BF198 (cross posted).

.............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.

Reply to
ian field
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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

Reply to
John Larkin

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

Reply to
ian field

Oops,

^^^^^^^

Increases, of course.

Reply to
John Larkin

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...

formatting link

John

Reply to
John Larkin

Thanks - I was hoping for something to download though.

Reply to
ian field

formatting link
has some early transistor books, along with books about radio..

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Reply to
Michael A. Terrell

On Sat, 18 Oct 2008 16:26:48 +0100, "ian field" put finger to keyboard and composed:

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

formatting link
(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

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Reply to
Franc Zabkar

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

Reply to
ian field

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.

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Reply to
Michael A. Terrell

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.

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Reply to
Baron

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.

Reply to
ian field

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.

Reply to
ian field

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 !

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Reply to
Baron

Yeah, $1.95 is a lot of money.

John

Reply to
John Larkin

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