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-11 vdc at point j ? - Printable Version

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-11 vdc at point j ? - don hinkley - 09-17-2018

1929 Philco model 76 filter caps replaced, resistors checked and replaced.  While trying to repair I discovered that the 3872 push pull input
 transformer was open, I ordered and tried to install a replacement transformer a stancor A-53 OR PT-156 from AES . On my radio , point
J where the old transformer was soldered in has a dc voltage of -11 using the chassis as a meter ground point. Using the original wires from 
the old transformer and the new wires  hooked together.
 
1.red wire  (power) -11 volts 
2.blue wire to 27 1st af plate,   Did not hook up
3.green wire #1 to 45 plate 2nd af
4 green wire #2 to 45 plate 2nd af 
5 black wire  to screen grid of 27 1st af.     this wire is a center tap in new transformer between green wires.
 
the radio now has sound from speaker, not good or loud sound but mush mouth understandable sound.
The question is this minus dc voltage normal?  The voltage comes from the power transformer, hv centertap and goes through lug #2 of the
filter caps on the negative side. when changing the caps I used mylar caps NOT electrolitics.
    I am using the philco service manual for schematics and information.  Variac and amp meter also in use. 
                                                                                                       Don

RE: -11 vdc at point j ? - morzh - 09-17-2018

There is no negatives in this radio with the sole exception of the power transformer's centertap (rect negative).

Is your speaker's field coil ok?

RE: -11 vdc at point j ? - Radioroslyn - 09-17-2018

Hi and welcome,

>1.red wire  (power) -11 volts 
2.blue wire to 27 1st af plate,   Did not hook up
3.green wire #1 to 45 plate 2nd af
4 green wire #2 to 45 plate 2nd af 
5 black wire  to screen grid of 27 1st af.     this wire is a center tap in new transformer between green wires.
 
Red wire should have abt +150vdc. If not as Mike mentioned check the field coil in the spkr. Simple test is to unplug the spkr w/ set OFF and use an ohm meter to test the resistance across the 4 pins on the spkr plug Any combination of two pins should show less than 2 or 3k. If much higher coil is bad.

Blue wire is the plate lead but it won't work if you don't connect it.

Green wires connect to the control grid (1 ea) of the 45 tubes, not the plates.

Black wire connects to B which is the center tap of the hv winding. 27 tube has no screen grid as it's a triode. The original transformer has a center tap too.

GL

RE: -11 vdc at point j ? - don hinkley - 09-17-2018

guys, checked my speaker field coil out and sure enough, not good, one pin was almost 4k ohms. the green wires are in the
correct position and the black should be on the other side of the 500k resistor. as you have guessed my tube theory is 0
so now to find a usable field coil, A.E.S. I guess. but still my - dc must go. thanks again.
Don

RE: -11 vdc at point j ? - Radioroslyn - 09-17-2018

Opps I made a boo boo.
Did a little research on the G spkr to find that  the coil is abt 3.2K. So your is probably ok.
Unfortunately the schematic is somewhat difficult to read, lots of wire crossing over each other
but can't tell if they are connected or not.
Is the screen grid voltage on the 24a's ok??