[MrFixr55]

Hi DConnant,

The reason why C127 is isolated from ground is because sets of this vintage did not have the center tap of the HV (B+) power supply grounded. Instead, they grounded the cathodes of things like the output tubes and developed a negative voltage for the grid bias. They developed the grid bias or "C-" by a voltage divider network. Between choke #122 (the speaker field coil) and R 121, there is 378Ohm between Ground (B-, C+) and the B Supply center tap (C-) Note that the can (- terminal for Cap #123 is also not grounded but is connected to the center tap of the HV supply like C127 is. The + terminal is connected to Chassis ground. Measure between the + and - terminal of this cap and you may see the missing voltage.

I looked at the Philco Service Info and the Nostalgia Air schematics fdor this set and am missing the voltage documents. However, reading the RCA Receiving Tube manual, Specs for a 6B4 are the same as for the 2A3 except for Filament voltage.
Voltage between plage and cathode is 300V max. Grid bias at this voltage should be 62V. So this is close to the 370V that you see between the terminals of C127. So it sounds like you are close, especially when you fire this thing up with the tubes and speaker in place.

Hope this helps.

[dconant]

Hello MrFixr55,

Thank you for taking the time to explain. I measured 50 volts where you say. Today the 370 volts at cap 127 is reading 310 volts. So based on what you say I am close enough. Can you tell me if I need to find a replacement choke (#120) or if I can just use a 400 ohm resistor. I currently am running with a 400 ohm resistor. After adjusting the hum pot I hear very little if any hum.

Thank you,
Dan

[dconant]

Hope you guys are still with me.

I have moved on to the alignment. I completed the IF alignment which was pretty much spot on. I'm stuck on the fidelity selectivity alignment. Instructions say to set that control fully counter clockwise and then sweep the generator between 460 and 480 to find a peak on each side. There is no peak in that range on either side of 470 just a steady decline of the tone. Yet when I listen to music there is a definite difference in the treble. What am I doing wrong and is the peak going to be greater than what is at 470?

As for the magnetic tuning adjustment I don't think I'm going to mess with that because it is working rather well pulling in the stations when pretty far off frequency on both sides.

[morzh]

You're doing great.
I wish I could be more involved, but my mind is right now preoccupied with the reason I am here in the first place.

Magnetic tuning is way cool, and watch it work (especially demonstrating it to others) is even cooler.
I think what we today call PLL was patented by Philco, a year before and used in a few radios next year, that is 1937.
In other countries later they used motorized auto-tuning (some even in the Soviet-build sets).

[dconant]

I am having a heck of a time doing the alignment on the AM band. I can't seem to dial in the 1500 KC oscillator. I can hear the tone on multiple places around 1500 KC. Closest I can get is 1520 KC. I am adjusting 42, 24 and 6. 24 and 7 seem to peak up. Adjustment of 42 comes in a various spots on the compensator. The 580 adjustment is spot on although station 740 comes in best at 750. Station 1550 comes in best at 1560. I expect better with this radio. Is this radio really that hard to dial in. I had a very hard time with the SW bands too but got them good enough.

Mag tuning is off and all tube shields are in place as well as the bottom plate. I am using a frequency counter along with my signal generator.

Maybe I should have changed those mica caps in the RF section???

[MrFixr55]

Hi DConant
Yes, you can replace chokes with resistors.  You do stand the risk of increased hum.  the solution is to increase the capacity of capacitors.  This is often done when one cannot repair a Field Coil speaker and replaces it with a speaker that uses a permanent magnet.  If you are describing the 400Ohm choke, labeled #120 on the schematic, you may want to increase the capacity of Cap #62B to compensate for the filtering that the choke provides.  My rough guess (the more engineer-oriented experts can chime in) would be to double the value.  Based on the location of this choke, there is no need to change the capacity of other caps.  The issues and pros vs cons of changing or upping the capacity of filter caps is this:

• PRO- The more capacity the less the AC "ripple" of the supply.  Newest generations of tube equipment have caps as high as 50 - 80 uF.  Power supplies for solid state equipment have capacities in the hundreds or even thousands of uF.
  
• CON- Since caps draw current when used as a filter, especially as the first cap after the rectifier, increasing the capacity increases the voltage, and the current.  Too large a capacitor in the "input" position (directly across the rectifier with no choke or resistor) can cause a current draw that exceeds the capacity of the rectifier or power transformer.  
  
• PRO- the higher the capacity, the less need for expensive chokes (inductors).  Just price caps, resistors and chokes on the resale market.  The first filters were paper caps, which have the lowest capacity / size ratio.  Wet electrolytics had a higher capacity / size ratio.  The latest electrolytics have the highest capacity / size ratio.
  
• CON- Increasing the size of the capacitor increases the output voltage of the power supply.  This may affect bias, violate maximum voltage specifications for a tube, cause excessive current draw, etc.
  

Since the specifications for electrolytics 'back in the day' was -20% / +80% increasing the cap value by 50% is likely not an issue.  Voltages should be within 20% of specifications.  For some (but not all) circuits, measurements with modern DVMs may not match the published readings that may have been taken with meters with sensitivities and loading of as low as 500 ohms per volt, etc.

Regarding the "Selectivity / Fidelity" adjustment and peaking the IF, I don't think that a dip between 2 peaks will cause poor fidelity.  the concept was to have as wide an IF bandpass for fidelity, but a narrower bandpass for selectivity.  People on this Phorum who have experience as Radio Station engineers can comment with more authority than me, but I believe that the fidelity of AM stations have decreased on purpose with the movement from music programming to talk programming.  It is likely that the set was designed to narrow the IF bandpass to attain greater selectivity on shortwave.  Sets having variable selectivity / fidelity settings either by a separate switch or incorporated into the band selector were designed to either widen the IF bandwidth to gain better fidelity at the expense of adjacent station rejection or to narrow the bandpass to reject adjacent channel interference.  

My comments are based on general knowledge of electronics and audio, especially vacuum tube circuits, and analysis of the schematic.  Sadly, I do not own one of these sets because it looks like a great set both aesthetically and electronically.  Owners of the set, please add additional comments.

[dconant]

Thank you MrFixR55, I appreciate your comments very much. I do not detect much hum if any so I will be staying with the 400 ohm resistor. These 37-116 radios are very difficult to work on at least for me anyway. I have lots of hours on that chassis especially the RF section.

Mike the magnetic tuning in this radio works very well.

Dan

[462ron]

Hi Dan, it’s been 10 years since I restored the electronics on my 37-116 so I’m going on some foggy memories. I remember the variable IF alignment being rather tedious. I was working with the typical service grade Heathkit RF generator of my dad’s from the late 50s and monitored it with a modern frequency counter. The constant setting and resetting of the frequencies 460-480kc and adjusting the alignment settings was touchy. I never did get exactly the same peaks but fairly close. The end result is it seems to work fine according to my ears so I don’t know how finicky you need to be. I imagine if you have a modern generator that you just punch in a frequency would make the task a lot easier and quicker. You’re right about the tuner, what a monstrosity to work on. As far as getting the dial right on frequency, my set can be off by 10kc depending on what part of the band you’re in. It is possible to tweak some of the blades on the tuning cap as the outer blades are slotted but then this being a 5 band radio, you risk throwing alignment off on some bands so it’s best not to try bending any plates at all. I did that on an AM only radio once and had good results. Good luck with your restoration, it is good performing and sounding set!

Ron

[dconant]

Hi Ron,

Thanks for the input. I think I'm going to call it good enough. It does seem weird I can usually dial in other Philcos of the era a lot closer than this one but their dials are not spread out near as much as this one. So being a little off seems like a mile here.

Dan

[dconant]

Ron and Mike I mentioned a 2 meg resistor not on the schematic. I stumbled on a changes note in Riders that mentioned the 2 meg resistor. Riders Volume 8 - Changes Page 3-8. It is to prevent the clicks. Funny I have that resistor and still hear the click once in a while when I change the bass.

[462ron]

Well then Dan I guess this old man’s memory wasn’t too foggy  although I couldn’t remember where I saw that upgrade!

Ron

[dconant]

Looks like I made a mistake. I painted my tube shields silver. Now the reception is poor and can't really zero in on most stations. Not saying that is what the problem is, could just be weather related since it is rainy and foggy here. Did I mess up?