Phantom 19 IF Alignment Help

[Joe Miller]

Just finished recapping it, powered up and it sounds great.  I noticed that they eye tube doesn't close on a strong station (see link from ARF below on details)

http://antiqueradios.com/forums/viewtopic.php?f=6&t=199147&...

 

Anyway, I want to check the IF alignment to see if that will raise my gain, increase the AVC (negatively) to the eye tube, and cause it to fully close on the strong station.  I'm somewhat skeptical that I could be losing that much gain because the radio sounds so good and loud on the strong station already.

 

Does anyone have experience in aligning a Phantom 19?  Would appreciate any hints anyone might have.  The instructions I have are a bit vague.  I think I can figure it out, but would like any input any of you might have.

 

Thanks,
Joe

[Norman S Braithwaite]

Bill:

The Q of IF coils having a ferrite core for adjustment will be higher when adjusted to the lower end of the coils frequency range.  I would be surprised, however, if the variation in overall IF selectivity due to Q variation over the range of 465 to 480-khz would be noticeable without sensitive instruments.

The gain of an IF amplifier is primarily related to the gain in the tubes and secondarily to the gain in the IF transformers, if any (cheap radios necessarily employed amplifying IF transformers but Scott did not embrace amplifying IF transformers during the pre-war chrome era).  The gain of the IF transformer is related to the turns ratio, electronic proximity of the coils, and how well the resonant frequencies of the primary and secondary are matched.  With respect to IF amplifier gain, the only place coil Q comes into play is when the resonant frequencies of the primary and secondary are not matched in which case misalignment of high Q coils will result in much greater loss (or less gain for amplifying IF transformers) than misalignment of low Q coils.

Joe:

Peak the IF at 480 where it appears to have been aligned before.  If the performance of the receiver is not quite up to par and there is no other obvious reason, you can try the lower IF frequency.  The easiest way to change the IF frequency from 480-khz to 465-khz is to walk it down as you have described.  Scott did not peak their IF amplifiers at 455-khz during the time that the 19-tube Phantom was being produced.

[Bill Liers]

Norman,

 

You may be right about the Q variation over the small frequency variations we're talking about. It is interesting though that the Phantom alignment info published on N/A says to use 455 kHz.

 

regards, Bill

[Joe Miller]

Bill, Norman,

Completed alignment at 471KC--only needed minor tweaking.  Still have low AVC of -1.6V at the detector and -.6V at the grid of the eye tube (with eye tube removed).  Local station plays loud and clear and can pick up WLS (120 miles away), but no action on the eye tube given the low AVC voltage.

I noticied that I had to have my sig generator on high to get a signal through the IF train.  I inserted at grid of the converter tube.  I've swapped out all the IF tubes, detector, and RF tubes but don't remember if I also swapped out the converter (it tested good though).  Given the need to insert high level of signal to do IF alignment maybe something is wrong with the 6L7 tube??  I will swap it out to see if that helps.

I measured the following voltages on the tubes:

Plates of RF, converter, IFs are all about 215-225 volts (seems reasonable)

Screens of RF, converter, IFs are all about 110-150 volts (seems reasonable)

Cathodes of converter and IFs are all about 10-15 volts (seems reasonable), but cathode of RF is ony 1.4 volts (seems low)??

Given all the gain available in this set, I would expect it to be a boomer, so I am going to keep looking for the loss of gain.  I'll go back and recheck to make sure all the cathode bypass caps (.05s) were properly replaced.

My next thought is to insert a modulated 1400 kc signal from my signal generator and use a scope to measure stage to stage gain (i.e. grid to plate for each tube).  What should the gains be for the RF and IF sections?  Will this method work?

Oh BTW, I checked the RF AVC section and it appears to be working fine.  It generates -4.5 v of AVC on the strongest station (feeds back to the RF and converter grids).  Could the RF AVC be fighting with the normal AVC prevening it from reaching expected levels?

Thanks for all your help.

Joe

[Bill Liers]

Joe,

Something is definately not all there. With your plate screen and cathode voltages close, I would check the grid circuits to see if you don't have something open like a grid cap lead. also, the 2nd IF amp biases the grid through the selectivity switch so that path must be verified (how does selectivity switch affect?). The RF AVC won't affect the IF significantly. Also, double check all related resistor values. You did say that you were able to go through peak on all trims, No? The 6H6s very rarely give trouble but if you have another, try it. Rough order of magnitude of 10:1 per stage from antenna (of course without avc). I'm sure you'll find it...

regards, Bill

 

[Joe Miller]

May have found the problem

The 3rd IF transformer secondary (between 2nd and 3rd IF tubes) appeared to be open.  Took off the can and the grid lead fell loose, so it may have not be connected.  So that is an easy fix.  But when I checked the rest of the secondary winding that connects to the selectivity switch (broad, medium, sharp on the schematic), I found the winding connection to the broad option (yellow wire on the schematic) open. the coil is okay to the medium and sharp options (blue and green). So I have 2 options--either chase down the coil connected to the broad option and hope it is broke close to the surface or find a replacement IF transformer.  I prefer the later, so will begin looking ASAP.

Are all three IF transformers identical and similar on other Scotts?  Any ideas on where to get one?

Joe

[Bill Liers]

Joe,

 

Since it's not a burn victim, the break is likely close to the surface and I've repaired a few. Give it a shot before you entertain finding a replacement. There were a number of Scotts made with the switched IF transformers but I'm not sure which would be interchangeable without looking at p/ns. Sounds like you are close...

 

Bill

[Joe Miller]

Got It fixed!

 

I was able to chase the open winding back 1/2 turn and found the break...soldered a piece of wire in and fixed the grid cap lead and all is good.

Lots' of AVC and eye tube action.

Will do one final IF alignment check to make sure I'm dead on and then It's all done.

Bill and Norman thanks for your help.

Joe

[Bill Liers]

Joe,

Excellent work! It's amazing that these radios have so much gain that they can seemingly work well even with the IF chain broken. Do you have plans to align the IFs for the broad settings as well? that is where the real "fidelity" is found.

regards, Bill

[Norman S Braithwaite]

Bill:

I checked my Scott published Phantom service data and see that Scott specified an IF frequency of 455-khz for the Phantom from the start.  Regarding alignment for the broad IF bandpass, this should only be done using sweep alignment methods and even then only making very minor readjustments of the IF trimmers to balance the bandpass characteristic.  Otherwise, the receiver may loose modest sensitivity when used in the narrow bandwidth position.

Norman

[Joe Miller]

I aligned it in Sharp mode per the specs.  I don't plan to do any further alignments, but out of curiousity how would one do the broad alignment and what is the objective?  Is it all about shaping the bandpass curve in broad mode?

[Bill Liers]

Joe,

You would need to sweep the IF (sweep generator) and monitor the detector output on a scope to see a representation or the bandwidth, adjusting accordingly. There are other methods, but as Norman mentioned, you stand the chance of ending up worse than you started. If you have or can borrow the equipment, I can run you through it over the phone even.

regards, Bill