The AM Forum

The Valiant had a BIG zorch... I finally got some bench time to look at and repair the damage.

The little firecracker looking thing is a molded choke at 4.7uh. The caps were 0.01uf at 1.5kv.  Looks like a plasma cutter sliced it open.

I replaced the caps with .002uf 6kv jobs I had lying around.  The choke I am not sure about, but it is a LOT beefier than the one that was in there.

How critical is the value of that choke anyway? As long as it is not self resonant in an operational frequency?

Ed,

I recall reading somewhere that the little choke was designed to act as a choke at VHF and probably addresses a problem that does exist so you might want to try to get a pretty close replacement.  I imagine you can find a current equivalent from Mouser or Digikey.  Did you come up with a cause for the "Zorch event"?

Rodger

amazing that the valiant has a plate choke about 2X teh size of the stock choke in the gonset amps.....fix looks good ed. make sure you got a awesome ground under that thing when you screw it back down. might even want to tin it up a bit.

Is that a National R-100 choke? It looks like the National R-100 or a BUD.

Derb,  I'm not  sure about the origins of the big choke.  It just has a johnson part number in the book and there's nothing printed on it.  It is Heavy Duty though. It weighs about a half pound or so!  I was surprised that they would put this Monster PI choke in there and then put this TINY molded choke in the same line.  I guess as long as the current handling capacity was ok it would work, as it did for the past 50 or so years I guess.

Rodger, Yea Mouser has a 0.661amp 4.7uH choke that I will get, cheap enough too, about a buck before shipping.  I was just impatient!

Hi Ed,
   I had the same Zorch, see:

http://members.dsl-only.net/~rsoennichsen/radios/JVII/JVII.htm

Note:  I did not use the three turn homemade inductor shown.

Cheers

Rich

Interesting. I wonder if it was just an old cap going to ground and dumping all the HV through the little choke or if there are parasitics involved?

I plan on sniffing around with my Grid-dipper and seeing what might be there.

I also found a blackend pin in the accesory plug on the back.  Charred up pretty good.  Of course the inside is encased in a lovely can that you have to remove all the filter caps & coils on to get out.   

I may just remove that whole assembly (accessory plug & scocket)  en mass anyway.  Looks like just another failure point right now.

The Ohmite Z-50 was recommended in many older designs to be inserted in the B+ line before the bigger pi-net plate choke.   It was also specified to reduce VHF parasitics.

Bought one but didn't put it in my 813 rig.  Thought that a 50 ohm/ 10 watt w.w. resistor had some inductance and used it in place of the Z-50 but more importantly, it helps to protect the tube in case of very high plate currents.  And yes, another 500 pf/15kv doorknob cap to ground was placed on the B+ side of the wire wound resistor.

I always thought that doorknob caps made piss poor bypass caps, they were to be used for DC/AC voltage/rf blocking only. Wrong?

So this little pi network was to filter VHF parasistics from the B+ supply? 
In addition to the Resistor/coil combinations mounted on the plate caps of the finals?  This filter is on the supply side of the big Pi-choke, where would these parasitics be coming through? B+ wiring is right up on the other wires in the bundle, coupling through the cabling?

Was all this due to using a large Pi-wound choke? I had thought that Pi-wound chokes had an issue with having multiple resonant frequencies, which would explain the issue of potential parasitics.

I thought that you just wanted to make sure that the self-resonant frequency of the B+ rf choke was outside of the band(s) of interest, usually wound to be up around 48-56MHz or so.

Anyway, learning a lot on here as usual!

HRO,

Are you sure that L12 is supposed to be a splatter filter in the Valiant?  I am familiar with what Collins did in the KW-1 with their high level splatter filter which is just off of the secondary of the modulation transformer and it was responsible for modulation transformer failures in that transmitter.  However, in the Collins application it is a 1.5 Henry reactor in a pi network with 6000 pf capacitors.  I don't think the little 4.7 uh choke in the Valiant is going to do much for splatter and it seems much more analogous to what Collins referred to as their HV TVI filter which I believe is probably the reason for the choke in the Valiant.  This makes sense as the large choke is probably not terribly effective at VHF frequencies.

As such, if the Valiant owner is in a cable/satellite TV area the choke could probably be removed without concern.

Rodger

Whelp...  applied power, blew up something else... don't know what yet.
Realized that the Accessory plug was not in. (I had not yet removed the socket as I wanted to fix the rig before making changes). Don't know if that contributed to the pop...

Something is still shorted in the B+ side somewhere. There is about 150 ohms between the B+ and ground somewhere.   Might be the modulation transformer, as everything else checks out ok. filter caps, diodes and wires all look ok.  I guess I'll pull the leads and check the resistance to ground/frame.

Ed,

Didn't you mention a burnt pin at the accessory plug earlier?  You might have a carbonized path (or a shorted bypass cap) where the plate supply is normally looped through this plug.  You might try pulling the plate feed to this plug and see if your low resistance to ground goes away.

Rodger

ed,

use your nose. make sure there's no voltage on that acc socket, then stick your nose right up on it. If there's carbon there you'll smell it. If you do smell ionized carbon smell just cut it all off the old socket,   throw the old socket away and and rebuild it from scratch using new junk. I've nver owned a Valiant, so I dont know what he sockt is made out of, but if B+ is on it anywhere, try to get a ceramic replacement offa ebay.

(if you do find a zorch thats visual, take a pic! I LOVE zorch photos!  :D

Ok guys.  The socket is either bakelite or brown plastic. Yea one of the pins was carboned up pretty badly, and the corresponding hole is black too.  I don't have any ceramic 9-pin sockets, so a complete socket-ectomy is in order.
 

I am just hoping that the problem is an carbon arc across the socket, and not something worse...    I will have to make some time to get back to the bench.  After the last flash-fizzle I kinda got disgusted and put everything up for a bit. 

Sometimes you just have to back away and let things cool off.

Ed,

I just bought a 9 pin socket for you offa ebay, it's a micalex socket and should work FB for you. A thank you for sending me that board for Yeasu #2. I'll turn it around to you when I get it.  ;D

Derb,

That was a very nice gesture proving once again AM'ers are far above average!

Your post reminded me that another common "Zorch point" in the Valiants which is dust leading to carbon tracks at the interface between the tube base and socket for the 866A rectifiers.  Ed, check whatever you are using for rectifiers to make sure you don't have any carbon tracking there.  Of course the other very common failure point in early Valiants is where the cathode leads to the 866A shorts to chassis ground because Johnson used basic hookup wire in this circuit for the first several years of production.  Since this lead is at DC plate voltage above ground it will arc over and burn the insulation leading to failures of both the plate supply and the low voltage transformer which supplies the 866A heater voltage.  Over the years I acquired a couple of parts Valiants that turned in to parts units due to these fireworks.  Later Valiants used properly insulated wire for these connections, the wire should be replaced or sleeved in the early units.

Rodger

Well, I found the HV problem.  Nothing spectacular (greatfully) it seems that the HV diodes I used to solid state the B+ weren't up to the task.  I'm kinda surprised with this as they were Microwave Oven units rated for ~12kV and lotsa current.  One had opened up, and one had become a 25ohm resistor.   It appears that the other components in the HV supply side survived intact.  Odd.  Maybe those diodes aren't as good as I thought, or ???

The socket-ectomey went well too.  All gone.  I did find a small burnt portion on one of the magnet wire coils, but that's history now. 

I also found that the 6C4 cathode resistor had one end just FLOATING in space.  I've been under this thing so much and so often, I don't know how I could have missed that!  But I did...   I guess it pays to be using the specticals and the illuminated magnifier!!!

Oh and in the for-what-it's-worth department, my Valiant had the heavier cathode lines for the 866's, and there was a burn through on one of those wires where it rubbed against the chassis. I found it when I was taking the tubes out as I removed the sockets for space.  The previous owner must have never found it. He just installed a pair of 120V fuse holders in the front panel for easy access.   :o

I guess he was big into that easy access stuff, drilled holes so he could reach the nuetralizing cap through the front panel too, as well as a few others so he could reach various rheostats for adjustment...  One of the reasons I don't mind the drilling and blasting on this rig.

the first name of that person was Hammy, The last name was Hambone. Trained his hole drilling skillz on my matchbox, then went up to yer Valiant.

Yea, Hammy Hambone strikes again, and again.

I'm stuck in Verminont this weekend so Next week I will hopefully get that righ finished, and incorporate some more audio mods as well. Might as well as long as I've got her opened up like a gutted fish...

now is the time.....not in November when you got 12 poeple on and they say 72 and over to you....you drop that maul and you get a flash n some smoke.... :-\ :P

A posting elsewhere got me thinking (ohh verry dangerous...) about applying the phase splitter idea.

What if I eliminated the driver transformer all together and ran a phase splitter to feed the Modulator grids direct.

Should I use the current driver tube (12au7) in the paralleled triod config or go with something else?

I also want to replace teh 6146 mod tubes with Sweep tubes too, though this idea shouldn't make that one too hard to accomplish either right?

Wouldn't getting rid of the limited response of the driver xformer help out the whole audio chain thang?

If you need some gain out of phase inverter stage that 12au7 would work fb as a long tailed phase inverter ... 73 ..John