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 Post subject: H.H. Scott 350 early MPX 38 kHz oscillator
PostPosted: Apr Tue 30, 2019 4:52 pm 
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Joined: Jun Sun 23, 2013 9:03 pm
Posts: 432
Location: Canandaigua, NY
A friend brought over a 350 tuner for alignment. We couldn't get stereo and realized that the MPX oscillator wouldn't free run at 38 kHz, but would only adjust down to about 42 kHz. The cap values in the tank tested OK (5600 + 1000 pF). I ended up putting another 1500 pF across them, and then everything worked fine. Instant lock and good stereo separation. I can't think of anything that would cause the oscillator to run fast unless something was wrong with the associated coil/slug. I don't think it was being loaded by a 10X scope probe, at least not nearly that much. Could a tube cause this? The values didn't match any of the schematics we had, but I doubt the factory would have shipped it not working in stereo. Any ideas on what was really wrong?


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 Post subject: Re: H.H. Scott 350 early MPX 38 kHz oscillator
PostPosted: May Wed 01, 2019 3:49 am 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 538
Location: Fayette County, Pa
I would suspect the coil or slug. Possibly a crack in the core which would effectively lower the inductance value. This would raise the frequency of resonance.


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 Post subject: Re: H.H. Scott 350 early MPX 38 kHz oscillator
PostPosted: May Wed 01, 2019 1:53 pm 
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Joined: Jun Sun 23, 2013 9:03 pm
Posts: 432
Location: Canandaigua, NY
Thanks- I was afraid of that. I think we'll call it done, as it works fine and I don't really want to remove and disassemble that can. The slug can't be removed without disassembly.


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 Post subject: Re: H.H. Scott 350 early MPX 38 kHz oscillator
PostPosted: May Sat 04, 2019 2:41 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20064
Location: Warner Robins, GA
Would an out of tolerance resistor cause that?


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 Post subject: Re: H.H. Scott 350 early MPX 38 kHz oscillator
PostPosted: May Sat 04, 2019 4:45 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20390
Location: Utica, NY 13502 (USA)
Conrad Hoffman wrote:
Thanks- I was afraid of that. I think we'll call it done, as it works fine and I don't really want to remove and disassemble that can. The slug can't be removed without disassembly.
I had a similar problem with the detector coil in my Scott LT-110 tuner. But, in my case, it was silver-mica disease. After removing the mica plates inside, I installed an external trimmer cap across the winding and was able to bring the detector into proper alignment. I called it "done" also.

Dave


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 Post subject: Re: H.H. Scott 350 early MPX 38 kHz oscillator
PostPosted: May Sun 05, 2019 8:02 pm 
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Joined: Jun Sun 23, 2013 9:03 pm
Posts: 432
Location: Canandaigua, NY
That prompts an interesting thought. The caps I know about are external, and that's all the schematic shows. But, if there are any caps inside the can, they could easily be bad. If it keeps working, we'll never know, but if it drifts off I'll probably end up pulling it apart. Regardless, very pretty gold faced tuners, and the THD performance was far better than published. I got it down to near 0.6% instead of 2%.


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 Post subject: Re: H.H. Scott 350 early MPX 38 kHz oscillator
PostPosted: May Mon 06, 2019 2:28 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20390
Location: Utica, NY 13502 (USA)
The way Scott drew their schematics, caps inside the cans of coils and transformers are shown but not marked with values or part numbers. Looks like the coils in the MPX section of my LT-110 don't have internal caps. I think the 350 is the same but I don't have a schematic for the no-letter suffix version that you have. I agree that the gold-faced Scotts are the best looking and that their performance far outshines their specs.

Dave


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 Post subject: Re: H.H. Scott 350 early MPX 38 kHz oscillator
PostPosted: May Mon 06, 2019 4:25 pm 
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Joined: Jun Sun 23, 2013 9:03 pm
Posts: 432
Location: Canandaigua, NY
This one didn't quite match any of the available schematics in terms of cap values, but it seemed factory original. There are a lot of secret details nobody mentions about doing alignments. One of them is to put a good 19 kHz notch filter on the input of your THD meter, as the pilot residual will contaminate the measurements. Then, adjust by looking at the THD residual on a scope, before reading any numbers.


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