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 Post subject: Selecting a osc coil replacement
PostPosted: May Wed 08, 2019 11:50 pm 
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Joined: Jan Wed 07, 2015 10:39 pm
Posts: 223
Location: Jamison PA
On the other board I have posted about my Coronado 36L chassis,.

Specifically The original osc coil is mouse eaten, It is part 2849. It looks like a Meissner coil make. Has the square bottom and trimmer on board

It is 456KC, So I found a Meissner 4034 that looked like mine. Long story short, I cannot get it to bring in anything, it does oscillate, but way off when a digital radio is used to test. I tried a universal one that was 455KC and that did nothing at all.

So Now I am back to the drawing board. I was able to remove the coils from the original form after melting the wax.The original osc coil top section (unstranded) is OK and I could reuse it. The bottom is litz 10 strand looks like awg 38, so I have some coming, cause even though I was able to resolder a eaten piece, I cannot get it back on the form, unless I unwrap it and rewrap on form. I have never done one of these before. It looks like it is 5 rows in a diagonal, then changes diagonal for I think another 5. I can see the imprint on the wooden form. As the wire is thicker, its a larger coil in circumference than the 4034 coil. The top coil is thinner than the 4034 too.

Not sure what to do, some advise greatly appreciated. See my posts on this radio for clarity.

THank you

Mike


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Thu 09, 2019 6:22 am 
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Location: Near Portland, OR
The schematic says 456 KC, but the alignment instructions say 465 KC, followed by "this must be accurate."

Looks like this was originally a Sentinel 36L. That schematic has 465 KC handwritten on it, while the Coronado schematic has 456 KC typed on it. Perhaps it's a typo by Rider's.


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Thu 09, 2019 1:25 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 8479
Location: Long Island
Looking at the schematic, it does not appear that you radio has a common 365 pF/165 pF split tuning capacitor. Those became more or less standardized some years after the set was built. Therefore, “universal” oscillator coils will probably not work at the right frequency. An exact replacement, or rewinding the original, will probably be necessary.

Since the set has separate coils for the SW band, you could verify that everything else is working correctly if the SW band operates normally.

As for rewinding, you already have a leg up because you know how many turns it originally had and the approximate size and shape of the winding. Don’t worry about the winding pattern; that is not likely to make a big difference.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Thu 09, 2019 1:35 pm 
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Joined: Jan Wed 07, 2015 10:39 pm
Posts: 223
Location: Jamison PA
Thank you for your help.

I learned something, that is that the tuning cap pf value has everything to do with osc freq.

I guess that is why I read that the universal ones seem to work with many radios. I guess this one is too early and specific,.

I did try adding pf caps to the osc but didn't help much at all.

I'm glad the weave isn't critical, but wanted to confirm:

If the form is vertical, do you wind going to the right? ie clockwise?. I think that is the way its wound. In addition, would you unwind whats left of the original to get the approx number of turns?. A mouse got it and ate into it. That left a couple turns to come off......

Thank you again

Mike


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Thu 09, 2019 1:59 pm 
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Location: Annapolis, MD
delco32 wrote:

If the form is vertical, do you wind going to the right? ie clockwise?. I think that is the way its wound. In addition, would you unwind whats left of the original to get the approx number of turns?. A mouse got it and ate into it. That left a couple turns to come off......

Thank you again

Mike

The direction of an individual winding usually does not matter---what counts is the phasing between TWO windings---primary and secondary. Depending on the type of oscillator circuit, the primary and secondary might be in phase or out of phase. The low-tech way of getting it right is to simply copy the way it was.....

Take a look at the wiring diagrams for this "universal" oscillator coil:
https://www.tubesandmore.com/products/coil-oscillator

For this coil, winding 5-4 is "in-phase" with winding 1-3. This means that--if 5 to 4 is wound clockwise, 1 to 3 would also be clockwise.

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-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Thu 09, 2019 5:08 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
The number of turns in a coil determines its inductance, so if you change the number of turns it will oscillate at a different frequency. If the dial calibration is reasonably good on the shortwave band, and nobody has changed the trimmers or other parts in the RF section, you can put some extra turns in your new coil, then remove a turn or two at a time until broadcast stations come in right on frequency, at least in the middle of the band. Center the trimmers in their ranges before you proceed so if you have to do a little fine-tweaking at the top or bottom of the band, you've got the room to do it.

As for phasing, one of the secrets of RF coils and transformers is that most have considerable capacitive or electrostatic coupling between the windings, as well as the expected electromagnetic coupling. Due to this, the phasing of the windings might not make as much difference as one would think. But it is easy enough to tell. If the phasing is affecting things the wrong way, the oscillator may have difficulty starting or won't run, or it may only oscillate over a small part of the band. In that case, simply reversing the connections to one of the windings should fix it.

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"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Thu 09, 2019 7:09 pm 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 3646
Location: Florida
delco32 wrote:
.................. I tried a universal one that was 455KC and that did nothing at all. ................Mike


I haven't checked lately but AES used to have a universal slug-tuned coil that could be adjusted for different tuning caps, frequencies, etc.

RRM


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Thu 09, 2019 8:23 pm 
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Joined: Jan Wed 07, 2015 10:39 pm
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Location: Jamison PA
I did try that particular coil. It would do nothing for this radio. I'm sure it works, just not in this configuration... :(


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Thu 09, 2019 8:43 pm 
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Joined: Jun Sat 09, 2007 8:14 am
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Location: Florida
I was curious since you stated 455 coil. Did it not oscillate at all or just way off?

RRM


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Thu 09, 2019 9:00 pm 
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Location: Annapolis, MD
Retired Radio Man wrote:
delco32 wrote:
.................. I tried a universal one that was 455KC and that did nothing at all. ................Mike


I haven't checked lately but AES used to have a universal slug-tuned coil that could be adjusted for different tuning caps, frequencies, etc.

RRM

That's the coil I linked----If the IF is even close, it should work in just about any circuit.

Note: In my experience the phasing always matters---at least for BC and most SW bands.

_________________
-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Thu 09, 2019 9:51 pm 
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Location: Long Island
The oscillator coil and the tuning capacitor form a parallel resonant circuit that sets the frequency of the oscillator. So the values have to match up, otherwise you’ll end up somewhere else in the RF spectrum than you expected.

Now it is true that from about 1940 on, coil/capacitor pairs for the AM broadcast band became fairly standardized, as did radio IF transformers and antenna coils. So-called universal replacement coils became common fare for radio repair shops. But prior to 1940 every radio manufacturer used different tuning caps with different coils. “Universal” replacement coils might work in such sets or they might not.

It might also be noted that the term “455 kHz oscillator coil” is technically incorrect. For one thing, the oscillator frequency varies with dial setting. It does not stay fixed on one frequency in normal usage. For another, oscillators in most broadcast radios run above the frequency by the IF. So if you set the dial to 1,000 kHz and the radio has a 455-kHz IF, the oscillator should be running at 1,455 kHz. What the designation means is that the coil is meant to be used with a 455-kHz IF. So you could find any number of “455 kHz oscillator coils” but only ones with the same primary and secondary inductances as the original will match up with your tuning cap and tune to the right frequencies.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Fri 10, 2019 12:13 am 
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Joined: Jan Tue 16, 2007 7:02 am
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Location: Lexington, KY USA
The "universal replacement" coils still being manufactured are all adjustable, to match different tuning capacitors. If they still have too much inductance with the tuning slug completely gone, turns might be removed to reduce the inductance.

A new coil will not always fix all of your problems. These old sets sometimes have multiple failures.

The universal replacement coils should at least make the oscillator run, if possibly not at the correct frequency. Use a counter, scope, or second radio to see what frequency you have.

Ted


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Fri 10, 2019 4:38 am 
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Joined: Jun Sat 09, 2007 8:14 am
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Location: Florida
The schematic and PL don't give values for the variable cap. What does it look like physically (big, small, # plates, etc.). Picture?

RRM


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Fri 10, 2019 11:55 am 
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Usually Lurking wrote:

The universal replacement coils should at least make the oscillator run, if possibly not at the correct frequency. Use a counter, scope, or second radio to see what frequency you have.

Ted

+1 The coil in question---if wired correctly, and if there are no other issues, WILL cause the oscillator to run.

_________________
-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Fri 10, 2019 1:50 pm 
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Joined: Jan Wed 07, 2015 10:39 pm
Posts: 223
Location: Jamison PA
The 4034 osc coil I have did oscillate. I cold get a zero beat with a digital radio, but dial way off. digital was lets say 1100 and radio 1440.

Only once could I get a station, and that is after I removed the trimmer in the pic from the ratty original, and alligator clipped it to the 4034. If I held my thumb on the trim plate just so, I actually got our local 1210 burner station. The combo of my grounding it, the alligator leads probably kludged in the signal just right.

I figured that if I added pf caps to the trimmer it might bring the frequency closer, but it never worked. When turning the dial, Id get whispers of what might have been off freq stations, but who knows?

Trying with the universal coil netted nothing at all. Reversing connections didn't help either.

Here is the original coil.. The upper fine wire is still intact, though it had broken off close to the inner side. It will be a real chore to tack to it, but it is showing continuity with my ohm meter.. I'll post pics of the radio this eve. I have since removed the lower coil off the form. It was bit through and there is at least two turns bitten out, that I have. What a mess.....

Thank you

Mike


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Fri 10, 2019 11:19 pm 
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Posts: 223
Location: Jamison PA
Here is the chassis and the other osc coil


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Sat 11, 2019 2:27 am 
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Joined: Jun Sat 09, 2007 8:14 am
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Location: Florida
I'm one of those who is puzzled about the universal coil doing zip. Have you verified all the other components in the oscillator circuit (padder, trimmer, variable cap, other caps, resistors, etc.)?

The variable cap doesn't look unusual. Having both sections is the reason a padder is required.

RRM


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Sat 11, 2019 3:10 am 
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Joined: Jan Wed 07, 2015 10:39 pm
Posts: 223
Location: Jamison PA
I think after finding another sentinel schematic like this, that had to top side shown, that I may have the ant and osc tuning caps reversed at the bandswitch connections. One side is supposed to have pin 5 and the osc tuner goes together at bandswitch. The Ant tuner goes to pin 4 and includes top cap of 6A7. :shock:

Perhaps that is the smoking gun here?

Mine is the 36L chassis. The other one is laid out much like this.


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Mon 13, 2019 1:36 pm 
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Joined: Jan Wed 07, 2015 10:39 pm
Posts: 223
Location: Jamison PA
After swapping the ant and osc tuner cap wires, its about the same. I can get 2 stations with the 4034 with trimmer attached. 1060 is 1320.

Universal tried in several configurations and nothing at all.

I will rewind the original as the inductance must be something special to this set.....


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 Post subject: Re: Selecting a osc coil replacement
PostPosted: May Mon 13, 2019 2:18 pm 
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Location: Annapolis, MD
The Coronado 36L--and the the two schematics you just linked--all use a conventional Armstrong circuit**. The required inductance in all cases is determined by the net effective capacitance of the tuner + trimmers and padders. There is nothing special or unique with yours.
If a universal coil does not work, I would not expect anything to work.

Oscillators 101: Take a tube stage with gain. Put a tuned L-C circuit on the grid. Connect a feedback winding with the correct phasing to either cathode or plate (AKA oscillator anode in something like a 6A7).
Apply power, and there will be oscillation at some frequency.

**Armstrong = tuned circuit (tank) wired to the grid. Separate feedback winding in the plate circuit. Coil Phasing: grid and plate OUT of phase

_________________
-Mark
"Measure voltage, but THINK current." --anon.


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