Philco 41-680 What is voltage on plate of 7C6(Det/1st Audio) - Printable Version

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Philco 41-680 What is voltage on plate of 7C6(Det/1st Audio) - Harlan - 02-05-2008

Hi,

Philco 41-680. I only have very faint sound from local station and nothing
else. Checking the plate of the 7C6 (Detector / 1st Audio) and the voltage
is only about 7 volts. The B+ on the other side of the 220K resistor, #63,
is about 150volts. I replaced the resistor which was cracked but voltage really didn't change.

Just wondering if it should be that low and is there somewhere I can find what votages should normally be at each element of the various tubes in this model.

Thanks much!
Harlan

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- Ron Ramirez - 02-05-2008

Hi Harlan

There should be around 165 volts on the plate of the 7C6 when the set is in "radio" mode (pushbutton, AM or shortwave); 140 volts in "phono" mode.

(I say "around" because the voltages were originally measured at the factory with 1000 ohms/volt voltmeters which are nowhere near as sensitive, or as accurate, as today's high-impedance meters which do not load down the circuits under test as the old meters did.)

Check to see if mica condenser (66) is shorted. Barring that, the 7C6 itself could have an internal short, or a short on/in the 7C6 tube socket itself. The original resistor probably cracked because of the short which you must now find.

There is a voltage table in the Philco Service Bulletin for this model. You should get a copy from Chuck...available for a reasonable fee.
http://www.philcorepairbench.com/schematics.htm

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- Harlan - 02-06-2008

Thanks Ron,

First ... sorry I had a brain freeze on the model number. You obviously knew I meant a 41-608 rather than 680.

Your information makes good sense. I will do a check on that mica and rule out the tube by just unplugging it and see if the voltage pops up to supply level. Then we can see which is the culprit. Guess I just could have done a little more logical checking but was getting kinda tired of the task for a while.

Will try to reply with the results next chance I get to follow up on the project.

Thanks again and best regards,
Harlan

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- Harlan - 02-06-2008

Hi again,

Just wanted to let you know I found the cause of the low plate
voltage on that 7C6. It was a very leaky coupling cap #67 (.01)
going to the output/s. Replacing it brought about 167volts to the
plate and there is very good volume plus much more reception
across the broadcast band.

Just tried the shortwave band and have absolutely nothing coming
in there though. I just have a length of wire clipped in for my
antenna on one of the screws as it is hard to hookup the built
in large loop just while working on the bench. Could this temp
antenna method be the reason for the no SW band?

Thanks again!
Harlan

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- Ron Ramirez - 02-06-2008

Good detective work!

Quote:Could this temp antenna method be the reason for the no SW band?

To quote Bugs Bunny: "Mmmmyyyeahhh, could be..."

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