[Collins] 75S-1 Problem (update)

[GBabin73 at aol.com]

Thanks Jerry (good name),
I went through the voltage/resistance checks again and all readings were  
nominal. I noticed that the '61 edition of the schematic listed the IF gain set  
pot as 1K vice the 500 ohm listed in the '59 edition, so I replaced it with a 
1K  (it did help a little). I also swapped out the IF tubes with 2 different 
ones  and now am able to set the gain to S-9 at 50 micro volts (albeit with 
almost 1K  on the gain pot, but it tamed down considerably). Without the 
excessive gain,  the crackling also went away.
The PTO problem is  definitely in the PTO. I swapped out the PTO and the 
problem went away. I prefer  the PTO that jumps because it is otherwise more 
stable and linear. The comment I  made about shot gunning the PTO caps was slightly 
exaggerated. The problem does  not seem to be heat related. It has jumped 
freq as soon as I turn it on, or it  might not jump for 4 hours. I cleaned and 
lubricated the lead screw a couple of  years ago and it still looks good. The 
negative coefficient and disk ceramic  caps don't seem very likely candidates 
for problems (the dipped mica either).  The cap I think is giving me the problem 
is the 200 pf molded mica (postage  stamp) that couples the screen to the 
frequency determining network. I have  traced more problems to "postage stamp" 
mica coupling caps than I can count. I  have heard that the silver leeches over 
time and they go squirly. I don't know  for sure why, but I do know I have had 
to replace quite a few of them, mostly in  older receivers. There are 2 other 
mica caps in the PTO, both dipped. I have no  idea why they used a single 
molded S/M cap. 1% or 2% 200 pf 500 V  dipped  mica caps are not that rare. Both 
of my PTO's are very early models, maybe they  switched to dipped silver mica 
caps in later models.

Thanks again and 73!  DE N5MCJ, Jerry

In a message dated 2/17/2005 7:29:30 P.M. Pacific  Standard Time, 
geraldj at ispwest.com writes:
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