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September 19th 10, 10:18 PM
posted to rec.radio.amateur.boatanchors
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AR88D - Very Low R.F Gain
"timlad28" wrote in message ... Richard Knoppow;718630 Wrote: "timlad28" wrote in message ...- Hi, The R.F gain on my AR88D has to turned almost fully clockwise to hear any stations. I can pick up stations across the bands but can only hear them clearly when the R.F gain is full on. I thought the pot (R46) may have failed. I detached the RF gain pot and tested, the pot measured from 0K to 68K. The two resistors to ground are both in tolerance. -- timlad28- Have you checked the tube voltages, if not do that first. I think there are also stage gain values in the handbook. These will help in finding where the signal is disappearing. Using a good scope as a signal tracer will also help. RCA draws schematics with the tube bases shown as they are which often makes them difficult to decipher. Sometimes re-drawing a part of the circuit helps. -- Richard Knoppow Los Angeles WB6KBL Hello Richard, Thank you for answering my post. Today, I managed to check the tube voltages and V12, Anode 3, measures -1.2v, when it should be 30v. All other voltages check out. - Tim First of all which handbook do you have? The ones I have show V-12 as the BFO with 40V on the plate (pin-3). The BFO must be turned on for this voltage to be present. If its turned off there will be nothing there. In any case, even if there is a problem at this point it should not affect the overall gain of the RX. The symptom is so general its hard to point out any one thing. It would be helpful if you could borrow an oscilloscope (I assume you don't have one) and use it as a signal tracer to see where the signal drops off. Also, don't assume tubes that check good on a tube tester really work, sometimes tubes which check good don't work in some circuits. Unfortunately, RCA does not seem to have provided either socket resistance measurements or stage gain measurements in their data. You can find a lot of additional info on the AR-88 and its relatives by doing a web search, There are Canadian and British military manuals available with more info than the RCA handbooks. If your receiver has an S-meter installed its indication may give you a clue. -- -- Richard Knoppow Los Angeles WB6KBL |
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