[Radioroslyn]

The flat copper wires are from the hum bucking coil. All of the other resistances seem to be OK.

[Kurt P]

Terry, gotcha, thanks for the clarification.  That’s probably why the speaker seems shorted.  Assuming the OT/FC/speaker work correctly, I guess I need to look for a short somewhere else.  I’ve double checked my work and I don’t see any obvious mistakes.

The 80 rectifier tube was destroyed when I got this set, someone probably plugged it in and blew the cathode bonding wire off since there were loose pieces rattling around inside.  I have another very strong 80 tube and I want to make sure I protect it.  I’m thinking of bringing the set up slowly on a variac (with an isolation transformer & bulb current limiter) with no tubes and seeing if there’s a short.  If there’s no shorts I’ll try it with the rectifier tube.

That’s the process I’ve tried with success before on some AA5 sets, just want to make sure that’s the best way here.  This set just seems more important somehow and I don’t want to skip a step and lose a part that’s tough to source like a transformer.  Thanks again!

[Kurt P]

My dim bulb tester is lighting up when I turn on the set with only the 80 rectifier tube inserted, so there's probably an issue with the rectifier circuit.  One of the two 8uf caps tests correctly in circuit but the other one tests 14uf... maybe too much dwell time cooked that cap.    Gonna have to take it apart and see and then probably test every connection in that circuit...

[Radioroslyn]

You can just disconnect it for now from the can. It will just add some 60cy hum but no damage to the circuit. Something else you can do is use an ohm meter to look for low resistances on the B+ line. Should see upwards of 5 to 10k ohms, if you see 100's of ohms I would investigate.

[Kurt P]

Well finally got back to this and found that I had connected R18 to the negative side of the filter caps instead of the top of R38.  The filter caps are fine, not shorted and I don't see anything else mis-wired in the power supply section.  Still getting a short (some brightness on the dim bulb at 60V but full brightness at 100V and pulling .6A) so it looks that wasn't the only issue...

Is the humbucking coil normally shown in the schematic?  I wonder if this speaker came from a set that needed that coil, while this one (according to service bulletin 146B) doesn't seem to.  Might not be an issue.  I have a clearer schematic now and I can now see that the primary of the OT should be 7100Ω but I'm only measuring 460Ω.  Not sure but that seems way low. 

I can't think of a way to test the power supply without the speaker section since the B+ runs through it.  I'd like to eliminate possibilities of issues in the whole speaker section but other than the resistance readings I don't know of another way to verify it's working.  There's gotta be a basic troubleshooting approach here but I'm missing it, any troubleshooting tips would be appreciated!

[morzh]

The HB coil is often shown in the schematic as an inductance symbol in series with the speaker after the secondary of the audio xfmr.
I suspect you might want the speaker with the humbucker, as the coil is used as the filter coil.
7100 Ohm is probably a mistake (I see it in the Code 123). It should be in hundreds of Ohms.
Anothe 89 sch shows 450 Ohms; this is more like it.

[MrFixr55]

Hi Kurt,

Impedance of the output transformer primary is about 7KOhm, but resistance will be lower, and 460Ohm is not out of the question.  In fact it sounds fine to me.  Hum bucking coils are in most electrodynamic speakers (Speakers with an electromagnet for a field as opposed to a permanent magnet like a modern speaker).  Its purpose is to cancel any 60Hz fluctuation in the field coil by having a magnetic field exactly opposite it (Think the concept of noise canceling headphones, common mode rejection, etc.)  If the transformer secondary, hum bucking coil or voice coil was shorted to ground, this would cause low or no sound but is not the kind of short that would cause excess current draw.

Now a short to chassis ground in the field coil or the output transformer primary would be a different thing entirely.  The field coil being shorted to ground would be catastrophic.

By R38, do you mean C38?  

Accidentally connecting R18 to the negative of the 2 filter caps would cause no power to be delivered to the plate circuits of the convertor and RF, but would not load the power supply down.  

Try the following:

• Disconnect the speaker, remove all tubes.  
  
• Ohm between the green /white wire (junction between cathode of rectifier and R37) and ground.  The reading should be infinite (after charging of the Cap R37.  Repeat the test measuring between the green / white wire and the HV secondary center tap.  Again, the reading should be infinite after the cap charging.  A low resistance or short indicates a short in the wiring, C37 or a grounded Rectifier filament winding.  Look for frayed wires, solder splashes, etc.
  
• Leaving speaker disconnected, measure between white wire (junction of C38, the field, output transformer B+ Radio B+ circuit, etc.  the reading should be the sum of resistors 30, 31, 32 and 33, approximately 90K Ohm after C38 charges.  A low resistance indicates a short in the power supply or these resistors. (Short in R32 or r33 would contribute negligibly to the issue but would cause bias problems in operation.
  
• Again, with speaker disconnected, measure between plate pin of the 43 (output) tube and ground.  Reading should be infinite after charging of bypass and tone caps.  Low reading or O Ohme indicatres a short in wiring or the tone control cap C47) or switch or HF bypass cap (C48)
  
• Leave speaker disconnected.  Run current / dim bulb test.  Current should be negligible but monitor voltage between green / white wire and chassis.  Do not allow the voltage to increase to a level higher than the "WV (working voltage) of the capacitors.
  
• Leave the speaker disconnected, install all tubes other than the 80.
  
• Repeat the dim bulb test.  Current should be substantial, I would think at least half or more of the rated value, as the filament of each tube draws 0.6A except the 42 which draws 0.7A, so the total load on the secondary is 3.1A at 6V. so 0.3A transformer current draw is not out of the question.
  
• With the speaker disconnected, remove all tubes and reinstall the 80.  The 80 filament draws 2A, therefore you will draw 0.2A just from the filament.  Measure between chassis ground and the junction of the 80 Filament, C37 and the green / white wire (goes to the field coil).  You should get about 350-450V.
  
• Power down, touch the green / white and white wire together.  This will discharge the charge on the cap through R30, 31, 32, 33 (voltage divider).  Verify. 0V between the green / white wire and the transformer HV secondary center tap.
  
• By this point, with only the 80 installed, you should only be drawing 0.2-0.35A.  Connect the green / white and the white wire together.  Install all tubes other than the 42 (I will tell you why later).  Repeat the dim bulb test.  It should be less than specifications.
  
• OHM between Field coil and speaker frame.  Ohm between output transformer primary and speaker frame.  Both readings must be infinite.
  

IMPORTANT NOTE:  
         NEVER run a pentode tube with the output transformer disconnected but B+ supplied to the screen.  With no plate current, the tubes
         entire current would dissipate to the screen, causing it to glow bright yellow to white and burn out, likely shorting to the suppressor grid,
         rendering the tube useless and possibly damaging other components.

Once you find and correct the issue, connect this way:

• Cable from junction between rectifier cathode and C37+ (should be green / white) should connect to the field connection that the output transformer primary is NOT connected to.
  
• Cable from junction of C38, R30 and B+ (should be white wire) should connect to junction of one leg of the speaker field and one leg of the output transformer primary.
  
• Cable from plate of 42 (output tube, should be green) should connect to the leg of the output transformer that is not connected to the field coil.
  

Hope this helps.

[Kurt P]

Hello!  MrFixr55: thanks for your detailed troubleshooting tips.  My results are as follows...

By R38, do you mean C38?
Yes, I meant C38.

Ohm between the green /white wire (junction between cathode of rectifier and R37) and ground.  The reading should be infinite (after charging of the Cap R37.  Repeat the test measuring between the green / white wire and the HV secondary center tap.  Again, the reading should be infinite after the cap charging.  A low resistance or short indicates a short in the wiring, C37 or a grounded Rectifier filament winding.  Look for frayed wires, solder splashes, etc.
Cap continued to slowly charge but after a minute or so it was at 2.8MΩ and I figured that was close enough to infinity.  Could this be the issue?

Leaving speaker disconnected, measure between white wire (junction of C38, the field, output transformer B+ Radio B+ circuit, etc.  the reading should be the sum of resistors 30, 31, 32 and 33, approximately 90K Ohm after C38 charges.  A low resistance indicates a short in the power supply or these resistors. (Short in R32 or r33 would contribute negligibly to the issue but would cause bias problems in operation.
89.3KΩ.

Again, with speaker disconnected, measure between plate pin of the 43 (output) tube and ground.  Reading should be infinite after charging of bypass and tone caps.  Low reading or O Ohme indicatres a short in wiring or the tone control cap C47) or switch or HF bypass cap (C48)
Infinity.

Leave speaker disconnected.  Run current / dim bulb test.  Current should be negligible but monitor voltage between green / white wire and chassis.  Do not allow the voltage to increase to a level higher than the "WV (working voltage) of the capacitors.
0V.

Leave the speaker disconnected, install all tubes other than the 80. Repeat the dim bulb test.  Current should be substantial, I would think at least half or more of the rated value, as the filament of each tube draws 0.6A except the 42 which draws 0.7A, so the total load on the secondary is 3.1A at 6V. so 0.3A transformer current draw is not out of the question.
No glow on the dim bulb tester, all filaments glowing and pulling .125A.

With the speaker disconnected, remove all tubes and reinstall the 80.  The 80 filament draws 2A, therefore you will draw 0.2A just from the filament.  Measure between chassis ground and the junction of the 80 Filament, C37 and the green / white wire (goes to the field coil).  You should get about 350-450V.
Bringing power up slowly the dim bulb begins to glow at 80V and glows brightly as I increase the voltage.  At 110V I'm only getting about 16V at this junction.  Tried another 80 tube and got the same results...

This tells me that 1) the speaker isn't the issue and 2) there's still something shorting out or overloading in the 80 circuit.  All the wires are in pretty good shape (especially considering their age) and ohm out fine.  The transformer leads to the filaments of the 80 tube measure about .2Ω when the schematic calls for .1Ω, close enough.

37 filter cap is reading 14µf, though their may be something in parallel that's increasing that value from the 8µf I put in there.  I guess I keep looking at those caps, hoping for operator error!  Not sure what else to try at this point...

[MrFixr55]

Hi Kurt,

The resistance readings all sound good. Any frying noises?

Confirming that your DVM was set to DC Voltage, the speaker was out of the circuit and only the 80 was installed, all other tubes out and the Green / white wire was not connected to anything other than your DVM. Confirming that the dim bulb started lighting brightly at 80VAC in and output is only 16V. Confirming that the 80 filament was lighting at a decent level, considering only 80VAC in.

I suspect that either C37 is breaking down or something starts arcing over when the voltage is increased. However, if something starts arcing, you should hear it.

I am not worried about the cap reading 16uF; this is not enough extra capacity to cause an issue. That is where I would start.

Next step: disconnect the 80 cathode from C37 and the green / white wire (only the filament and plate windings connected to the 80), remove all tubes except the 80, connect the chassis to the dim bulb tester and CAREFULLY measure between the 80 cathode and chassis ground. The output should be unfiltered DC, about 250-350VDC and the dim bulb should be dim or not lit. If you receive these results, then replace the cap and verify your wiring. If it weren't for the dim bulb, either the 80 would have a heluva light show or the cap would blow to the middle of next week.

If this resolves the issue, then reassemble and continue testing.

Hope this helps!!