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 Post subject: Allignment questions
PostPosted: Feb Thu 19, 2004 11:41 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 529
Location: Dallas, TX USA
I have got a Westinghouse H-122a that I've been tinkering with. Seems it had a fire on the top of the chassis and melted the plates of the tuning cap. It's had a whole host of other issues including open IF trnsfomers and osc coil, but its starting to come around. Now I need to allign the thing (I don't have much experience with this) and I'm running into questions.<P>The Riders schematic, 15-5,6, says to attach the hot lead of the signal generator to the "control grid" of the 6SF7 tube. I'm guessing that this means pin 2, but I'm not sure and the tube manuals I have access to don't use the term "control grid." The next tube is the 6SA7 and again I am to connect the hot lead to the "control grid." My guess is pin 5, but again, I don't know for sure. Could someone let me know if I am right? Also, how does one identify the "control grid."<P>Second question: I am using an Knight RF signal generator. When I connecected the ground to the radio chassis, I got sparking and a lot of noise. I have now looked at the manual and it says to connet the ground through a .05 mfd cap for AC/DC sets. Is this normal with signal generators?<P>Thanks in advance,<P>Jim<P>------------------<BR>


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 Post subject: Allignment questions
PostPosted: Feb Fri 20, 2004 12:07 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 27516
Location: Detroit, MI USA
Hi Jim, to answer your second question first, you have just discovered why it is recommended to use an isolation transformer for the radio while servicing, but connect all test equipment directly to the power line. The Knight RF generator has one or more capacitors connected from the AC power line to the chassis to reduce RF radiation into the power line. There is some measurable AC voltage between the chassis of your radio and that of the signal generator. <P>For safety reasons, an isolation transformer is mandatory when repairing and testing, and especially when connecting any test equipment to the radio. Many AC-DC designs have one side of the power line connected to the chassis directly or through parts of the circuit having very low resistance. If by chance one of the caps inside the signal generator were shorted, and you had no isolation transformer connected to the radio, fireworks could result when making the ground connection to the radio chassis, or you could be badly shocked or worse if you touched both chassis at the same time. <P>The name "control grid" is most often applied to the G1, or first grid from the cathode, because in most tubes it has the greatest effect on the conduction of the tube.<BR>In the 6SF7, that is indeed pin 2.<BR>However, pentagrid converter tubes such as your 6SA7 are a different story. In that tube, the G1 is used in the oscillator circuit, and the RF signal is applied to the G3, which is pin 8, and considered the control grid. <P>Sometimes the hot RF connection from the signal generator is connected in series with a capacitor to the control grid when aligning the radio. This would be specified in the manufacturer's alignment instructions.<P>------------------<BR>Dennis


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 Post subject: Allignment questions
PostPosted: Feb Fri 20, 2004 12:17 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 529
Location: Dallas, TX USA
Thanks Dennis!<P>jim<P>------------------<BR>


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