Philco 84B Power Transformer
Posts: 82
Threads: 14
Joined: Mar 2014
City: Philadelphia, PA
Hello Phorum,
Well I just finished the rebuild of the 84B. I had previously checked all coils, but realized today upon fire-up that I still hadn't checked the transformer, except for the dim-bulb test which passed. From F-F on the rectifier I get continuity=0 Ohms and no voltage. The voltage across the rectifier plates is correct however, although a little high ("630V", actual =680V). Is it worth rewinding the power transformer or should I buy another? Based upon what I've seen for other coil types rewinding doesn't seem that bad. Any suggestions? Thanks guys,
bdi
Posts: 1,114
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Joined: Feb 2013
City: Irvington, NY
Its pretty unusual for a filament winding of the transformer to go bad. Did you measure continuity with the 80 tube installed in the socket or was it removed. If you measured with the tube installed, the continuity reading could have been caused by the tube filament itself.
Check the solder connections from the filament winding wires to the tube socket terminals. A bad connection could cause the problem. There could also be a bad connection inside the transformer from the filament winding to the leads.
If all else fails, you could probably use a solid state replacement for the 80, which requires no filament voltage.
Posts: 214
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Joined: May 2010
City: Kokomo, IN
The filament windings are only a few tenths of a ohm and don't read very good on an ohm meter I would remove the 80 and turn it on and measure the AC voltage on the filament of the 80 socket should be 5 volts AC.
Posts: 82
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Joined: Mar 2014
City: Philadelphia, PA
Hi Vinzer... I checked the AC on the rectifier socket without the bulb and it does read 5V AC (F-F). P-P was 720V AC. When I turned it on, all bulbs were light red on the filaments (I think) - no glowing at all, which I assumed meant not much juice was getting there. Where did you find the 5-volt spec? The voltage info I have says 50Volts AC on the rectifier socket...is this wrong? The ohms only read 0.2. Thanks for any help as this is my first set.
Posts: 16,505
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City: Jackson
State, Province, Country: NJ
I hink you have a good transfomer. Read up a bit more about your tubes etc....you will get your specs there.
http://www.radiomuseum.org/tubes/tube_77.html
http://www.radiomuseum.org/tubes/tube_80.html
Posts: 599
Threads: 24
Joined: Jan 2014
City: Edmonton AB CA
The description you just gave says the rectifier winding is good and operating. It should be 5 volts AC (F-F) not 50 and it will read a very low ohm value. It is the least likely winding to fail in a power transformer and I have never seen a bad one in 30 some years of doing this.
A light red on the rectifier tube filament is very normal.
Gregb
Posts: 82
Threads: 14
Joined: Mar 2014
City: Philadelphia, PA
Greg- You are correct about the 5 Volts; I checked the tube info. My 84 schematic says 50V for the "Filament Volts - F to F" under "80" which is what threw me off... am I reading this wrong?
Follow up info- I checked all filaments for all tubes - all are good- about 6.3V. I also probed from the first filament to every place the other filament connected (caps, resistors, tubes) and all show the same voltage (5.7V). If my tubes were not glowing but had red filaments, does this indicate the type of problem it may be? I'm not sure what to check next. Thanks guys-
bdi
Posts: 82
Threads: 14
Joined: Mar 2014
City: Philadelphia, PA
I just saw a previous comment that I missed. Greg - you said that a very dim red in the tubes is normal? I saw the same very dim red in all tubes. I thought more blue and brighter? I am only comparing to online pics...never seen them working yet (or any).
Posts: 599
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Joined: Jan 2014
City: Edmonton AB CA
Most of the photos on line are over exposed to highlight the filament and the blue glow of a power tube. Some tubes, depending on construction, are hard to even tell if they are glowing or not.
I am sorry but I went back in the thread and I don't see where you describe what the issue with your radio is. Does it not play or what is the issue? Maybe I just missed it.
Gregb
Posts: 82
Threads: 14
Joined: Mar 2014
City: Philadelphia, PA
Hey Greg-
When I got it, it was completely untouched, no speaker, and powered up. My FC and output trans were both open so I got a new 8ohm speaker with a Hammond universal output trans, with an 1800Ohm resistor, and increased cap (40uF). I just got done rebuilding the set. When I powered it up no sound came at all. I noticed when probing around though, that when I touch any rectifier connection and the cap of either 77, the speaker makes a loud buzzing sound, which I am taking as good news of the sound section, although I'm not completely sure. I also have a 15 ft antenna attached. Absolutely no sound at all as is. Any suggestions?
Posts: 214
Threads: 16
Joined: May 2010
City: Kokomo, IN
could be open osc coil or ant coil or both very common on philcos of this age can be checked with a ohm meter.
Posts: 599
Threads: 24
Joined: Jan 2014
City: Edmonton AB CA
I would start with some voltage checks, check the plate and screen voltage on both 77 tubes. Also do an ohm check on the antenna coil and let us know what you find.
Gregb
Posts: 82
Threads: 14
Joined: Mar 2014
City: Philadelphia, PA
Hey guys- The osc and antenna coils are OK. After rechecking all caps and resistors, I found a resistor that was 1K instead of 1M (my fault of course). After replacing, it started making sound...buzzing, which changes sound with tuning. I checked the voltages for the Output tube; all normal. The 77s seem to be a little low on voltages; Plate =69 (should be 100), screen=25 (should be 60), if I'm checking it correctly. The tube circuit still confuses me. I am probing Pin#1 (hot) with Pin#2 (for Plate, as per scheme), Pin#3 (for Screen as per scheme), etc, with DC voltage check? I checked the Rectifier using AC voltage since it is still AC at that point. Also, I cannot check the 77 caps at all because the speaker starts screaming at me if I touch either one. I appreciate all of your input. Thanks
bd
Posts: 599
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Joined: Jan 2014
City: Edmonton AB CA
You will need to sort out those low voltages but you are well on your way to a working radio.
Gregb
Posts: 82
Threads: 14
Joined: Mar 2014
City: Philadelphia, PA
Sound like a bad tube Greg? I tracked the low voltages far back and it doesn't seem like there is a bad connection anywhere. Thanks again- I'll keep troubleshooting it.
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