Johnson "Whiteface" Problem :( Please help!

[broncoman529]

I need help to get my Johnson "Whiteface" running again. Here is my problem, It worked awesome for almost a day and then out of the blue it blew a fuse in the power cord. A friend of mine who is pretty good with electronics found out the problem is the rectifier 12bw4 solid state tube replacement that consists of 2 1N4006 diodes and a 1/2 watt 10ohm 10% resistor, It is called a Vibratrol SRW4. We started by taking the rectifier out to verify that it was the reason the fuse in the power cord was blowing and it was. I rebuilt the solid state rectifier two times with exact replacement parts, The first time I turned on the radio I was successful talking on it for approximatly 2 minutes before hearing a low tone RRRRrruuUUUMMP and then the fuse popped. You should have seen the black crusty mark that was left inside the rectifier. The second time I rebuilt the rectifier, As soon as I turned on the radio I heard the dreaded RRRRrruuUUUMMP immediatly, The fused popped and the rectifier was destroyed again. I ordered a 12BW4 tube off of ebaY but I do not dare install it as I am afraid of blowing it as soon as the radio gets turned on. Does anyone have any ideas on what the problem could be, What could be causing the rectifier to self destuct every time I turn the dial to the "ON" position. Please help!

Here are the schematics:

http://f4.grp.yahoofs.com/v1/gM2qQ2...SFK4bB6X815YZw/Johnson Messenger 1/MESS1S.JPG

http://f1.grp.yahoofs.com/v1/gM2qQ4...5BfPHUUEmrpCTQ/Johnson Messenger 1/MESS1T.JPG

Thanks,

 

[nomadradio]

Sure. Shorted electrolytic filter capacitors. That radio is old enough to require that each and every electrolytic be replaced.

In theory, the rectifiers should have been capable of withstanding the surge current long enough to blow the AC fuse. Makes me wonder if you didn't use a fuse larger than the one they recommend?

That radio is around 40 years old. Electrolytics are not meant to last half that long. The "worked for a (day, week,)" routine is exactly the way an old electrolytic will tend to behave. Works for a short while after a long shelf visit, then it fails.


Oh, and the links you provided to a Yahoo group 'files' section won't work unless you are logged into Yahoo Groups.

Uh, which Yahoo Group are those files stored in? The link is "hashed", masking the identity of the group.

73

 

[broncoman529]

Actually I put in the exact same fuses that were originally in the fused plug(two 2amp fuses 250 volt). I cant say for sure about the 250 volt part, But according to the schematic it is supposed to have two 2amp fuses(One on each side of the fused plug). Here is the link to the Yahoo Group website:

http://groups.yahoo.com/group/tubetypecbradiocollectors

Thanks for your help with answering my questions. I am new to restoring tube type CB's and it will take me a while to learn the in's and out's.

 

[nomadradio]

Whoa, the "Eagle" two-prong fuse plugs. Takes me back. All the Johnson stuff came with those. They tended to provide a measure of lightning protection that an internal fuse holder doesn't. And it was cheaper, to boot.

If you get tempted to put a 3-prong cord on that radio, just be sure to install a fuse holder inside the radio, so that it has SOME protection. Just be nice to that old AC power plug. If somebody steps on it, another one may be tough to track down.

Sure enough, I had downloaded those two pics of the schemo when they were posted a year and a half back, I think. I had forgotten just how old that radio is. The date on the schemo is 1959. This means that there will be a number of tubular-shaped capacitors made with waxed paper as the dielectric.

ALL OF THOSE HAVE TO GO ! ! !

Modern tubular capacitors of that range of voltage/capacitance are made with synthetic plastic for the dielectric, and won't just go bad from age. But the paper tubular caps WILL.

And that's on top of the electrolytic capacitors, too.

One way to be sure that your problem is with the B+ section of the power supply is to remove your rectifier diodes and see if the fuse still blows with them out of the circuit.

If it does, there are two of these tubular paper caps C57 and C58 in parallel with the B+ winding of the power transformer where it fed into the old 12BW4 rectifier tube. These so-called "buffer" capacitors were there to smooth out the rough AC waveform that the vibrator produced when using it from DC power. They don't do much when you're using it on AC, but they're still in parallel with the B+ winding of the power transformer. In the bad, old days, if you replaced the vibrator in a radio used on mobile DC power, the instruction sheet would tell you to replace the buffer cap or caps. Didn't say "test" them, but to replace them before ruining a new vibrator. Very often, when a vibrator went bad, it would be one of those caps that killed it.

Likewise C59, the 80 uf 450 Volt filter should be checked, and if it looks original replace it anyway, even if it doesn't check shorted. That kind of filter can "heal" a short-circuit because of the chemistry used inside it. Checking it with the low voltage coming out of an ohmmeter will show that it's okay. UNTIL you put the normal high voltages on it, and the internal short-circuit comes back and pops fuses. If it's old, it can't be trusted.

So long as you stick with the 2-Amp fuses, you shouldn't damage the power transformer. Bigger fuses probably will.

This is the kind of job that a variable-voltage AC transformer makes a LOT easier. Called a "Variac", or "Autotransformer", a radio that has power supply problems can have its line voltage turned up slowly from zero. This way, any overloads that still remain don't get the full "surge" current that they would if you just flip the power switch "ON" to make smoke tests.

Remember, the more smoke that you leave IN the radio, the better it tends to work.

73

 

[broncoman529]

Yup,

Work in progress is what I call it. There is a guy here in town that has a pretty good knowledge base of tube equipment. He is not an expert, But he seems to know quite a bit and has helped me so far. He also suggested that we take out the rectifier to see if the fuse blows, The fuse does not blow when the rectifier is removed and the radio powers up fine. I also do have a Variac, But I thought it really was for a "First Time" power up. I never thought of using the Variac to trouble shoot, Now I know. Well, I'm on my way to go get some more parts and to hopefully troubleshoot this radio. Right now this radio is the one I want to get most on the air. AudioShockwav was very nice to me and hooked me up with some crystals, So needless to say it is not going to sit around. Some of the tube chimneys are missing from inside the radio and only one is left in there, It should be fine without the chimneys, Right? Nomadradio, Please keep the suggestions coming. Anything will help us along. My guess is this radio is 1958 as there is no red paint around the knobs and the Johnson Viking symbol on the top left corner also is not painted. I have been led to believe that these were the earlier of the johnson white faces. Do you have any idea? Is this a fact?

Thanks,

 

[broncoman529]

GOOD NEWS! The Johnson is up and running, It was blowing fuses because the 12BW4 solid state replacement plug that goes into the socket of the radio was shorting out from the inside of the plug, Don't ask me how. I got the actual 12BW4 rectifier tube in today and put it in the radio and she fired up. All I can say is "OMG" Slamming clean clear audio. My friend that helped me get it going talked on it so I could hear it and "WOW". Well now I am trying to figure out if I have an audio receive problem on it, I can hear people fine but depending on how far away people are I have to adjust the volume to hear them. The weaker the signal the louder I have to turn up the volume knob to hear them. If the signal is strong and they are close by and key I need to turn the volume way down because they are super loud. What do you think? I never hear any static, But super far away stations that nobody else hears I can hear without static, But I need to turn the volume way up. It really sucks when I am talking DX and talking to the locals at the same time. I turn up the volume to hear the DX and then when the locals key in I get blasted out of my chair from the volume being way up. please let me know what you think.

Thanks again,

 

[nomadradio]

Hey, congratulations, Larry.

Don't have any "inside" knowledge about production details, like what year they painted the "Viking" logo and such. That radio was old when I took this job 30 years ago. Probably haven't fixed more than ten or fifteen of them total, in all that time.

If you plan to use it regularly, the precautions about wax-paper and electrolytic capacitors still apply. If it shuts down after a week or a month, don't be too surprised.

But if it's working, at least you don't HAVE to fix it today.

Except for the receiver sensitivity. The reason you're having to turn up the volume is that the receiver's sensitivity has dropped. It should hear a noise level. If you haven't aligned it, this is where you should start. A signal generator is nice, but peaking the receiver coils and IF transformers for max noise level from the antenna may work well enough.

If the receiver is still weak, and you're SURE that all the tubes are okay, looking for DC voltages that are too low on the receiver tubes would be next. The radio is full of carbon-composition resistors, and they can change value with age/wear-and-tear. Sometimes they will LOOK bad, sometimes they'll look perfect, and measure wrong. Typically if the resistance climbs too high, it will cut the voltage to one or more receiver tubes, making it behave as if you'd turned an RF Gain control down too far. (On a radio that has one).

Have fun and be careful with the high voltages in that thing. It's a small radio, but the power supply can pack a BIG punch if you're not being cautious.

73

 

[broncoman529]

Yup, Positive...The tubes have all been tested and are good. The plan is to replace all the old wax-paper and electrolytic parts. We already started to make a list of components that we need to buy to continue the longevity of the radio. We will try the alignment first as you suggested. We do have a signal generator at our disposal so we will use that. I will keep you up to date on whats going on.

Thanks,

 

[DTB Radio]

If you are receiving from a good distance with little noise, I'd say thats a good thing. I haven't looked over the schematic for that radio so I do not know how effective its AGC is, or if it even has one, but if it does, I'd start looking in that part of the receive circuit. If it doesn't have an AGC, get one of those devices that is available for keeping TV channel volumes at the same level. I'm not sure where to tell you to look, but from what you've posted, I'd go that route rather than try to change something in the radio. If you're getting that kind of distance on receive without the extra noise, thats a GOOD thing!

Something else just occurred to me. You could, in theory, tap into the audio somewhere before the main output amp to the speaker, and wire an insert jack into it. Plug an ordinary vocal compressor-limiter unit into the insert jack, and use that to level out the receiver volume. Turn the radio's volume all the way up and use the compressor to control your volume.

Along that same line, you could take an SP-1A and wire it into the receiver audio using the same idea as the vocal compressor. The effect would be the same. in the case of the SP-1A, it would need to be wired before the volume control.

 

[vswr]

I learned something about old caps pre-transformer input. The 2 at the very input of the radio. If any one of those shorts, and chances are they will, you wind up with a hot chassis. Very unhealthy to test 115VAC between ground of your coax connector and the chassis itself.