Troubleshooting (or just shooting) a Johnson Valiant

[John Poet]

(Also posted this to a Johnson email list, but I have no idea how active they are..)

I have a Johnson Viking Valiant 1 with no drive/oscillation, and  the low-voltage fuse blows in about 6-7 seconds.  When it first failed, I had just finished fully loading the amplifier on the first tune-up after a move, when the 1.6 amp fuse went.

This Val has been recapped, the power supply has been solid-stated.  I don't find any fried parts or loose connections.  I just finished switching out every tube with known good tubes from a working unit (except the 6146s), including the two inside the VFO, and tried again but no change.  Inside the VFO, there is one slightly charred red wire running from the 6AU6 to a small coil, which had led me to believe perhaps that tube was shorted, but guess not. No idea how long it had been like that, I've never been inside the VFO before.  The Chernobyl Resister has already been redone with something like 30k ohms.

(I have tried this on VFO AND Xtal both, and in different band settings and makes no difference.  No filament current on the meter.)

I did already spend a lot of time metering for direct shorts in caps and resistors, after this went down five years ago--- it's been on the shelf for that long. Now  I mean to start systematically checking resistances on the tube sockets per the manual, but fear I may not be able to interpret the results.

Has anyone run into this situation before?  And do the symptoms suggest some specific things/area to check?

Thanks,

[redhat]

You could have a shorted (or leaky bypass cap somewhere.  Sometimes you can get carbon trails along tube sockets, phenolic ones are particularly bad.  If you have one, bring the unit up on a variac slowly and start looking for smoke.  The old carbon resistors they used in older tube gear would change value with age, and in some cases go open.  Double check everything, sometimes you will have to remove a suspect component from the circuit to be sure.  It is also possible you have a leak in your mod transformer, or one of the input chokes to ground.

Good luck!

+-RH

[ka1iic]

First of as REDHAT said... bring it up slowly using a variac... never plug vintage anything directly to the mains without one... big rule of thumb.  Lacking that many years back I use to use a 100watt filiment type light bulb in series with the AC input but that was quiet another story... but I digress...

My gut is telling me that you should disable the High voltage power supply first... then bring it up slowly via the variac.  If all goes good check your solid state diodes on the high voltage supply...  The inrush of current because of this modification might had fried some thing.

Also check the sockets of any tubes that are feed by the high voltage supply... what to look for is charring on the socket to the chassis (ground).  That rig had a 6146... right???  check that socket...  the screen resistor on the 6146 needs to be checked and check the plate lead of the 6146 for charring where it may have come in contact with the chassis or other components.

Right now I am praying for that rig because... I hope the high voltage windings haven't shorted to the core of the transformer...  Oh please Radio G-d don't let it be so.... arrrrghhhh!

Take what I say with a grain of salt... I only worked on one of those rigs about errrrrr 35 years ago... never owned one tho... nice rig tho...

I think on this for the next few days to see if I can come up with more ghosties and such...

Good Luck Sir!

[John Poet]

Guys, this is on the low-voltage side, protected by the 1.6 amp fuse, which is the one blowing.  At the time, there was no smoke, no drama whatsoever, just "phhhht" went the slow-blow low-voltage fuse, after full tune up.  I even checked the wrong fuses first, since that thing had never gone out on me.  The 8-amp fuses covering the high voltage side never blew when it broke down. 

This happens about 7-8 seconds after turning on the filament switch.  In that amount of time I would normally see the meter showing oscillator drive, but now there's nothing.

The solid state diodes supplying the high voltage don't come into play until you throw the plate/high  voltage switch.  I've had those go out before.  Since there's no drive, I never get to the point where I'd be throwing that switch.

I've since found that the 5763 multiplier/buffer tube is dead, (also on the low-voltage side),  but the oscillator should normally work without it (as it did in Valiant #2 which I had traded all the tubes with).  I will check/clean up that tube socket, as well as all the others with some DeOxit and see what happens... I've been checking other parts for shorts but haven't found anything.

It does seem to me that there's a short somewhere, probably on the low voltage side, but I suppose a filament short on the other tubes could do it also.

Don't own a  Variac, that's something I never picked up.  What I did pick up were spare T-1 power and modulation transformers.... but I don't think those have anything to do with this.

[redhat]

During that brief period before the LV fuse goes, what does your LV look like?  All of what I read said that it should be around 300V.  If its low, something is pulling it down.  Did you replace the LV rectifier with diodes as well?  if not, look for a gassy rectifier tube, otherwise, shorted (or leaky) diodes are possible.  Worst case, a short in the LV transformer could also be a cause.  Try and pull all of the tubes out of the unit and see if it still blows fuses.  If so, you may have an issue with the transformer.  If the fuse holds, add tubes one by one until you find a tube or combination of tubes that cause the fuse to blow.  I'm not sure how the replacement high voltage stuff was redone, but check too that nothing has happened to the filament connections to the HV rectifier tubes, as those are run off the LV transformer and are relatively high current capable.

Dumb question, but do the filaments and pilot lamp light when you turn the filament switch on?

+-RH

[John Poet]

Gosh, you know, I've never looked at the tubes to see if they lit, it would barely be enough time.... but the pilot and dial light do come on.

The low-voltage side has not been solid-stated.

"what does the low-voltage look like"?
It looks like the pilot lamps are on!

I've never done live voltage measurements....
and I'm not really anxious to start if I can possibly avoid it.

Thanks for the tube-swapping and other advice.

[radiogaga]

X2 on the dimbulb/variac and pulling of all tubes.
the filter caps in lv swapped, correct?
read somewhere about the lv wires coming from xformer are sometimes too close to chassis and arcing.

bust out that vtvm....
best o luck

rgg

[James Brownyard]

At first I was thinking the 5v4 was shorted, but you said the LV is solid stated. Had that problem before.

Was it a kit or factory wired unit? If the tube sockets are riveted, it is factory built. If it is a kit, was the work sloppy? Check every tube socket for any extra blob of solder that may be causing a short. Mine was factory built, but my earlier mods were rather sloppy. I had the exact same problem. I did all kinds of things, swearing, pulling my hair out. Still,  blowing LV fuse. Turns out a tiny blob of solder was shorting the Filament (green wire) to chassis on one of the small tubes, I forget which one. But those connections are small and hard to see. We are all getting older and like I did, I swore I looked everything over 100 times.

[ka1iic]

could it be a short between the filament winding and the 300 volt winding of the low voltage supply?

I'm thinking but coming up short so far... keep at it OM ;-)

[John Poet]

At first I was thinking the 5v4 was shorted, but you said the LV is solid stated. Had that problem before.

Was it a kit or factory wired unit? If the tube sockets are riveted, it is factory built. If it is a kit, was the work sloppy? Check every tube socket for any extra blob of solder that may be causing a short.

Thanks, James.  I'll be sure to check out around those small tubes, specially near my audio mods.
The LV is NOT solid-stated, only the high power... which needs resisters added inline.  It IS factory built.

I haven't had a chance to do anything more on this yet.  First thing I need to clean all the tube sockets with deoxit and a toothbrush...  and run resistance on the transformers before checking all the other points on the list.

It IS possible that something was knocked "loose" or "together" when this got moved between residences, but I haven't found anything yet.  Like I said, it went down on the first tuneup after having sat under a sheet for about 9 months.

[John Poet]

All right.  Not good...

I haven't done most of these checks yet, but maybe I don't need to, now....
(I finally relocated the short I found 5 years ago, that I hadn't traced out)

here's the schematic, for whoever doesn't have one, click to enlarge:
http://freeradiocafe.com/images/Valiant_schematic.jpg

On tube V20 (with AND without the tube pulled), 5V4 low voltage rectifier,
I have direct shorts to chasis  at pins 2, 4, 6, 8 which should have resistance, according to the manual.
(There were no signs of carbon on the tube socket, but I cleaned it anyway).

The schematic shows  all of these leading directly to the T2 low voltage transformer

(Both on the lower right-hand side of schematic)

Now, my question is, is there any way that this is NOT shorted turns in the transformer?
Can this be something else INSTEAD of shorted turns in the transformer?

Or, is this enough to consider the transformer definitely wrecked?


I don't see how it can mean anything else instead, but I'd like to hear from those with more expertise.

(I've since read the LV fuse is mainly intended to keep the T2 from cooking itself, so I guess it performed its function there, although that doesn't really help much other than to keep the house from burning down or something.  I suppose that's not an unimportant consideration. Ha ha.)

There are resistances listed in the manual lead-to-lead to check on the transformer, but the insulation is so faded it's a little difficult to tell for certain which is which--- particularly between "yellow" and "red-yellow", and tracking down where the rest actually attach isn't that easy, either, other parts tend to hide the wiring-- so I haven't been able to really check much on that yet.  But maybe that isn't really necessary, either.

Even if that's the big problem, it'll be somewhat of a relief just knowing it... and I can just move on.
Rewinding of these may be available for about $157, but that won't be my problem.  Truth is, I was looking to sell one of these anyway.  The other has only a minor problem.

((I found this through trying to check electrolyitcs 96A & 96B, now separate, which initially look like they are shorting to ground-- but disconnected from their ground point, they show some resistances))

[redhat]

Pull V21 and see if your short clears.  If not, T2 is shorted.  The same LV winding is used for both tubes.

Got skype?

+-RH

[John Poet]

Pull V21 and see if your short clears.  If not, T2 is shorted.  The same LV winding is used for both tubes.

Got skype?

+-RH

Well, pulled V21 and everything on V20 still shorts.  I guess that's that.  Thanks, Redhat.

Under the chasis, V21 was so overlapped by other parts that I couldn't even read any pin numbers to check there...

No, I don't Skype. Spose I should get one.

And the moral to the story appears to be,
"if its working, don't move your Johnson!"

[redhat]

You could have one of the leads shorted to ground some place.  Of course, you'd have to pull the transformer to know for sure.

Sorry dude.

+-RH

[refmo]

"if its working, don't move your Johnson!"

Words to live by!.... 

sorry to hear about the transformer.