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philco: Transitone; TH-14 (TH14): No output, help!

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Forum » Radios and other type of sets (Physics) etc. » MODELS DISPLAYED » philco: Transitone; TH-14 (TH14): No output, help!
           
Michele Denber
Michele Denber
 
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19.Apr.09 03:14

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Hello, dear Radio Collectors!  Here's a new mystery - I'm working on a Philco TH-14 Transitone.  I replaced all the capacitors and all the rotten rubber insulated wiring.  I applied power and got nothing, not even a hum in the speaker at any volume level.  Here's the details so far:

B+ = 146 v.

Audio output tube (35A5):

grid, pin 6, normal audio waveform that varies with the tuning capacitor

plate, pin 2, 146 v. DC, zero AC (shouldn't I see an AC waveform here?)

130 ohm R21 measures 144 ohms.

Audio output transformer primary: 183.8 ohms

Across speaker terminals: 1.8 ohms

C22 (which I just replaced anyway) is OK

I tried a different output tube with the same results.

Does anyone have an idea what might be wrong or how to proceed from here?

Also, can someone tell me what the notations on the schematic by the speaker mean?  They look like "V.E." and "D.C.", but I have no idea what they mean there.

Thanks / danke sehr!

 

 

 

Thomas Albrecht
Thomas Albrecht
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19.Apr.09 09:34

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Hi Michele,

Check the voltage on the screen grid (pin 3) of the 35A5.  If for some reason there is no voltage here, the tube will be cut off, providing exactly the symptoms you describe -- signal on the grid, DC plate voltage on the plate, but no signal to the speaker.

Check the voltage on the cathode (pin 7) as well.  You should see something like 5 to 10 volts on it.  If it is zero, that is another indication that no plate current is flowing (possibly due to no screen grid voltage).  No plate current also results in high B+ voltage (should be more like 90 to 120 VDC with everything running as usual).

Often the screen grid connection (pin 3 of 35A5) is where the B+ and the hot side of the output transformer are connected, so if that is the case here, and you see voltage at the socket, you may want to confirm that it is actually reaching the tube pin by touching your voltmeter on the tip of the tube pin coming through the bottom of the socket (if accessible with the type of socket you have).

If the screen grid voltage is OK but the cathode voltage is significantly higher than 10 volts, check to make sure both ends of the 130 ohm resistor are properly connected, and the cathode pin is making good contact.  No cathode connection to the circuit common would also cause the symptoms you see.

If cathode, screen, and plate voltage are OK, confirm that the control grid (pin 6) has the correct DC bias on it (zero or somewhat negative), and also test your speaker.  Since the secondary winding of the output transformer is connected in parallel with the voice coil of the speaker, you'll see a low resistance between the two terminals even if one of the two is open.  Disconnect one of the voice coil wires and then check both the voice coil and the output transformer secondary for continuity.  You can "click" test the speaker by touching wires from a 1.5 volt battery across the speaker (you have to do it with the radio turned on, so that the field coil is energized).

The one remaining component where there could be a problem is the output transformer.  A short in the primary or an open secondary could be the source of your problem (although if the only problem were an open secondary, you should see signal at the plate of the output tube).

You should be able to find the problem in one of the above suggestions.

Best regards,

Tom

P.S.:  The markings near the speaker are "V.C." for "voice coil" and "B.C." for "bucking coil."  The bucking coil is for hum bucking, to reduce 60 Hz hum from the field coil (which is being used as a power supply filter choke).

This article was edited 19.Apr.09 09:39 by Thomas Albrecht .

Michele Denber
Michele Denber
 
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19.Apr.09 22:00

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Once again, Tom comes to my rescue!  The cathode voltage on the 35A5 output tube was indeed zero, and the fact that the screen and plate voltages were identical confirmed no plate current.

I wanted to take another look at R21, the 130 ohm cathode bias resistor.  Although I had measured it, it seemed odd and looked burned.  On my way to doing this I noticed a white wire that connects the 7C6 audio plate to the 35A5 screen via R17.  Philco parked R17 between pins 3 and 4 on the 35A5 using pin 4 as a tie point for the wire.  The wire runs from there back to the 7C6.  Philco routed this tightly around the 35A5 socket past pin 6 before taking off for the 7C6.  By now you can see where this is headed.

When I was replacing all the bad insulation that was literally flaking away, I decided to lift the 7C6 end of this wire because it was easier to reach and slide heat shrink tubing over the now bare wire from there.  This radio is very cramped and from all obvious appearances, the wire goes to the output tube pin 6, so I slid the tubing over it that far and called it done.

What I didn't see was that the wire does not connect to pin 6.  Instead it dives under R21 there before going on to pin 4.  That insulation was also gone, the wire was touching the pin, so the bottom line was that the screen and grid were connected to each other via R17!  Fortunately, that didn't cook anything else and once I fixed all of the insulation, it powered right up and now plays fine.  B+ is now down to 110.8 V with a 115 VAC input.

You can see what a tangled mess this all is in this photo.  It's just as hard to see in real life what goes where.  The black arrow points to where I thought the white wire ended, the white arrow points to where it really does end.  The wire originally ran under R21 and behind pin 5 on its way to pin 4.  You can't even see R17 here - it's buried in there somewhere :-)

I guess the moral is you can't be too careful replacing wires in these old Philcos.  Lifting one end and using shrink tubing over the original wire does save time, but be very careful that you can see exactly where the wire really ends.  Hopefully this exercise will be of some use to the next person who comes across one of these little radios.

Anyway, all is well now - thanks again!

This article was edited 20.Apr.09 07:13 by Michele Denber .

Paul E. Pinyot † 2013
Paul E. Pinyot  † 2013
 
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28.May.09 03:31

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Good find Michele,

I had to chase a few bad wires I created over the years.  I often just bite but bullet and replace the rotten wires in these old Philcos with new wire. 
 
It helps (me) to clip one end leaving a little insulation as a landmark and feed the new wire along the original path.  Then solder the new wire in place.  At that time I remove the little cut ends of the original wire.
 
If the phone rings then all bets are off. 
Paul.
 

This article was edited 28.May.09 03:31 by Paul E. Pinyot † 2013 .

Michele Denber
Michele Denber
 
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28.May.09 06:56

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Hi Paul.  Good suggestion.  I do that often with capacitors.  But even that can backfire if you have a radio that someone else already reworked before you.  One time I snipped out a capacitor and when I came back with the replacement noticed that there was another pigtail sticking up nearby from some other long-gone component.  And darned if I couldn't tell which one was the one I had snipped.  Fortunately, I always take photos of everything I work on before I start.  I looked at the picture and was able to get the right wire.

And if my phone rings while I'm in the middle of some critical operation, I just let it ring :-)

  
rmXorg