EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

I don't see any mention of how to adjust the RF diode cap #283 in the service manual. I have the IF's all lined up.

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Right.  EH Scott Radio Laboratories did not provide directions for aligning the RF AVC diode.  With the selectivity/fidelity control set approximately 1/3 from the high selectivity position, feed a STRONG RF signal to the antenna terminal and adjust for MINIMUM output as measured at the input to the volume control.

Norman

So, this is a wave trap for preventing RF signals at 465 from entering the IF just like most superhet radios that followed suit?

I do not believe the circuit is considered a wave trap.  It is definitely not a 465kc wave trap.  It is an AVC circuit reducing RF gain on strong stations to prevent overloading the IF amplifier.  The source for AVC voltage is after the converter therefore the RF AVC diode is tuned to the IF frequency but in a manner that attenuates strong RF signals rather than peaking weak signals.

Thanks Norman, I'll make that adjustment.  i think this will be the last thing to do and I am done with the restoration.

Well, I tried that adjustment but there was no effect when turning the  adj. screw on either the audio at the VC or the AVC line.  I used a very strong signal ay the antenna at 1100 kc.  I tried different levels but no change.  there is a new 6B7 and 6A7 installed.

Can you post a sketch of how the RF diode is wired into the circuit?  There is a common restoration error that will defeat the RF diode function.

Norman

Taking a close look for a mis-wire I discovered no continuity to plate of 6B7.  After energizing there was no plate voltage to 6b7.  My fear was an open coil in the RF diode can.  Turns out the 1K resistor had broken off the terminal.  I was surprized to see this since it was under no stress during the rebuild.  Looking at the break under magnification it was a very square separation.  I tried to take a close up.  It looks like it just separated from the solder joint, I made a red pointer to the spot.  So it's been replaced ad now the adjustment does work.  I'm glad it wasn't the coil and I didn't just dismiss the non adjust problem.  Thanks to Norman and this Forum.

Cold solder!  FYI, I have plenty of extra diode coils if anyone finds their set to be in need.

Norman

That's good to know, I thought a coil would be very hard to find!  Here is a pic of the dial.  Was there any type of screen over the strength meter or bulb?  There is a 44 lamp in there now

The dial escutcheon held a piece of isinglass on which both the dial scale and meter needle were projected.  Without a front panel and escutcheon, a piece of frosted mylar or other similar translucent material is required for viewing the dial and meter.

Norman

I installed the radio back in the cabinet a couple weeks ago and thought everything was fine.  I got a call this morning that the volume control is apparently bad.  It was a replacement probably done in the late 50's so I thought it was ok with just a shot of cleaner.  But as soon as you come off zero the volume jumps up and with as much power as this radio has it's pretty bad.  I noticed this radio uses a fairly low value, 100K.  I wonder if others have had this trouble and if there were any mods to reduce the gain a little since there is so much power available in this set.  Anyway the radio is on the third floor of an old house so I will try and replace the control at the site.  I'll check my parts for an audio taper 100K pot, it's too bad I don't have my stuff better organized as it may take a few hours to look for it, ha ha!

Update to old post, I did have a NOS control and installed it in the set at the owners house.  That was 7 years ago so I guess my work has held up(along with 23 tubes)!

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