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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 17th May 2011, 11:07 pm   #1
1955APREN
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Default Star Sr550

Can anone help with a loud hum on this set present as soon as the set is switched on before the valves have had time to warm up. I have replaced the main res/smothing cap this as improved the fault but is still present.
It looks like that is set as been got at by a unknown person, there is a blue lead that comes out of the primary side of the audio output transformer
that does not go any were infact it as a peice of tape on it . The ht smothing resistor is w/wound 250r. I would appreciate help in solving this problem .
Thanks Derrick
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Old 18th May 2011, 10:55 am   #2
G8HQP Dave
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Default Re: Star Sr550

Rectifier fault?
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Old 19th May 2011, 8:56 am   #3
1955APREN
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Default Re: Star Sr550

Thanks Dave
Both ht diodes havs good forward resistance and no back leakage but i will replace with a couple of by's and see what happends
regards Derrick
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Old 19th May 2011, 10:48 am   #4
G8HQP Dave
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Default Re: Star Sr550

Does the audio pentode have a capacitor from anode to ground? Many radio sets have this. Is this capacitor leaky?
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Old 20th May 2011, 3:57 am   #5
PEHamel
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Default Re: Star Sr550

The unused blue lead just taped off indicates that the output transformer was replaced with a push-pull transformer.
Check for miswiring and poor solder joints. The replacement transformer may have to be rotated if it is near the main power transformer. Did they drill new mounting holes for it?
Good luck
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Old 9th Jul 2011, 8:44 am   #6
1955APREN
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Thumbs down Re: Star Sr550

Thanks to Dave and Pehame
for the advice sorry that i have not replied b4. the wire from the output tx was not a center tap but was about a 3rd of the way from
the ht conection, but looks seemed to indicate that it had been changed at some time . "new by133 made no difference and as i did not have circuit /dig
I returned unit to the owner G6cob Thanks again . This thread Can now be removed from the list. Regards Derrick
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