Stromburg-Carlson zener diode location

[wheelhorse]

Hello,

I have a Stromburg-Carlson Model 1543 I've been playing around with.  I currently have it attached through an OBI100.  Because the OBI doesn't recognize DTMF, I've ordered and installed a rotatone.  On hook voltage shows around 45v, but off hook voltage drops to 3.8-4.5v which causes the obi to behave erratically.

I've been reading that this can be due to both low voltage output at the obi, and the carbon receiver fitted in this phone.  When I remove the receiver the voltage off hook goes up closer to 5, and the obi seems to work more reliably.  My understanding  is that a zener diode should help keep the voltage from dropping below the correct amount, but I'm trying to figure out where I should install it.  I THINK it's supposed to go between the #6 terminal and the receiver.  I'm not completely sure however.  Perhaps someone with more experience with these has some insight?

Thank you.

I've attached the schematics for the phone

[tubaman]

A Zener can clamp the voltage lower if it is higher than you need, but it can't stop it dropping below what is available.
I don't think this will be the answer here - you need to find why the phone is pulling the line voltage so low.
I'm a bit puzzled that you mention a 'carbon receiver' as I've only ever come across carbon transmitters.
Do you mean the microphone (transmitter) or the earpiece (receiver)?

[wheelhorse]

Sorry, I meant transmitter.  My brain was still trying to start this morning. 

As per the rotatone site:

"Rotatone needs about 5 volts minimum across the Orange/Red and Blue wires in order to operate correctly
and reliably. If the phone is fitted with an electronic transmitter, this level of voltage will be present in all
circumstances of line current, as the electronics of the transmitter unit will insert a resistance into the line in
order to obtain this level of voltage for the operation of its own electronic circuits. If a carbon microphone
is fitted, it is possible that the low transmitter resistance may cause the voltage to fall below the 5 volt Rotatone minimum. A simple solution to this potential problem is to either fit the phone with an electronic transmitter, or to install a 5.1 volt Zener diode anywhere in the telephone series circuit (with due
consideration given to polarity.)  Invariably the easiest place to put the 5.1 volt Zener diode is between the wires that were previously connected to the pulsing
contacts on the dial"


I had unhooked the microphone from the handset and checked the voltage, and the voltage does raise up to more acceptable levels.  I could fit an electronic microphone and see if it makes the operation of the rotatone more reliable.

[tubaman]

I'd try the electronic mic route first as I can't understand how the zener is supposed to work in this context.

[wheelhorse]

I wonder if it's supposed to clamp the voltage through the microphone.  I haven't checked the resistance across the mic yet.  I should mention, the earlier voltage readings were taken at the tip and ring  after the hook, not on the microphone. 

I'll maybe stop at a thrift store later and see if I can find an older phone for parts.