[fenbach]

my model 60 speaker makes a popping noise when turn it off. a sharp and quick snap, so to speak. like the click of a switch but much louder.
otherwise, it works fine. any thoughts on whether the noise is to be expected or what might cause it? and, of course, how to eliminate it.

[Radioroslyn]

Could be that the contacts in the switch are arcing as they are opening up into the off position. Leave the set on and plug it into a switched power strip. Use that switch to turn it off to see if you get the same results.

[fenbach]

i tried just unplugging the radio. popped all the same. ??

[RodB]

When the loud pop happens it's probably from a capacitor discharge through the output transformer. Could be the value of the power supply output filter cap is higher than what the schematic calls for or try replacing the power output tube.

[MrFixr55]

Interesting issue. This is a rather mature design, the basis for all transformer powered AA5s with the exception that the 42 Cathode (and the cathode of some of the other tubes) is referenced to Chassis ground, and the bias is from a "C-" developed between Chassis and the return to the power supply through R47.

Are the tone control caps, C38 and C39 new? if not replace with 630V mylars or polys. If this does not fix it, power up, warm up and power down with one tube out, power down, install the tube, and pull the previous stage tube starting with the 42, then the 75, then the 78, then the 6A7. There is likely going to be no noise when the 42 is out. If there is noise with the 42 in and the 75 out, troubeshoot the power supply and all components associated with the output stage (transformer, caps, resistors, etc.) sounds like a spark when the magnetic field of the output transformer or field coil collapses. I am not really thinking arcing but it is possible, possibly in the field coil. Temporarily replace the field coil style speaker with a PM speaker, a resistor and bigger caps. If this solves, then the field coil is at issue.

Good Luck.

[fenbach]

fixr,
all the caps are new. the electrolytics are 10 uf compared to 8 originally. close enough, i have to think.
with a permanent magnet speaker i did not get a pop. i also tried turning the radio off in the dark [with the field coil in the circuit] and didn't see any arcing but only hear the popping sound maybe half the time, if that, with no apparent rhyme or reason.
i jiggled the tubes and even tried another 42. i poked everything with a wooden stick and found one solder connection that wasn't good. correcting that didn't seem to make a difference.
i also tried putting cap 43 and resistor 44 in series between the 2 electrolytic caps. apparently they were not used in later runs. anyway, the radio popped all the same with them in the circuit.
for what it's worth, i hear a similar popping sound when i connect or disconnect an antenna.
the radio is certainly useable. i probably won't spend much more time on it unless someone knows definitely what's amiss.
thanks for all the suggestions.

[MrFixr55]

Hi Fernbach, 

If you left the PM speaker and resistor in the circuit and cycled the power many times without having a pop, and if disconnecting and connecting the antenna while using the PM speaker did not yield a pop, it still sounds like the issue may be in the field coil or in that part of the circuit. When you subbed the PM speaker, did you sub the transformer also.  I have had them arc.  Arcing in the output transformer is usually visible, but if the issue is further inside the transformer, the arc is not always visible until it actually starts to burn out.  

I only have a few radios with a field coil and will have to fire one or 2 up to duplicate the issue.

Although I would think that the arcing may be visible, if the issue is further inside the field coil, it may not be visible.  However, you may hear the click (if your hearing is better than mine) from the arc.

I hear you about not going crazy if the issue does not occur while playing.  It will either work or smoke.   Unless the arcing is from field coil or transformer primery to ground, the issue may not be harmful.  However, if the output transformer primary opens, the output tube may fail from an overheated and melted screen grid.

Good Luck.

[radiorich]

Hello Fernbach,
it sounds like you fixed the issue and Mrfixr55 has some good points .
Good luck !

Sincerely Richard