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City: Cumming, GA
I have a Truetone D 726 but the better print is from Allied A9775 (Riders 8-5) link here apparently it was made by Continental , Admiral? Allied anyway.
http://www.nostalgiaair.org/pagesbymode ... 000752.pdf
I have tested the tubes and they are good.
Field coil measures good at just over 1100 ohms
Output xformer was open and replaced with a new one PT-291(Antique Electric supply) I can use a 9v battery and can get sound through the speaker and output xformer.
6.3v on the filaments
80 tube pins 3-2 380vac, 3-gr-212ac, 2-gr 174ac. pins 1&4 5v Power transformer good.
I have replaced the filter caps
I have not fully recapped as I wanted to power it up and at least hear static but I get nothing dead air.
One question I have concerns the filter caps. On the print it shows 150v off the 4mf (I only get 88v Very low) but only get 7v off the 8mf ( nothing on the print, can this be correct?
What am I missing?
Eric.
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City: Roseville, MN
Maybe the filter caps have a bad ground. Double check the chassis ground versus B-. Remove the 80 rectifier and temporarily solder in a couple 1N4007 diodes.
Posts: 7,286
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Joined: Dec 2009
City: Roslyn Pa
I'd also have a look @ the 250 ohm resistor from the hv ct to chassis gnd. If that has gone high that will lower the hv and raise the bias voltage.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
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Joined: Nov 2011
City: Cumming, GA
The 250 Resistor reads 268 ohms (seems ok less than 10%)
The neg of the filter caps is .1 ohms to the chassis same from the end of the 250 ohm resistor and other ground points to the center tap
what would a couple of diodes look like can't seem to visualize it?
Posts: 7,286
Threads: 268
Joined: Dec 2009
City: Roslyn Pa
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
Posts: 987
Threads: 40
Joined: Feb 2015
City: Roseville, MN
The diodes replace the function of the rectifier tube. Put the striped end of both diodes on the 80 tube pin that the 8 mfd filter cap is connected to. Put the other end of one of the diodes on pin 2 and the other one on pin 3. Now you have a solid state power supply. If you get slightly higher than normal voltages then your 80 tube is bad. After all, it is 80 years old.
Posts: 451
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Joined: Nov 2011
City: Cumming, GA
I have other 80 tubes and I get the same, the more I look at the power side from the 42 through the 80 I'm seeing issues like the 5 pin (Cathode ) from the 42 is not going to chassis as the print shows but is routed through 2 resistors 250 ohms each I get 514 ohms total. The center tap which is supposed to go through a 250 ohm resistor is tied to the chassis directly. So I need to go through this line by line to see what they have done.
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City: Roseville, MN
The 250 ohm resistor in the power supply is to bias the 42 grid at -10 volts. The way you explained it, it sounds like the 42 is now wired for cathode bias. To me, 500 ohms is too high to produce 10 volts. But you can check the 42 tube characteristics to get the right value. Probably in the range of 250-300 ohms and it should have a bypass cap of 5mfd 16v.
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Joined: Nov 2011
City: Cumming, GA
I haven't been able to work on this for a few days but I'm back at it. What I have found.
Center tap was tied to the chassis directly.
The cathode of the 42 pin 5 was not tied to the chassis and also had resistor 250 ohm tied to it which put in series with the other 250 ohm resistor so it was a 500 ohm resistor.
The negative of the filter caps was tied to the chassis.
I have corrected these and decided to go pin by pin with a highlighter and have already found another but I'm not sure on this and that is on the 42 Pin 2 Plate has a .005 400v cap between pin 2 and pin 3 and the print shows a .01mf but the cap looks original.
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The cap between pin 2 & 3 of the 42 filters out some of the highs giving more bass to the speaker. Increasing the value gives more bass. I generally use a 0.01 or 0.02 mfd cap. Some radios will have a Tone switch to add a second cap to the circuit.
Posts: 451
Threads: 85
Joined: Nov 2011
City: Cumming, GA
Success, made a few more wiring changes per the print and decided to go ahead and change all the caps and resistors that were out of spec. About half way I thought I would power it up and I got static and then I went and did the rest and powered it up and checked voltages and got the proper voltages on the caps and the other spots that showed voltages and picked up stations. I swear I have no idea what they were thinking when they did what what they did!
Thanks to you guy's that gave me feedback
Eric
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