Hi:
Newbie question, Recapping the above radio, lost all sound 2-3 times after replacing most caps, but seemed to have traced the problem to either tube 7B7 or the 508X. There was no sound but when I jiggled tube 7b7 the sound came back. Further, when I tap the 508X tube the there i a popping sound in the speaker indicating that there might be a problem there as well. There seems to be an intermittent short perhaps. Please help me diagnose the problem so as to resolving the intermittent sound loss. Thanks for the help. Also, part C7 a capacitor, can't seem to find it on the schematic, what size cap is it?
God Bless,
Dave B.
Dave B, when you say there is no sound and then you have sound do you mean the radio is receiving or are you referring to noise out of the speaker? As far as C7 goes, I don't think there is one. Usually the caps are numbered either clockwise (start at left [antenna] and move right to speaker then down and left to plug or from left to right top to bottom. In the schematic I have they are numbered the latter way and no C7 appears. This can be due to a change that removed the C7 as it served no practical purpose vs. cost and they did not need to re-number all caps on schematic. PL
PS Just re-read my post and what I meant by the "clockwise" is that in most, if not all, schematics the antenna to speaker curcuits are displayed horizontally across top half of page and the power (B+, heater) curcuits are usually on the lower half of page to the right so clockwise would be making a u turn and down.
What is a 508X tube?
Loctal sockets are notorious for developing bad contacts. One way to clean them is to squirt the tube socket holes full of contact cleaner, then quickly plug the tube into the socket, plugging and unplugging it a few times. A better way is to use a tiny file, available from welding supply companies. Don't over-do it with the file, clean gently - just enough to knock out the crud without damaging the socket.
... I have also found that most old loctal-type tube pins are always corrosive, and a good way to clean them is by using a xacto-knife blade or equilavent ( olfa knife w/ snap-off blades), and take time to hand-scrape all tube pins to shiny metal appearance. Good quality contact-cleaner sprayed into tube sockets helps when re-seating tube pins several times by repeating tubes inserted in their respective sockets. Sometimes, it also helps by checking "squeeze" factor on underchassis loose tube pin connections, by carefully "tightening" those loose socket pin connections with needlenose pliers with tube out of socket. Be careful to not apply too-much pressure with pliers!! Loctal type tubes are notorius for not making good pin connections, opposite of what they were originally designed for!
