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 Post subject: 1930 Sparton 400 series, low volume issue
PostPosted: Jun Tue 11, 2019 4:55 am 
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Joined: May Sat 07, 2016 12:46 am
Posts: 159
Location: Las Vegas NV
Hi..Ive been working a Sparton 400 series.. I have replaced all the caps, checked the resisters, and all tubes are good.. able to pull in a few stations when tuning (but real low volume) when the volume is all the way up.
Did a signal trace and found something interesting (never ran across this before) so not sure of my findings... in the diagram shows the input transformer the green dots I can hear the stations and changes when I move the dial... however, at the red dot I get a lower volume as the same that coming out of the speakers. so I went down and purchased a 124A transformer and changed it out.. still with the same results. could it be the volume control? or ??????

I was able to do an aliment (little different then I'm used too) reset the antenna screw to the loudest out put...put 1200 signal on the antenna post and dialed in the RF adjustments

link:
http://www.nostalgiaair.org/pagesbymode ... 029725.pdf


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 Post subject: Re: 1930 Sparton 400 series, low volume issue
PostPosted: Jun Tue 11, 2019 5:32 am 
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Joined: Nov Tue 14, 2017 5:09 am
Posts: 1864
Location: Austin, Texas
You are not supposed to get any signal at the red dot. That is the B+ supply and it should be a DC level.

Jay


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 Post subject: Re: 1930 Sparton 400 series, low volume issue
PostPosted: Jun Tue 11, 2019 6:15 am 
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Location: Las Vegas NV
Hi Jay.... Im not sure.. on that one... I placed the signal tracer on it and I get a low volume broadcast which is the same going out to the speaker.. The signal is strong the other side of that transformer. so my thinking that the transformer may be shorted. When I replaced it with a new 124A Transformer, I got the same results. never really worked with an input transformer before.... (just thinking out loud) could the output transformer on the speaker be my problem?

I just was looking over the diagram again.. the first tube off the antenna (24) Im going to re-look at the Pin2 again since that feeds that side of the transformer


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 Post subject: Re: 1930 Sparton 400 series, low volume issue
PostPosted: Jun Wed 12, 2019 4:34 am 
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Joined: Nov Mon 02, 2009 7:01 am
Posts: 3726
Location: Lincoln City, OR
Greetings to binaryxy and the Forum:

I would measure the DC potential between the 183 filaments and ground. There should be approximately 60 volts here. If this voltage is incorrect, it indicates that one or both output tubes are bad or the output transformer primary is bad.

If the voltage is correct, additional causes could be an open secondary on the output transformer or the speaker voice coil may be open. The speaker may also be frozen mechanically, although this is usually accompanied by severe distortion.

The nominal grid bias for the 183 is given as 60 volts for class A operation. I would expect the audio drive signal to swing at least 30 volts on either side of this value and probably more at high volume. If you look at the audio peak to peak voltage on either 183 grid (the green dot points on the secondary of your driver transformer) with a 'scope, you should see at least 60 volts peak to peak.

If anyone else on the Forum has curves for the 183 or can provide a value for the grid swing at or near full power output, then we could be more certain of whether or not the tubes are being adequately driven. But do measure the bias first as I requested above.... it will at least establish that the tubes are operating at the correct DC parameters.

Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: 1930 Sparton 400 series, low volume issue
PostPosted: Jun Wed 12, 2019 7:38 pm 
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Location: Las Vegas NV
Hi guys.... I walked back though the chassis and voltage check every thing again... it all comes down to the audio circuit... this unit has two 71A tubes which feeds the output transformer on the speaker... Pin3 on the 71A (Both tubes) the signal is clear and able to turn it up and down with the switch.. However, on Pin2 (On both tubes) I get a buzz with the signal (weak).. both tubes have been checked out... for what I'm seeing its ether weak tubes or the output transformer on the speaker feeding back to the tubes.....other then Ohm checking the transformer, What other method I can use to rule it out?


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 Post subject: Re: 1930 Sparton 400 series, low volume issue
PostPosted: Jun Sun 16, 2019 12:14 am 
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Joined: May Sat 30, 2015 11:49 pm
Posts: 837
Location: Keystone Heights, FL, USA 32656
So, yes, measure the ohms of the output transformer. Radio unplugged, pull both output tubes, measure pin 2 to pin 2. Then measure each tube pin 2 to center tap; what do you get?

Just for reference, measure the resistance of the primary and secondary of the input transformer, the one you replaced, just for information.
Jeff


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 Post subject: Re: 1930 Sparton 400 series, low volume issue
PostPosted: Jun Mon 17, 2019 6:44 pm 
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Location: Las Vegas NV
HI Jeff, I was trying to load the pictures on this tread and the website was kicking them back.. sorry for the confusion... I have the measurements requested

pin2 to pin2 (without tubes) came in at 497 ohms
pin2 (1) to tap was at 234 ohms
pin2 (2) to tap was at 259 ohms

however the 71a filaments to ground came in at 132 Vac
71a grid bias was at 1 vac


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