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Author Topic: Heathkit DX-60 "key down" with no key in!  (Read 9354 times)
K3STX
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Posts: 961




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« on: November 16, 2012, 05:36:30 PM »

This is a brand new problem. When I turn my DX-60B to AM or CW position, EVEN WITH NO KEY DOWN (or even plugged into the TX), I transmit!? Very strange. Like a partial short across the key jack. I have not opened her up yet, but has anyone else ever heard of this? Looking at the schematic it is not clear to me what could cause this.

paul
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KA3IWU
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Posts: 20




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« Reply #1 on: November 16, 2012, 07:06:23 PM »

Sorry if this sounds like a silly thing to check, but several old radios I have messed with before had normally closed switch contacts used on the key jack so that if no key was plugged in the key jack was shorted so that when you flipped to cw position it could be used as a "tune" setting.

Try plugging in a key and see if it stops.
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K3STX
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Posts: 961




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« Reply #2 on: November 16, 2012, 07:12:26 PM »

thanks, i guess i was not clear. It does it with a key plugged in (but not closed!) or not. It is not FULL output, like a "partial" short. I checked all tubes (except the 6146, don't have a tester for it) and all seems fine. I recently replaced the electrolytics (2 days ago), but right after the install all was perfectly fine (at least for that 15 minute smoke test/sample QSO). I wonder if the "good" electrolytics allowed enough current to fry something? (all caps are disc ceramic, no paper caps).

p
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K1ZJH
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« Reply #3 on: November 17, 2012, 07:02:36 AM »

Are you technically inclined? If you are comfortable digging into that rig, I'd suggest checking the negative bias voltages which are used for grid block keying. Are you using an external VFO that is tapping into the grid bias for VFO keying? It could be a partial short on the keying line... been a long time since I was a novice with a DX-60 and HR-10.

Pete
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K3STX
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Posts: 961




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« Reply #4 on: November 17, 2012, 07:23:21 AM »

This happens when I use the external VFO or if I just use a crystal. In fact, even when I put the crystal position knob to a position that does NOT have a crystal installed. You are right, sounds like partial short on key line, I will trace this out. i guess it COULD be one of the disc ceramic caps, but I thought they were pretty reliable.

paul
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AC5UP
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Posts: 3825




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« Reply #5 on: November 17, 2012, 08:03:38 AM »

...the truly talented troubleshooter assumes nothing, relies on logical testing to find the issue, and doesn't play the odds exclusively. If the last nine things you worked on had a bad electrolytic that doesn't mean the tenth one has a bad electrolytic. Every repair is a fresh roll of the dice and every part has a unique reliability curve. If a ceramic condenser has an average life expectancy of XX years that means half of them will fail before then.

The exception to this is an All American Five with a bad hum in the audio... Then you test the electrolytic filter cap first. But don't assume that's the only thing wrong with the radio. If it's old enough for one part to fail, it's old enough for multiple issues. And if it's a Heapkit, there will be multiple passive components out of spec 'cuz it was probably born that way.   Wink
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K3STX
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Posts: 961




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« Reply #6 on: November 17, 2012, 09:06:10 AM »

I replace electrolytics always. I was planning to use this TX all the time, so didn't want to take chances. I am gonna look and see if somehow I installed something backwards.

I was just playing with it again, when I turn the TX to "CW" I should ONLY hear noise in my RX when I key down. What I hear, even with no key down, is a BUZZ. Actually sounds a bit like old electrolytic hum I used to hear on old RXs. I'll check it out.

One question, when I put the TX on CW, the meter reads that there is about 100 mA plate current already (but no grid current). When I key the TX the plate current goes to 150 mA and now the grid current goes to about 2.5 mA. Is this normal to have plate current even with the key open?

paul
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K1ZJH
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« Reply #7 on: November 17, 2012, 09:17:31 AM »

I suggest you measure the DC voltage across the key contacts, with key UP. What is the negative DC voltage you are seeing across the key contacts?  The rig uses grid block keying. Unless you can verify that the negative bias supply is working, I suggest sending it out for repair.

pete
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K3STX
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« Reply #8 on: November 17, 2012, 12:25:54 PM »

I suggest you measure the DC voltage across the key contacts, with key UP. What is the negative DC voltage you are seeing across the key contacts?  The rig uses grid block keying. Unless you can verify that the negative bias supply is working, I suggest sending it out for repair.

pete

Negative 35 volts across the key when open, when I close it it goes back down to zero.  Thanks for your help. I sure as hell am not sending this OUT!! I gotta fix it myself. I will deal with this after SS is over.

GO PVRC!!!

paul
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KB3WYZ
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Posts: 4




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« Reply #9 on: November 18, 2012, 08:04:50 AM »

Hello Paul,

I am having the same problem as you are having..
Mine is a Heathkit DX-60A. I got mine on ebay about
three weeks ago. Mine seemed to work fine till one day
I noticed the key seemed to be stuck down in the down
position, causing the 6146 to glow bright red. Pulled the
key out still kept glowing and was getting a buzzing sound
from the Rec. Turned it off right away and tried to figure out
what went wrong. So I thought, I would change all the Electrolytic
filter caps and any out of spec. resistors. Fired her up and everything
checked out alright. Had nice clear tone from the Rec. then after about
ten minutes the key started sticking down, even with the key pulled
out... Angry I am getting -70vdc across the key.
Maybe we can work together on this problem?
Gary, KB3WYZ
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K8AXW
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Posts: 3683




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« Reply #10 on: November 18, 2012, 08:19:31 AM »

Paul, Gary.... You have one clue staring you in the face.  One has -35v and the other -70v bias.....  but that isn't what I wanted to comment on.

Hopefully, you both have the assembly manual for the rig.  The schematic will show all the proper voltages at the pertinent points.  I understand reading the manual is un-American but if you don't tell anyone, it never happened!   Roll Eyes

I would suggest also replace the disc caps in the bias circuit.  You have to remember this is an OLD rig and ALL caps are suspect.

Working together on this problem is a great idea.
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K1ZJH
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Posts: 901




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« Reply #11 on: November 18, 2012, 09:20:27 AM »

I think the grid block voltage is around 115 volts. If someone has a working DX-60 some feedback on the open key voltage would be helpful.

Another thought that has occurred to me is that it might be possible for a tube that is part of the grid block keying is going gassy?  That is a WAG on my part.

Pete
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KE3WD
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Posts: 5694




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« Reply #12 on: November 18, 2012, 12:18:40 PM »

Just a hunch, try reading across the key contacts with your DMM set to AC voltage. 

You are looking for excessive AC ripple. 


73
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K3STX
Member

Posts: 961




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« Reply #13 on: November 18, 2012, 06:49:13 PM »

Not sure my manual is complete. Now that SS is over I will check this out. It DOES sound like the power supply ripple from bad electrolytics. I'll get it figured out.

Paul
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KB3WYZ
Member

Posts: 4




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« Reply #14 on: November 19, 2012, 06:09:33 AM »

Do you have a schematic??
I changed all electrolytics...Still have the problem,
I'll check for AC ripple across the key today.
My schematic shows test point voltages with pin
numbers.  Guess I'll check that out too.

Gary
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