[Milsurplus] ScotT SRLM: Hackery Quackery

[David Stinson]

Morning.

Main projects right now are reviving a BC-669 for a retired Navy tugboat 
skipper who used them and modding an Icom IC-735 for the 630 meter 
band.  Still deciding how to approach the GO-9 power supply issue.
While the BC-669 is sitting in the hot Texas sun, drying from its bath,  
used a bit of the "mean time" to take a first look the latest addition 
to my "big beast receiver" harem, a Scott SLRM AC/DC Marine set.
https://photos.app.goo.gl/SnKGqMxjN6d9UNaL9
https://photos.app.goo.gl/kKHn5Yup9xH6X53v9

The receiver is very clean and all power supply filter caps check 
"good."   Replaced the line cord with a three-circuit and, since it's 
AC/DC with series tube filaments and no transformer, double-checked that 
the "AC Hot" wire went to the right place. Bringing up slow with a 
variac and the receiver plays, but the audio sounds a bit distorted and 
the AVC doesn't work well.  Noted the pilot light, which checked "good" 
did not glow and the filaments strings seem unbalanced.  Here's a 
diagram of the power supply and audio sections:
https://photos.app.goo.gl/Y2UYi6eHpUMrm2Ct8
The brown wires indicate the connection to "Neutral/Ground" in the 
typical U.S. 120 VAC service fashion.

Noticed an unoriginal 1/4 inch "stereo" jack on the front panel, curse 
his drill.
https://photos.app.goo.gl/tScKY6X3izipLXVq5

Tracing back, I found that Mr. Brainiac had attempted to tap audio 
across the Volume control and bring it out to the front panel.  In doing 
so, he'd used shielded cable and connected the shield braid to the pot, 
and from there to the Negative terminal of C-148, assuming that was at 
Ground.  Well... It ain't.
https://photos.app.goo.gl/mwWU5wz7X9xypzeaA

By doing this, he placed a whole swath of stuff in the receiver at 
Neutral/Ground, including the Push-Pull Audio PA cathodes, berefting 
them of grid bias.  The red lines in this diagram show the circuits now 
at Ground:
https://photos.app.goo.gl/jJBj3Ud1G92ALWvBA

With a moment's thought, he could have done what he was after by 
grounding the panel end of his cable and leaving the pot end of the 
braid open.  This kind of "Hackery Quackery," half-thought-out and it 
would seem without even looking at the circuit diagram, has sent most 
such fine equipment to the scrap yard.  No wonder he discarded the 
receiver after listening to the squishy this atrocity did to his 
receiver's audio.  Some time today I'll pull-out this nasty little mess 
and see what other "golden screwdriver" nit-wittery has been done to the 
poor thing.

  A link to the full schematic:
https://drive.google.com/file/d/1BCPxlJzVOEK9ZAJPay-tB1V-z83acxz7/
Link to the Scott SRLM Manual:
https://drive.google.com/file/d/1APoGbHK6exFzpQzlSOtfduRF74T0Rqfw 
<https://drive.google.com/file/d/1APoGbHK6exFzpQzlSOtfduRF74T0Rqfw/edit>

GL OM ES 73 DE Dave AB5S



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