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Author Topic: 75S-1 Motor Boating only at one Frequency - SOLVED  (Read 2362 times)
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NY2KW
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« on: March 07, 2021, 12:13:45 AM »

After some basic repairs and alignment my 75S-1 is working really well except for one odd finding.  Regardless of band selected, mode (AM, USB,LSB,CW) selected there is a spot on the PTO dial at ~155kc mark where I find a rapid motorboating, sound like a fast ticking clock.  Few Kc up or down and it disappears.    I thought it was a PTO problem but I loosely coupled the PTO tube to another receiver and could pick up a clean PTO signal varying the 75S-1 PTO from 0-200 kc and listening on the second receiver from 2.7-2.5 MHz.  There was no motorboating component when the PTO was tuned to the 155kc spot (but still heard on the 75S-1).   I pulled V3 (Crystal Osc and 1st Mixer) and motorboating still there.  I also pulled V9, V1 and V2 and still there.  I did a complete replacement of all tubes, one at a time, and no change.  One clue, as I was swapping tubes I discovered I could make the motor-boarding completely disappear when  my hand was hovering over the area between V1 and V2.  Even after pulling everything from that area (V9, 100kc xtal, V1, V2) I could make the motor-bordering disappear with my hand hovering over this spot.  Looking under the chassis this is one of the most un-cluttered areas in the radio.  I tightened all chassis screws, nuts holding the bandswitch shield cans, etc and no joy.  As one last hope I reflowed all the tie points for the ceramic caps and chokes in that area but no joy.

Any advice how to further test to localize the issue would be greatly appreciated

Thanks

Jerry NY2KW
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w4bfs
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« Reply #1 on: March 07, 2021, 08:24:10 AM »

Hello Jerry and welcome to the forum.  The motorboat osc generally occurs in audio stages due to dried out electrolytic decoupling cap.  Ur problem sounds (ugh!) like something further toward the rx front end.   U might try bridging decoupling caps thruout to see if it quits.  an interesting problem

I have also seen this in old rx with converter tubes like 6K7 etc.  again try more decoupling caps .. gl  
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Beefus

O would some power the gift give us
to see ourselves as others see us.
It would from many blunders free us.         Robert Burns
NY2KW
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« Reply #2 on: March 07, 2021, 10:40:47 AM »

Hello Jerry and welcome to the forum.  The motorboat osc generally occurs in audio stages due to dried out electrolytic decoupling cap.  Ur problem sounds (ugh!) like something further toward the rx front end.   U might try bridging decoupling caps thruout to see if it quits.  an interesting problem

I have also seen this in old rx with converter tubes like 6K7 etc.  again try more decoupling caps .. gl  

But if in audio stages, why is this happening only at one spot across the 0-200kc PTO spread (2.5 - 2.7MHz) and no other frequency?  I checked on voltages and resistances and all good.  I think it has to be after the RF and before Audio so IF amps??  There is one old electrolytic between plate and cathode in the Prod Det which follows the IF amps.. I will replace that and if that doesn't work I will try to bridge key 0.01uF decoupling caps in the IF Gain amp circuits.

Thanks for any advice

 
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WD4DMZ
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« Reply #3 on: March 07, 2021, 05:52:20 PM »

That is similar to a problem I have with an HQ170A. The motor boating occurs when I narrow the passband. Apparently that is common in HQ receivers and happens when a mica cap in one of the IF cans go bad. Very hard to troubleshoot and replace so I just leave the bandwidth at 3Khz. You may want to focus on the IF cans to see if there is an open or shorted mica cap.

Rich
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NY2KW
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« Reply #4 on: March 08, 2021, 11:20:02 PM »

I found the problem, one of the .01uF bypass caps between the PTO and 2nd Mixer had a broken lead to ground.  Must of happened as I was moving the cap back and forth to get to probing the PTO.  A little solder and no more motorboating. The 75S-1 sounds great.

thanks for the helpful comments.  I used a0.01 cap jumper and started testing caps systemically from the 2nd Mixer and didnt have to go far before the problem disappeared when I jumped the suspect cap and saw on close inspection the barely noticeable wire break right at the terminal strip.

Jerry NY2KW 
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w4bfs
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« Reply #5 on: March 09, 2021, 08:12:36 AM »

good job and thanks for sharing your diagnostic journey  Grin

on the 75sx .... check your filament voltage .... with 120Vac here I measured 6.7Vac afaik .... I added a 5Ohm 5W resistor in series to the line fuse .... it also softens the turn on
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Beefus

O would some power the gift give us
to see ourselves as others see us.
It would from many blunders free us.         Robert Burns
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