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40-155 first look
#5

Hi Bridkarl,

What looks like a solder bridge may not be; verify that there is no solder bridge or other short. Looking at the green resistor soldered to the nut holding the capacitor #61, that resistor looks very burned and likely open. BTW, the case of that cap is NOT at ground potential; it should be insulated from the chassis. The case of Capacitor #58 should be at ground potential.
  • With the 84 out of the socket, set meter to ohms and measure resistance from the cathode of the 84 to the center tap of the power transformer.  There should be a relatively low resistance at start but the resistance should steadily climb to tens of kilohms as the filter caps charge.  If there is a short, fix it.  in addition, check the resistance of the filter choke and resistors 59 and 60 (I suspect that there is an open resistor here.  If any of these are open, there will be no B+ to the radio.  Resistors 59 and 60 complete the B+ circuit and grid bias voltages are developed across it.  Next, perform the following steps in sequence very carefully.  
    With power on and the rectifier and output tubes in their sockets, and with the speaker plugged in:
  • With the meter on 700VAC, measure between the junction of the HV secondary center tap and resistor #60, and each of the legs of the secondary at the 84 socket.  there should be somewhere around 300VAC give or take.
  • With the meter on a range higher than 300VDC, measure between the junction of the HV secondary center tap and resistor #60, and the junction of the cathode of the 84 and the speaker field coil.  There should be approximately 270V.  If much higher, then there is no load.  if 0V then the 84 is bad.  However, other than a bad solder joint in the pins or the filament (heater) being open, there are few failures that should cause the 84 to be so dead that there is no output.  I do not believe that there is anything akin to a fusible link between the cathode of the 84 and the cathode pin.  If the output of the 84 was shorted to ground, there would be some fireworks in the tube or power transformer.  Perform the next step.
  • Measure between the junction of resistor #59 and chassis ground, and the junction of the cathode of the 84 and the speaker field coil.  The voltage should be about 18 volts or so less than the previous voltage reading.  If 0V or close to it, then Resistor #59 0r 60 are open.  Move the negative lead to the chassis.  The reading should be the same value as that obtained at the junction between the resistor and chassis ground.
  • Measure between chassis ground and the junction of capacitor 58, the speaker field and B+ to the rest of the radio. There should be approximately 180VDC at this junction.  If 0V, then the speaker field is open or mis-wired.  If much higher than 180V, then there is no load.  Check wiring to the output transformer and measure the junction of the output transformer and the plate of each 41 output tube.  The value should be close to that at the junction of the speaker field and capacitor #58.

If all the measurements are good but there is absolutely no sound, not even a slight hum, then suspect the speaker voice coil.

Hope all of this helps.

"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis

Best Regards, 

MrFixr55


Messages In This Thread
40-155 first look - by bridkarl - 09-24-2025, 01:24 PM
RE: 40-155 first look - by RGK20m3 - 10-14-2025, 07:52 PM
RE: 40-155 first look - by bridkarl - 10-14-2025, 08:11 PM
RE: 40-155 update - by bridkarl - 11-01-2025, 05:49 PM
RE: 40-155 first look - by MrFixr55 - 11-01-2025, 06:41 PM
RE: 40-155 first look - by bridkarl - 11-02-2025, 05:50 AM
RE: 40-155 first look - by morzh - 11-03-2025, 11:30 AM
RE: 40-155 first look - by bridkarl - 11-03-2025, 11:44 AM
RE: 40-155 first look - by bridkarl - 11-15-2025, 06:01 PM
RE: 40-155 first look - by RodB - 11-16-2025, 10:47 AM



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