[propflux01]

Hello,
I have aMitchelll 1264 that I acquiredd while waiting on my Philco 42-340 to come in. The guy that gave it to me said all it did was buzz when plugged in. It came without the Masonite backing/antenna. I cannibalized one from a junk Philco I had and wired it in. I took it home, got the schematic from:

http://www.nostalgiaair.org/Resources/952/M0010952.htm

It is a four tube model. I replaced all the caps (sans one Ceramic cap - Cap 8 ), and most all the buzz went away. The problem I have now is it has bigtime static. I can get two stations, one at 970, one at 1400, but they are weak and well into the noise (static noise). The only other thing I noticed is there is virtually no noise or stations on the lower end, and there are approximately 4 areas from low end to high end that buzz loudly, sort of like a resonance thing. I replaced the tubes one at a time to make sure it wasn't one of those, with no change (except one 50C5 tube was somewhat brighter than the other). The resistors seem to check within spec. The "replacement" antenna ohms good, and I used new wires from the variable capacitor to the antenna, and double checked my solder joints. Can anyone steer me in the right direction here? I can take a video of it in operation if needed. I got the clock working good and would like to keep this one going. BTW, I am a newbie at this, and its been 30 years since I repaired my last TV so I'm kinda green here. Tim

[Radioroslyn]

Boy that a cheap set. Well this thing doesn't have an IF amp stage and it use a pentode mixer both of these will make your set have poor sensitivity.I would suspect that your static trouble is from local interference or not having the proper antenna loop. It needs to be the right inductance the resistance doesn't matter so much.
Terry

[propflux01]

Its cheap but it's cool too! lol. Any suggestions on how I can better tune that antenna? it came from an onld philco f-809 I used for parts.

[rghines1]

First, be very careful assuming no isolation transformer is available... shock hazard!

Try shorting out the antenna (L1) or C2 to see if the interference is greatly reduced. Otherwise noise is likely internal to the radio.

To tune antenna, move the dial to 1400 and adjust the rear gang trimmer for maximum background noise. This is basically Step 3 in the alignment procedure w/o benefit of a signal generator.

[propflux01]

I am using an isolation transformer on this unit. I was too afraid of getting shocked! Lol

When you say "rear" gang, are you talking about the smaller one or the larger one of the two (I am assuming you are referring to the tuning gang)?

[Radioroslyn]

It would be the trimmer connected to the loop ant.
GL
Terry

[rghines1]

Good on the isolation transformer.

When you say "rear" gang, are you talking about the smaller one or the larger one of the two

Should be the larger, the smaller is for the oscillator.

(I am assuming you are referring to the tuning gang)?

Correct.

The trimmer is normally part of the tuning gang.

[propflux01]

OK, here is what I have done and gotten:
Turned on radio, very little noise at all, no stations picked up. I just got a Heathkit SG-8, and having no lead for it yet, just decided to turn it on, set it to 450KC and see if my radio would pick it up. It picked up a squeal with the gang closed. I used a plastic driver and turned the IF coil until it got really loud. I then shorted the antenna and squeal went away. tried to tune upwards with the generator, following it with the radio. Didn't work out that well at all. pretty much nothing. Closed the gang, set the SG8 to 450 again, and started to open gang. At about 1000KC (radio knob indicated) I could barely hear the sqeal again, had to really get down by the speaker to hear it. heard it again growing ever fainter on approx 1400 and all the way open. During this whole time I never got one single station. only the squeal of the generator. I am waiting on test lead for it to come in the mail. I tried to adjuast trimmer but didn't really have much background noise change at all.

Also would like to thank you guys for replying, and for your patients and help.

Tim

[Image: http://i6.photobucket.com/albums/y240/pr...G_2864.jpg]

[Image: http://i6.photobucket.com/albums/y240/pr...G_2865.jpg]

[Radioroslyn]

The IF adj shouldn't change with tuning the tuning condenser. It should only change with adj the slug in the IF transformers.
Terry

[propflux01]

Tried realigning again. Got good strong signal at the low end and adjusted both upper and lower IF adjustments for loudest squeal. Put signal generator to 1600 but squeal wasn't as loud. Adjusted gang for loudest tone. I took radio in my bedroom and tried to tune, got one station fairly loud but still seems its not very sensitive, as my click radio picked up more stations than the Mitchell would.

[rghines1]

I'm wondering if the oscillator is running. Make some basic voltage checks on the 12AU6 convertor tube. The grid (pin 1) should read about -3 volts or more if the oscillator is running, otherwise it will read zero.

Another method is to use a working radio tuned to around 1455 nearby. Tune the radio in question to 1000 and should hear zero beat in the working radio. Oscillators are designed to run 455 Khz above the incoming antenna signal.

If the oscillator is running then the loop antenna taken from a Philco as an ad hock replacement may have an inductance to far out of tolerance to resonate with the gang/trimmer capacitance. Hate to think an LCR meter is called for.

It's also good practice to make basic voltage checks on all tubes after a restoration.

[propflux01]

I'm getting a -4V from the pin. I tried the next method and I get a good squeal from the 'good' radio with the Mitchell tuned to 1000.
My next question is should I just try another antenna, by either making one or maybe off another junk unit I come across? Or is there some kind of fuzzy math formula to add a component or something?

[rghines1]

The oscillator is working for sure.

Could try adding capacitance in 10 pF increments across the antenna up to 50 pF. If no better that would mean the loop has too much inductance to start with. Inductance of the loop could be reduced by removing turns but will likely be a tedious process.

The math is involved is straight forward if you want to take that approach. Will need to have a way to measure the capacitance of the gang section and the donor loop inductance. This is where a LCR meter is useful.

Typically for a radio like this the antenna gang section will about 100 pF at 1000 Khz on the dial. Loop inductance works out to around 253 micro Henries.

[propflux01]

Newbie question: Would the capacitor be installed in series with the loop or in parallel?

[rghines1]

Connect in parallel with the loop.