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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Gentlemen,
I've got this RF sig gen I'm testing at the moment. It was made in 1955 and is all valve (I say "all valve" but there are only two tubes in it and only one of those generates the RF - the other's 400Hz for the modulation). Anyway, it has 8 ranges in total covering 100KHz to 240Mhz. The oscillator tube is a double triode, a 12AT7. One half handles range from 100Khz to 30Mhz and the other takes care of 30 - 240Mhz. Now, it all works great EXCEPT for one range (the 3rd lowest) which has appreciable distortion present on the output. It looks a bit like it's being over- driven on my scope. My question is, does a valve like a 12AT7 require different DC biasing points for every range of frequencies? Obviously the range switch is switching in different combinations of coils and capacitors, but is it likely to be also switching in different cathode- grid DC biasing at the same time? TIA -- This message may be freely reproduced without limit or charge only via the Usenet protocol. Reproduction in whole or part through other protocols, whether for profit or not, is conditional upon a charge of GBP10.00 per reproduction. Publication in this manner via non-Usenet protocols constitutes acceptance of this condition. |
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