Author Topic: AA5 miseries  (Read 562 times)

TC Chris

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AA5 miseries
« on: October 15, 2019, 07:39:57 PM »
I'm in the same boat as Greg with his AA5 frustrations.  A friend asked me to fix one for her--an old Detrola table radio.  It was humming and had low output so I figured the filter caps were shot.  Tonight I put the new ones in and  yes, it still hums and has low output.  So I started replacing suspicious-looking paper caps in the signal path.  Still hum/low output... aargh!!  I may run the 50L6 through the tube tester again to see if it has a little short or something.  I hate being outsmarted by one of the simplest electronic devices ever!

Chris Campbell

electra225

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Re: AA5 miseries
« Reply #1 on: October 15, 2019, 10:34:36 PM »
It really grinds my grits when I can go thru a hacked Stereo Theater and make it work, but a simple AA5 kicks my butt.  I feel your frustration.   :-[ ;)


Have you isolated the hum? Is it affected by the volume control?  If you have a known good 50L6, you might sub it just for grins.  Then get a wood stick and start probing and prying.  You could be suffering from poor lead dress.  Clean the volume control just because.  Verify the filter caps are connected properly.  I'd probably finish replacing the rest of the paper caps if you haven't already done that.  Did you try reversing the power plug?  Does it have line isolation caps?  Have those been replaced?  You may have one of the other tubes with a heater to cathode short.  Do you have tubes you could replace the ones there one at a time to see if that makes a difference?  Good luck.
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TC Chris

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Re: AA5 miseries
« Reply #2 on: October 16, 2019, 03:55:41 PM »
I'm thinking the heater/cathode short is a best prediction.  I'll try subbing tubes from another AA5 and see if that helps.  The new filter caps had a somewhat ambiguous negative designation--an extended oval or flat-sided O--but as installed, the B+ is about right, the rectifier plates don't glow orange, and the new caps don't get hot, so that must be some graphic designer's idea of "minus."  The hum moves with the volume control (up/down) and modulates the audio.

Chris Campbell

TC Chris

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Re: AA5 miseries
« Reply #3 on: October 16, 2019, 05:52:02 PM »
OK, it's raining and nasty in northern lower Michigan so I called off the daily after-work walking route and decided to work on the AA5.  I was going to brush on a coat of dull varnish over the existing failed finish, a quick improvement for a radio that doesn't demand perfection, but the cabinet joints were  loose so it's gluing up right now.  Then I turned to the electronics.  I pulled out an old RCA AA6 with octal tubes.  It had a 12SK7 and 12SQ7.  An old Philco AA6 had a 50L6.  All subbed in with no improvement.  Then I searched my tube stash for the last one, a 12SA7.  No luck (of course).  So I pulled out the tube tester for that one.  Whoa, the "short" indicator, a neon bulb, was intermittently flashing, then pretty steadily (if weakly) on.  Maybe it is a heater-cathode leak. 

So what do you think... if the first RF tube has that kind of short, it might explain the fact that  receive one station, VERY weakly, the big local one with its transmitter two miles off.  But it is also the oscillator/mixer, so part of my brain says the superhet wouldn't work at all if the oscillator were faulty.

I see that AES has a tube sale right now.  Maybe I'll see if they've got a 12SA7 and buy one anyway.

Chris Campbell

19and41

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Re: AA5 miseries
« Reply #4 on: October 19, 2019, 07:32:52 PM »
Another possibility might be a open into your AF amp portion.  an amp with an unloaded input can hum with the volume control varying it.
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