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Author Topic: Help with R390A audio section  (Read 8066 times)
N3DT
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Posts: 474




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« Reply #15 on: January 25, 2013, 09:16:56 AM »

After much digging around I think I've got it figured out, if anyone cares.  Looks like in the 2 AF modules, which include the 3 chokes for the PS, I have 3 good chokes and 3 bad chokes.  One of the bad chokes is actually swollen up.  So I've installed the 3 good chokes on one module.  I found the DC resistance values of the chokes in the book as well as printed right on the choke. The module that has the bad input choke (low resistance) turns the PS into effectively a capacitive input filter which in operation gives a higher output voltage.  So it looks like to make the other receiver work properly, I need at least 2 new chokes for the IF/RF B+ and another for the AF B+ if I want to use the AF section.  It may take a while to find these items even if I wanted to afford them.  All I can do is keep a look out on ebay and several of the places that sell items like this (Fair Radio Sales).

Anyhow, I'm on to the IF sections.  They both seem to work to some degree or another, trouble is, one of them has some bad mechanical filters.
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G3RZP
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« Reply #16 on: January 25, 2013, 09:33:20 AM »

>trouble is, one of them has some bad mechanical filters.<

Murphy suggests that is the one that has the three good chokes....
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N3DT
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« Reply #17 on: January 25, 2013, 01:21:19 PM »

The chokes are on the AF module.  The filters are on the IF module.  Luckily Murhpy hasn't got me there, the bad filters are on the funky IF module, the good filters are on the better IF module.

Trouble is I lent a perfectly good (the best one I had) R390A to someone that took off with it and I've never seen it again.  Some people....
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KE3WD
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« Reply #18 on: January 25, 2013, 04:23:45 PM »

...All I can do is keep a look out on ebay and several of the places that sell items like this (Fair Radio Sales).

Chokes can be found new at places that specialize in parts for old audio and radio rebuilds. 

http://www.tubesandmore.com/ -- Formerly known only as Antigue Electric Supply, may be able to supply them. 

Chokes aren't all that critical, well maybe for the purists, but I've even subbed some by just finding one in the junkbox of the same size or a bit larger. 


73
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N3DT
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« Reply #19 on: January 25, 2013, 08:02:15 PM »

Yeah, I've got my eye on a Hammond, 4H 250ma.  I'm sure it would do for the 2 large ones.  $25 ea.  Looking on ebay these guys want $45 and up to well over $100 for some old junk.

I was looking for a power transformer too, the ebay ones are out of sight for either old or used stuff, I found a nice new Hammond one for $88.  Fair Radio has an R390A transformer for a reasonable price too.  It was too low in the voltage department for my use.
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G3RZP
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« Reply #20 on: January 26, 2013, 06:27:56 AM »

The only R390A I ever worked on had a bad mains transformer, too. Was it a weak link?
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N3DT
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« Reply #21 on: January 26, 2013, 07:24:34 AM »

I don't think the main PS transformer was a weak link. Been my reading that it hardly ever goes bad.
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KC4MOP
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« Reply #22 on: January 27, 2013, 04:51:06 AM »

The main thing I get from this is that the higher voltage is making more ripple.
Maybe the filter caps in the other module are breaking down from the higher voltage.
The higher voltage may have damaged the filter network. You mention swollen chokes.
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N3DT
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« Reply #23 on: January 27, 2013, 08:30:55 AM »

Actually I think what is making the ripple is mostly the chokes which are about 1/2 the DC resistance they should be, which is probably indicative of shorted turns.  With a much lower value of H, the PS filter will look more like a capacitive input filter which raises the voltage, and also probably makes for more ripple, but given that both of the high current chokes are bad, I really can't do much until they get replaced one way or the other.  I can get new Hammond ones in the range for about $70 for all 3.  The capacitors are all new and they check out.

The AF module with all 3 good chokes is working pretty good now.
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KE3WD
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« Reply #24 on: January 27, 2013, 09:16:30 AM »

Actually I think what is making the ripple is mostly the chokes which are about 1/2 the DC resistance they should be, which is probably indicative of shorted turns. 

I think you can safely drop the "probably" from that sentence. 

When much younger, still living with my parents and trying to learn and build up a modest ham shack for myself, I taught myself via trial and error how to take a choke apart at the E core laminations, remove the bobbin, cut out the old wire and scatterwrap a new coil onto the thing.  The first few, I would try to unwrap the old bobbin one turn at a time and keeping count, simply because I wasn't exactly sure on the number of turns thing, but that proved to also be educational as coming down in the wraps where the insulation had given up, finding the turns that had annealed, finding the turns where the copper had fused together from the wayward current and examining same brought some insight into failure mode.  One day, in the interest of speed, I decided to just try quickly cutting the old wiring out with an Exacto blade knife and rewrapping with same gauge enameled magnet wire to the point where the bobbin had just about the same size wraps or a little more - and that worked as well. 

From those early lessons it was not a long trip to tackling filament transformers and even some audio transformers.  What all this taught me was that it is very good indeed to investigate the coil, magnetics, from 60Hz AC right on through to VHF/UHF. 

The coil, no matter if found in a choke, transformer, relay, buzzer, doorbell (!), RF circuit, or anywhere else, is apparently little understood and far too many look upon the thing as being a replacement part that you pay someone else for.  That's good, don't get me wrong, but in cases where the part may be unobtanium, or where as in my early days, money is tight, why throw away the most expensive part of the component, the core and casing if any?  The part that is in need of replacement is always only going to be the copper wire. 

Rigging up a variable speed drill with a mandrel to hold the bobbin, a simple spooling device for the feedwire, wearing a cloth glove on the feedhand to both control the wire wraps and guide the layering might be a bit more scatterwrapped looking than what can be done with a lathe turned into a wire spooling machine, but has always proven to work just about as well here, if not identically as original in a lot of cases. 

Just something to think about.


73
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N3DT
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« Reply #25 on: January 28, 2013, 10:17:10 AM »

I rewound an old 90V 300W transformer to give me a 20A 13V power supply, so I too have dug into these old transformers.  I just may whack apart one of these chokes to see if I can re-wind them, but they may be potted, not sure until I get one apart.  They're sealed, like the Freed transformers, but maybe I can heat it up and get it apart.  Nothing ventured, nothing gained.
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KE3WD
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« Reply #26 on: January 28, 2013, 10:27:50 AM »

I have found that for the most part, potting in the older chokes and transformers, while messy, is usually an organic such as tar or shellac.  I can only recall one instance where a transformer was potted in something resembling a hard plastic that resisted all chemical or heating efforts, a light brownish colored full potting that was hard as brick.  This was inside a milspec "can" transformer and presumably was an effort to make something sea or jungle worthy.  It still did not stop me from going for the rewind, though, I simply used power tools, the drill press and the Dremel tool, to grind away both the potting compound and copper wiring. 

The shellac compounds often will soften by immersing the whole bobbin in denatured alcohol for a day or two. 


73
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N3DT
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« Reply #27 on: December 20, 2013, 09:50:58 AM »

Just to finish this thread, I happened to get 3 new chokes for the bad AF/PS module and now it works fine.  However, I'm wondering if just one is bad, apparently according to one of the R390A gurus low resistance in the chokes is not an indication it's bad.  I wonder if it's just the for sure bad one choke that supplies the AF B+  that's causing the problem, but I somehow doubt it.
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N3DT
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Posts: 474




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« Reply #28 on: December 21, 2013, 12:57:46 PM »

I checked both PS coils and they are bad too, so all 3 chokes were bad, one total failure (audio B+), one just plain bad (the swinging choke) and the other one marginal (4 H), but I'm not going to use it.
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G3RZP
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« Reply #29 on: December 22, 2013, 01:42:51 AM »

I don't think that I've ever had a choke go bad, so it's interesting that you've lost three. Could be someone trying to 'cheapen' things in production without reference to engineering......Someone in purchasing doing that in buying the dynamotor/cooling blower motor in a radio led to about 7 RAF jet fighters being lost back in the 1960s....
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