Greetings to all,
I have a question about my radio I need help with if possible. I have recapped this and replaced resistors and it seems to play fine. But when I check voltages I find 3 that are way off. My plate on the 71a tubes show 110v instead of 215. Also the plate on the 24 detector is 100v not 35. All other voltages match.
Is this of worry? Radio works fine so I am not sure.
Thank you
Hi and welcome,
Hmmmm. Does seem a bit odd that the voltage feed point is the same for the two 24A rf tubes and the 71A's yet they are significant lower in voltage.
If you measure the voltage from the chassis (-) to the grid pin (+)(#3)how much do you have? Should see about -50vdc. Without that voltage the the plates of the 71A's are going to draw excessive current and cause a voltage drop across the output transformer's primary.
If the -50v checks out ok then connect your meter to measure the dc plate voltage on one of the 71A tube. Remove one of the 71A's. Does the voltage go up to it's normal 200v or so? Insert it back in and do like wise with the other 71A. If the voltage does rise up then your 71A is suspect.
Odd indeed. I only know of one place in 20 (having restored two of them) that shows very different (I think much higher) voltage, and that is the plate detector's plate. The rest, even with today's DMMs are very much in line with what's written.
Uhm.....is your meter OK? And is the AC power OK?
I remember the bias resistor #25 section 1-2 (part of the big enameled one) being open in mine. But that's not likely what you are having.
And much lower plates on 71A - 1) as Terry said, could be 71As are conducting too much, 2) have you checked your D and B voltages, are they OK?
Thanks for the reply. I have checked pin 3 on the 71a tubes and get -8vdc. Also have pulled each tube on the set one by one and no effect. With the 71a tubes out the plate pin goes up maybe 10 volts. No difference with second meter.
I get -8vdc on the B and 320vdc on D. All transformer volts match the specs.
Ran it yesterday for a few hours and sounded great. No overheating issues. Tempted to just leave it be but I will give this radio to someone as a gift so trying to be as Thorough as possible.
Welcome to the Phorum!
Ok lets check the dc voltage at terminal 6 on the power transformer.With - probe to the chassis and + to the terminal should see about -50vdc. If you do your audio driver transformer is suspect #21. To check it with the set off measure the resistance from terminal 6 to pin 3 of each 71A. Should see a fairly low resistance like less than 1K. If your seeing anything up in the 10's or 100's of K transformer is kaput! AES has a suitable replacement for abt $15.
https://www.tubesandmore.com/products/tr...tage-10-ma
http://www.nostalgiaair.org/pagesbymodel...029582.pdf
Stupid questiobs: are the tubes good?
Well I have fixed my problem with the plate voltage on the 71a tubes. It was my fault I will admit it. I had the leads reversed to the speaker. J & D. Voltage is now 220vdc. I still do not have the -50vdc but I don't know if my Meter is just not reading it right since it is a dmm. Checked the pin 6 on the transformer and it's -8vdc. Resistance from the centertap of audio transformer to pin 3 on the 71a is 2.3k and 2.7k. This was the resistance I got before recap. Could this transformer be suspect anyway? Also tubes test good.
Also I checked current and I get 10 ma going to the 71a pin 3 but when I checked the center tap I get 500ma. Sorry for double post edit button not working on my phone.
Don't mean to hijack this thread but I just received a model 20 and, upon pulling the chassis from the cabinet, found there's a heavy transformer just dangling by the wires from under the chassis. It appears it was put in to replace #27 the filter choke. Can anyone recommend a suitable replacement for this choke from AES or Radio Daze? Terry? Thanks.
Unfortunately Philco doesn't give us much to go on. The resistance and inductance isn't super critical on this part as it's not used to developed the negative bias. Judging from the value of the caps around it it needs to be a fairly high in inductance to filter the ac hum out. I'd go with this:
https://www.tubesandmore.com/products/fi...ma-300-ohm If the level of hum is unreasonable add some capacitance to filter cap on the right side of this choke.
Thanks, Terry. Will give that one a shot if the one that's dangling under the chassis doesn't do the job. I appreciate your help!