[AVSorrell]

Hi Folks - I have a 38-15 that is giving me a bit of trouble.  I have done the regular things - I have recapped it and replaced out of tolerance resistors - checked the transformers and tubes.  On the schematic,  off of the volume control (part #20) one leg is connected to between a  250 ohm 1 w resistor  (p# 33) and a 70 ohm 1/2 watt resister (p # 34). Problem is that 250 ohm resistor keeps dying (i.e. burning hot then smoking)- way too much current through it. it is connected to the transformer (p#36) center tap of the secondary winding.  I also am getting no sound from the speaker.    Attached is a photo of my work so far. I have been mindful of the polarity of the 10 and 12 uf electro caps in part # 31.  Any ideas? -- Andy     

[Radioroslyn]

I would think that you have a short from B+ to B-. Measure the resistance across the output of the power supply. Should see something higher than 5 or 10K. Much lower you've got a solder bridge or wires touching that shouldn't be.

GL

[MrFixr55]

Hi Andy, an interesting problem.  R33 and R34 form a voltage divider to provide the cathode bias for the 75 1st Audio, but more importantly, for the 41 Audio Output tube.   All of the DC current for the radio passes through those 2 resistors.

If there is a B+ short to chassis ground, then both of those resistors should fry pretty quickly.  Try the following:
Pull the 41 from its socket and fire up (sorry for the pun) radio.  R33 and R34 should not get hot.  If they still do, then there is a short between B+ after the Field Coil and ground.  Check C27 (Audio Output bypass cap) and the 4uF section of C30 (Between output of field coil and ground).  Also check for solder splashes.

B+ Voltages should be rather high die to no current being drawn by the 41.  Measure between ground and the common Negative of C31.  It should be a rather low voltage.  If this is the case, reinstall the 41 and check the voltage between chassis ground and the common negative of C31.  It should not be above about 18-20V.  If it shoots up, then there is excessive current being drawn through the 41.  An internal short in the 41 can do this.  One cause of the internal short could be an open Output transformer.  This will cause the screen grid of the 41to absorb all of the tube current, glow white hot, burn out and possibly short to the suppressor, internally connected to the cathode and therefore to chassis ground, and creating the dead short againes the output of the filter and putting about 250V across R33 and R34.

Hope this helps.

Best Regards, 

John, MrFixr55

[AVSorrell]

John and Terry - Thank you! I pulled the 41 and R33 and R34 did not get hot. Put 41 back in and and R33 got hot again. Replaced C27 and R33 no longer gets too hot and voltages are getting closer to where they should be - voltage between chassis ground and common negative of C31 is now 14.85 VDC.  Biggest issue now is that 41 tube plate voltage reads 292VDC when it should be 200.  There is non-tunable hum from the speaker now (nothing before).

Everything else is about right - 41 screen voltage is at 226 VDC (should be 220). 84 rectifier cathode is at 290vdc on the nose. 4 uf C30 is at 220vdc and 2 uf is at 196vdc. For C31 between 10uf and 12 uf it read 290VDC and between 12 uf and black (neg) it reads 304 VDC.

What is making that plate voltage on the 41 output run so high?  If there is hum from the speaker, does that mean the output transformer is still ok?  This is a chairside - so the speaker is a S19 - In the below, the new green wire goes to field coil yellow new white to terminal on the output transformer (now I am thinking this may be wrong given my issue with the excessive voltage on 41's plate) and new black to the black from the field coil.  -- Andy

[Radioroslyn]

So just for grins what do you have for resistance between pins 2 and 3 on the 41 tube? Should see about 500 ohms. Higher than that it's time to find an opt. With 27 shorted the primary of the opt was passing a fair amount of current from the power supplies hv side to gnd.

https://philcoradio.com/library/download...%20291.pdf

[AVSorrell]

Thanks Terry. I get 650 ohms between pins 2 & 3 on 41. I forgot to mention in my previous post, that the white wire from the output transformer terminal is connect to the plate pin (pin 2) and the black wire to pin 3 on 41. the green wire is to field coil connection. I replaced the two electro caps of C31 (the 10 and 12) and now get 240 vdc on pins 2 and 3 tube 41. I think I should also looks at the 4 of C30 now too. Still have non-tunable hum from speaker.-- Andy

[Radioroslyn]

If the voltages seems good on the chassis go ahead and connect a 470kc modulated signal to the Ant and gnd wires. And adjust 14a,14b, and 15a with the bandswitch set for the bcb and the dial set for the low end of the band.

[AVSorrell]

I figured it out. Where tube 41 connects to the chassis, and particularly where the plate pin from the tube connects in, there was a a break (fatigue I guess) between the pin connection and the wire connector behind it. I soldered a connection between then and voila! works great now! Thanks for helping me chase down the issue! -- Andy

[MrFixr55]

Congrats Andy! If you found an"open" between the OPT (Output transformer) andthe plate of the 41 at the socket, that may explain the high plate voltage. One time I found an open output transformer by watching the inside of the tube glow a lot brighter. It was the screen grid getting too hot, hot enough to glow. However, with the cathode connection open, strange to find that the plate was high and the screen was normal, as with the cathode connection open, there should have been little current. Wonder if some of the readings caused mechanical disturbance that affected them. The Screen is fed by the same level of B+ that the OPT, therefore the readings should be the same or the plate slightly lower.

Glad you got it working. Infortunately, the speaker wiring colors don't match the schematic. Based on your pic, the white should go to the 41 plate. The Green appears to be the output of the filter, so should go to the main B+. The black wire should go to the 84 rectifier tube cathode. If the green and black are reversed, audio level will be lower, B+ and hum levels are likely to be higher.

When observing the OPT, not theat there are 3 connections. The OPT connections are on one side, the lone Field connection from the Rectifier Cathode is on the other side by itself. The connection that is only to the primary winding goes to the Output tube plate. The connection that is both the OPT Primary and has a field connection is the Radio B+ (Output tube screen, B+ to rest or radio). The connection on the other side of the transformer, for the field coil only, is from the cathode of the Rectifier.. Phasing of these connections is also important. If there is too much hum, it is possible that some of these connections were reversed. If the phase of the hum bucking coil or field coil are reversed, hum will be greater.

[AVSorrell]

John,

Thanks for the info on the output transformer connections.  I did have the output tube 41 screen connection wrong - fixed that and now hum is further reduced and all my Pandora stations are sounding great ( via my AM transmitter!)  here is a LINK to it playing on the Philco Facebook site.  -- Andy