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Hallicrafters S-76 Reception Problem on Broadcast AM




 
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Author Topic: Hallicrafters S-76 Reception Problem on Broadcast AM  (Read 566 times)
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KK6LPW
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« on: March 03, 2018, 12:04:57 AM »

I work for a radio collector learning to repair old radios. My boss had me inspect a Hallicrafters S-76 he bought on Ebay from a guy who gave it up after meticulously re-capping and replacing resistors on it but it would not play. I traced the problem to an open Rf choke, L7. After repairing the choke, it now works and so I aligned it. It was a difficult alignment since there are two IF frequencies to deal with. The 2nd IF, 50khz is done first and the response is sometimes vague. Then the 1st IF at 1650khz is done and then the alignment of the frequency to the dial. I've aligned it twice using two different methods but I still get bleed through from a strong radio station on 1650 khz. The same frequency as the 1st IF. This station seems to show up on several spots on the last third of the AM broadcast band. I've tried using an antenna trap but it was broad enough to suppress this station. Anyone have a clue on what the problem might  be?

Fortunately we have a second S-76 that is still mostly original that we can use to compare the circuitry. Maybe something got mis-wired or maybe there is a bad component. This other S-76 does not exhibit this problem. It seems to play OK but needs a full restoration because of the old caps etc. Any suggestion is appreciated.
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w4bfs
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« Reply #1 on: March 09, 2018, 02:04:55 PM »

sounds like a parasitic oscillation in the ist mixer .... I have seen this when recapping old receivers .... a lossy old wax cap could keep it snuffed and a low loss replacement would allow it to bloom ... a sticky wicket .... you may have to experiment to get it to settle down .... using a scope may help you to see
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Beefus

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to see ourselves as others see us.
It would from many blunders free us.         Robert Burns
AJ1G
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« Reply #2 on: March 12, 2018, 06:44:41 PM »

Try using a good short direct ground on the receiver along with the wave trap in the antenna circuit, also check the ac line bypass caps, if the station is strong enough it may be bleeding in via the AC line.
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Chris, AJ1G
Stonington, CT
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