My Econo Keyer is fun but not working right.

Started by vwflyer, May 14, 2014, 06:21:47 pm

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Hi guys. I decided to finally learn how to use paddles. Because of my tight budget I bought what came up at good prices. I wound up with a Ham-Key iambic key and an MFJ-401C keyer. I've been using the keyer's sidetone off the air to just practice using a paddle for the first time. I can tell it's going to be nice having QSOs with this when I get a little better. Hopefully others will understand me better with this than my straight key too but right now I still make too many mistakes with the paddles. I often key to many or too few dits. As it stands though, I can't get on the air with this even if I wanted to. It continually keys up my Kenwood TS-140S as soon as I plug it into the key jack. I measured the resistance of the MFJ's key output jack while not keyed and it is about 1500 ohms. Apparently that is too little for the Kenwood and it sees it as a closed circuit. I know that a few of you here are kind of electronic savvy so I thought I'd throw this problem out there and see if anybody has any obvious answer to my frustrating situation.


I can't find the schematics for the 401C but if you go to MFJ here (link) you can download the manual with schematic for the 401D.  I bet there's very little difference.
In their crappy, really low resolution schematic I see 2 jumpers.  They select how the output to the rig works.  It sounds like yours is set to "grid block" (both jumpers in the G position).  Change JP1 and JP2 to "direct" (both jumpers in the D position).  This is on pg-4 of the manual.

Keep at it with the paddles.  They take some getting used to but paddles and a keyer REALLY clean up your sending.  Yes, getting the number of dits right is the hard part for me too.  But remember to not send faster than you can copy.


Hi cockpitbob. Thanks for the encouragement. I think that I am probably trying to send too fast. If I slow it down to what I can reliably copy I will probably struggle less with the dits. I've tried playing around with the weight adjustment but I don't really notice much difference when adjusting it. Of course I don't really know what I'm listening for.

The C model is a bit different from the D model in that rather than one output jack with internal jumpers to select direct or grid-block it has two seperate connectors. I've tried connecting to both of them and the radio keys up continually on both. The manual says it will do that if you connect to the wrong one for your radio but it shouldn't do it on both as far as I can tell. Any thoughts on putting a resister in the line?


With the very little I know about the problem I'm thinking there might be something blown in the output.  Do you have the schematics for the 401C?


I've found the schematics for the D model but not the C model. I've found the manual in a couple of places but the page labeled schematic is blank. The board is pretty simple looking. Nothing looks wrong just by looking at the top but then you'd never see a blown transistor just by looking. I will remove the board and flip it over to see if there is anything obviously wrong with the solder joints. Does anybody know how to test a transistor with a DVM?


Not much to go wrong there so this should be simple.  It's either the diode or the 2N7002 transistor nearest the direct keying jack.  That diode is really hard to blow, but I wouldn't put it past MFJ to put it in backwards.

1) Verify the diode isn't in backwards.  With the ohm meter verify there's zero ohms between the center of the DK jack and the cathode (stripped end) of the diode nearest the DK jack.  It should be less than 1ohm.

2) With power on, measure the resistance between the DK jack's center and outer conductors.  Be sure to put the meter's red (+) lead on the center and the black (-) lead on the outer.  If it reads anything less than 1meg ohm the transistor is probably blown.  Those fets can be fragile.

Also, if you can do straight keying with it, or slow the beeps way down, measure the voltage at the gate (middle lead) of the 2N7002.  It should be going between 0V and 5V with the beep off and on respectively.  If so, then the circuitry driving the transistor is OK.

I might have a 2N7002 in my parts bin.  Let me know.


Given that this is MFJ, I would check the soldering as well...



QuoteGiven that this is MFJ, I would check the soldering as well...

Wise advice Gil :-\

Cockpitbob, I don't know how you can know all that just by looking at the photo but I'm glad that you do.

I'll give you the results of the test in reverse order from how you gave them to me.

First, the voltage at the gate of the transistor behaves exactly as you say it should. 0V unkeyed and 5V keyed. That's good news.

Second... First let me explain that I don't have an auto-ranging ohm meter. It has a a range selector switch with scale settings of 200, 2k, 20k, 200k, 2M and 20M. Set at 200 I get infinite, so at least the resistance is more than 200 ohms (good). I do start to get readings at the 2k scale setting though (not so good). However the exact amount of resistance continues to climb as I go up through the scale ranges. At 2k it reads 1.3, at 20k it reads 1.8, at 200k it's 2.1 and so on. These values are the same whether the unit is turned on or off. If I reverse the red and black leads the values are a bit higher. 

Finally, your first suggested test: The resistance between the center of the jack and the cathode of the diode is exactly the same as the above values found in checking the resistance of the jack itself. When I check between the diode and the outer-cunductor of the jack then I get 0 ohms. It is 0 ohms no matter which way I have the meter leads. Obviously the diode goes to the outer-conductor and not the center. Resistance from the outer-conductor to the other side of the diode is infinite unless I switch the leads, then it goes up to 154k on the 200k scale.

That's all I know ???


I've been doing this kind of thing for 30yrs.  It's my job.

Looking carefully at your picture I can see some of the copper traces on the back side and I see my assumption about the circuit being essentiall identical to the 401D's is wrong.  It still sounds like a blown FET.  To be sure I think it's time to remove the board.  You're going to have to do that eventually anyway.  If you can post a pic of the back side I'll have everything I need to know.

Be careful to not zap it with static. Touch the outer part of an RCA jack before touching the board.  I don't think you'll ever find another Curtis 8044A keyer chip. 


Here it is, for your viewing pleasure, one flipped board and one backlit board. Thanks for your help on this.


May 16, 2014, 03:08:13 am #10 Last Edit: May 16, 2014, 03:10:37 am by cockpitbob
Those 2 pics were perfect.  As I'm sure you figured out by now, I had you looking at the wrong diode.
So, there's only 2 things connected to the center of the jack:  transistor drain and diode cathode.  Neither should conduct when power is off.  It must be the transistor.  Pull the transistor and verify the ohm meter reads open with only the diode.
I can't find the 2N7002 in non-surface mount.  I don't think they make it anymore.  I have a BS170 in my parts bin.  It's identical to the 2N7002 except the leads are reversed so you install it rotated 180deg.  PM me your address and I'll mail you one if you want.


Alright, I'll do it. I'll have to see how well this $20 pencil iron does on a board. I'll pm you with my address too.


vwflyer, I got the PM and the BS170 goes out in Saturday's mail.  Hope this does it.


Bob, you're a genius! I popped in one of the new transistors you sent me and she works like a charm. I think I'll practice with it for a day or two again and then attempt my first QSO with a keyer.  Do you take American Express?


vwflyer, that's super!  I'm glad I could help, and I got my depression era dad's disease of liking to take something busted and fix it because you take junk and turn it into something with value.

I don't take plastic.  I'm just pay'n it forward.  Glad I could contribute to the ham community that's helped me so much.  Besides, the parts were free; left over spares from a work project years ago that I effectively pulled out of the trash.

As far as "genius" goes, a genius is just a person at work in his own back yard.