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Old 18th Dec 2019, 10:36 am   #81
allan
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Default Re: RCA AR88D Comms Receiver.

I'm getting confused now. The osc is packing up at the low end ie.Range 5 = 16MHz and Range 6 = 22MHz.
But it's working at the high end where the Q is less.
It must be the ratio of C to L that's upsetting the propensity to oscillate?

Can you remove the slug from Range 5 coil and see if this performs differently in the Range 6 coil? And ditto with the Range 3 slug.
I know that different grades of ferrites have different frequency specs so is it possible that there is more than one type of AR88 slug? If so could the wrong slug(s) have been fitted in the factory?

Finally will the AR88 slug work in your Advance oscillator and will the Advance slug work in your AR88? If so swap them over. At least for Range 6.
Allan
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Old 18th Dec 2019, 10:49 am   #82
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Default Re: RCA AR88D Comms Receiver.

Found these if any use?
5 ot appears to be range 5 osc from google
6 rft, i dont know.
Rob
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Old 18th Dec 2019, 2:55 pm   #83
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Default Re: RCA AR88D Comms Receiver.

Rob.
These two coils could prove conclusively whether or not I have a coil problem.
How do I get them?
Please pm me if you can help.
John.
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Old 18th Dec 2019, 5:15 pm   #84
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Default Re: RCA AR88D Comms Receiver.

Hi John, there are 2 osc coils in one box and 4 of the rft in the other. I have 2 more identical boxes, ie making a total of 4 osc and 8 rft.
PM me your name and address and I will post.
Without boxes they can go as a large letter, otherwise boxes make them a small parcel!
Let me know what you need.
Rob
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Old 18th Dec 2019, 5:54 pm   #85
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Default Re: RCA AR88D Comms Receiver.

Are those coils for an AR88LF rather than the D model?
Range 6 should be 22-32Mc/s and Range 5 16-22Mc/s
The slugs would be OK though?
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Old 18th Dec 2019, 6:09 pm   #86
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Default Re: RCA AR88D Comms Receiver.

Sorry Allan,
only know what is on the boxes and what google turned up.
Rob
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Old 18th Dec 2019, 10:15 pm   #87
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Default Re: RCA AR88D Comms Receiver.

Allan.
The 5 core is 1mHz better than the 6 core.
The 4 core is the same as the 5 core, although the 4 core is half as long again.
The Advance slug gives better coverage than both AR88 slugs.
The Advance is not complete, just a part chassis with coils, switch and valve bases, so cannot try the AR88 slugs in it.
Robin is sending me a 6 coil and slug to try.
That will move me on a stage.
John.
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Old 18th Dec 2019, 11:11 pm   #88
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Default Re: RCA AR88D Comms Receiver.

One other interesting thing about this whole problem is that at these frequencies many manuafcturers built L/O coils without slugs. So the presence of them is not a requirement to sustain oscillations.

The advantage of a slug tuned coil, normally is a higher Q than a non slug coil as the windings can be much shorter and there is a lower resistance, and you can adjust the L value for the lower end of the band and not require to manipulate the padder cap value for the low end frequency setting.

So the really odd thing here is that while the defective or incorrect slugs have raised the inductance to the about correct value (the tuning frequency range is correct) they have somehow affected the Q, without altering the resonant frequency and added a "loss" component. If that is true the loss can only be a hysteresis loss or an eddy current loss. Its hard to imagine how the slug's B-H curve could have changed (unless say magnetized), so in some way groups of particles in the mixture must have become electrically connected and there must be circulating currents in the material.

It seems far more likely it has the wrong grade of slug, but if that were true, why would it have been working in the past with those slugs.

Also, none of this would be detectable at all if the valve had a higher gm, so I would double check that again with at least 5 NOS valves to be 100% confident it wasn't that.

So it is one of the better mysteries we have seen for a while.
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Old 18th Dec 2019, 11:20 pm   #89
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Default Re: RCA AR88D Comms Receiver.

There could also be factors which limit the maximum indicated Q, so those similar top values may hide differences.

David
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Old 19th Dec 2019, 12:17 am   #90
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Default Re: RCA AR88D Comms Receiver.

Just looking at the schematic again posted on this thread, it indicates that the 6J5's grid resistor is 56M, obviously a typo, in the actual set it is presumably 56K and is that the value you have been using in your test circuit and, not 5.6M ?
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Old 19th Dec 2019, 12:35 am   #91
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Default Re: RCA AR88D Comms Receiver.

There's an old US convention of using Roman 'M' where we're used to seeing 'k' for the 1,000x multiplier, it can make for few double-takes at first!
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Old 19th Dec 2019, 12:55 am   #92
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Default Re: RCA AR88D Comms Receiver.

I'd noticed the use of M for K but as John can physically see the resistors I don't think this is an issue.
I know for certain that the triode section of a 6K8 for example can lose emission and I changed an EF91 in an RA17 because the VFO only got halfway through tuning.
I guess the 6J5 might be a bad batch and gone gassy but John reckons they test OK.
Maybe the iron powder coating has deteriorated and the iron dust insulation has failed in parts? It may explain the problem especially if it only happens after 70 years. I do know that bits of me are failing since my arrival early in WW2 so why not bits of an AR88?
I'd swap the slug for another (less ancient) even it entails adding a plastic insert or wadding to make it fit.
Did I mention that the HF coils in my Moreton Cheyney receiver do not oscillate. The LW and MW coils without slugs work fine but the HF ones WITH slugs don't oscillate. That receiver is 72. Coincidence?
Allan
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Old 19th Dec 2019, 10:25 am   #93
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Default Re: RCA AR88D Comms Receiver.

R4, 6J5 grid leak in the owners manual is listed as 56,000 ohms, I figuered out at an early stage that M means K. e.g. 5,600,000 ohms is listed as 5.6 meg
I have 5 6J5's, all work the same.
John.
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Old 19th Dec 2019, 10:53 am   #94
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Default Re: RCA AR88D Comms Receiver.

Hi John, I sent 2 of the rft and one osc. Made sense to use the 100g small packet to capacity.
Posted first class at a post office so should be with you before Xmas.
I have been following most of this thread and in the early stages thought that I had some coils, but wasn't sure they hadn't gone to a radio rally and been sold. That's why I kept quiet as didn't want to raise your hopes.
Anyway, found them now. There are various types, but the ones pictured are the nearest frequency wise to what you are using.
Rob
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Old 19th Dec 2019, 3:51 pm   #95
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Default Re: RCA AR88D Comms Receiver.

I have been conducting more tests with my osc lashup.
Using the 150v and 6.3v from the set the band 6 coil gives the same result as I am getting in the set.
I used a new valve base and new components and a fm vc with the 3 gangs wired in parallel and an air spaced trimmer in parallel with the 3 pre-sets on the gang.
No osc at the low end using the AR88 cores.
However, using a 1" piece of ferrite rod and the Advance core pushed down the former I can get the coil to tune from 33 meg down to 23 meg with not bad amplitude, but with a bit of a dip in the center of travel.
I think that I have now conclusively proved that I have a frequency dependant slug problem. Unless you prove me wrong??
I eagerly await Rob's coil in the post now for a final test.
Any ideas on how I make my own slugs, iron dust in a clay mix?
There could be, buried in the RCA archives, the spec for maufacturing AR88 slugs.
I find it difficult to believe that I am the only owner of an AR88 with this problem.
John.
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Old 19th Dec 2019, 4:27 pm   #96
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Default Re: RCA AR88D Comms Receiver.

There are dust-iron toroids available in many sizes and several materials. I'd look for 'Amidon' cores from amateur radio parts suppliers and also the G-QRP club sales page on the back of Sprat. They're really made by Micrometals inc. Anaheim California, But Bill Amidon is a parts retailer selling to amateurs, and his name seems to have become strongly associated with them.

A group of small toroids epoxied together in a stack, and with the screw epxied in the centre ought to do.

Toriods are available from 0.15 inches up to over 3 inches od.

-2 or -6 material is worth trying.

David

Oh.. this is how Yaesu musen make the permeability tuned preselector core for their top end HF transceivers. You know, the ones with names like FTDX70000000MPDXDXDX MkXXIV. They have a stepper motor positioning the core in the coil.
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Old 19th Dec 2019, 5:45 pm   #97
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Default Re: RCA AR88D Comms Receiver.

An easy answer John.
Your new coils from Rob will have dust cores you can swap over.

For my MC receiver the IF dust cores had rusted and broken off their brass screws so I filed down some threaded cores that almost fitted. When they slid into the coils I superglued them to the end of the threaded brass adjusters and they worked a treat.
The new cores had a hex hole through them so I fitted them onto a drill to make the job of filing off some of the outer thread easier.
Allan
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Old 19th Dec 2019, 5:56 pm   #98
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Default Re: RCA AR88D Comms Receiver.

Quote:
Originally Posted by turretslug View Post
There's an old US convention of using Roman 'M' where we're used to seeing 'k' for the 1,000x multiplier, it can make for few double-takes at first!
Anyone who's fluent in Italian will be familiar with "Mille" as 1000.

It can be perplexing at first to see this in 1930s/1940s schematics/circuits, but it always reminds me that the science of radio owes rather a lot to a certain Italian gentleman....
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Old 19th Dec 2019, 9:01 pm   #99
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Default Re: RCA AR88D Comms Receiver.

Just a point,are those valves you have tried from the same batch?There is an article re EF91,s in latest BVWS mag which state about variations in valves despite being in spec.
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Old 19th Dec 2019, 10:58 pm   #100
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Default Re: RCA AR88D Comms Receiver.

There are all sorts of RF coils and cores available from Surplus Sales Nebraska, they might possibly have the exact size & type slugs.
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