[Collins] 75S-1 Problem (update)

[Dr.Gerald Johnson]

Gain too high. 
1. Tubes replaced with high gain tubes instead of 6DC6, 6U8, and 6BA6.
2. Low AVC voltage from leaky AVC bypass and time constant capacitors.
3. IF gain pot wrong value (too low a resistance).
4. Low negative bias supply from bad selenium rectifier and/or filter 
capacitor.
5. Open RF gain resistor (3.3 megohms).
6. Cathode bypasses on first audio stage open.
7. Circuits modified.
8. IF and RF tubes with grid emission to counteract AVC control voltage.

Audio crackle.
1. Tubes.
2. Leaky coupling capacitor between driver and audio output. Leads to 
damaged output tube.
3. Way too much audio output power to high impedance load driving the 
plate voltage below the screen voltage frying screen of the output tube.
4. Audio output tube plate bypass capacitor (HF roll off) breaking down with 
excessive audio voltage.

PTO jump.
1. The sideband switching voltage that drives that 1N34.
2. Any capacitor in the PTO, but not all.
3. Grunge on the lead screw or just dried grease.

Remember many of the capacitors in the PTO are selected both for 
calibration and for temperature compensation and replacing the whole 
batch is not necessarily going to improve the lot. Why not attack with a little 
finesse, applying alternating heat and cold to the whole assembly to see if it 
is temperature sensitive (for the jump) and then work at isolating the part by 
localizing the heating and cooling using hot air and freeze mist?

There is no guarantee that the jumping IS the PTO, it could be the crystal 
oscillator, or the BFO crystal even. You can check for PTO separate from 
crystal oscillator by tuning to 055 or 155 on a low band and finding the 
calibrator leak through to the first IF. That will be strongest with the RF tuning 
set for minimum frequency.

73, Jerry, K0CQ, Technical Advisor to the CRA
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.