The PHILCO Phorum

I have an 87 that I rebuilt several years ago that worked great.  Never aligned it as the reception was very good. However, one day when it was on I heard some loud popping noises.  At first thought it was electrical interference since it was still playing.  After a few minutes, the popping suddenly got louder and more constant, the music died.  When up to find an 80 tube pinkish/purple with fireworks inside the tube.  So replaced the 80 as it was showing shorts.  Reception now very poor.  Found and replaced two bad 26's and the 27.  Helped but not much.  Went over the radio again, all caps & resistors, tubes showing good, good voltages.  I've decided to check the alignment now to see where that is.  Following the document here on philcoradio, "Neutralization of Philco Neutrodyne-Plus Receivers" I've been able to improve reception, but not to where it was in the past.  In that document, it has the following statement:

              >>> Adjust the 3rd neutralization condenser (number 6 in Figure 2) for minimum signal.    <<<

What exactly is minimum signal?   If by meter, is this when it's at '0'?  If by listening to the speaker, when the signal generator tone is no longer heard?  The process has improved reception, but not close to where I think it should be at this point, and I'm thinking my interpretation of minimum signal may be incorrect. 

Any thoughts on this or something I may be missing, please let me know.  Thanks!

https://philcoradio.com/library/index.ph...receivers/

Probably the best description of minimum signal is at it's lowest point. The 87 has no avc so you can dump a whole lot of signal into it if you need to.

Got an 87 myself, it's a heavy brute!

>>min·i·mum
/ˈminəməm/
noun
noun: minimum; plural noun: minima; plural noun: minimums

the least or smallest amount or quantity possible, attainable, or required.


So, there it is. It is the minimal value you could achieve. Say, from 10V, 5V, 4V, 5V, 7V sequence the value of 4V would be the minimum.

Are you using a dud to neutralize?

Yes, using a dud.  However, I don't know if it's too much of a dud. . .  It was good at one time, but has a dead filament now.  After reading some of the documentation, if it has problems outside of the dead filament it's not good to use.   So this weekend I'll make the adapter to change a good tube to a dud tube.

It is unlikely the tube has more than one problem.
If the filament opened, the tube is likely a good dud.

Quote:So this weekend I'll make the adapter to change a good tube to a dud tube

Use the tube with an open filament or take a tube that has too low an emission and cut off one of the filament pins.

Creating an adapter will add a few pf to the circuit, that will affect the neutralization such that it will not be null when the good tube is reinstalled...

Watching a meter will only distract when neutralizing. The ear is ample enough instrument and no leads be connected.

Theoretically, forcing a strong signal to neutralize may not be a good idea as Miller effect may come into play and cause an error. Be sure all shields are in place and any bottom plate too.

If the set had a tendency to oscillate in one area of the band, note that area and neutralize at that frequency. If the area of oscillation has moved and set still oscillates split the difference in the neutralizing frequencies and try a third time.

If still no joy then component placement, RF tube(s), voltages, grounds or wiring may be an issue, hopefully not as they are even more "cut and try".

GL

Chas

I am a little worried as to why you had a fireworks display in the 80. This usually occurs only when their is a short across the power supply or in the power supply, usually a shorted filter cap.

You may be able to tape the filament lead of the tube that you actually intend to use in the set after neutralization, and then neutralize the set on that tube. When the neutralization is complete, remove the tape.

Was the fireworks around any festive holiday? That could explain why.

Skol!

Make sure all your RF coils are good and no open primary windings.
Good luck