on left & S42 on right separately)
Another thing which leads me to believe that some of the wiring has been altered is the quality of the soldering, some wires look like they have only just been tacked on, and there are 2 rubber insulated stranded wires (all the rest are solid copper with black sleeving on) one is connecting C4 to the terminal, and the other is connecting the wiper of VR1 to the ground terminal on the transformer. (sorry I also forgot to identify bottom terminal of T1 primary is grounded)
So at the moment if I connect a DMM on the phones terminals, and a 100 ohm resistor between the terminals marked R, I do get a null which seems to give an appropriate reading on the scale, (vibrator is also still working at this point)
But I dont know what the other variable resistor VR2 is for, it is marked P.F. Bal, could it be power factor, they have used for µµF for pF on the face plate so I dont think it stands for pF. It is wired in series with the 1µF capacitor C3.

I would be very grateful for any help with confirming the circuit.
thanks
Mike

p.s.
What year do you reckon for this instrument, I entered it into the Radio Museum as 1935 (guess) but I now think about the same age as my early KB radios so late 20's  say about 1928. https://www.radiomuseum.org/r/standard_p...r1185.html

[Pat Pending]

Mike.
You note that DC could be used for a resistive bridge, this is true, but as an AF signal is needed for the phones, that's what's used for both the capacitance and resistance ranges, I'm stuck on a project here at the moment and as I've some stc operating manual scans, I'll look for your instruments one this afternoon.

Is there any more information on it?

As that vibrator works, its a gem.

Andy

[Crackle]

Hi Andy, if you can find some official information on it that would be great.
thanks
Mike

[Pat Pending]

I've had a look at the circuit over breakfast and can say that I think you're right Mike, it looks like someone has tried to restore it and got in a bit of a muddle, if you've sketched how its wired at the moment accurately, then the balance pot isn't doing anything on the capacitance range.

The PF control should I believe be used to balance components with bad power factors, even then they had bad wax caps! It was so the null point could be made more defined.

It also looks that the components originated over several decades, confirming that it's been got at.

I also have a book that was written by W.T.Cocking that describes construction of a simple R/C bridge, that may be of interest to some.

Andy.

[Diabolical Artificer]

Mike, the nearest thing I can find in my schematics etc, similar to yours (if you take out the active components)is a Taylor RC bridge, see attached.

Try looking at the Heathkit bridges, should give your some ideas.

Andy.

[Terry]

... if you look at S3, a DSDP switch (or change over switch) intended I believe to switch from resistance measurement to capacitor measurement, you will see that the 2 central common terminals have been bridged together. There are 2 other wires, one from R3 R2 R1 common and one from the switch itself which are soldered together. I cant work out how these would have been fitted to the switch.

The basic function of S3 is to connect the common point of the range selection resistors or capacitors to the appropriate point in the circuit. The bridge between two central terminals is correct.

The soldered joint in mid air seems to suggest that a component has been removed at some time. I wonder if it might have been a preset to adjust the accuracy of the readings?

You've done an excellent job with that drawing but it would be more helpful if it was also converted to a normal schematic - I don't think you would want to work on one of your radios armed only with a wiring diagram, would you?

[Crackle]

Did you find the circuit in your STC stuff Andy?
I have checked the resistors and they seem to be quite accurate. I have also measured the capacitors values but not done any leakage tests as yet. I suspect that  C3 will need re-stuffing, I have come across many of these green TCC caps in my early KB radio restorations.
Yes please, I would be interested in some other simple RC bridge circuits.
I find it difficult to follow the Taylor circuit Andy, but thanks for posting it.
Mike