BF198 (cross posted).

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?

Oops,

^^^^^^^

Increases, of course.

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...

John

Thanks - I was hoping for something to download though.

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:

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

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.

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.

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.

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 !

Yeah, $1.95 is a lot of money.

John