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 Post subject: A sick Philco PT-2 on the bench
PostPosted: Nov Fri 24, 2017 10:21 pm 
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Joined: May Fri 06, 2016 3:41 am
Posts: 245
Location: Johnstown, Ohio
Hi Folks,
I am tackling another ebay wonder on the bench to learn how to fix these old radios like you guys do. I have replaced all the caps and one resistor that was a little crumbly. The tube lineup in this one is the 7A8, 7B7, 7C6 (replaced with a 14B6), 35Z3 and 50L6. All of these tubes checkout on my tube tester (Precision 912). I checked the voltages and all are ok except for the 14B6 which has a voltage of 93 VDC instead of the schematic voltage of 30 VDC. I tried touching a wire to the center terminal on the volume control and got no sound (no static) at all. At this point I am scratching my head. I tried using the signal generator on all the grid locations but nary a sound out of any of them. I think I am right that the 14B6 can replace the 7C6 but am not so sure that the voltage would be equivalent. Thanks in advance for any guiding hand you can provided.
Regards,
Don


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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Nov Fri 24, 2017 10:39 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Livermore, CA
Hi Don

Yes, a 14B6 can replace 7C6 in your AC/DC radio.

Problem is in audio stages 14B6, 50L6. What voltage do you have on pin #3 and #4 on the 50L6? Pin #3 of this tube drives your speaker.

Plate of 14B6 will read higher due to loading of the meter. Older meters loaded down voltage more.

With the radio unplugged connect and disconnect a battery to wires running to your speaker. You should hear noise doing this. A 9 volt battery can be used but should hear something even with 1 1/2 volts.

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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Nov Sat 25, 2017 1:53 am 
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Joined: May Fri 06, 2016 3:41 am
Posts: 245
Location: Johnstown, Ohio
Thanks Norm for responding.

On pins 3 and 4 of the 50L6 I get 93 and 96 VDC, respectively. I used a 9 volt battery and clips to attach to the speaker and heard static through the speaker.

What should I check next?

Don


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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Nov Sat 25, 2017 3:43 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 36085
Location: Livermore, CA
Don

Something between the volume control and pin #5 of 50L6. Is there a cap between pin #2 of 14B6 and pin #5 of 50L6? Is there a cap from center terminal of volume control and pin #3 of 14B6?

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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Nov Sat 25, 2017 9:09 pm 
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Joined: May Fri 06, 2016 3:41 am
Posts: 245
Location: Johnstown, Ohio
Hi Norm,

There is a 0.01 uF cap on p1n 2 and pin 5 of 50L6 and a 0.01uF cap between the center terminal of the volume control and pin 3 of the 14B6. They are both new caps as are all the others.

Don


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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Nov Sat 25, 2017 11:25 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 36085
Location: Livermore, CA
Touching pin #3 on 14B6 should cause hum from your speaker. Probing pin #5 on the 50L6 should also cause a little noise from the speaker.

Pin #8 of 50L6 should measure around 5 volts. Other voltages are reasonable.

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Norm


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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Nov Sun 26, 2017 1:03 am 
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Joined: May Fri 06, 2016 3:41 am
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Location: Johnstown, Ohio
Hi Norm,

I did get sound through the speaker at both points. Pin 8 on the 50L6 reads 6 VDC.

Anything else?

Don


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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Nov Sun 26, 2017 2:36 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 36085
Location: Livermore, CA
Don

In your first post you didn't get noise touching center of the volume control. You did get sound touching pin #3 of 14B6. There should just be a cap between these two points. If you do hear a loud buzz sound problem is before these audio tubes.

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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Nov Sun 26, 2017 2:46 am 
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Joined: May Fri 06, 2016 3:41 am
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Location: Johnstown, Ohio
Thanks Norm, I will check those tubes in the morning and let you know what I find. I will look for anything else that may be there.

Nite,
Don


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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Nov Mon 27, 2017 9:33 pm 
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Joined: May Fri 06, 2016 3:41 am
Posts: 245
Location: Johnstown, Ohio
Hi Norm,
Did a bit of checking through the radio starting at the tuner and working toward the 35Z3 and 50L6. I know you said the problem likely is present before these tubes. I do not have enough experience to know how to test for any deficiencies in this area. When I plug the radio in it does not even have that very faint hum that a working radio usually has. Power is getting to all the tubes. The loop antenna shows continuity so no breaks there. Am at a loss at this point. Could you suggest something?

Don


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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Dec Sat 02, 2017 10:12 pm 
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Joined: May Fri 06, 2016 3:41 am
Posts: 245
Location: Johnstown, Ohio
Hello Folks,
Norm Leal was helping me with this radio problem but he is probably very busy with the forum. Is there anyone else out there that could read the previous posts and possibly give me some direction on what to do next. I would very much appreciate it.
Thanks
Don


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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Dec Sat 02, 2017 10:46 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 36085
Location: Livermore, CA
Don

If pin #3 of 14b6 cause a loud noise in the speaker that tube along with 35Z3 and 50L6 are operating.

That leaves 7A8 and 7B7 tubes. 7B7 should have positive voltage on pin #2 and #3. 7A8 also pin #2 and #3. Pin #5 will also be positive. From your 1st post believe you have these voltages.

Pin #4 on 7A8 will be a couple volts negative when the oscillator operates. If less than 1 volt there is a problem here.

http://www.nostalgiaair.org/pagesbymode ... 014125.pdf

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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Dec Sat 02, 2017 11:45 pm 
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Location: Victoria, Australia
I would expect a DVM to read higher on 7C6 plate but by that much seems excessive (check plate resistor for correct one) . With the meter referencing to the negative (negative of filter caps) measure the voltage of the cathodes Pins 7 for all but 50L6 (pin8). I would expect around 7V for 50L6 and zero for the others.

Why check cathodes? If the cathode is open all of the tubes elements will be positive and have a voltage close to that of B+.

Marc


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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Dec Sun 03, 2017 11:22 pm 
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Joined: May Fri 06, 2016 3:41 am
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Location: Johnstown, Ohio
To Norm and Marcc. Thanks Norm for jumping back in here and for Marcc just starting. The voltages pins 2 and 3 of 7A8 were 1. 2 and 1.3 respectively and for 7B7 the voltages on these same pins were 1.3 and 0.7, respectively. These were all DC measures. Pin 4 on 7A8 was 0.04 which is probably just 0 volts. Not sure how accurate the digital meters are as they approach zero. And for the readings that Marcc asked for and the readings on Pins 7 for all tubes except for 50L6 which was pin 8 the readings were all less than 1.0 (0.3 to 0.6) except for 35Z3 which was 2.9 and on pin 8 of 50L6 the reading was 0. I do not know what to make of them.
Regards,
Don


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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Dec Sun 03, 2017 11:56 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 36085
Location: Livermore, CA
Don

What happened to those voltages? All are low. You should have around 90 volts on pins mentioned. Be sure negative lead of your meter is on negative side of electrolytic caps.

Marc has a good point about plate of 14B6. Voltage should be lower than 93 volts. Ideal would be about half of this. Like he said either the tube is bad or plate resistor wrong value, too low?

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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Dec Mon 04, 2017 12:54 am 
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Joined: May Fri 06, 2016 3:41 am
Posts: 245
Location: Johnstown, Ohio
I found the problem with those readings. The 14B6 I was using as a replacement for the 7C6 was not doing the job. I had changed out the 7C6 because it was not seating correctly in the socket and would move slightly. Fixed the socket now the 7C6 fits deeper into the socket and apparently does a better job that the 14B6. So now those readings are better. The readings for the pins 2 and 3 of 7B7 were both 95 and 60 respectively, while these same pins were 95 on the 7A8. Pin 4 on the 7A8 was 0. The pin 7 reading that Marcc requested on 7A8 was 16, on 7B7 15, on 7C6 0, on 35Z3 16 and on pin 8 of 50L6, 6 volts. Sorry this happened but although the 14B6 tested good on my tube tester it did not do well in the radio.

Don


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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Dec Mon 04, 2017 2:03 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Livermore, CA
Don

Good to hear you have reasonable voltages now.

Pin #7 is cathode on both those tubes. According the schematic both pin #7's are connected directly to B-. Could a wire be broken from pin #7 to negative of filter caps?

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Norm


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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Dec Mon 04, 2017 7:49 pm 
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Joined: May Fri 06, 2016 3:41 am
Posts: 245
Location: Johnstown, Ohio
Hi Norm,
I see that pin 7's on the 7A8 and 7B7 are connected with a white wire and it has continuity. I am not sure where the B- is you are referring to. This is my basic problem, too little knowledge.

While I have your attention here, is there a book or online source to learn about how to test these AA5 radios. I have some books and online downloads that tell me how they should work but not how to ascertain what it is that may be wrong. The problem is that I think these things probably come to you through experience, years of experience. I am 72 and I hope you are not going to say years of experience for the obvious reason.
Don


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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Dec Mon 04, 2017 10:11 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 36085
Location: Livermore, CA
Hi Don

Those pins are connected together but may not be to B-? B- is negative leads on electrolytic filter caps. B- can also be found on pin #7 of 7A8, 7B7 and 7C6 (14B6) tubes if that's easier to find. A wire may be missing from pin #7 of 7B7 to pin #7 of 7C6?

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Norm


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 Post subject: Re: A sick Philco PT-2 on the bench
PostPosted: Dec Tue 05, 2017 1:44 am 
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Joined: May Fri 06, 2016 3:41 am
Posts: 245
Location: Johnstown, Ohio
I checked the pins. There is only one wire going from pin 7 of 7C6 and it goes to pin 4 of 7B7.
With regard to my knowledge of B- it seems to be always on at least the ground of the electrolytics. I see it noted on some schematics but I just do not see it from looking at other schematics. Just do not recognize it from its wiring structure. Getting back to the wire issue, should I add a connection?

Don


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