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AR-8506-B Local oscillator




 
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Author Topic: AR-8506-B Local oscillator  (Read 1361 times)
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k7mdo
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« on: June 01, 2020, 11:03:30 PM »

I have been presented with a 1944 (SN 44359) AR-8506-B receiver.

At first glance it looks great but previous owner says that it is "dead"!  I checked it out and sure enough "dead".

It has been 99% re-capped, good tubes, filaments light, all voltages seem OK (have manuals etc.)

but on checking further, I am able to get some audio out of the speaker by injecting a modulated signal at the detector.... so that end is ok

What doesn't work are the two oscillators, the BFO and local.  Both 6J5 simple triode oscillators... seemingly.

The two oscillators do "nothing"....  any hints or kinks anyone has.... one other thing I have noticed is that the "gimmick" of the BFO is actually soldered to the IF tube... makes me think that the signal was found weak at some time?

73, Tom
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Detroit47
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« Reply #1 on: June 02, 2020, 12:38:02 AM »

Is the neon light lit?
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"We are all born ignorant, but one must work hard to remain stupid."  Benjamin Franklin
k7mdo
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« Reply #2 on: June 02, 2020, 08:31:07 AM »

Yes!  We have hv.  There is one capacitor way down in the NFL can that is original.... I may look at getting to it but it is major disassembly.

I may breadboard up a 6j5 to see what a working osc. requires to go or not go!
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WB6NVH
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« Reply #3 on: June 03, 2020, 12:28:58 AM »

My receiver has a working oscillator but it dies on the lower half of Band 3.  And only Band 3.  I have changed the mica capacitors including the padder in that circuit with no result.  The B and other voltages are all fine.  About the only thing left is removing the oscillator coil and melting the wax off and inspecting it and cleaning it, or rewinding it.  I have had communications from other 8506B owners who also are hassling with oscillator problems.  Oh, and yes, I have tried lots of different oscillator tubes.
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Geoff Fors
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k7mdo
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« Reply #4 on: June 03, 2020, 07:17:37 AM »

Today I got the LO working...  Seems it's output is very touchy!  X10 probe on a tek scope swamps it completely out!

Now I have traced to first IF can....  Goes in but not out!

Moving along....

73, Tom
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k7mdo
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« Reply #5 on: June 03, 2020, 11:57:13 PM »

1st IF can has a problem...  One of the two variable caps has its plates in contact because the nut that tensions the moveable shaft was a press fit and over 75 years it simply split under that tension....  I think I can make one but maybe someone knows where they can be bought?

It is a 5/16 nut hollowed to press fit on the shaft.  I measure the shaft at .154"

 thoughts ?  I can't know what undersize i would need to ream the hole in the nut...  Maybe .001" or so. ... A little tricky.

Tom
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k7mdo
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« Reply #6 on: June 04, 2020, 11:31:41 PM »

Built a replacement lock but on the lathe with a little piece of brass bed rod....  Pressed it on with a C clamp!  Works fine.

Got the set aligned on band 1 but dead on others...!! However the process is encouraging!
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WB6NVH
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« Reply #7 on: June 09, 2020, 04:28:33 AM »

I had the same issue.  Pressed a nut off a junkbox capacitor and then put it on with the bench vise.  This seems to be a now common problem, the "cracked APC trimmer nut."
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Geoff Fors
Monterey, California
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