EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

Hello,

     I am trying to bring a dead Phantom Deluxe (AM) back to life.  I have 2 problems that are stumping me.  First, the RF amp doesn't.  It attenuates the incoming signal as evidenced by scope traces on the grid and plate, with the plate trace being a much lower amplitude than the grid.  The 6U7 tube tests good on my Hickock 600 tube tester.  The whole set has been re-capped and out of spec resistors replaced.  Socket voltages on the 6U7 are as follows: Plate +230V reference chassis, grid 1 -1.5V reference to cathode, Cathode +6.5 V reference chassis, grids 2& 3 (tied together) +105 V reference chassis.  

The oscillator, 6J5 voltages are as follows: Plate +150V reference chassis, Grid +4V reference Cathode and 0V reference chassis, Cathode +4.8V reference chassis.  My frequency meter and scope measure no output from the oscillator.  

I am fairly certain that the converter works since I can tune in stations by injecting an unmodulated signal at the grid of the 6L7 at a frequency 455KHz above the desired tuned frequency.

Your thoughts?  Thanks

Tom

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Take a very close look at the air cap for the oscillator coil circuit and make sure the 2 sets of tuning cap blades are not shorted.

I have had two instanced where the press fitting failed. These air caps are spring loaded and about 80 years old now.  I had to get replacements from a junk chassis.

I will do that and report back.  It has also been suggested that I test switch contacts for continuity, even though I g=have sprayed them with cleaner and exercised them thoroughly

and if you wind up needing odd parts, let me know, I have plenty of parts chassis to help.

Kent

Thanks for the offer.  I do have a Phantom parts chassis that may have a working variable cap.  Will check that one if I run into trouble.

it is a little cap - the trimmer (or is it the padder - terminology !!)

Thanks



David C. Poland said:

it is a little cap - the trimmer (or is it the padder - terminology !!)

I checked the three sections of the variable tuning cap for shorts and found none.  

Thomas - I think you misunderstand. 

I was not addressing the big 3 section tuning cap. (But you do need to loosen and re-tighten the mounting screws for the big tuning capacitor to assure good grounding)

Look at the instructions for adjusting the oscillator and find the two adjustment screws for the Broadcast band (noted as B.C.). Refer to Riders Vol 14 page 41 for alignment and see the illustration to locate the Broadcast trimmer and padder adjustment screw heads. One of those two adjustments (C1, I think) is a miniature air capacitor (a very small version of one section of the big tuning capacitor.)  If the press fit brass flat spring has loosened, the air cap blades will touch, shorting out the oscillator coil. (I no longer have an example to photo for you.)

Yes, I now know what you are talking about.  I will check that one out.  Thanks

Checked both the trimmer and padder for BC operation,  Both seem good as far as shorts go.  Operated through the full range with ohmmeter attached and no shorts.  That grid bias shouldn't be positive, should it?

The grid of oscillators is normally a bit negative when measured with a high impedance meter such as a VTVM under normal operating conditions.  

Got it working!  Seems It was a problem between the soldering iron and the seat. :)  When I replaced the coupling cap between the oscillator grid and its section of the tuning capacitor, I either read the value of the original wrong or grabbed the wrong value from the parts drawer.  Either way, I had installed a 3pF capacitor in place of the 50pF capacitor that the schematic called for.  To make matters worse, I did something I try never to do and that is install a new part so that the label cannot be read easily.  I had placed this one with the label towards the back and only realized my mistake when I had to move it out of the way to make a voltage measurement.  

I'm proceeding with alignment now, but I have one question.  After the IFs are aligned it talks about maximizing output of the RF diode.  I can't find this anywhere on the schematic, nor can I find reference to an adjustment for this on the diagram showing all the trimmers and padders.  Are they referring to the detector?  Thanks

Tom

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