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 Filament voltage problem - AK 9C

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atwaterkent
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Join date : 2018-06-17

PostSubject: Filament voltage problem - AK 9C   June 17th 2018, 6:13 am





Joined: Sep Sun 17, 2017 9:23 pm
Posts: 779
Location: Plymouth, MI


Hi everyone! I've just joined your community.  
 I'm working on restoring an AK 9C . I've replaced the transformers in the T/A, taken apart and cleaned the T/A rheostat and the on/off switch mounted on the breadboard and installed a new battery cable.
When I apply the A voltage (6v), only the detector tube lights up. I removed the T/A top from the radio and applied filament voltage directly the posts, all the tubes lit up. I started checking out the rest of the filament voltage circuit and found that when I checked the voltage at the ends of the battery cable (where it is soldered into the circuit underneath the breadboard) the voltage checks .7 volts. The A-/B- cable goes to the on/off switch, so I am thinking it's something to do with the positive cable and what I believe is a bypass condenser on the underneath side of the breadboard that the A+ ties into. The connection at the lower right of the photo is the incoming A+ from the battery cable.
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FrankB
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Join date : 2010-11-22

PostSubject: Re: Filament voltage problem - AK 9C   June 19th 2018, 1:39 am

Hi,
 Welcome to the Forum.

 OK It appears that the det tube runs off a separate winding.

 I think you are correct in that the capacitor block is likely bad/leaky, drawing the voltage down.
Disconnect one end of the block and apply fil voltage briefly. Fil's should now light.
 You can also ohm the capacitor to ground, with it disconnected from the fil side.
 My AK Factory manuals are in storage, but I found a schematic online.
 AK marked their caps oddly, with their own part numbers.
 As a WAG, you might try a .1 to 1mfd mylar type (non-'lytic) in place of it also, just to see if it works. I'd use a 600+V cap.
 Please post if this was the problem.
Best,
FrankB
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chas
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Join date : 2017-04-09

PostSubject: Re: Filament voltage problem - AK 9C   June 19th 2018, 9:03 am

Take voltage drop measurements both negative and positive, from the K101 up to the tube pins. If the drop is in the cable then there is a rotten spot in the stranded wire, get a new cable... Do not trust riveted or other mechanically screwed connections. Solder together or re-fasten.

Do take a look at the meter test leads. Got fooled the other day by the Simpson leads, strands broken at the test leads plugs, (no wonder why zeroing was unstable)...

Chas
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atwaterkent
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Join date : 2018-06-17

PostSubject: Re: Filament voltage problem - AK 9C   June 19th 2018, 12:33 pm

Frank - Did as you advised, all tubes lit up! I'll replace the .2 MFD capacitor and hopefully that will do it. 

  My thanks to both you and Chas for the advice. After I get the new capacitor in, I'll let you know if I have a working AK or if I'll be bothering you both with another problem!!  Shocked
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FrankB
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PostSubject: Re: Filament voltage problem - AK 9C   June 19th 2018, 4:04 pm

Shiny! Cool Smile
 On other models of the AK radios, ( And some other mfg),often the capacitors are encased in tar or metal cans. Multiple ones and sometimes coils, xformers, and resistors in the tar & cans too.

Might be good to replace all the old paper caps while you are at it, for reliability.

NOTE: In the "Early Days" they did not have 'lytics, and used regular caps. There were a couple of exceptions, though.
 It was not uncommon to see up to a 4 mfd cap in just paper and no polarity. One radio I repaired decades back had a plethora of from .1 to 2 mfd paper caps in it. Often the radio would work fine with off value caps too, like a 1 mfd instead of a .1 or even a 2 mfd.
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atwaterkent
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Join date : 2018-06-17

PostSubject: Re: Filament voltage problem - AK 9C   June 22nd 2018, 4:55 pm

Good news!  Frank - you were right, capacitor was pulling down the filament voltage. I received the replacement today and soldered it in. The radio is working and  picking up several stations. You solved it Frank and I am grateful, thank you! 
 One problem though, one of the cans (left side tuning capacitor) does not seem to make any difference when tuning in a station. It checks good for continuity.  Any ideas?

  Update: I have been informed by someone, who is an expert on these radios, that the left tuning can is only for impedance matching on this radio and has little to do with the tuning.
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FrankB
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PostSubject: Re: Filament voltage problem - AK 9C   June 22nd 2018, 6:33 pm

Glad that your problem is solved.  cheers cheers cheers
 That's what this forum is about, helping others when we can.

 All we ask is you let others know about the forum, post fixes, hints, ideas, and such.

 I learned years ago that no question is stupid. We all have to learn and start someplace.

 Many of the posts I make are really applicable to newbies. No one helped me much when I got started, as there was no such thing as the internet.
  Most of my knowledge came from an old tattered 1966 ARRL handbook and the Ham Radio Study Guide, until I got into tech school. (That's a story on its own. Let's say it was a prime example of "Those who can't do- teach".)
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atwaterkent
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PostSubject: Re: Filament voltage problem - AK 9C   June 22nd 2018, 7:15 pm

Time to order the display case. Some photos.
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FrankB
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PostSubject: Re: Filament voltage problem - AK 9C   June 22nd 2018, 11:53 pm

Nice radio.
I might suggest fusing both the AC inputs for the transformer and the secondaries.
  As you experienced, those old 70+ year paper caps can short and call all sorts of mischief.
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