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 Post subject: Re: Predicta Holiday Jitter
PostPosted: Sep Mon 20, 2021 10:07 pm 
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Forgot to mention that the instability is accompanied by a rhythmic audible thrum, with pulses about one second apart.

This is a new one for me.


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 Post subject: Re: Predicta Holiday Jitter
PostPosted: Sep Tue 21, 2021 1:57 am 
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Location: Ann Arbor, MI
Is the thrum coming from the speaker or from some component?

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 Post subject: Re: Predicta Holiday Jitter
PostPosted: Sep Tue 21, 2021 4:20 pm 
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Something that comes to mind is too much AGC. But I don't know if this applies to a Predicta. I have two of them, but have not started working on one yet. Just a thought, and those who know a lot about this set will have an answer.

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 Post subject: Re: Predicta Holiday Jitter
PostPosted: Sep Tue 21, 2021 4:56 pm 
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Since it is OK for 3 minutes something is probably heating up. Maybe get a can of freeze spray and see if you can locate what it is in the horz section. Just be careful not to hit a hot tube. I can tell you that doen't end well. :lol:


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 Post subject: Re: Predicta Holiday Jitter
PostPosted: Sep Tue 21, 2021 6:02 pm 
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The electrolytic caps filtering the various B+ voltages also act as bypass caps. When a circuit draws variable amounts of current, the caps prevent the B+ from varying. When this bypass action fails circuits can act very strangely and circuits can interact. In this set we have both the horizontal and audio circuits acting strangely at the same time.
I wonder if one of the new filter caps is failing or if there is a bad solder joint causing a cap to become disconnected.
If you have an oscilloscope you could check the B+ lines. Or you could just double check the solder joints.
Possibly just try gently wiggling the capacitors.

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 Post subject: Re: Predicta Holiday Jitter
PostPosted: Sep Wed 22, 2021 5:57 pm 
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Joined: Mar Mon 30, 2020 12:50 am
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Tom, the thrum is coming from the speaker.

madlabs, I will do some component cooling, and will try not to destroy any tubes in the process.

Thank you all.


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