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Author Topic: Heathkit DX-60 "key down" with no key in!  (Read 10163 times)
K1ZJH
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Posts: 1005




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« Reply #15 on: November 19, 2012, 07:40:16 AM »

I'd trying swapping the PA tube if you can. But, until the problem is fixed, don't leave a good spare tube in the PA except for the brief test.

Pete
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K3STX
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Posts: 983




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« Reply #16 on: November 19, 2012, 09:22:31 AM »

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

You are looking for excessive AC ripple. 


73

Hmm, I read 60 volts AC across the key terminals with the KEY UP!! I think we are getting somewhere. Maybe one of my "new" electrolytics is bad.

p
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K8AXW
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« Reply #17 on: November 19, 2012, 09:30:47 AM »

At the risk of insulting everyone's intelligence, note that the "new" electrolytic caps are to be installed "backwards" so that you have a negative voltage at the output.

With that being said, note also that the voltage at the grid of the 6146 is supposed to be
-38vdc.  If that voltage isn't there or incorrect then it's time to check components C12 the .005ufd cap, resistors R11 - 12.  I'm leaning toward either a bad filter cap or in wrong and then the .005 cap to ground.  Not necessarily in that order.
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K3STX
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« Reply #18 on: November 19, 2012, 10:05:09 AM »

... note that the "new" electrolytic caps are to be installed "backwards" ...

WHAT?? My electrolytic can had two 40 mfd caps in there, the central "pins" were marked + (on the case) and the cap case was ground. So I hooked the + of new electrolytics to each pin, hooked the - of the electrolytics to ground, and then SNIPPED of the lead to the electrolytics to take them completely out of the circuit.

Was this not correct?

paul
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AC5UP
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Posts: 3875




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« Reply #19 on: November 19, 2012, 10:48:03 AM »

On a NEGATIVE power supply (relative to ground) the hot lead is negative and ground is positive.

I have a hunch you wired the electrolytic backwards and it tried to work for a while. Then it puked & died.

See the link below for the squidmatic of a voltage doubler / differential power supply. It is more complicated than needed to make this point, but does illustrate some of the clever things that can be done with a center tapped transformer:

http://tubecad.com/2010/12/15/OTL%20power%20supply.png

'Ground' can be positive or negative relative to the leg of a power supply, and note how the two 100mfd electrolytics below the 0 volt center line have their + side to 'ground'. Think of that point as 'common'... Less confusing.

Note to N4NYY:  Didja' click the pic and notice how all the diodes point to positive, even if that's 'ground' on the negative side?
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K3STX
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« Reply #20 on: November 19, 2012, 11:02:03 AM »

I'll go back and read schematics again, but I hooked the NEW electrolytics based on what the case of the cap can said! Surely they did not indicate + on the can when they really meant the pins were to be grounded!

Bottom line, I'll check this against schematic.

paul
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AC5UP
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« Reply #21 on: November 19, 2012, 11:23:28 AM »

Per the schematic of a DX-60B: Take a close look at C37 and C38... 20mf each. That's the ripple filter for the negative bias supply. (110 volt winding @ 30 mils)

The positive side of C37 goes directly to ground, the positive side of C38 goes to ground through R31. Either way, the 'hot' side of that voltage source is on the   negative   side of those two electrolytics. A negative power source has reversed polarity relative to ground.
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KB3WYZ
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« Reply #22 on: November 19, 2012, 12:18:20 PM »

I'm getting 50mv ripple across the key..
Paul, I think the CAN electrolytic were in right in the
first place. The only cap going to + is c-37 and C-38 has
a 220 ohm resistor between them. I re-stuffed the can with
two 47uf 450v caps tied both neg. leads to grnd.
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K3STX
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« Reply #23 on: November 19, 2012, 06:55:10 PM »

The only cap going to + is c-37 and C-38 has a 220 ohm resistor between them.
PROBLEM SOLVED!!!

Do I HAVE to tell you all how I screwed up?









OK:
When soldering in new electrolytics, it is advisable to solder them in EXACTLY as the old ones were. For example, if the common negative of C37/C38 goes to the transformer via lug #2, when you replace it, do NOT use lug #3!!!  I tied the common negative of C37/C38 to GROUND by mistake!! Thanks goodness I had the Heathkit manual. Moved all over to lug #2, now all great!

Thanks guys, it was very helpful. Now you can all continue laughing. Grin

paul
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K8AXW
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« Reply #24 on: November 19, 2012, 09:02:45 PM »

Paul:  This mistake is easy to make.  However, while we're talking about this wiring situation, note the can is mounted on an insulator.  This is for a reason.  This way the electrolytic can be wire any way the designer wishes.  Most of the time it's wired in "backwards" as for a negative supply.

When an electrolytic can is used for a positive supply, like most are, then the can will be mounted with the can attached directly to the chassis. 

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AC5UP
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« Reply #25 on: November 19, 2012, 09:28:10 PM »

...if the chassis is at DC ground potential.

Not to nitpick, but one reason K3STX took a detour through the tall weeds is that he assumed a chassis is always negative. Usually is, but the DX-60 bias supply is an exception. Likewise, the assumption that a Twist-Lock electrolytic can usually be mounted directly to the chassis because it's at DC ground isn't always true.

On some newer stuff like audio gear and power supplies (IIRC) it has become fashionable to establish a single point ground which might not be tied to the chassis. The chassis could be at AC wall socket "green wire ground" for safety, but the DC supply has its own ground. Basic concept is one well-defined point that's absolutely at zero potential including noise. Audio amplifiers using this technique (allegedly) do have a lower noise floor / wider dynamic range and you'll see ground wires from every sub-section of the chassis connected to that single point.

BTW:   One other feature of Twist-Lok electrolytics mounted on a fiber insulator is that the can might be  HOT  relative to the chassis. K3STX will demonstrate this some day when he accidentally brushes against C37 and gets his fancy tickled........  Grin
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K3STX
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« Reply #26 on: November 20, 2012, 05:24:38 AM »

You guys are giving me too much credit, this has nothing to do with knowledge. I simply soldered the leads to the wrong solder lug!! I should be more careful when I snip a lead!

Paul
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W4OP
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WWW

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« Reply #27 on: November 20, 2012, 07:34:19 AM »

If you are not 100% positive about what you are doing- Take a close up photo of the area you are working on before removing anything.

Dale W4OP
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K8AXW
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Posts: 3860




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« Reply #28 on: November 20, 2012, 10:18:13 AM »

I have ALWAYS had this problem of forgetting what lead came from where!  Way back before the day of digital cameras I made it a habit of drawing on a piece of scrap paper what lead goes where, including the size (large-small) and color. 

Now that I'm 76 years old I continue this practice without fail.  Now if only I can remember what I do with the damn solder after I finish soldering something! Roll Eyes
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KD0REQ
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Posts: 950




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« Reply #29 on: November 20, 2012, 11:50:05 AM »

long as you can remember to put the iron down when you smell burning meat, keep on haywiring!
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