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 Arvin 551T oscillator drift.

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frankradio1



Posts : 4
Join date : 2010-10-17
Age : 80
Location : Sunny Southern Illinois

PostSubject: Arvin 551T oscillator drift.   Wed Oct 20, 2010 7:51 am

Hello,
This radio drifts off frequency over a period of about a half hour. and then remains stable. I've tried a different 6BE6 with no improvement. I don't see any components in the oscillator circuit that would cause this other than the coil. It has been recapped, etc. and has excellent sensitivity for a 5 tube. What am I missing?

http://www.nostalgiaair.org/PagesByModel/246/M0001246.pdf

Tom Ackerman
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marcc

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Posts : 30
Join date : 2010-10-12
Location : Victoria, Australia

PostSubject: Re: Arvin 551T oscillator drift.   Wed Oct 20, 2010 8:03 am

6BE6 is a bit prone to doing that more than others. It is a normal characteristic for tubes to drift until the set stabilises in temp. Normally this is upwards in frequency.

There is an off chance that something like a plate, cathode or screen resistor is getting a bit hot. were the resistors checked as the caps were changed?

Marc
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frankradio1



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Join date : 2010-10-17
Age : 80
Location : Sunny Southern Illinois

PostSubject: Re: Arvin 551T oscillator drift.   Wed Oct 20, 2010 12:24 pm

All the resistors were within 10%. I tried a used 6BE6, but I'll try a new one. I didn't think it was the tube, as it drifts for a half hour or so. I'll try checking the resistors after it warms up good and see if they change from their cold values.

Tom
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marcc

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Location : Victoria, Australia

PostSubject: Re: Arvin 551T oscillator drift.   Wed Oct 20, 2010 7:57 pm

I would expect natural change in the resistance with temperature, carbon decreases resistance with heat metal increases.

Voltage could be fluctuating, I doubt that there is too much wrong.

Marc


Last edited by marcc on Fri Oct 22, 2010 4:24 am; edited 1 time in total (Reason for editing : typo)
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Dave Wise



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Join date : 2010-10-18
Location : Hillsboro Oregon

PostSubject: Re: Arvin 551T oscillator drift.   Thu Oct 21, 2010 12:21 pm

Drift over a half hour is not the tube unless it's weak and near dropping out. How much does it drift? The tuning coil will expand as it warms up, which increases the area enclosed by the turns and therefore the coil's inductance, driving the frequency down. Some manufacturers use higher quality coil forms or include a temperature-compensating cap, but most don't.

Dave Wise
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