The AM Forum

Left power strip on and R390 was on for a week with no antenna hooked up.   Came back from trip and Its dead.  No RX. 

Audio works, I can hear hiss at Wide open volumn
AGC works, If I flip to slow, med and fast, I can see the needle pop and settle like normal.
Put sig gen in, No RX, NOTHING.

Checked tubes, even swapped IF tubes.  NADDA.   Cant find anything wrong so far, NO band works. Tried them all.  It wont even hear the T368 in the room at full tilt.

BReak in is working, If I mute, it, the hiss goes away from speaker. 

About out of ideas here.. Probably overlooked something simple.. Any ideas guys? Where would you dig first???

C

Clark,

My first thought is the ballast tube has died during your absence so check to see if the filaments fed by the 3TF7 are lit.  Also, this is an R-390 and not the A variant?  If it is an A make sure that you have replaced the "killer cap" that can take out the mechanical filters before going further since in an A when the cap fails switching bandwidth will sequentially take out the remaining filters.

When I have a receiver that powers up normally but does not receive signals the first thing I check are the various oscillators.  You can do this without disassembling anything or attaching test probes so it is a simple test that often will lead you directly to the trouble or at the least exclude parts of the circuit.  Pull out your R-390 manual and the block diagram will indicate the signal path.  It has 3 mixers so you are looking at two crystal controlled oscillators and the PTO.  Select a band on the receiver and determine where the crystal oscillators should be operating for that band and listen for them by putting a shortwave receiver set for SSB mode very close to the receiver (I use a Grundig YB-400).   If you don't hear the oscillator tune the test receiver a couple of Khz. above and below the target frequency in case the oscillator is slightly off frequency; you should clearly hear the hetrodyne from the oscillator in your test receiver.   Do the same for the PTO, for this I would choose a target frequency in the middle of the PTO range and tune the PTO around this frequency to search for the signal.

If one of the oscillators isn't found, then you have zeroed in on the problem stage.  If all of the oscillator stages are working my next step is signal injection and tracing.  I start at the audio range to make sure the audio stage is working and then work my way up through the lower IF range towards the front end.  Some people prefer jumping in with voltage measurements first but I prefer using a signal source and scope-it is a matter of personal preference and what works best for you.

I agree with Rodger.  Ballast tube - it'll kill the PTO and BFO.  If you turn on the BFO and get no increased hiss, it's almost certainly the ballast tube, or the PTO tube or BFO tube (they're all three wired in series).

Easy fix - replace both the 6BA6s with 12BA6s and jumper the ballast tube pins - or, rewire the ballast tube socket for a 12BY7 fils and plug in a BY7.

ITs a brand new balast tube.  Its been in there less then 6 months.  I will yank it today and test it on the Tv7 and I will check the other tubes. There is NO change when going to BFO On.  I can hear the hiss change when I switch bandwidths! so thats a good sign. 

C

100% ballast tube.  Don't mess with replacing it, just bypass it via one of the methods described above.

This radio came with Two big resistors stuck into the Balast socket. It seemed to work fine but I put a balast tube in as I wanted it original.  Can I go that route for testing? I dont have any 12 tubes here.

C

You needs to drop 12V at 300 mA, so just figure the resistor accordingly.

The 3TF7 is a giant PITA and is probably the single most unreliable component in the radio.  It's expensive and does virtually nothing for stability unless you're running it on a generator with a widely varying voltage.  It's a huge improvement to throw it over the shoulder and never have to worry ab out it again.

Ok.. I am looking for some 12 volt tubes now. Do you have the pinout or picture of what pins to jump?

OK, maybe you missed the OR in John's post.
Boolean Rools

Sounds like you tested the two 6BA6s.  The filaments light up, right ?

SO keep the 6 volt tubes in there (if they test good) and get yourself a 12BY7. One tube to find, rather than two. The 12BY7 serves only to provide the series voltage drop allowing the filaments to again light up on the existing 6BA6s in there. You're not using the tube for anything else but as a glass enclosed resistor that lights up JUST LIKE A REAL TUBE>

As for pinout, put your ohmmeter across the pins where the ballast tube (or big fat resistors) once sat. Find the two that have resistance (this will include the resistance of the two 6BA6s that are waiting for you to complete the circuit).  Put a dot of black sharpie pen around the two socket holes (in case you ever want to go back).  

Now find the two filament pins of the 12BY7; look inside the glass to save time.  Put a dot of red sharpie pen on each of the two matching socket holes. Rewire the underside of the ballast tube socket so that they match the pinout of the filaments of the 12BY7. (The red dots minimize the chance for confusion when you go upside down.)

The 6BA6 are untouched.

If you're putting 12BA6s in there, same deal -- hang your ohmmeter across the holes in the ballast socket until you find two that are low impedance. A proper mil-spec paper clip, and (clap once and rub palms) you're done !

I dont want to rewire anything.  I am going to get two 12ba6s and put them in, then put the paper clip in place of the balast.  Does anyone know the pinout of where the paper clip goes?


C

An ohmmeter is your friend here. And while waiting for the 12BA6s to come in, it gives you something to do.

Pins 2 and 7 of the ballast tube socket. Use a short piece of insulated solid hook up wire. Strip about 1/8 inch of insulation off of each end.

PS: Use the money saved to purchase 2 bottles of cheap scotch ;D

Craig,

THanks Craig...  I apretiate it.

C

Another suggestion:  Use a 42 ohm resistor between pins 2 and 7 of the ballast socket. 5 watts minimum, 10 watts is mo'bettah. Slip some shrink tubing over the legs if you're concerned with it shorting against anything. Leave everything else alone, just swap out the 3TF7 and you're set.

Quick, easy, effective, been done forever. And rapidly reversible if the Collector Gestapo shows up at your door in the night checking for the dreaded..... originality. Dallas Lankford even says it's more stable than the 12BY7A approach, though I doubt any AMer would notice or care about any detectable difference.

"The standard 3TF7 substitutes, (1) using a 42 or 43 ohm 10 watt resistor in place of the
3TF7, and (2) using an appropriate tube, like a 12BY7A, with a 12.6 volt filament in
place of the 3TF7 are both acceptable substitutes. Using a 10 MHz rubidium standard I
determined, somewhat to my surprise, that the power resistor is generally a more stable
substitute than a 12BY7A."

I had a couple through my hands maybe 10 years ago that were out of the St Julian's Creek pile, one had a nifty metal heat-sink resistor installed and screwed to the chassis. It's the route of choice for me because it's simple (like me) and it woiks!   ;D

I hate mods like this.  I am going to try the resistor thing right now.. Wish me luck

C

Clark,

Do the mod CORRECTLY :  Pack up the radio carefully, and send it to my QRZ address,  and I will send you $100 -- after all it IS broken.   har har.    Good Luck  Vic

Clark,

Have you actually verified the tubes fed by the ballast are not lit?  If those tube filaments are lit your ballast is OK-optionally you should be able to see the ballast glowing slightly in a darkened room (and it will feel hot) if it is operating.  If not lit, check the ballast tube for continuity between pins 2 and 7 to make sure it is the ballast tube that is bad and not an open filament in one of the two tubes fed by it.

Lots of talk and little work :(  I have been so busy with work and life.  I did see that all the tubes on the top are lit.  I checked the IF tubes and swapped them one by one. Dead.  My TV7 wont check a 3ft7 so I stopped there. THe thing is in a case atop the T368. The next time my brother comes over, we will pull it out of the rack on the table and have a better look. There are lots of tubes on the underside. Just not easy to get to it.  I also need to put that new on and off switch in while I am checking it out.

C

Clark,    I stand corrected
it sounds like you need not worry about modifyng the radio as there was already something done with the resistors replacing the stock 3TF7 sooooooooooooooo you have a choice to replace said ballast tube or plug in the 12BY7 and rewire. The IF strip removes easily. Do you have the military manual? It gives easy step-by-step removal. Ya'll make it sound like the end of the Earth buying a 3TF7.....geesh their around and I have two for spares. Antique Electronics near YOU, Clark.
In the 18 yrs of owning my '390A, a substitute IF deck had a burned out ballast tube. The trick is to have spares of most of the tubes and you won't have any problems. When you don't have spares, then Murphy steps in to ruin your day................hi

Fred

Clark - you don't need a tube tester to test the 3TF7.  An ohmmeter to test continuity is all you need.  The 3TF7 only has a filament; consider it just a series dropping resistor.

This one has the 6080's? Not the 7-pin VR tube then. That VR tube opened on one of mine and it seemed like almost every tube was worn out. Hope you can fix yours.

My friend Francesco Ledda fixed my "A" model and it works FB. I still have a very intermittent AGC issue which he informed me of, but it seems to have gone away lately. I felt bad because he did not charge me a dime to repair it, refused to be paid! I owe him big time.

What's not to like? If you go the resistor route, it's as simple as yanking the ballast, stuffing the resistor into pins 2 and 7, and you're done. Best of all, it's just as easy to reverse. The 12BA6 version is a bit more involved, and the 12BY7A version requires some rewiring of the tube socket. You can pick the level you want to dive in to.

Something to keep in mind is the original purpose of the ballast tube: to compensate for some wild swings in line voltage and the resulting current fluctuations. This was due to the military's use in other countries where line regulation wasn't too hot. Not only that, the ballast was one of the items slated for removal in the cost-reduced A model. but someone in a position to do so decided to override the decision. Otherwise it would be a moot point. Present day, it serves no real purpose beyond aiding in monetary transfer.

THis is an R390. Not an R390A. 

Todd, I would rather the rig run like it was supposed to.  Its just preference. I will do it if my tube is bad. I have to work tomorrow but I will devote some more time to this thing and report back

C

Your choice, but be prepared to drop $35 for a new 3TF7 on a regular basis, for no gain in performance.

My R-390A came with a couple of spare 3TF7 tubes but when those are gone it will get the resistor modification and I am about as anti-mod as anyone out there.  I pull out modifications when I buy and restore a piece of gear. 
 
With your AZ QTH I would seriously consider also disconnecting the oven heaters since you don't need to be dumping any extra heat in your receiver/radio room.

I finally did the 12ba6's and a jumper on my R390A's....no rewiring and took all of a half hour or so.

I'll bet I spent at least $100.00 on those ballast tubes over the years.

Original some want but hell it's behind the front panel and under the top lid....out of sight out of mind.

And the R-390As were often used in RTTY circuits where frequency stability was critical.

Good to see you know the difference. That puts you ahead of many.  ;)


And that's just it - you'll be running it the same either way. The ballast concept was only added to deal with the 'what ifs' of overseas duty. But as I said, it's nice to stuff a resistor in there and be able to swap it back to the 3TF7 in a few minutes without hassle if the mood hits you. Otherwise it perks away just fine, a hair more stable, and you won't be having to haul it out of the cabinet in a year or so for another $35 band aid as Johnny sez. Think of the 3TF7s you'll be helping to conserve!!  ;D


Ohyestheywere. As with their predecessors. In fact, ASA had a field mod which added those nifty neat-o verniers to the BFO knob for more accurate retuning. Really slows down the BFO tuning rate a lot. They carried it forward when the A models hit the scene, but most of the ones I've come across were on R-390s.

The nomenclature escapes me at the moment, but they had a specific RATT/RTTY set up that utilized a pair of autotune R-391s with a matching antenna/preselector tuning unit, power supply, and diversity unit. FRR-33 or something like that, Johnny probably knows. A rack full of dials spinning away whenever you changed frequency.

 

I think the reason why some folks posting are reporting that they have replaced the 3TF7 a few times is because they are running the radio at today's 120-130 volts supply. The HUZMAN advised me a LONG time ago to use an isloation transformer and adjust for 110V.
That's the way it's been for the last 15 yrs. I have replaced a number of tubes that get tired from 24/7 operation during the winter months. About every two years I go through my radio for a tune-up.

Got strange AGC problems???? Isolate the troublesome, hard to replace filters used for bandwidth selection with a .01 good quality capacitor at the input AND output of the filter assembly. That was my strange coming and going AGC problem. Sometimes it would be a perfect radio and at a critical time of use strong sigs would overload the IF and massive distortion.
Rick Mish Myths: Excellent tech and loves the 390, 390A and another variant that uses 28 volt tubes (?)
He told me that most filter assy he services on the IF strip are bad and a little leakage takes place and will kill the AGC. And the .01 cap story killing the filters is plain BS.
And that running the radio at today's supply voltage is not a problem.

Fred

Yeah, it's good to put a bucking transformer in there to drop things back to 110-115VAC. I've got a lot of filament transformers out of scrapped Collins 300-Gs and I've used one to drop the voltage on the entire receive rack. (SP600, R390, R390A)

And the 28V variant is the R392.

Fixed.. What a bear. Spent 40 minutes unhooking cables and lugging that damn thing out to the table.  The 6ba6 by the pto was dark.  Stuck in two big resistors in the balast tube socket. Pins 2 and 7.  Still dead. Replaced the 6ba6 and she came back to life.  When the balast tube went it took the 6ba6 with it or vise versa.  Thanks for all the help.  Tomorrow I am finally going to replace that damn on and off switch with the new one I got. Also the panel lights are out. I used LEDs with resistors and they still burned out. I think I am going to stash a small variac under the table and turn the voltage down to 110 for both recievers.

Thanks again for the help. I hope this lasts!  Those resistors get hot as hell. Two 22 ohm 10 watts wired up for 44 ohms.

C

Clark,
 
Glad to hear your R-390 is healthy again and now you don't have to deal with the ballast tube in the future.  By the way, your ballast tube is probably still OK.  The 6BA6 filament is in series with the ballast so when it opens there is no current flow through the ballast either, kind of like the old series string Christmas tree lights of the 40's and 50's.

The resistors will generate some heat since they are dissipating close to 4 watts (13.2 v at .3 A) and even though the resistors you used will easily handle that power 4 watts of power is going to generate some heat.  Think of it as being the same as the heat generated by a couple of dial lights.  The ballast tube also generates the same level of heat.

Clark, the on/off switches are notorious for welding themselves shut. Some guys leave their set switched on and use a power strip or variac to operate it, others add some kind of soft-start. If you choose to continue to use it as-designed, you'll probably want to sock away another spare switch.

I found that this switch is a cheap switch used in AC units.  I posted the info on AMfone.  I think $5 or something.  Its exactly the same switch. I might as well put a new one in since its out of the rack on the table.  Using the power strip meathod is what got me into this trouble. I forgot and left the power strip on for over a week while I was out of town on biz. Came back and the reciever was dead. 

Also. I found a small compact tube audio amp. It says "Voice of the theater" on it.  I am going to wire up the diode pick up point to the small amp and set it on the case of the reciever so I have better audio.  Thats my biggest complaint with R390s. Lack of audio punch. 

C

What are my options for the lamps over the FREQ dial?  They burn out all the time.  I put LEDs in but they also burned out. I checked the voltage today and its 27.5 volts AC on that lamp assembly. When I put it on a variac it will be down to the 26.5 its supposed to.  Should I use a Dropping resistor?  My LEDS are labeled 12 to 14 volts.

c

Clark
The lamps I use are the exact replacement lamp used in the radio and at my shack I replace the lamps about every two-three years. And that's only because the glass gets dark from continous use. The 110 VAC might be the answer and less off-on action.
All these bad luck stories............hopefully you'll get up and running with more reliability.
Fred

WIred the LEDs in series with a diode and resistor.  Now its 12.5 volts per LED. That should fix it.  Also wired up the diode load to a "voice of music" mono amplifier.  It uses a single 6bq5.  Sounds great!  Now to put it all back together.

C

All done, Back in the rack fully functional.  It has a alot more RX now. The 6ba6 must have been weak.. I dont know.  Its strong. The audio is fantastic with the diode pickup. One thing I realized is that now the audio is not muted during break in mute. No big deal.  I have a remote RF gain knob at the operating position near the T3's remote key switch.  I can turn it down a bit and use it as a monitor.

One thing I realized this reciever has is "squelch" on the front panel.  This seems to do nothing on my radio.  What is this used for?  Does it work like a normal squelch and mine is not working?

THanks again for all the help guys!

C