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Author Topic: I might have fried my mfj-407c keyer.  (Read 3257 times)
N1RND
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Posts: 69




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« on: August 18, 2013, 05:32:58 PM »

I was playing around with my Swan 500C.  Making up cables to use the mfj407c keyer for the first time on the Swan.
I had it working good for a short while.  Then I think I accidently re-plugged the Swan into the DIRECT jack on the back of the keyer (DIRECT is for solid state rigs not grid block).
Now what happens is using the GRID BLOCK keying jack does not work with the Swan and using the DIRECT jack present a constant SHORT (keydown) with my Solid State rig.
What did I do?  Fry a transistor or diode?

I'm bumming because I had just finished inserting an audio filter into the speaker line to make the rig more usable on CW.

Oh Yeah, the technical support on the MFJ website is not good.  No schematic, no parts list, no diagrams, and it appears that the owners manual must be for a very early model.  It refers to moving jumpers inside the unit, that just don't exist.
« Last Edit: August 18, 2013, 05:39:22 PM by N1RND » Logged
K8AXW
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Posts: 3956




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« Reply #1 on: August 18, 2013, 08:37:37 PM »

RND:  I just spent a great deal of time trying to locate the schematic for the 407C without any luck.

It sounds to me that you probably wiped out the keying transistor.  It should be easy enough to find......just backtrack from the key out jack to the first transistor you find.  Hopefully, it doesn't have a "house number" on it and has a valid transistor number on it.

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N1RND
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Posts: 69




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« Reply #2 on: September 03, 2013, 03:22:44 PM »

GOOD NEWS!
The direct keying transistor is shorted.  All other transistors test fine.  Diodes also.  I called MFJ and they send me a pdf of the schematic.
With the shorted direct Q (2n7000) removed from the circuit, the grid block option now works.
I just ordered a lot of (5) 2n7000's on ebay for less than $4.00 including shipping.

Nice when you can fix something yourself and for cheap.  If I make the same mistake again, I know what to do.
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K8AXW
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Posts: 3956




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« Reply #3 on: September 03, 2013, 06:11:10 PM »

RND: Congratulations on troubleshooting and fixing the keyer!  Great feeling, eh?

Now about the 5 spares.  Murphy's Law #21 indicates that whenever you buy a spare component for one that you accidentally zapped, you will never zap that component again....ever!   Grin
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AC6CV
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Posts: 56




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« Reply #4 on: September 03, 2013, 07:46:19 PM »

I always check out Mouser for transistors, ICs, or diodes. That transtor is about 38 cents at Mouser. You probably saved on postage though.
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