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 Post subject: Hallicrafters S-76: 1650khz Station Bleeding Through
PostPosted: Mar Sun 04, 2018 7:13 am 
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My boss bought a Hallicrafters S-76 on Ebay. The seller meticulously re-capped it and replaced the resistors but he could not get it to play and gave it up for sell. I was able to find an open Rf coil, L7. After repairing it, the radio came to life. I went and aligned the thing following the Sam Photofact instructions. I soon found that a third of the AM band is playing a strong Los Angeles station on 1650 AM, KFOX. This is the same frequency as the first IF. I've tried using a wave trap to notch out that station but it didn't go well. We fired up a second S-76 to see if this is a common problem and it wasn't. It played just fine. Does anyone have any suggestions on what the problem might be?

I plan to go into work Monday and open up both radios and compare the wiring and components. Maybe this unit has a bad coupling cap or maybe I goofed up the IF alignment. Anyway, it's weird.


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 Post subject: Re: Hallicrafters S-76: 1650khz Station Bleeding Through
PostPosted: Mar Sun 04, 2018 11:01 am 
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Location: Costa Mesa, California
L7 is the BC band RF coil. It is a major part of how the radio "selects" the frequency to heterodyne with the oscillator frequency to create the first IF. The S-76 leaves out a range of frequencies from 1580 to 1720 to avoid being overpowered by a strong station at or near the first IF of 1650. As you saw with your other radio, a strong station at 1650 should not be a problem.

How did you repair the coil? Which side was open? When you did the alignment, did the BC band seem to align normally? Were you able to peak the top and bottom of the band with L7 and the trimmer adjustment A11. It sounds like L7 is not functioning correctly. There is a coupling capacitor, C10, that would allow signal to bypass the RF coil, so the radio may marginally work with that coil not fully functioning.

Norm

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 Post subject: Re: Hallicrafters S-76: 1650khz Station Bleeding Through
PostPosted: Mar Sun 04, 2018 3:59 pm 
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The Sams says L7 is the 15-40 MHZ band. If its correct the circuit looks odd,
as if there is no plate coil for RF amp on the lowest two frequency bands,
with only a very odd capacitance-resistance load on the RF amp plate. It looks like
the signal is being fed around the 3300 ohm resistors by capacitance in
the switch! BIZARRE ... if true. IS it true?


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 Post subject: Re: Hallicrafters S-76: 1650khz Station Bleeding Through
PostPosted: Mar Sun 04, 2018 4:42 pm 
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Location: Rochester, NY.
Sams uses a different order of reference designators than the factory manual.
L7 looks like an RF choke used to block the 50kc T3/T4 from backing up into the power supply B+.
http://bama.edebris.com/manuals/hallicra/s76/


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 Post subject: Re: Hallicrafters S-76: 1650khz Station Bleeding Through
PostPosted: Mar Sun 04, 2018 5:35 pm 
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I need to admit I was mistaken. L7 is not the BC band.

Norm.

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 Post subject: Re: Hallicrafters S-76: 1650khz Station Bleeding Through
PostPosted: Mar Sun 04, 2018 5:35 pm 
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If you are located close to a strong 1650 Khz. station there will be a major problem with breakthrough into the first IF and it will be worse on the lowest range where the front end offers less attenuation.

Your choices are either:

1. Build a very good shielded trap and make sure that the case and chassis have a good RF ground and that the line bypass, speaker leads, and anything else coming into the case are bypassed.

2. Realign the first IF to 1660 Khz. which should cure the problem, this should not create any major calibration/tracking issue with the S76 since it is a fairly small change. Of course the second oscillator must be aligned to 1610 instead of 1600 khz. to provide the correct conversion to the final IF.

I suspect the other S76 may have had its alignment changed to avoid this common issue. When these Hallicrafters receivers were designed 1650 Khz. was not part of the BCB.

My SX-101A required a trap when operating on 160 Meters to prevent a noticeable hetrodyne from a lower power AM station on 1650 Khz. located 100 miles away in Iowa. If this is a powerful broadcaster anywhere close then a suitable trap, shielding, and grounding will be critical if you want to take the approach of rejecting rather than avoiding 1650 Khz. RF coming in via the antenna and/or other external leads.

Rodger WQ9E


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 Post subject: Re: Hallicrafters S-76: 1650khz Station Bleeding Through
PostPosted: Mar Sun 04, 2018 5:52 pm 
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dtvmcdonald wrote:
The Sams says L7 is the 15-40 MHZ band. If its correct the circuit looks odd,
as if there is no plate coil for RF amp on the lowest two frequency bands,
with only a very odd capacitance-resistance load on the RF amp plate. It looks like
the signal is being fed around the 3300 ohm resistors by capacitance in
the switch! BIZARRE ... if true. IS it true?


Now that you pointed this out, I also am intrigued by how the signal is coupled and tuned by the RF stage for the two lower bands. I would suggest that the coils are receiving the stations inside the radio and coupling the signal forward, but you may be correct that it is bandswitch coupling. If it were the coils, then that would explain why a strong local station overpowers the band. Place your radio inside a metal box and see if that prevents the interference.

Norm

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 Post subject: Re: Hallicrafters S-76: 1650khz Station Bleeding Through
PostPosted: Mar Sun 04, 2018 8:05 pm 
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Maybe unrelated but... In my BC-348 receiver I had a very close by radio station at 920khz bleeding into the 915khz IF. My trap at the antenna was only partially effective until I mounted it inside a small metal box and mounted to the chassis. Then the chassis was slide back into the case. That made a bigger difference but still didn't take out the interference completely. I saw a number of long wires inside the chassis. I clipped a .05uf bypass cap in a bunch of places one at a time until WOO HOO, the interference stopped. I think it was in the high voltage power supply section. But as mentioned by rsingl, it could be coming from other wires in your radio.


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 Post subject: Re: Hallicrafters S-76: 1650khz Station Bleeding Through
PostPosted: Mar Sun 04, 2018 8:36 pm 
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dtvmcdonald wrote:
The Sams says L7 is the 15-40 MHZ band. If its correct the circuit looks odd,
as if there is no plate coil for RF amp on the lowest two frequency bands,
with only a very odd capacitance-resistance load on the RF amp plate. It looks like
the signal is being fed around the 3300 ohm resistors by capacitance in
the switch! BIZARRE ... if true. IS it true?


Note that L7 is the designation from Hallicrafters. Sam's will be different. That was a shock when I was bouncing back and forth between the two documents and discovering that the component designations are different between Hallicrafters and Sam Photofact.


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 Post subject: Re: Hallicrafters S-76: 1650khz Station Bleeding Through
PostPosted: Mar Sat 17, 2018 11:22 pm 
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This S-76 problem has been resolved. I had an issue aligning this radio. It was very insensitive on the 14 to 30Mhz band requiring a 50 to 200mV signal to hear a tone during alignment. My boss started tapping around that circuit and low and behold, the 400hz signal came blaring out of the speaker. I was able to reduce the generator's output down to about 5 to 10 uV, huge improvement. The culprit appears to be the temperature compensated cap, C54 or C56 I think. It's a trimmer cap that's in parallel with a 10k resistor, I believe. There's no information on it's tuning but it was very loose and barely fasten together. I removed it, tightened it up, then reassembled. The unit is now aligned and playing beautifully. There's a little bleeding from KFOX on band 2 but not nearly as bad as it was before. Band 1, AM broadcast, is playing beautifully. It evens picks up the only Oldie station in LA, it comes in fairly weak on most radios.


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