The Fine Things are Always Hand Made
AW23 alignment issue
Looking for advice and ideas. I'm restoring an early AW23 for a friend. I replaced all the wax caps and have replaced the resistors in the C divider (out of spec, now have good divider voltages). IF is aligned nicely, and the 3 SW bands are also aligned and work well.
AM is another story...it drifts. A lot. I sit here with it set to 1400, generator on 1400 and the signal fades in and out. It seems to drift up (when it gets quiet, if you go up to 1420-1430, you hear the signal again). I've tried several OSC tubes. Oh, the set does have the two caps strapped on the OSC filament and the shorter cathode ground - it went back to the factory for those at some point - work was done when I opened it up. Working through the alignment - everything "behaves" as you'd expect except for the massive drifiting...
Any ideas or suggestions would be appreciated!!
Kent
Replies to This Discussion
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Permalink Reply by mike hadley on
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I take it you have checked the HT after the regulatotr tubes is stable and the 75k grid resistor is ok, I once had one drift high when warm,
If the radio is ok on the other wave bands then it would suggest the oscillator circuit is ok, try swappping out the coil wheel just to see if the fault goes away then this would point to the mica caps being leaky,
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Permalink Reply by Kent King on
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Thanks Mike!
An update: the grid resistor was out of spec high, I replaced it. The regulated plate voltage is solid at 127V. I was wrong - all bands drift. It's about 20-30 KHz, so very noticeable on AM, but not as much at 10+ MHz. Even though they all drift some, I tried swapping the coil wheel - no difference. I also took the 4 front end tubes from one of my "daily players" (6A7, 6B7. 6D6 and osc) and swapped them out - still drifts. So it is something in the non-swappable part of the set, common to all bands. But...back to work tomorrow, so I'll have a few days off to think about it.
Could it be in the IF? I may try to realign it, but it was really tight - when I got done, I was only injecting a 100uV IF signal. I tuned to 465, no indication on the chassis that the IF is different. Any other ideas?
Kent
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Permalink Reply by Alex R. Whitaker on
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Flaky neon tube oscillator voltage regulator? Dirt in one of the neon tube sockets? You might want to get out the freeze spray for this problem. Check the mica caps that are in common to all the bands.....I've had the oscillator plate bypass cap (mica) go out in a couple non-Scott sets, as it is subject to high voltage and associated breakdown with age. You could put an oscilloscope on the oscillator output, and start spraying things with freeze spray, and looking for massive frequency change. That's how we used to do it in the radio shop I once worked for.
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As alex stated first thing is to check that the local oscillator is ok, a scope would be good but a frequency counter would be better as you only have a 20-30 Khz drift.
once this is checked start looking at the IF, there could be a problem with one of the micas but once you are past the converter it's all over as far as frequency drift you would notice the amplitude changin rather than the frequency,
have a poke around the 6d6 front end and the 6d6 tubes.
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Permalink Reply by Norman S Braithwaite on
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The problem will not be found in the IF amplifier. The IF amplifier simply amplifies the signal it receives from the converter tube. Problems in the IF amplifier will affect the degree of amplification, not the frequency of the received signal. The problem lies in the RF or oscillator circuits. Have you removed the tuner and re-tightened the stator lug screws? Also, have you cleaned and re-lubricated the rotor wipers?
Norman
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Permalink Reply by Bill Liers on
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My question is, does it only drift up in frequency as the set warms? This points to oscillator components. I would use a large soldering iron to heat micas or anything tied to the socket pins. Shouldn't take much. "fading in and out" suggests possible slight regulator variation.
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I haven't had radio time in the past several days, but it fades in and out, about a 1 minute cycle. While this is going on, I have a digital voltmeter on the osc plate line from the regulator tubes, it doesn't change. I'm going to start looking at the mica caps when I get back to it...work is in the way again - must retire....
Norman - I did have the tuner off the set, cleaned and lubricated and resecured to the chassis. All part of the process.
Kent
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Then your problem is almost surely a mica cap. I would think, if a resistor was changing back and forth like that, you'd get noise. But, once again, freeze spray is your friend. If you don't have any, and have a can of computer duster air, then you can turn it upside down and spray components. It's just as cold as freeze spray.
Kent King said:I haven't had radio time in the past several days, but it fades in and out, about a 1 minute cycle. While this is going on, I have a digital voltmeter on the osc plate line from the regulator tubes, it doesn't change. I'm going to start looking at the mica caps when I get back to it...work is in the way again - must retire....
Norman - I did have the tuner off the set, cleaned and lubricated and resecured to the chassis. All part of the process.
Kent
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