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50-922 restoration problems - Printable Version

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50-922 restoration problems - nash - 01-15-2011

I recently bought a 50-922 set. This is the model with the "sun dial" on top. I've re-capped it, new electorlytics, X1Y2 bypass cap, CL-90 thermister for gentle tube warmup.

The good news it it plays with good volume and the cabinet is in good shape with a fair finish and no chips or missing parts.

The bad is poor sensitivity, especially mid band (1MHz) and distortion on local stations. I suspect silver mica desease.

AVC voltage is very low - 0.7V on strong stations, 0.3V on weak. Resistors checked out in tolerance. The set uses a dual diode - triode (14B6. I rewired it to use one diode for the AGC (33pf to the plate of the IF stage and 470K diode load resistor.) Great AVC action, voltage jumped to 7V on strong stations. The IF distortion is gone. I do notice some distortion on very weak stations (volume control up most the way). On most stations it sounds great, but overall sensitivity is lower than it should be.

I aligned the set, the IF cans do peak, but broadly - about 1/3 turn to see a change. The RF stage is not tracking right. It aligns well at 580 and 1600, but is rather deaf at 1000. Re-peaking the RF trimmer at 1000 restores the sensitivity, but it is then increasingly deaf above 1200.

Any suggestions? My thought is to remove the IF cans and replace the silver mica caps - but what values to use? Anything else I should check?

Thank you

Re: 50-922 restoration problems - Radioroslyn - 01-15-2011

Hi Nash
Would double check the loop ant for shorts or improper wiring. Also a bad mixer tube. Any mica caps in the ant circuit would be suspect too. Check cap, coils and alignment of Z1.
As for the IF padders I've hear that 100mmfd is typical you may want use a GDO to confirm resonance
Terry,.

Re: 50-922 restoration problems - nash - 01-18-2011

The loop antenna measures 3 ohms, I will try swapping the 7A8 to see if that makes a difference. None of the coils are open. Z1 adjustment doesn't affect the reception with the limited range I can turn it (about 1 turn). The slug is pretty much frozen if I try to go beyond 1 turn. I certainly don't want to crack the slug or break the coil support. I'll have to disassemble Z1 to free up the slug.

I pulled the second IF transformer. Take a look at the photos. The mica had extensive silver migration. Looks like I'll be removing the old mica caps on the first IF and Z1. When I used 100pf capacitors across the coils, I could not adjust the coils to 455KHz. 490KHz was the lowest I could get. I should be able to get some 110pf caps today and see if that doesn't get me into the 455KHz range. I tested the transformer with my signal generator attached via a 47K resistor to the primary coil. The DVM then read the voltage developed by the diode connected to a 0.01ufd cap on the secondary coil.

[Image: http://www-rohan.sdsu.edu/~nash/philco/2nd%20IF.jpg] [Image: http://www-rohan.sdsu.edu/~nash/philco/2nd%20IF%20opened.jpg] [Image: http://www-rohan.sdsu.edu/~nash/philco/2nd%20IF%20mica.jpg]

Re: 50-922 restoration problems - nash - 01-19-2011

Update - I have the second IF transformer (Z3) working now. I found that 120pf capacitors resonated at 455KHz with the slugs in the middle of the their adjustment range!

I don't have a frequency counter, but I do have several digital radios. I adjust my signal generator around 455KHz until the second harmonic shows up at 910KHz on my digital radio (Grundig G5).

I pre-aligned the IF can prior to installing, taking time to mark which 120pf capacitor goes to what side of the IF transformer. The clip that holds the IF can in place prevents installing the IF can with the 120pf caps soldered in place. I used super glue to stabilize the solder pins of the IF can. Super glue was unfazed by repeated heating from soldering the pins. I tested the radio to make sure my work was ok before proceeding with repairs to Z1 and Z2. I found a noticeable improvement in sensitivity!

Thank you 7estatdef for your suggestions. I tried a NOS 7A8 but it had no effect on the tracking/drop in sensitivity around 900 to 1200KHz. Once I have the mica caps replaced in Z1 and Z2, I will go through all the alignment steps again.

I assume the coupling capacitor shown in Z1 is not a critical value. If the Z1 cap is leaky, it could be affecting the bias on pin 6 of the 7A8 converter. Anyone know what a typical voltage should be on pin 6? For those interested here is a link to the 50-922 schematic

Re: 50-922 restoration problems - Radioroslyn - 01-19-2011

Hi Nash
That cap is going to be a small value like 10mmfd or so voltage wise 200v or higher should be fine. Glad you got it working! Glad to help.
Terry

Re: 50-922 restoration problems - Ron Ramirez - 01-19-2011

Terry is right. Model 50-621, a set designed for use on AC or DC lines, or batteries, has a very similar RF coil. The 50-621 schematic gives the value of the little coupling cap as 10 pF.

Pin 6 of the 7A8 is the control grid and will have AVC voltage applied to it. I would expect this voltage to be in the neighborhood of 0 to -10 VDC, depending on whether the set is tuned to a weak station, a strong station, or no station at all when you make the measurement. The AVC should be more negative on a strong station.

Re: 50-922 restoration problems - Radioroslyn - 01-20-2011

Just as side note the bigger you make that 10mmfd cap the less effect the slug tuning will have. It blocks the hv dc and adds a bit of selectively to the rf amp.
Terry

Re: 50-922 restoration problems - nash - 01-21-2011

I finished removing the old mica capacitors in the 1st IF transformer and the RF transformer. From their looks it was needed. The the silver oxide looks like it migrated all the way to the transformer case! The RF transformer is the second photo with only one tunning slug.

[Image: http://www-rohan.sdsu.edu/~nash/philco/1st%20IF.png] [Image: http://www-rohan.sdsu.edu/~nash/philco/RF.png]

I pre-aligned the 1st IF before re-installing it, using 120pf capacitors. What a difference! I easily hear twice the number of stations now. The IF transformers now have a very noticeable peak when checking the alignment.

I really appreciate the comments on the value of the RF cap. I would not have guessed the RF transformer capacitor to be only 10pf. A 33pf is used for now. Even with the 33pf, the RF coil alignment now peaks easily in less than a 1/4 turn. There are some spurious noises (oscillation?) in a couple places on the dial when the tuning capacitor RF trimmer is aligned for max sensitivity. I think that may go away when the 33pf is replaced with a 10pf.

Now the only issue is what I would call a RF buzz between stations and on weaker stations (these could not even be heard before fixing the IF and RF transformers). Tuning in most stations, the audio is great, no hum or buzz. The antenna coil is not open, and the plug is making contact. The buzz varies with the volume control. Oddly, If I touch the external antenna screw with my test meter, the buzz is greatly reduced. I have a 0.01uf X1Y2 250VAC cap across the AC line, in the original location of C15. C15 was a 0.047uf originally. I don't think the value of C15 is the issue as when I unplug the radio, the buzz continues for a few seconds until the radio quits.

Re: 50-922 restoration problems - nash - 01-23-2011

The buzz turns out to be caused by a CFL in the kitchen. Turn off the lights and no buzz. Who would of thought? A portable radio helped zero in on the culprit. Replaced the bulb with a new one and the radio is buzz free.

The loop antenna on the Philco is more sensitive to this sort of electronic interference. My portable transistor radio barely picked up the buzz until I was in the kitchen.

I replaced the 33pf RF capacitor with a 15pf. Fry's was out of 10pf caps. The 15pf eliminated the spurious noises I was hearing. The alignment now tracks across the band.

The speaker voice coil was rubbing occasionally. Playing the radio a few hours seems to have cleared it up.

Thank you for your help.