Philco Model 70 -- Help - Printable Version

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Philco Model 70 -- Help - Guest - 10-02-2008

Starting to partially restore to operating condition a Model 70 . I replaced the filter caps and the coupling cap to the audio tube and realigned the set. All tubes test good. It now has very good sensitivity and selectivity but the volume could be much better. Seems that when I touch the volume control , the R.F.grid or the ant. clip, the volume almost doubles.
When I test voltages the screen grid volts are about one and a half times what they should be -- the control grids are about five times what they should be!! -- Cathode volts are about one and a half times . The even stranger thing is that the " 24 " second detector plate has over the double ( 230 v. ) what it should be!! The other plates are only sl. high.
The large w/w resistor tests O.K. Even with all these all these problems the set plays reasonably well! --- anybody know whats going on here????

Tom

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Re: Philco Model 70 -- Help - Ron Ramirez - 10-02-2008

diamondust Wrote:It now has very good sensitivity and selectivity but the volume could be much better. Seems that when I touch the volume control , the R.F.grid or the ant. clip, the volume almost doubles.

Do you have an antenna connected to the set? It needs at least 25 feet of wire; longer is better.

diamondust Wrote:When I test voltages the screen grid volts are about one and a half times what they should be -- the control grids are about five times what they should be!! -- Cathode volts are about one and a half times . The even stranger thing is that the " 24 " second detector plate has over the double ( 230 v. ) what it should be!! The other plates are only sl. high.

Did you check all resistor values? What is your line voltage?

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Re: Philco Model 70 -- Help - Chuck Schwark - 10-03-2008

Also make sure you re-build ALL the bakelite block condensers, not just the audio coupling. Very leaky B+ bypass caps can drag down the B+ and screen voltages. Resistors ALWAYS go up in value and they can lower voltages too. Check and replace ALL resistors as needed, like Ron says.

Also check the continuity of the "volume" control (dual pot unit), if you have the EARLY version (non-AVC) chassis. One section controls the grid bias (via cathodes) of two for the RF tubes. If open or defective it possibly will unbalance voltages.

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Re: Philco Model 70 -- Help - Guest - 10-03-2008

Thanks guys for the help -- I'll check it out

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