Can't see the wood for the trees...? Hallicrafters S20 AGC. - UK Vintage Radio Repair and Restoration Discussion Forum

ms660 · 14th Jul 2020, 4:42 pm

Anyone know the purpose of R13 in the schematic in the link below?:

http://www.nostalgiaair.org/pagesbym...6/M0008856.pdf

Its value is given as 100 Ohms, SW1 is the AGC on/off switch, so far as I can make out with the AGC switched off the AGC line is pegged to the +ve voltage that's developed across the 6Q7's cathode resistor (I get that bit) The other associated resistors are of a very high value compared to the value of R13.

Lawrence.

14th Jul 2020, 7:40 pm

There is a somewhat tortuous path directly across some capacitors, the small 100R would stop the switch being hit by a large current when switched. An old dodge (best practice) to lengthen switch life. I still see reset switches on microcontroller circuits directly across 100nF or so, they don't last long.

ms660 · 14th Jul 2020, 8:06 pm

The source voltage to the 100 ohm resistor will only be a volt or two at most.

Lawrence.

14th Jul 2020, 4:42 pm	#1
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Can't see the wood for the trees...? Hallicrafters S20 AGC. Anyone know the purpose of R13 in the schematic in the link below?: http://www.nostalgiaair.org/pagesbym...6/M0008856.pdf Its value is given as 100 Ohms, SW1 is the AGC on/off switch, so far as I can make out with the AGC switched off the AGC line is pegged to the +ve voltage that's developed across the 6Q7's cathode resistor (I get that bit) The other associated resistors are of a very high value compared to the value of R13. Lawrence.

14th Jul 2020, 7:40 pm	#2
Guest Guest Posts: n/a	Re: Can't see the wood for the trees...? Hallicrafters S20 AGC. There is a somewhat tortuous path directly across some capacitors, the small 100R would stop the switch being hit by a large current when switched. An old dodge (best practice) to lengthen switch life. I still see reset switches on microcontroller circuits directly across 100nF or so, they don't last long.

14th Jul 2020, 8:06 pm	#3
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Can't see the wood for the trees...? Hallicrafters S20 AGC. The source voltage to the 100 ohm resistor will only be a volt or two at most. Lawrence.