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Alignment/Oscillator Problems on 19B - Printable Version

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Alignment/Oscillator Problems on 19B - PAradiogeek - 01-22-2015

HI Everybody,

I restored a 19B, code 126 about a year ago with a lot of help from many people here.  A week or two ago, I powered it up and found that the radio went silent below about 720 KHz.  I remembered posts about the 36 tube cathode resistor (item 10) being too high of a value, and that this resistor could go to as low as 7500 ohms.  At the time I did the restore I had lowered this to 10K and it seemed to work.  So last week when the radio stopped working at the low end of the dial, I opened it up and put in 7500 ohms and it seemed to work fine.


Then I got this great idea that I should try another alignment.  I had done this radio rather early on and I wondered if I might be able to get it aligned a little better now that I had a little more experience.  I hadn't put it back together since changing the cathode resistor, so last night I tried an alignment.

I tuned up the IF cans, and also made adjustments at 1400 KHz as per the instructions.  I was making modest improvements and all seemed to be going well.  Then I came to the point where I was to make adjustments at 650 KHz.  I set up the signal generator and tuned the dial down to '65'.  The radio was completely silent.  I hadn't noticed it at first, but the radio cut out at around 950 KHz.  I went back and tried tweaking the IF and 1400KHz adjustments again, but no joy. 

Does anyone have an idea about what I did wrong, or how to proceed?  I was using very low signal strength from my signal generator.

It seemed like this was working with the change of item 10 to 7500 ohms, but now that value isn't working.  No other changes were made to the radio since modifying the cathode resistor value.  Radio still works above 950 or so.

Thanks in advance.

RE: Alignment/Oscillator Problems on 19B - Radioroslyn - 01-22-2015

If you rewound the osc coil You need more turns on it. If you didn't  you need to.
Terry

RE: Alignment/Oscillator Problems on 19B - PAradiogeek - 01-22-2015

I did rewind the outer coil connected to the cathode. It had 27 turns originally, I put 28 turns on my rewind. Is this the coil that needs more turns, or do all three of the coils on that transformer need additional turns?
Also, any guesses as to how many additional turns is enough?

RE: Alignment/Oscillator Problems on 19B - PAradiogeek - 01-29-2015

I have figured out why the oscillator in this set was misbehaving. In short, it was humidity.

I had had the radio on the bench in my basement workshop for over a week since changing the oscillator cathode resistor. In that span of time, we had had several days of heavy rains. I don't run my dehumidifier in the winter, and I think that the humidity level in the basement rose enough to cause moisture to be absorbed by the oscillator coil form.

A day or two after initiating this post, where I indicated the radio wasn't working below about 950, I went back down and retested. This time, the radio wouldn't work below about 1200. Nothing at all had been done to the radio in those two days and yet it was working rather significantly worse. That was when I remembered that this coil is susceptible to moisture absorption, which impedes oscillator function.

I did find an easy way to 'dry out' the coil though. We have forced air heating, so I decided to just lay the radio on top of one of the heater vents upstairs. Two days later, I retested, and the radio works all the way to the bottom of the band.

I know that, long term, I need to rewind that coil, but I just didn't feel up to it now. And I didn't even have to remove the coil and bake it. I will have to see if this method of drying the coil works for any extended period.