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Author Topic: Why does the 2IF amp act like this?  (Read 4376 times)
AC5UP
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« Reply #15 on: October 29, 2012, 02:26:49 PM »

Where can I find info on how to figure out how many turns?

The wire might be too fragile for this, but if you could count the turns as you removed the open winding that would be one option.

Since it's an adjustable coil the re-wind doesn't need to be 100% faithful to the original. Get as close as you can, but as long as you can find an obvious sweet spot in the slug position that's probably a good sign.
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KE4JOY
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« Reply #16 on: October 29, 2012, 04:06:43 PM »

No problem counting the turns actually I havent stripped it yet.

But doesent the diamater of the wire affect things?
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AC5UP
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« Reply #17 on: October 29, 2012, 05:04:19 PM »

Sure does... But probably has more to do with the Q of the transformer than the tuning range. Unless you radically change the wire size, I think you'll find the number of turns has more to do with the inductance value than the physical size. An ultra-fine wire will put more of the turns within a closer proximity to the core and give a better Q, but a super-fine wire also raises the DC resistance of the coil while reducing the current carrying capacity. If I can't match the wire size exactly I'd prefer a tad heavier to a tad lighter.

No one has mentioned this yet, but there's a reason why the open winding decided to go open. Could have been a flaw in the wire itself, could be the transistor on that winding has gone leaky and was pulling too much current. Could be a current limiting resistor has changed to a lower value and now the circuit draws more mils. I haven't looked at the circuit but typically the coil is in series with the associated transistor and B+ voltage... Short the transistor and the coil becomes a fuse.

Might want to confirm the rest of the parts are good before putting the juice to the new or rewound IF transformer........   Wink
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G3RZP
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« Reply #18 on: October 30, 2012, 04:30:32 AM »

It could also be corrosion - not an unusual problem with wires like #42 or #44 over the years.

What size is the former?
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KE4JOY
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« Reply #19 on: October 30, 2012, 12:43:30 PM »

The former tube is 7mm or ever so slightly larger than 1/4".

The open winding is most probably due to abuse. I had this coil open some time ago when I replaced the 330pf mylar cap that was in there. Its probably been like this for some time. The stage just never tuned right and I went all through the components of that stage. Even put in a low noise amp npn in there.

Is there any way to guestimate the number of turns required given the previous wire size and turns as compared to 30ga?
« Last Edit: October 30, 2012, 01:04:03 PM by KE4JOY » Logged
W8JI
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« Reply #20 on: October 30, 2012, 01:28:21 PM »

You probably do not want or need to change the number of turns, even if you change wire gauge.

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KE4JOY
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« Reply #21 on: October 30, 2012, 01:42:50 PM »

Well I unwrapped the sucker and here is what I got


Primary

x---10---x---24---x
==================
                           x---4---x

Secondary

It was kind of strange in that the secondary was wound on the 'end' of the primary winding and not over one another. Forgive me but I found that strange.

Let me see if I can get 38 turns of 30 ga on this former.  Wink
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AC5UP
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« Reply #22 on: October 30, 2012, 02:34:41 PM »

Not strange at all...

Some RF coils have a tap not-too-many-turns up from the cold end for a Lo-Z signal pick-off point. This achieves an impedance conversion without excessive loading of the Hi-Z (hot) end of the coil. You'll see this in crystal sets intended for a random wire antenna that's typically a very Hi-Z input. There are also times when you want loose coupling between two windings lest one detune the other so you stack them like donuts with a little space between them. For tighter coupling you wind one on top of the other, usually up from the cold end.

In both cases the windings are parallel but with stacked windings only a few turns at either end are in close proximity.

Years ago I experimented with resonant loops for AM BCB reception and started out thinking the secondary one-turn loop should be nested inside the resonant winding for maximum signal transfer... Not So. If one end of the receiver antenna connection is grounded, tight coupling one side to ground can detune / degrade the primary. Better to loose couple to a balanced pickup loop that's happy than tight couple to one that isn't.........
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KE4JOY
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« Reply #23 on: October 30, 2012, 04:50:01 PM »

tnx for the lesson, good to hear the voice of experience.

I am rewinding the coil, it is going slowly but it is going well. The patient may live.

Might even post some pics when its done.

TGF hotglue  Grin
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G3RZP
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« Reply #24 on: October 31, 2012, 02:05:24 AM »

330pF mylar? Presumably for tuning, but I wouldn't generally use mylar for that. Polystyrene is better for Q, and comes in small sizes.
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KE4JOY
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« Reply #25 on: October 31, 2012, 07:28:14 AM »

Actually it is a poly cap, my bad.  Tongue Your right a mylar probably would not even fit.

Well I got it rewound, fairly close to orginal, put it back in the circuit and.... well no change. Same broad tuning with no real 'peak'. After looking closer at the schematic the coil is picked off at the center tap and the far end is not connected to anything so I do not think the lack of continuity to the far end had any real impact to begin with!  Angry

Frustration!  Huh
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KE4JOY
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« Reply #26 on: October 31, 2012, 01:19:39 PM »

Resolution...

I decided to go through and check every flippin resistor involved in the circuit and they all checked good and had been replaced a few years back.

I checked one last resistor, the one leading from the bias output. It had not been replaced but was marked in my notes as being within tolerence. I dident suspect it because the voltage checks were okay and it was simply bias to the secondary of the 2nd stage which couples to the base of the 3rd amp.

So I lifed the resistor supposed to be 1K and it measured at around 1,400. I replaced it with a new resistor, soldered everything back down and fired up the rig.

The second stage now has an actual peak in it not nearly as sharp as the first and third stage but at least now the slug doesent run out till it hits the can!

Sometimes you overlook the obvious.

Thanks for your help everyone, at least winding a new coil was a learning experience!
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AC5UP
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« Reply #27 on: October 31, 2012, 01:31:34 PM »

So I lifed the resistor supposed to be 1K and it measured at around 1,400.

...and you're aware you look at that as a 40% tolerance error, not as "off by 400 Ohms". A radio like that probably had a design tolerance of 10% since Heathshkit wasn't exactly famous for tight tolerances, but a 40% error is seriously  w - i - d - e  even by Benton Harbor standards.

Congrats on finding the issue! Sometimes 90% of diagnostic skill relies on a pantload of perseverance.........    Grin
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KC4MOP
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« Reply #28 on: December 15, 2012, 03:45:28 PM »

Ya got one of those magnifying head band glasses?? You might be lucky and find the fine wire broken right at the terminal. You will have to dissect the can and do some surgery. Usually, if you're really lucky, the wire broke off the terminal inside. Someone might have a junque box with one of those units and you can buy one real cheap.
But you have found the problem to the no-tune IF can

Fred
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WB6BYU
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« Reply #29 on: December 15, 2012, 05:06:35 PM »

#32 magnet wire isn't hard to find (I have a couple spools).  You might be able to
salvage suitable wire from an old speaker.

Just count the turns when you unwind the original coil and replace with the same
number of turns.

If you know the Heathkit part number you might find that it was also used in some of
their other rigs, and may find a replacement from a basket case radio.  That's the same
IF used in the old HW-12 and several other rigs over the years - just a matter of whether
they have a coil the same size.

There will be a capacitor across the coil - if you don't get exactly the same inductance
you may be able to vary the capacitance across the coil to bring it to resonance.  If you
look at the circuit and find the capacitance across the coil, you can calculate the
required inductance based on the resonant frequency.
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