BF198 (cross posted).

[ian field]

..............and others for AGC use.

The result of numerous google searches has turned up conflicting answers,
I'm trying to find out how the discrete transistor AGC stages work in TV &
radio IF strips.

One answer states that varying the base voltage of the controlled stage
varies its input impedance to the previous stage by varying the B/E bias,
the other states that using variable bias varies the collector current - on
which the stage's gain is dependent.

As hardly any manufacturers still use discrete devices for this purpose
there seems to be very little I can find on this subject, please can anyone
help?

TIA.

 

[John Larkin]

On Sat, 18 Oct 2008 16:26:48 +0100, "ian field"
<gangprobing.alien@ntlworld.com> wrote:

.............and others for AGC use.

The result of numerous google searches has turned up conflicting answers,
I'm trying to find out how the discrete transistor AGC stages work in TV &
radio IF strips.

One answer states that varying the base voltage of the controlled stage
varies its input impedance to the previous stage by varying the B/E bias,
the other states that using variable bias varies the collector current - on
which the stage's gain is dependent.

As hardly any manufacturers still use discrete devices for this purpose
there seems to be very little I can find on this subject, please can anyone
help?

TIA.

Tube radios backed off the DC grid bias of several stages, uisng a
feedback signal from the detector. Negative on the grids reduces
transconductance hence gain.

Transistor radios worked the same way... reducing DC base voltage
reduces collector current and reduces transconductance hence gain.

The first of the cited "answers" is wrong... reducing base voltage
reduces transconductance and gain of this stage, but also reduces base
input impedance, which *increases* the gain of the previous stage
somewhat.

John

 

[ian field]

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:cj4kf49h8e0vih03dcolc8frkfgsgks0sp@4ax.com...

On Sat, 18 Oct 2008 16:26:48 +0100, "ian field"
gangprobing.alien@ntlworld.com> wrote:

.............and others for AGC use.

The result of numerous google searches has turned up conflicting answers,
I'm trying to find out how the discrete transistor AGC stages work in TV &
radio IF strips.

One answer states that varying the base voltage of the controlled stage
varies its input impedance to the previous stage by varying the B/E bias,
the other states that using variable bias varies the collector current -
on
which the stage's gain is dependent.

As hardly any manufacturers still use discrete devices for this purpose
there seems to be very little I can find on this subject, please can
anyone
help?

TIA.



Tube radios backed off the DC grid bias of several stages, uisng a
feedback signal from the detector. Negative on the grids reduces
transconductance hence gain.

Transistor radios worked the same way... reducing DC base voltage
reduces collector current and reduces transconductance hence gain.

The first of the cited "answers" is wrong... reducing base voltage
reduces transconductance and gain of this stage, but also reduces base
input impedance, which *increases* the gain of the previous stage
somewhat.

John

Thanks - any idea where I can find more reading on the technique?

 

[John Larkin]

On Sat, 18 Oct 2008 09:50:26 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sat, 18 Oct 2008 16:26:48 +0100, "ian field"
gangprobing.alien@ntlworld.com> wrote:

.............and others for AGC use.

The result of numerous google searches has turned up conflicting answers,
I'm trying to find out how the discrete transistor AGC stages work in TV &
radio IF strips.

One answer states that varying the base voltage of the controlled stage
varies its input impedance to the previous stage by varying the B/E bias,
the other states that using variable bias varies the collector current - on
which the stage's gain is dependent.

As hardly any manufacturers still use discrete devices for this purpose
there seems to be very little I can find on this subject, please can anyone
help?

TIA.



Tube radios backed off the DC grid bias of several stages, uisng a
feedback signal from the detector. Negative on the grids reduces
transconductance hence gain.

Transistor radios worked the same way... reducing DC base voltage
reduces collector current and reduces transconductance hence gain.

Oops,

The first of the cited "answers" is wrong... reducing base voltage
reduces transconductance and gain of this stage, but also reduces base
^^^^^^^

Increases, of course.

input impedance, which *increases* the gain of the previous stage
somewhat.

John

 

[John Larkin]

On Sat, 18 Oct 2008 18:07:50 +0100, "ian field"
<gangprobing.alien@ntlworld.com> wrote:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:cj4kf49h8e0vih03dcolc8frkfgsgks0sp@4ax.com...
On Sat, 18 Oct 2008 16:26:48 +0100, "ian field"
gangprobing.alien@ntlworld.com> wrote:

.............and others for AGC use.

The result of numerous google searches has turned up conflicting answers,
I'm trying to find out how the discrete transistor AGC stages work in TV &
radio IF strips.

One answer states that varying the base voltage of the controlled stage
varies its input impedance to the previous stage by varying the B/E bias,
the other states that using variable bias varies the collector current -
on
which the stage's gain is dependent.

As hardly any manufacturers still use discrete devices for this purpose
there seems to be very little I can find on this subject, please can
anyone
help?

TIA.



Tube radios backed off the DC grid bias of several stages, uisng a
feedback signal from the detector. Negative on the grids reduces
transconductance hence gain.

Transistor radios worked the same way... reducing DC base voltage
reduces collector current and reduces transconductance hence gain.

The first of the cited "answers" is wrong... reducing base voltage
reduces transconductance and gain of this stage, but also reduces base
input impedance, which *increases* the gain of the previous stage
somewhat.

John



Thanks - any idea where I can find more reading on the technique?

My GE Transistor Manual (7e, 1964) has a few pages on transistor radio
AVC. It's a good reference for discrete transistor design. Maybe
alibris or ebay can turn one up.

Hey, Amazon has it...

http://www.amazon.com/GE-Transistor-Manual-7th-Ed/dp/B000IW9ITE

John

 

[ian field]

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:gfakf49ebfpb7mhvagnobe77fdfuq15llf@4ax.com...

On Sat, 18 Oct 2008 18:07:50 +0100, "ian field"
gangprobing.alien@ntlworld.com> wrote:


"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message
news:cj4kf49h8e0vih03dcolc8frkfgsgks0sp@4ax.com...
On Sat, 18 Oct 2008 16:26:48 +0100, "ian field"
gangprobing.alien@ntlworld.com> wrote:

.............and others for AGC use.

The result of numerous google searches has turned up conflicting
answers,
I'm trying to find out how the discrete transistor AGC stages work in TV
&
radio IF strips.

One answer states that varying the base voltage of the controlled stage
varies its input impedance to the previous stage by varying the B/E
bias,
the other states that using variable bias varies the collector current -
on
which the stage's gain is dependent.

As hardly any manufacturers still use discrete devices for this purpose
there seems to be very little I can find on this subject, please can
anyone
help?

TIA.



Tube radios backed off the DC grid bias of several stages, uisng a
feedback signal from the detector. Negative on the grids reduces
transconductance hence gain.

Transistor radios worked the same way... reducing DC base voltage
reduces collector current and reduces transconductance hence gain.

The first of the cited "answers" is wrong... reducing base voltage
reduces transconductance and gain of this stage, but also reduces base
input impedance, which *increases* the gain of the previous stage
somewhat.

John



Thanks - any idea where I can find more reading on the technique?


My GE Transistor Manual (7e, 1964) has a few pages on transistor radio
AVC. It's a good reference for discrete transistor design. Maybe
alibris or ebay can turn one up.

Hey, Amazon has it...

http://www.amazon.com/GE-Transistor-Manual-7th-Ed/dp/B000IW9ITE

John

Thanks - I was hoping for something to download though.

 

[Michael A. Terrell]

ian field wrote:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:gfakf49ebfpb7mhvagnobe77fdfuq15llf@4ax.com...
On Sat, 18 Oct 2008 18:07:50 +0100, "ian field"
gangprobing.alien@ntlworld.com> wrote:


"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message
news:cj4kf49h8e0vih03dcolc8frkfgsgks0sp@4ax.com...
On Sat, 18 Oct 2008 16:26:48 +0100, "ian field"
gangprobing.alien@ntlworld.com> wrote:

.............and others for AGC use.

The result of numerous google searches has turned up conflicting
answers,
I'm trying to find out how the discrete transistor AGC stages work in TV
&
radio IF strips.

One answer states that varying the base voltage of the controlled stage
varies its input impedance to the previous stage by varying the B/E
bias,
the other states that using variable bias varies the collector current -
on
which the stage's gain is dependent.

As hardly any manufacturers still use discrete devices for this purpose
there seems to be very little I can find on this subject, please can
anyone
help?

TIA.



Tube radios backed off the DC grid bias of several stages, uisng a
feedback signal from the detector. Negative on the grids reduces
transconductance hence gain.

Transistor radios worked the same way... reducing DC base voltage
reduces collector current and reduces transconductance hence gain.

The first of the cited "answers" is wrong... reducing base voltage
reduces transconductance and gain of this stage, but also reduces base
input impedance, which *increases* the gain of the previous stage
somewhat.

John



Thanks - any idea where I can find more reading on the technique?


My GE Transistor Manual (7e, 1964) has a few pages on transistor radio
AVC. It's a good reference for discrete transistor design. Maybe
alibris or ebay can turn one up.

Hey, Amazon has it...

http://www.amazon.com/GE-Transistor-Manual-7th-Ed/dp/B000IW9ITE

John



Thanks - I was hoping for something to download though.

http://www.pmillett.com/technical_books_online.htm has some early
transistor books, along with books about radio..


--
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm


There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

 

[Franc Zabkar]

On Sat, 18 Oct 2008 16:26:48 +0100, "ian field"
<gangprobing.alien@ntlworld.com> put finger to keyboard and composed:

.............and others for AGC use.

The result of numerous google searches has turned up conflicting answers,
I'm trying to find out how the discrete transistor AGC stages work in TV &
radio IF strips.

One answer states that varying the base voltage of the controlled stage
varies its input impedance to the previous stage by varying the B/E bias,
the other states that using variable bias varies the collector current - on
which the stage's gain is dependent.

As hardly any manufacturers still use discrete devices for this purpose
there seems to be very little I can find on this subject, please can anyone
help?

TIA.

Here is the tuner page from the "Circuit Description" for a Philips
KT2A-1 TV chassis:

http://www.users.on.net/~fzabkar/ELC2060.jpg (1.1MB)

ISTR that some manufacturers use positive AGC, others negative, ie
increasing the AGC voltage may increase the gain in some tuners, and
reduce it in others. I'm not sure about this, though.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[ian field]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:wNadnSzMLvYt3mfVnZ2dnUVZ_jCdnZ2d@earthlink.com...

ian field wrote:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message
news:gfakf49ebfpb7mhvagnobe77fdfuq15llf@4ax.com...
On Sat, 18 Oct 2008 18:07:50 +0100, "ian field"
gangprobing.alien@ntlworld.com> wrote:


"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message
news:cj4kf49h8e0vih03dcolc8frkfgsgks0sp@4ax.com...
On Sat, 18 Oct 2008 16:26:48 +0100, "ian field"
gangprobing.alien@ntlworld.com> wrote:

.............and others for AGC use.

The result of numerous google searches has turned up conflicting
answers,
I'm trying to find out how the discrete transistor AGC stages work in
TV
&
radio IF strips.

One answer states that varying the base voltage of the controlled
stage
varies its input impedance to the previous stage by varying the B/E
bias,
the other states that using variable bias varies the collector
current -
on
which the stage's gain is dependent.

As hardly any manufacturers still use discrete devices for this
purpose
there seems to be very little I can find on this subject, please can
anyone
help?

TIA.



Tube radios backed off the DC grid bias of several stages, uisng a
feedback signal from the detector. Negative on the grids reduces
transconductance hence gain.

Transistor radios worked the same way... reducing DC base voltage
reduces collector current and reduces transconductance hence gain.

The first of the cited "answers" is wrong... reducing base voltage
reduces transconductance and gain of this stage, but also reduces
base
input impedance, which *increases* the gain of the previous stage
somewhat.

John



Thanks - any idea where I can find more reading on the technique?


My GE Transistor Manual (7e, 1964) has a few pages on transistor radio
AVC. It's a good reference for discrete transistor design. Maybe
alibris or ebay can turn one up.

Hey, Amazon has it...

http://www.amazon.com/GE-Transistor-Manual-7th-Ed/dp/B000IW9ITE

John



Thanks - I was hoping for something to download though.


http://www.pmillett.com/technical_books_online.htm has some early
transistor books, along with books about radio..

Thanks - it'll take me a while to go through all the stuff I downloaded.

 

[Michael A. Terrell]

ian field wrote:

Michael A. Terrell wrote:

http://www.pmillett.com/technical_books_online.htm has some early
transistor books, along with books about radio..

Thanks - it'll take me a while to go through all the stuff I downloaded.

You're welcome. A copy of a recent ARRL handbook would be a useful
addition to your collection, as well.

I wish that I still had the schematics for the Microdyne 700 & 1620
series receivers & combiners. The AGC system had 14 op amps, was a
linear AGC system, and used a 0 to +5 VDC output from each receiver to
control the combiner. I can't remember how many op amps were on the
video combiner board, but there were well over 30 op amps when the three
units ( 2 700 series, and one 1620 series) were used in the complete AGC
system.


--
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm


There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

 

[Baron]

Franc Zabkar wrote:

ISTR that some manufacturers use positive AGC, others negative, ie
increasing the AGC voltage may increase the gain in some tuners, and
reduce it in others. I'm not sure about this, though.

- Franc Zabkar

That is my recollection as well ! AGC reducing gain is probably the
more common.... Certainly in the interests of stability. Although I
have seen multiple AGC loops in some equipment.

--
Best Regards:
Baron.

 

[ian field]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:mrSdnSgxR8nu8mfVnZ2dnUVZ_ofinZ2d@earthlink.com...

ian field wrote:

Michael A. Terrell wrote:

http://www.pmillett.com/technical_books_online.htm has some early
transistor books, along with books about radio..

Thanks - it'll take me a while to go through all the stuff I downloaded.


You're welcome. A copy of a recent ARRL handbook would be a useful
addition to your collection, as well.

There was a copy on the alt.binaries.e-book.technical a while ago, the yenc
segments decoded into an iso CD file but after writing to a CD and running
the installer it failed with a checksum error.

The CD has various pdf files but looks to fill only a small portion of the
capacity.

 

[ian field]

"Franc Zabkar" <fzabkar@iinternode.on.net> wrote in message
news:cbikf4hun4l1pvsnuokmu86i40absbf55v@4ax.com...

On Sat, 18 Oct 2008 16:26:48 +0100, "ian field"
gangprobing.alien@ntlworld.com> put finger to keyboard and composed:

.............and others for AGC use.

The result of numerous google searches has turned up conflicting answers,
I'm trying to find out how the discrete transistor AGC stages work in TV &
radio IF strips.

One answer states that varying the base voltage of the controlled stage
varies its input impedance to the previous stage by varying the B/E bias,
the other states that using variable bias varies the collector current -
on
which the stage's gain is dependent.

As hardly any manufacturers still use discrete devices for this purpose
there seems to be very little I can find on this subject, please can
anyone
help?

TIA.

Here is the tuner page from the "Circuit Description" for a Philips
KT2A-1 TV chassis:

http://www.users.on.net/~fzabkar/ELC2060.jpg (1.1MB)

ISTR that some manufacturers use positive AGC, others negative, ie
increasing the AGC voltage may increase the gain in some tuners, and
reduce it in others. I'm not sure about this, though.

- Franc Zabkar

Back in the days when I serviced TVs for a living it was rare to get a tuner
schematic unless the setmaker manufactured their own tuners.

In late summer the thunderstorms kept us busy, if the tuner was faulty our
first port of call was the tea chest where we kept the tuners salvaged from
any chassis we scrapped, if that didn't contain the right tuner we'd look in
the next tea chest which contained faulty tuners we'd swapped out - as a
source of spare UHF transistors, as a rule if the picture was very snowy
with barely discernible video content, then the front end transistor was
dead and the noise was from the IF AGC running full whack. If there was a
dull milky white raster it was the LO transistor.

 

[Baron]

ian field wrote:

ISTR that some manufacturers use positive AGC, others negative, ie
increasing the AGC voltage may increase the gain in some tuners, and
reduce it in others. I'm not sure about this, though.

- Franc Zabkar

Back in the days when I serviced TVs for a living it was rare to get a
tuner schematic unless the setmaker manufactured their own tuners.

In late summer the thunderstorms kept us busy, if the tuner was faulty
our first port of call was the tea chest where we kept the tuners
salvaged from any chassis we scrapped, if that didn't contain the
right tuner we'd look in the next tea chest which contained faulty
tuners we'd swapped out - as a source of spare UHF transistors, as a
rule if the picture was very snowy with barely discernible video
content, then the front end transistor was dead and the noise was from
the IF AGC running full whack. If there was a dull milky white raster
it was the LO transistor.

You were lucky that you had any spare UHF transistors to salvage ! 9
time out of ten it was the front end transistor that had died ! Some
times there was a bad mixer. One or two had a bad joint on one of the
coils/chokes where it was soldered to the tin.

Happy days..... not !

--
Best Regards:
Baron.

 

[John Larkin]

On Sat, 18 Oct 2008 20:17:07 +0100, "ian field"
<gangprobing.alien@ntlworld.com> wrote:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:gfakf49ebfpb7mhvagnobe77fdfuq15llf@4ax.com...
On Sat, 18 Oct 2008 18:07:50 +0100, "ian field"
gangprobing.alien@ntlworld.com> wrote:


"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message
news:cj4kf49h8e0vih03dcolc8frkfgsgks0sp@4ax.com...
On Sat, 18 Oct 2008 16:26:48 +0100, "ian field"
gangprobing.alien@ntlworld.com> wrote:

.............and others for AGC use.

The result of numerous google searches has turned up conflicting
answers,
I'm trying to find out how the discrete transistor AGC stages work in TV
&
radio IF strips.

One answer states that varying the base voltage of the controlled stage
varies its input impedance to the previous stage by varying the B/E
bias,
the other states that using variable bias varies the collector current -
on
which the stage's gain is dependent.

As hardly any manufacturers still use discrete devices for this purpose
there seems to be very little I can find on this subject, please can
anyone
help?

TIA.



Tube radios backed off the DC grid bias of several stages, uisng a
feedback signal from the detector. Negative on the grids reduces
transconductance hence gain.

Transistor radios worked the same way... reducing DC base voltage
reduces collector current and reduces transconductance hence gain.

The first of the cited "answers" is wrong... reducing base voltage
reduces transconductance and gain of this stage, but also reduces base
input impedance, which *increases* the gain of the previous stage
somewhat.

John



Thanks - any idea where I can find more reading on the technique?


My GE Transistor Manual (7e, 1964) has a few pages on transistor radio
AVC. It's a good reference for discrete transistor design. Maybe
alibris or ebay can turn one up.

Hey, Amazon has it...

http://www.amazon.com/GE-Transistor-Manual-7th-Ed/dp/B000IW9ITE

John



Thanks - I was hoping for something to download though.

Yeah, $1.95 is a lot of money.

John