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 Post subject: superadio 3 selectivity mod from earmark
PostPosted: Nov Tue 22, 2022 5:56 pm 
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Joined: Jan Mon 24, 2022 8:30 pm
Posts: 51
Location: Kanabec mn
has anyone done this modification on their superadio? it involves changing r7 from 100k to 50k on the front end of the radio to increase selectivity.my radio seems to get a lot of splatter from other close stations when I'm trying to dx at night.just wondered what your results were?...............snuffy


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 Post subject: Re: superadio 3 selectivity mod from earmark
PostPosted: Dec Wed 07, 2022 4:12 am 
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Joined: Jul Thu 12, 2007 8:36 am
Posts: 913
Location: Boston, Ma
Hey Snuffy,

Just thought I’d touch base Maybe NONE of this is any help, but….

. I have an SR-1 and it has not been modded.

There may be one or two other things to MAYBE try before you try any mod if you are hesitant to do it as a first option.

Try rotating the radio on a plastic or wooden lazy Susan .There might be enough of a null if you can very carefully rotate the radio.

Maybe ( maybe) consider making a AM band tuned loop. Generally, strong stations don’t benefit from loops.But even a moderate effort loop may ( may ) begin to null adjacent station interference .

I use a 660/46 litz rectangular loop tuned by a 4 gang 500 of ceramic variable cap wired “ wiperless” . This delivers 500 pf of high Q variable capacitance.

Some of the best results come from quite unexpected geometries of which way my radio faces vs the loop direction.Sometimes you need some real fine trial and error re directions, distance, etc. But the results could be impressive if you find an available sweet spot.

My loop “at times” can actually deliver two stations on the same frequency.

Also it can knock down adjacent frequency interference much of the time.

A moderate effort loop can work quite well . Few radio projects deliver as much bang for the buck compared to the effort to build it.

Again, maybe of NO help to you in the end, just putting it out there.

If you search GE Super Radio , there are a few very good articles available to learn more about this COOL radio.

K


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 Post subject: Re: superadio 3 selectivity mod from earmark
PostPosted: Dec Wed 07, 2022 5:36 am 
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Joined: Jan Thu 01, 1970 12:00 am
Posts: 41950
Location: Canyon Country, CA
snuffy wrote:
my radio seems to get a lot of splatter from other close stations when I'm trying to dx at night.

Sounds like the IF filters aren't peaked to their maximum. Do you have a signal generator?

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 Post subject: Re: superadio 3 selectivity mod from earmark
PostPosted: Dec Wed 07, 2022 9:26 am 
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Joined: Mar Tue 27, 2012 12:24 pm
Posts: 1614
Location: Seattle, WA
It gets splatter because the SRIII uses 4 IF cans, instead of a ceramic filter. It's also why you get better sound. You probably would get splatter even if you had a communications receiver. Splatter is something we all have to deal with. Some radios can cut it down a lot -- the tradeoff is the sound is muddier.

Try using an external loop with your SR3. And experiment with placing it away from the radio. That can help with selectivity.

R7 being reduced may increase sensitivity, but if you're getting splash from locals already boosting sensitivity by reducing the value of R7 may not help much.

I've reduced the value of similar resistors on my SR1 and SR2, which helped (the actual value of these resisters seemed to vary from the factory), but I didn't see a change in splatter from local stations. That is pretty much a given with most radios.

Try using an external loop. They're less than $50 online (the Eton AN200 is a good one). Or you can make one, if you have a tuner capacitor, a frame of some sort (a milk crate works wonders) and 110 ft. of wire.

All that said, if you're really good with a soldering iron, you could always try reducing the value of R7 and see what happens. If you solder another 100K resistor in parallel it should be easy enough to undo if you don't get the results you want.


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 Post subject: Re: superadio 3 selectivity mod from earmark
PostPosted: Dec Wed 07, 2022 8:17 pm 
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Posts: 41950
Location: Canyon Country, CA
Renton481 wrote:
you could always try reducing the value of R7 and see what happens. If you solder another 100K resistor in parallel it should be easy enough to undo if you don't get the results you want.

In situations like this, a good method is to remove one leg of the resistor (or both), tack solder leads going to a 100K pot, and try reception with the pot at different levels.

Once you get the best performance, measure the pots resistance, and get a fixed resistor close to that value to use as a replacement.

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 Post subject: Re: superadio 3 selectivity mod from earmark
PostPosted: Dec Thu 08, 2022 1:41 am 
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Joined: Jan Mon 24, 2022 8:30 pm
Posts: 51
Location: Kanabec mn
thanks guys for the feedback.in response to your suggestions I have a loop that I have used but not much with this radio. Been using it as a bed side radio and a four-foot loop is inconvenient :lol: but will definitely try it when I can come up with a spare condenser for smaller loop.no I don't have a signal generator, it's also on the list of things to get. quality control was to supposed to be poor with some of these radios so tune up of the ifs may be in order. The earmark article said it was a selectivity mod not a sensitivity mod. Which is it? he said it increases the Q of the coil. Which increases the selectivity? i don't know.im not so worried about big wide sound as intelligible speech. I like the idea of a variable resistor. I wonder if it could be placed outside the radio without ill effects. Thanks again for the feedback will keep you posted..........snuffy


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 Post subject: Re: superadio 3 selectivity mod from earmark
PostPosted: Dec Thu 08, 2022 2:12 am 
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Joined: Jan Thu 01, 1970 12:00 am
Posts: 41950
Location: Canyon Country, CA
From my understanding, increasing Q increases sensitivity. The idea of peaking the IF's is to make the gain on any one station sharper vs co-channel interference.

You could get either a Selec-A-Tenna, or a Tecsun/Grundig/Kaito AN-200 tunable loop, which are smaller units to use on your nightstand. I've found that placing a tunable loop at different positions and/or angles around the radio can make a big difference.

I would suggest testing a pot at different resistances for awhile, before committing to drill a hole in the case to mount it for external adjustment. You may find that one resistance value only is all that's needed.

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 Post subject: Re: superadio 3 selectivity mod from earmark
PostPosted: Dec Thu 08, 2022 3:17 pm 
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Joined: Mar Tue 27, 2012 12:24 pm
Posts: 1614
Location: Seattle, WA
The SR1 and SR2 have similar resistors. Varying the resistance can, in some situations, improve the sensitivity of the radio. On the SR2 and SR1 it's R13. The resistors in all three cases are on the line that goes from the IF Amp inside the chip to the antenna coil.

But changing the value of the resistor can, in some cases, also improve selectivity somewhat. And there's apparently an adage that improving selectivity also improves sensitivity -- due to some rule or law of electronics that I don't completely understand. The two characteristics (selectivity and sensitivity) are related in some way.

Just rest assured that varying the resistance of that particular resistor can alter the performance of the radio.

If I remember correctly I tried lowering R7 in my own SRIII and it made it worse. Added a lot of noise or something. It was years back, so i don't remember. I was just glad I didn't wreck the radio while soldering and then removing the parallel resistor in to try it out.

My decreasing the values on the SR1 and SR2 worked a bit better. But then, I had seen some schematics where those radios had lower R13's than the ones that were in my radios.

My SRIII is plenty sensitive and selective enough without any mods. If I need a boost -- I just add the external loop.


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 Post subject: Re: superadio 3 selectivity mod from earmark
PostPosted: Dec Thu 08, 2022 9:56 pm 
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Joined: Jan Thu 01, 1970 12:00 am
Posts: 41950
Location: Canyon Country, CA
Renton481 wrote:

My SRIII is plenty sensitive and selective enough without any mods. If I need a boost -- I just add the external loop.

Sound advice, unless one wants to tinker.

_________________
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 Post subject: Re: superadio 3 selectivity mod from earmark
PostPosted: Dec Tue 27, 2022 3:52 pm 
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Joined: Jan Mon 24, 2022 8:30 pm
Posts: 51
Location: Kanabec mn
well i attempted the modification this week without success. Was unsure of location of the r7 resistor on the board. Found what i think is the location but it's all jammed under other components under wax and no way to jump it from the top of the board.and i didn't feel like removing the board with the delicate antenna wires next to it. i might try it in the future when i get a signal generator and have another go at checking the am rf.what i did find was that the radio was full of saw dust.i dont know how it could have got so much saw dust inside.it must have been a shop radio for someone. i blew it out and cleaned it the best i could and it seems to sound better. still has some birdies and dial misalignment but I'll live with it for the time being. Will keep you all posted...........snuffy


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