doel van R13 in B5X85A ? (Techniek Radio/TV)
door Pandu Rajan 
, 31.12.2014, 14:34 (3618 dagen geleden)
Dear Friends
In the B5X85A the detector diode filter is formed by C37,R11 and C38. Diode load resistance is R13-270K connected as the diode load. Since generally the volume control is considered to be the diode load will this resistance R13 not decrease the total effective value of the volume control because in a way the resistance is connected in parallel to the volume control? So what is the purpose of the resistance R13?
Regards
P.Rajan
![[image]](images/uploaded/2014123120495254a46170ce592.jpg)
doel van R13 in B5X85A ?
door Ouwe Schipper , Sassenheim, 31.12.2014, 19:03 (3618 dagen geleden) @ Pandu Rajan
As far as I can see....
The volume control pot has the option of being connected to a tape recorder input (switch SK7)
So it should not carry any of the DC coming from the detector
This R 13 prevents the detector circuit from floating
The only thing that really amazes me is the value of C42; how on earth would low frequencies pass through there??
Nico
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een kinderhand
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doel van R13 in B5X85A ?
door John Hupse † 
, Schoonhoven, 31.12.2014, 20:42 (3618 dagen geleden) @ Ouwe Schipper
Bewerkt door John Hupse †, 31.12.2014, 21:03
will this resistance R13 not decrease the total effective value of the volume control
Yes, it will decrease. The value of the volume control potmeter is 1 Mohm. The value of R13 is 270 Kohm. Together their value is 212 Kohm, which is less then 1 Mohm.
This is done to prevent the radio-signal sounding too loud compared to the signals coming from the pick-up and from the tape recorder.
In the fifties and sixties many radio manufacturers provided a physiological volume control on their sets. This type of control demands well-defined signal levels at the volume control potmeter.
how on earth would low frequencies pass through there
With s simple calculation you should be able to calculate the -3db point by yourself. The time-constant (product of R and C) is 3,3 x 1 = 3,3 ms. So the -3db point is 159 : 3,3 = 48 Hz. This is sufficient low for this set.
doel van R13 in B5X85A ?
door Ouwe Schipper , Sassenheim, 01.01.2015, 09:32 (3617 dagen geleden) @ John Hupse †
No need to teach me about RC combinations
But nobody can predict what manufacture or type of recorder will be used. The input impedance of that object may be anything between 10K and infinite; this has very much influence
N.
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doel van R13 in B5X85A ?
door John Hupse † , Schoonhoven, 01.01.2015, 16:50 (3617 dagen geleden) @ Ouwe Schipper
But nobody can predict what manufacture or type of recorder will be used
Right. So Philips did not predict this. I am sure you already know that Philips assumed that her customers always used the latest Philips products and technologies. Doing so Philips could guarantee that their products worked as promised. Without this assumption this guaranty is not possible.
doel van R13 in B5X85A ?
door Pandu Rajan , 02.01.2015, 04:52 (3617 dagen geleden) @ John Hupse †
Dear Friends
Thanks for the reply.
Regards
P.Rajan