Lat week I asked about removing the potting compound from a 425E Network
block in a Western Electric 500 telephone. I have no successfully repaied
this unit, so I wil ldescribe what the problem was.
On my line simulator, the phone worked properly apart from the fact that
the microphone (mouthpiece, transmitter, whatever) didn't do anything.
Checks shows 0V between the terminals (its a carbon microphone, of
course, so you'd expect a standing DC votlag here). Also, the votlage
drop across the phone when off-hook (the simiulator provides a constant
line current) was much lower than I expected.
With thr telepghone unplugged, I could measure 22 Ohms (or so) between
the microhone cup contacts with the microhopne removed. The same
resistance was found between the R and B termainasl on the Netowork, even
with the wire on the B terminal disocnnected. And yet agai nthe same
resistance bwtween R and B witl all the wiring disconnectd from the Network.
There's a 22 Ohm resistor inside the Network with one end conencted to
the B terminal (and nothing else). Therefore I suspected a short from the
other end of this resistor to something connected to R.
at this point I decided to open up the Network, hence by posts last week.
After getting rid of that potting compound, I found tha, yes, the 'other'
end of that resistor did terst as a dead short to the R terminal. A bit of
careful inspection showed that one of the enamelled transformer wires
connected to the 22 ohm resistror passed over the lead of a 68 ohm
resistor conencted to R, and that there was a short there.
No, I didn't rewind the transformer. I desoldered the 68 Ohm ressitor
lead and put a bit of sleeving on it. Tests showed the resistances
between the Netowkr module terminals were now more reasonable, and after
reassmbling the phone it works fine.
Yes, I _could_ have bought a repalcement Netwrok block, but this is much
mroe fun :-)
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