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Model 41-287 volume issue - Printable Version

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RE: Model 41-287 volume issue - RodB - 12-10-2025

OK we're on our way. Isn't there a third white wire going to the other 41? It should be connected to the same pin as the black wire on the first 41. It's the other end of czp 57. Measure the voltage there.

RE: Model 41-287 volume issue - mikeyg - 12-10-2025

Yes I have zero volts there as well.

RE: Model 41-287 volume issue - RodB - 12-10-2025

OK, if you check the schematic, you can see the problem is an open primary coil in the output transformer. At the junction of the field coil and filter cap where the schematic shows point F, you measured about 200 volts of B+. Follow the wire up to the center tap of the transformer primary. The voltage passes through the primary to both plates of the 41 tubes. If there's zero volts on the plates then the primary is toast, which I'm just getting ready to have with coffee. Further, I'm pretty sure that the connection of the black/white lead of the transformer has been somehow disconnected from the primary coil since there's no voltage on either tube. If the coil itself is open then we would see one half of the primary open and the other still good.

RE: Model 41-287 volume issue - mikeyg - 12-10-2025

Ok I think I am tracking with you. What do you suggest?

RE: Model 41-287 volume issue - RodB - 12-10-2025

If I had this problem I'd replace the transformer. The wire inside is unreliable, so if you fixed a broken connection there's a good chance a problem somewhere else in the coil will occur. But if you are cramped for cash then take it apart and see if the open can be repaired.

RE: Model 41-287 volume issue - mikeyg - 12-10-2025

So I’m looking at this. I have 218 v on that white and black wire from where it connects on the 41 tube.. going two spots away to pin 2 I have a -9.5 volts which I assume is my grid bias? Does this sound right?

RE: Model 41-287 volume issue - RodB - 12-10-2025

Nope. I'm not finding a voltage chart so I checked the tube characteristics for the 41 tub in push-pull configuration. It says it should be -25 volts. You should check resistor 61, all 3 sections. This is probably a candohm wire wound resistor fastened to the chassis or a wirewound resistor on a form. That low grid voltage can cause higher current flow in the output tubes causing overload to the transformer primary.

RE: Model 41-287 volume issue - RodB - 12-10-2025

I'm assuming you previously replaced cap 52. If you didn't, then if it's leaky it would cause the grid bias to be low.

RE: Model 41-287 volume issue - RodB - 12-10-2025

I'm putting more thought into this. If the output tubes are not conducting because the transformer is open then much less current is flowing through resistor 61 producing a lower bias voltage. If resistor 61 is OK then there's a good chance the bias will be normal when you replace the transformer. Now you can see the importance of having a dim bulb or variac.

RE: Model 41-287 volume issue - mikeyg - 12-11-2025

Ok cool I will try and locate resistor 61 and check it when I get home. I will also look up how to make a dim bulb tester. I made one years ago but it got tossed during a move years ago. I am going to put that audio transformer on order unless you have a better one to recommend. If there is a better one please let me know.

RE: Model 41-287 volume issue - RodB - 12-11-2025

$30 is a good price for a new transformer. Can't always wait for the right transformer at the right time when the moon is full.

RE: Model 41-287 volume issue - Arran - 12-11-2025

Sometimes you can find replacement audio output transformers on fleabay, new or used, but Antique Electronic Supply also sells them, generic Push-Pull units as well as the Hammond brand ones, make sure you get one with the correct plate load impedance measured in Ohms, for 6K6 tubes, 6F6s and 6V6s are different. With regard to the audio output transformer failing, this era of Philco is known for this so I think it may be a corrosion issue, that or they skimped on the gauge of wire used. I have a chassis from a 30-180 and it has a replacement Hammond output transformer from the 1950s or 60s judging by the label. These sets used 6K6s in push pull, but wired them with a screen grid inversion setup, so I wondered if that may have something to do with the failures. However Philco was using the same setup as early as 1937 in their 37-640 and 37-650 sets, and those were not known for output transformer failures.
Regards
Arran

RE: Model 41-287 volume issue - MrFixr55 - 12-11-2025

My 41-255 has a replacement output transformer also. Most Philcos of this era that have Push Pull output used the "screen grid resistor tap" method to get the inverted signal needed for the other tube in the Push Pull pair. Most of the 1941 sets used the '41 pentode, which is electrically the same as a 6K6. The B+ on these sets was only about 180V. I don't know why this was the case, because 250V is more common for tubes of this vintage.

One other thing is to check the outputs for shorts, especially that tube for which the leg of the output transformer primary leg was open. especially in a class A circuit, but possibly in Calls AB, the quiescent (no signal) plate current is relatively high. If the plate circuit opens, then the total cathode current is absorbed by the screen, which will overheat to the point of incandescence, burn up and short to either the grid, suppressor or plate, further damaging circuit components.

RE: Model 41-287 volume issue - mikeyg - 12-11-2025

So I am putting the p-t291 output transformer on order now. Will go ahead and build a dim bulb tester before it gets here and get those resistors checked and replaced if necessary. I will need a little more help hooking this up when it arrives. It’s still a bit of a challenge for me correlating what I see on the chassis versus what the schematic is telling me.

RE: Model 41-287 volume issue - Vlad95 - 12-11-2025

My work with similare output transformer problem:

Rogers Majestic R-646 restoration #1
Rogers Majestic R-646 restoration #2