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 Post subject: R390A RF tuning slug
PostPosted: Jun Mon 03, 2019 12:55 am 
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Location: Clayton, Ind.
I just acquired for free (!) a Motorola 56 order r390a, S/n 360. Very clean with just a few quirks.

One is that one of the 18 RF tuning slugs has a slotted screw for adjustment. The slug looks just like the others, though I have not pulled that rack to check length. I read that slugs for R390 have slotted screws and different permeability. Possible someone lost the Bristol screw and put the loose slug on a slotted screw. Does it really matter if one slug is from a R390?

The other question is on IF deck, it's marked Col. Ser. No. something, does that mean Collins? Early version without ceramic trimmers on top of mechanical filters.

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TDane WA9YI

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 Post subject: Re: R390A RF tuning slug
PostPosted: Jun Mon 03, 2019 4:53 am 
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Joined: Sep Thu 01, 2016 3:56 am
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Location: San Jose, Ca.
The slug with the slot adjustment is not original to a 390A, it's probably from a 390 (which is probably different than what is needed). However, the real proof is whether it tunes correctly or not. Tune the high and low end of that band, and if it tunes correctly, then it's ok.

Regards, Larry


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 Post subject: Re: R390A RF tuning slug
PostPosted: Jun Mon 03, 2019 7:40 pm 
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Upon further review, I see that the slug in question is more silvery than the others, so I conclude it must be from a r390. Rather than awaiting tuning experiments, I will look to buy or trade for one from a r390a. I wont turn thus on until after I recap, etc.

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 Post subject: Re: R390A RF tuning slug
PostPosted: Jun Tue 04, 2019 3:27 am 
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Joined: Jul Thu 29, 2010 5:35 am
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Location: Simonton, Texas
Be sure and get the correct one. The type used in the IF section are different permeability then the ones used in band tuners. Check the others in the same strip as the slotted one and see if there is a color slash or check the color of the slugs them selves. On the 390A I acquired I could not get one of the IFs to tune up correctly and I figured out that some one had installed the band type in the IF strip. I got the correct ones from Fair radio sales and if I remember correctly they were about $7.00 each. Every once in a while a set will show up on Ebay but they normally want about $60.00 for a set. Good Luck!

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 Post subject: Re: R390A RF tuning slug
PostPosted: Jun Tue 04, 2019 4:44 am 
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Joined: Sep Thu 01, 2016 3:56 am
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Location: San Jose, Ca.
It might be worth while to post a request on the R390 reflector asking for one. Put WTB and R-390A at the beginning of the subject line and include which ever slug you need, RF or VIF. Good luck.

By the way, do you have a variac to power it up slowly? If not try to borrow one. This is especially important since your unit only has 1 fuse. Some times recaps can go astray and cause B+ problems. Better to be safe than damage a choke or transformer.

Regards, Larry


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 Post subject: Re: R390A RF tuning slug
PostPosted: Jun Tue 04, 2019 4:59 am 
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Joined: Jul Tue 21, 2009 1:38 pm
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Location: SW WA state
TDane wrote:
The other question is on IF deck, it's marked Col. Ser. No. something, does that mean Collins? Early version without ceramic trimmers on top of mechanical filters.


Yes, Collins... The filters can be adjusted, but it's a PITA: Each mechanical filter has a fixed input and output capacitor, which probably isn't ideal. I built a "gimmick" capacitor with about 70 pF of fixed capacitance, in parallel with a 5-50 pF. I remove one side of the fixed capacitor, tack in the gimmick, tune as needed, then remove and measure the gimmick. I then replace the old, fixed cap with the value arrived at. Then wash, rinse, repeat seven more times.
Let me know if you can't find an RF slug. I have some parts cars. On the road for work, won't be back for several days.
Best of Luck with it! And congrats on the price. Had a similar situation with an R-388 a few weeks ago. Yes, mine needs work (usual dust, dirty pots & switches, PTO EPE and matchstick caps), but it functions!

-Tom


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 Post subject: Re: R390A RF tuning slug
PostPosted: Jun Wed 05, 2019 11:28 pm 
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Location: Clayton, Ind.
Thanks for the suggestions. It will be awhile before I plug this in. I need caps, a bucking transformer to get line voltage down to 110, clean overall and the RF gear train, etc. And haven't finished checking the many things in the WHRM site, plus all the other literature out there. And I need to download or print the Y2K manual, etc.

I do have some twinax to play with - about 600 feet of it. That also was free. The receiver belonged to a ham who died; his widow called up our club, and we gathered up everything, and our club treasurer knew I liked boatanchors, so he gave it to me.

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TDane WA9YI

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 Post subject: Re: R390A RF tuning slug
PostPosted: Jun Wed 05, 2019 11:42 pm 
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You don't need to use the Twinax as you normally ground one of the terminals and run your antenna to the other terminal.

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 Post subject: Re: R390A RF tuning slug
PostPosted: Jun Sat 08, 2019 7:08 pm 
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Interesting what you find. I am starting to pull tubes for testing and inspecting all soldered connections. I pulled the two filter cap cans, and instead of a 30x3 and a 47x2, I have two 30x3s. (One is marked 1957 and one 1961.)

So does someone out there with two 47x2s want to swap? :D

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TDane WA9YI

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 Post subject: Re: R390A RF tuning slug
PostPosted: Jun Sun 09, 2019 1:18 am 
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Joined: Jul Thu 29, 2010 5:35 am
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Location: Simonton, Texas
Some one on epay sells direct new replacements for about $89.00. Those can be cut open and re-stuffed with modern caps.
I also would think Hayseed Ham-fest would sell them also. Hayseed sells very nice canned electrolytic s and has made custom types for me for old HP equipment.

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 Post subject: Re: R390A RF tuning slug
PostPosted: Jun Sun 09, 2019 2:56 am 
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I'm going the restuff route unless I can fit them underneath, but things look pretty cozy under there.

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TDane WA9YI

Radio is a divine gift; God made us able to do one thing while listening to another.


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 Post subject: Re: R390A RF tuning slug
PostPosted: Jun Mon 10, 2019 1:08 am 
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This is fun. Pulled slug racks to clean the rollers. The odd slug has reddish brown around the top, I think one of three r390 types.

Pulled the 18 RF tuning coils. One damaged socket and what's that black stuff under T203? Yep, the coil is charred and a quarter inch of the phenolic tube is burned away. So add that to the list of things to find.

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TDane WA9YI

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