[AMRadio] R-390A Help


W2XJ w2xj at w2xj.net
Mon Jul 7 23:46:22 EDT 2014


The ballast tube is a series regulator for the PTO filaments. It is not a likely candidate for popping fuses since it has no direct path to ground. 

Here is how I would proceed. First determine the manufacturer ( there were about 20 ), get the serial number off the nameplate and research any idiosyncrasies about the specific build you have. Then look at the back and see how many fuses the are. The early models had one, F101, it is after input filter mention previously mention could not case it to blow, it would pop the wall breaker and maybe let out some smoke. 

If you are lucky enough to have a model with 3 fuses a lot of pain can be avoided. F102 is the main B+ coming directly off the power supply module which is really just a transformer and rectifier. F102 feeds unfiltered DC to the audio board where the DC filtering is done for the whole radio. After the filters, F103 protects filtered B+ to the rest of the radio. I would pull F102 and F103 if the radio has them. Turn. On the radio and if F101 blows it is either the power supply module or something in the 25.6 or 6.3 volt distribution which is primarily filament power. If F101 does not blow, replace F102 and repeat the test. If F102 blows without F103 being in place then it is quite possibly the filter caps. 3-30 MFD and 2-45 MFD, check those. If F103 blows, there is something in the radio shorting the B+ and you have to go module by module.  

So far you will not have had to even open the case of the radio. But here are some mods you should do. Replace the tube rectifiers with solid state. This will require the addition of a 200 ohm resistor in series to keep voltages normal. That mod is an official technical bulletin. If F102 and 103 do not exist on the back of the radio, add them. This could avoid a burned harness and other bad things. Next, although not a mod, replace the 5 filter caps on the audio module.  This falls under the general shotgun rule. 

Finally eliminate the ballast tube. It is simply a regulator scheme for the PTO filaments. Any standard solid state regulator that can handle the current will work. I would use a half wave rectifier to feed the regulator which can be mounted externally and connected through the tube socket once the tube is removed the wall will serve as a heat sink for the regulator. This option solves a few problems but most importantly that tube is the most expense one in the radio if you can even find it. There are materials about replacing other tubes if necessary. Mostly being due to the filament voltages used.

Hopefully to solution to your problem will be relatively simple. 

Sent from my iPad

> On Jul 7, 2014, at 9:33 PM, "Jay Bromley" <jayw5jay at cox.net> wrote:
> 
> Be sure to have a close look at the ballast tube.  73 de w5jay/jay..
> 
> -----Original Message-----
> From: AMRadio [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Bill
> Guyger via AMRadio
> Sent: Monday, July 07, 2014 5:46 PM
> To: Discussion of AM Radio in the Amateur Service
> Subject: [AMRadio] R-390A Help
> 
> Hi All
> 
> I acquired a 390A over the weekend from a friend who can no longer see
> to work on his gear. He made me promise to not sell it or trade it while he
> was still alive which I wouldn't do anyway out of respect to a genuine
> gentleman, a fine ham and one hell of a builder of radio related gear.
> Returning it to the air is my way of honoring a long time friend.
> 
> The last time he fired it up the line fuse blew preceded by a sound like one
> might experience after too much Mexican food and a wisp of smoke. He
> suggested something in the power supply which seems pretty basic other than
> Collins' use of 26Z5's for rectifiers. There's not much else there except
> for chokes and a couple of caps which date from the 1960's. 
> 
> I haven't had time to really dig into the unit, and those old caps seem a
> logical place to start in addition to suspecting one of the rectifiers.
> Hopefully lightning etc. didn't get into the radio and fry something when he
> wasn't looking.
> 
> My question to some of you R-390 gurus out there is. Is there a history of
> Power Supply issues and does anyone know of a source of 26Z5's. Or....if
> it's not heresy could I replace them with Silicon rectifiers after doing
> something about the additional B+ that will create? Not sure I want to
> pillage a great radio by doing that anyway if I can find the correct tubes
> but it's a consideration. 
> 
> Many thanks and 73 Bill AD5OL
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