1953 Arvin 8213 Restoration

[jr_tech]

Great work! congratulations!

I am wondering about this “bias” control... does it set cut off of the CRT? Where is it located in the circuit?

jr

[Crist Rigott]

Originally Posted by jr_tech

Great work! congratulations!

I am wondering about this “bias” control... does it set cut off of the CRT? Where is it located in the circuit?

jr

Here is a section of the schematic from Sams. It is R9. Below it are the Alignment instructions that talk about setting the Bias Control. Frankly, how or why its there is above my pay grade.

[Electronic M]

Originally Posted by Crist Rigott

Here is a section of the schematic from Sams. It is R9. Below it are the Alignment instructions that talk about setting the Bias Control. Frankly, how or why its there is above my pay grade.

It is probably an AGC level control. AGC is a feedback circuit that (if keyed) measures the sync pulse amplitude in the video stages (otherwise it just measures average video amplitude over roughly a frame) and varies the gain (via IF and or tuner grid bias) of the IF and or tuner to hold the video level constant...This compensates for differences in signal strength between stations and holds contrast constant in fading signal conditions (such as fringe or DX conditions).....Basically, it makes the contrast control a set and forget adjustment rather than a keep a hand on it at all times and throttle it adjustment.

[jr_tech]

Originally Posted by Crist Rigott

Here is a section of the schematic from Sams. It is R9. Below it are the Alignment instructions that talk about setting the Bias Control. Frankly, how or why its there is above my pay grade.

Interesting, thanks for posting! I suspect that they are driving the first two stages of the if amp with a different level of agc (through the dc amplifier, v11) than the rf stage (direct from the local/distant switch) to optimize signal to noise ratio at a given total gain. This is just a WAG.

jr

[old_tv_nut]

Originally Posted by jr_tech

Interesting, thanks for posting! I suspect that they are driving the first two stages of the if amp with a different level of agc (through the dc amplifier, v11) than the rf stage (direct from the local/distant switch) to optimize signal to noise ratio at a given total gain. This is just a WAG.

jr

I haven't traced it through, but if what you surmise is the case, it's quite normal for TV AGC design.

This is called tuner AGC "delay." This doesn't mean a time delay, but that the tuner runs at full gain for weak signals so that the noise figure is maximized [EDIT: I meant minimized, of course]. The tuner AGC gain reduction starts only when the IF AGC gain reduction has already had some effect. In the low signal range (I seem to recall, typically below 1 millivolt RF) the IF AGC is active by itself. Then, above that level, AGC gain reduction is also applied in the tuner.

An alternative, which was never used in TVs as far as I know, is to run both the tuner and IF at max gain for as large a range as possible and use a PIN diode attenuator at the input to adjust signal level.

TVs actually get pretty poor noise figures when the signal is strong and tuner AGC is active, but as long as the video signal-to-noise ratio is great enough to make the noise invisible, it's OK.

[Crist Rigott]

The alignment is done.

Here is what the IF curve should look like.



Here is what the initial curve looked like. Notice that the curve looks good but it is "off" frequency.



Here are the individual points after alignment. Not perfect but pretty close.









Then some pics from this morning.