Did someone replace the external speaker jack with a shorting jack? That would prevent NFB from having an effect.
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Screaming Bright Switch Cap??? - 1974 Fender SF Twin Reverb Master Volume Push Pull Switch
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about 60 or 120 Hz hum, should I be hearing that hum?
The superimposed low frequency makes your signal tops move periodically up and down.
What do you see if you change the horizontal setting (time deflection coefficient) to 10ms?- Own Opinions Only -
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Originally posted by TelRay View Postthx guys for the posts, I have just checked the wiring making sure there was continuity from one point to the next.
additionally made the following schematic to clarify the connections (hope it is clear enough):
If the 820R actually measures 820R and its other end connects to the 100R node, it MUST make a difference if you lift it. Your output signal should be noticeably higher without the 820R.- Own Opinions Only -
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Caveat: In your model/version of the SF Twin none of the master volume lugs must be grounded. Rather the MV sits on top of the 100R (NFB shunt resistor in the PI tail).
If you grounded the MV, this would short out the 100R and kill NFB.Last edited by Helmholtz; 04-25-2019, 06:59 PM.- Own Opinions Only -
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conventionally, the jack is represented something like this (click to zoom in):
With the sleeve part of the jack, which contacts the larger oval, shown in a type of 'shield' symbol
and the tip contact shown as the dot.
Note that if you look at it on-axis, the dot is in the center of the larger circle.
Does that help?
edit: of course, there's the 'other' way to represent a spring-contact phone jack, but that's not what we're going for here.If it still won't get loud enough, it's probably broken. - Steve Conner
If the thing works, stop fixing it. - Enzo
We need more chaos in music, in art... I'm here to make it. - Justin Thomas
MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey
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Originally posted by eschertron View Postconventionally, the jack is represented with the sleeve part of the jack, which contacts the larger oval, shown in a type of 'shield' symbol and the tip contact shown as the dot.
WHITE and GREEN wires connected to the TIP of EXT and SPKR jacks
BLACK wire connected to SLEEVE of EXT jack
sorry guys for the unintended confusion
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Originally posted by TelRay View Postyou are right! thx... in my defense... it was 5:30AM
WHITE and GREEN wires connected to the TIP of EXT and SPKR jacks
BLACK wire connected to SLEEVE of EXT jack
sorry guys for the unintended confusion
We are still left wondering why the NFB does not seem to be working. Helmholtz has made a good point to verify that the 100R resistor is indeed lifting the tail of the PI from ground.If it still won't get loud enough, it's probably broken. - Steve Conner
If the thing works, stop fixing it. - Enzo
We need more chaos in music, in art... I'm here to make it. - Justin Thomas
MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey
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Originally posted by Helmholtz View PostIf the 820R actually measures 820R and its other end connects to the 100R node, it MUST make a difference if you lift it. Your output signal should be noticeably higher without the 820R.
SPKR tip to EXT SPKR tip to 820R to 100R to GND
the 820R is dead on and a new resistor (as the other ones you see in the photo)
Originally posted by Helmholtz View PostIn your model/version of the SF Twin none of the master volume lugs must be grounded. Rather the MV sits on top of the 100R (NFB shunt resisistor in the PI tail)
checked that with the schematic when I was putting the new coax cables on the potentiometer a couple of days ago. here's the schematic:
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the connections tested good:
SPKR tip to EXT SPKR tip to 820R to 100R to GND
You did test the 100R node to ground and measured 100R with your meter? Actually, with the 820R lifted it will measure 100R. With the 820R in circuit it should read about 90R. If you haven't, please do these tests with one clip of the meter on a ground reference. I usually use the chassis to verify that there is a common ground that runs through the circuit, and I don't have to move that lead around - it just stays there for all my checks.
The way you've drawn your schem, it looks like the COAX shield is carrying signal. That's all I'm going to say about that.If it still won't get loud enough, it's probably broken. - Steve Conner
If the thing works, stop fixing it. - Enzo
We need more chaos in music, in art... I'm here to make it. - Justin Thomas
MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey
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Originally posted by TelRay View Postthe connections tested good:
SPKR tip to EXT SPKR tip to 820R to 100R to GND
the 820R is dead on and a new resistor (as the other ones you see in the photo)
yes, that actually puzzled me as I did not understand why were not grounded and went through the 100R that is connected to ground.
checked that with the schematic when I was putting the new coax cables on the potentiometer a couple of days ago. here's the schematic:
- Own Opinions Only -
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Originally posted by eschertron View Postjust to verify:
You did test the 100R node to ground and measured 100R with your meter? Actually, with the 820R lifted it will measure 100R. With the 820R in circuit it should read about 90R. If you haven't, please do these tests with one clip of the meter on a ground reference. I usually use the chassis to verify that there is a common ground that runs through the circuit, and I don't have to move that lead around - it just stays there for all my checks.
100R to ground with 820R connected = 89 Ohm
100R to ground with 820R lifted = 100 Ohm
820R lifted = 817 Ohm
820R to ground (with 820R lifted) = 917R (logical)
Originally posted by Helmholtz View PostWhat is the DC voltage at the 820R/100R node?
Actually measured all 3 nodes wired to the MASTER VOLUME:
Last edited by TelRay; 04-26-2019, 12:08 PM.
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