Viking Valiant Problems hoping for some assistance

[radionut]

I have a fully restored Viking Valiant of an unknown year of manufacture. When it runs I love it when it does not I want to throw  it out the window.   Any assistance, comments, and or suggestions as where to look for the problem would greatly appreciated.
I apologize in advance for the lengthy email but I wanted to provide as much pertinent information as I can up front.
Back ground info:
A restored Viking Valiant; all mods have been removed back to factory condition. This radio appears to be a kit radio, as witnessed by the tube sockets being mounted to the chassis with screws rather than rivets.
I am using a Palstar AT2KD antenna tuner (a 2Kw tuner that handles anything that this radio can throw at it)
Dry Dummy load
Running a tuned dipole tuned real close to 40 meters
Usually run the TX with 8 mA grid current and 330 mA plate current
Rig is always ran with the case off to minimize heat levels.
Used exclusively in AM mode
Typical modulation level (according the Valiant meter) 150-200 peaks at 250
Most often I use the crystal sockets and sometimes the internal VFO.
Band switch position is at 40 meters.
Often use a low pass filter in the antenna line. I have tested and it has no effect on the current or past problems.
The clipper is set to its widest setting. Which is 10 on this radio. I have been told that this control is wired backwards. In the past when the radio was running well if I set the clipper control on 0 I get very little output. At 10 I get maximum output and a wide signal.

Disliked issue with this radio
At times it appears that the switches on the front panel don’t always make contact on the first attempt (sometimes I have to cycle them). No surprise I guess for a radio that is probably 50 years old. I’m not sure if there are switches of current manufacture out there that could be used to replace these old ones.
 
Repair history:
I have had one previous failure in early September. The cap network (4 ea. 500 volt, 150 pF, C38, a, b, c, and d, on the schematic) attached to the band switch (used for 40 meters and below I believe) these were found to be shorted out. One of the caps had a nice brown crack running down its side. I replaced these with 4, 1000 volt 150 pF caps mouser #598-CDV16ff151J03F CDE Mica caps the part description indicated these caps would work well in a RF application.  This seemed to resolve the problem and I got about 5-6 hours of TX time until the current failure occurred.

The event
I was warmed up and tuned up at the usual grid and plate currents. I began making contacts with all running good and the usual great sounding audio. After about 20 minutes started to get reports that my audio began to sound dirty, distorted, or, over driven. I checked the plate current and it was well over 330 mA it was set a 330 at the begging of the broadcast. I shut down as I shut down a fuse in the AC power cord blew.  
Since this event I have replaced the fuse and have done some test tune ups on the dummy load and antenna. During these tests the fuse in the power cord has not blown again but I have not been txing anything but a carrier during these tests. The result pretty much follows what is said below in “the current state of things”

The current state of things:
Warm up for 15 minutes. I set the grid to 8 mA. When I power up the final (via the PTT switch on the front panel to the MAN position) and attempt to adjust plate current I am unable to move the plate current more than about 10 mA if at all. The plate current level is usually around 420 mA in this situation. The manipulation of the final control and the coarse and fine coupling controls have no effect on the plate current. In this circumstance the rig puts out less than a 10 watt carrier as seen on the antenna tuner. With the plate current this high I usually do not keep the rigged keyed for more than 20-30 seconds.

All thoughts and comments are welcome!

Thank you!

[WA1GFZ]

Check the final wafer on the band switch for burned contacts
Check the plate coupling cap to the pi network. it could be open.
Make sure you didn't blow any of the padders if they were switched in.
They may not be able to handle the RF current.

[radionut]

Thanks! What is a padder?

[WA1GFZ]

The new caps you installed

[KC4VWU]

Wouldn't those new caps be a little on the light side just being rated at 1 KV? I suppose that would be a DC rating, but were talking about RF potential in this case.

Phil

[W1AEX]

I would agree with Frank and Phil that the replacement fixed loading caps might not have been up to the challenge. You might try tuning the rig up on 10 meters just to see how it behaves when those are switched out of the tank circuit. It's my understanding that lack of ability to handle RF current is a major factor in the failure of fixed loading caps.

[Jim, W5JO]

I don't have my manual in front of me but, aren't those caps you replaced supposed to be 300 pf. each.  Two in parallel then those in series with two more in parallel?  Also check the last wafer of the band switch as GFZ suggested but also look for carbon tracks between the little rivits on the rotating part. 

Do you still have the turnstile cap in the ouput leg of the pi net?  You may have toasted either one or more sections of it?  I forget the designation of that one, but I believe it is 300, 600 and 900 pf that is switched in  different combinations.

[ke7trp]

Most of your trouble is in the switch wafers.   Stop now and clean and tension then. Here is what I do:

Purchase some deoxit cleaner
Get an old toothbrush. Its prob time you get a new one anyways.

Spray the entire wafer and contacts down and work the control. Take tooth brush and scrub the hell out of each contact. Rotate and scrub again. Notice that there are carbon tracks from wafer rivets to the contacts or mounts. Scrub it all off.

Rotate the switch so each contact is "open" and not touching another contact point. GENTLY push the contact down with a small all or screw driver. This way when you switch it, There is slight upward movement of the contact.

All 5 of my valiants had trouble until I went through all those contacts.  Good luck

C

[Ed/KB1HYS]

I have been through the same frustration with my valiant.  I recomend some things:

1- sounds like you have the original 2-wire fused cord. remove that and install a 3-wire grounded cord, with a single fuse on the line (not neutral) wire.

2- Replace the "special" C-35 cap with door-knob types at 5KV. If you remove the fixed loading cap and hardware, you can mount an aluminum plate behind the wafer with the DK's on it.  Works FB.

3- clean up the contacts like the guys here have said.

[ke7trp]

I want to add the fact that every valiant I have ever been around needed new Drive control and Audio gain pots.  2 of them needed new Clipper pots. The clipper might not be an issue if you have bypassed it.

I found the pots at a higher wattage rating made by clarostat on ebay.  They will fit with little trouble.  On valiant number 1, I had the same issue with the audio and this turned out to be the pot.  Once warm, The pot would not make contact internal. I have had no real luck cleaning these and when the old ones where out, I opened them to find a real mess.

I agree on the three wire cord.  This must be done. I would also suggest checking voltages in various places such as screen. I had alot of wasted time repairing one of them until someone here suggested I stop and check the voltages. I found the screen supply was non stock and needed repair.

Make sure to set set the Clamper tube adjustment correctly and the bias for final and modulators.  There are two manuals out there. One has negative voltage values for the bias adjustment and that is the way I do it.

I suggest you put the thing back in the case but do not screw it all down. This way, The rig will operate at the normal temp. You can slide it out and adjust the VFO and bias if things change when its hot. One two of my valiants the VFO would really go out of wack once in the case. Probably due to NPO cap trouble. However, Once I adjusted the vfo so it was on track in the case when warm, The rig stabilized.

C

[W1ATR]

Might not be a bad idea to lay off that plate current just a bit too. The mains voltage out of the wall is a bit higher these days than it was in 1956 when they designed the Valiant. My Val is perfectly happy right around 250 mils.

Sounds to me like you blasted off one of the mica caps on the rearmost wafer on the bandswitch shaft. I think they are 500 puffs and 1000volts, but not too sure at this moment.

If it still has the 866 rectiflashers in place, you might want to can them too while ur in there. I'm not a fan of solid stating the Val, so I slammed in a pair of 3b28's (xenon gas rectifier) in place of the 866's.

[radionut]

Thanks to everyone for the fabulous response! I really appreciate it. I will get to work checking all these items out and will report back with the results.

73's,
Radionut

[W2VW]

As Rob said padder caps must be able to handle R.F. current. Not a lot of voltage across them at 50 ohms.

[Ed/KB1HYS]

As Rob said padder caps must be able to handle R.F. current. Not a lot of voltage across them at 50 ohms.

I spent big bucks for caps designed for High RF current applications, they had 1/2 inch ribbon leads silver plated etc.  They exploded (litterally).

Doorknobs have been in place for a couple years now, no issues, not much cheaper, but the reliability is nice.

At one time I had a stack of mica chips 2-3" square.  I thought about splitting them up and restacking the original Special cap but with a bit thicker mica bits.  Then I recovered my sanity...

[The Slab Bacon]

Also check your PA biass. Valiant uses both fixed and grid leak biass. IIRC the resistor that feeds the biass to the 6146s is only 1/2W. No biass = soaring plate current. Also check the postage stamp mica capz that feed the RF drive to the finals, If they are leaky the PA grids will try to go positive and smoke the tubes.
It might not hurt to check the clamper circuit as well, if working properly, as your drive falls off, the plate current for the finals will as well.

The only crapout I have ever had with mine was when the mica cap between the buffer and the 5763 driver shorted out and put +300v on the grid of the driver. It proceeded to launch the 5763 to the happy hunting ground

As far as the 866s go, I have never seen them flash over in a Valiant. the voltage is too low (660v or so) They were used for their current handling capabilities in this application.

[WQ9E]

I had a flash over once due to a microphone switch failure resulting in immediate application of HV along with filament power.  No damage was done but I don't like MV rectifiers and that was my motivation to remove them.

If you have an earlier build Valiant, the wiring from the LV transformer to the 866A cathodes is suspect.  Original units (both factory and kit) were built with standard hookup wire for this connection and the insulation is not suitable since the rectifier plate voltage also appears on this wiring.  If you have any doubts, either replace these leads with new higher voltage rated wiring or sleeve the existing wiring.  Failure of these leads can take out the LV transformer.  Early build Valiants also had an undersized blocking cap of 500 pf.  This resulted in low output on 160 and 80; later builds used two 1000 or 1200 pf doorknobs in parallel.

If your Valiant is a late build (or modified for use with the 6N2 transmitter) it will have a pair of pots on the side to adjust the modulator bias and final bias in AB1 mode.  Earlier models make the adjustment via taps on a resistor.  Be very careful when moving these taps as the fine resistor wire is very easy to damage.  Of course if you have not already replaced the bias caps be sure and to this before going any further as failure here will result in a whole lot of collateral damage.

[Ed/KB1HYS]

Ditto on the 866 filament wiring.  I found chaffed/burnt spots on mine that caused intermittent pops-n-sparks.  I like the MV look alot (I built a Full Wave Bridge for my HB rig), but in this case I just SS'd the Plate volts.

[Rob K2CU]

Did you check the bias on the finals? The meter actually displays cathode current and loss/reduction of correct bias would cause the current to run up.  I assume key means switching the manual transmit switch.

[radionut]

Thank you very much for all the great responses! I am very sorry that I have not replied until now. I replaced the caps coming off the band switch and that seems to have resolved the problem.

Thank You, again!

[Ed/KB1HYS]

Nice!  Hope to hear your Valiant on the air.