Valiant go boom -- again

[WB5IRI]

Was yakking with James, AF5EH, last night on 3880 (all my problems seem to happen on that frequency), when the plate current pegged, power out dropped to 5 watts, and tuning the final had no effect -- no dip in plate current, nothing. Dropped the drive down so the plate current wasn't pegged, but still could not get a plate current dip. Went up to 40 meters and everything tuned normally. So, I assume I lost a capacitor in the 80 meter tank circuit, but before I unship the behemoth from its case yet again, anyone have a clue as to exactly which component failed this time, and am I on the right track?

BTW, I have to say that this group has saved me many hours of troubleshooting time by either telling me I am headed in the right direction or so far off the path that I need to call in the Texas Rangers to set me straight. You guys are fabulous!

Doug, WB5IRI

[WQ9E]

Doug,

It is most likely one of the 4 330pf series/parallel capacitors that form C39, these are used on 160 and 80.  But make sure it it isn't from a coarse loading capacitor (where was your coarse loading set on 80 and which other positions did you try?)

[ka4koe]

Welcome to my world.....you are now a member of the Valiant PING club....the sound of the plate current slamming the RH side of the meter movement. Good luck.

[WQ9E]

If you have one or more vintage transmitters and/or amplifiers one thing you should own is a MFJ-259 or similar "antenna analyzer".  You can use these in reverse to safely (all work is done with transmitter power off and unplugged) and easily check and substitute components in your tank circuit.  Calculate what the operating load should be for your finals and connect a non-inductive resistor from one of the plate connections to ground, a standard half watt resistor is fine and leave the tubes in place so that their capacitance remains in circuit.  Connect the analyzer to the coax output connector and you should be able to adjust the load and tune controls to their normal position for that frequency and achieve a 1 to 1 indication on the analyzer.  If you suspect a component is bad you can tack in a low power substitute while doing this test and then order the more expensive proper power level substitute after your suspicion is confirmed.

I first used a MFJ analyzer for this when I built a homebrew amp about 12 years ago and the settings I logged for the vacuum capacitors and roller inductor were all nearly dead on when i powered up the amplifier.  This is a great little instrument both for development and subsequent troubleshooting.

[ka4koe]

I really need an analyzer. I hesitate on the MFJ line due to QC issues.

http://www.rigexpert.com/index?s=aa54

[WQ9E]

I don't care to get mired in the typical online debate about MFJ quality but I have had no problem with my MFJ analyzer, audio filters, or keyers.  I stay away from their antenna tuners (besides the little one I bought for portable use with my FT-817 and FT-897 which has worked flawlessly) but I build instead of buying my high power tuners and few commercial tuners will stand up to legal limit AM on 160.

Internet myths about MFJ are nearly up there with the BS that gets posted over the differences in 6146 tube variants and much of what it out there is a repeat of something heard rather than actual experience.

The Rig Expert is a nice analyzer but overkill for most people.  My MFJ 259 is around 15 years old and still works perfectly.  I think I would rather spend the extra dollars on 3-500Z tubes or bourbon

[KB2WIG]

When you feel lucky, the Millen Grid Dipper is yer friend.

http://www.isquare.com/millen/manuals/90651-manual.pdf

I've used it for ball park tank adjustments.


klc

[Ralph W3GL]

My MFJ-249 is 21+ years old, covers 1.8 to 170 MC  (receipt dated 4/17/1993 @ $189.95)   and, other than battery replacement thru the years, has been flawless...  In use every time I change bands on my 200" center fed ZEPP sky wire...    

Yes, I have a calibration chart for the Match Box but keep forgetting where I put it and the 249 is just a switch away from the MB so it's quicker...  

Oh yeah, one other thing, it replaced my long lost Heath Grid Dipper as well...  

[WB5IRI]

I have had my MFJ-259 for about the same amount of time, and it has always worked perfectly. Always warned to find a way to key it, because it does put out a signal I can hear on my receiver. I also use the MFJ remote antenna switch, but in reverse, that is, I have one antenna and 5 transmitters, so I can switch between them without having to disconnect anything. Also have the 45-amp switching power supply. Never had a problem with anything I bought from MFJ. I'll try your tip on using the 259 to check components.

When I get home from work tonight, I will first try a different coarse loading position. If that works, then I'll know what to look for. If not, then it's probably C39. Will be using my Elmac station for awhile.

Doug

[W2VW]

My MFJ-249 is 21+ years old, covers 1.8 to 170 MC  (receipt dated 4/17/1993 @ $189.95)   and, other than battery replacement thru the years, has been flawless...  In use every time I change bands on my 200" center fed ZEPP sky wire...    
 

Well you didn't loan one to a neighbor who hooked it up to his antenna and transmitter with a Tee fitting

[W2VW]

The 259 is FB but the procedure mentioned works fine with any wideband ricebox too.

Used TS-440 to cold tune amp tank circuits as far back as 1993. Light it up and be within a few percent.

The one thing is the resistor used to make the dummy load that clips from plate leads to ground must be within it's dissipation and can smoke really quickly at just a few watts.

Always used a series/parallel combination to get the right number and did not burn one out at 2 - 5 watts on the TS-440.

[WQ9E]

Doug,

Take a look at the schematic for the coarse loading because different combinations of the 4 combined capacitors (if you still have the original turnstyle looking cap) are selected in the different positions.  In the full CCW position all 4 are in parallel (3,000 pf total), the next step drops out C41 (300 pf leaving 2,700 pf in circuit), and it continues selecting different combinations of capacitors so as to decrease the total 300 pf per step and the fine load interpolates the difference within step.  It is a decent system but it can become confusing trying to troubleshoot.  

If you have a capacitor tester that you trust you can measure both the plate tune padders (used only on 160 and 80) and the coarse load capacitors pretty easily as they can be isolated by selecting different positions of the band and coarse load switches, next best would be using your MFJ analyzer and a load resistor in the final, the other approach would be a dummy load and use the chart in the back of the manual to tune up on different bands which use different levels of the coarse loading padder to isolate the bad unit(s).

[W2VW]

Good info. Add to that a padder or padder switch contact failure might not show up cold.

[Tim WA1HnyLR]

Hi Doug,
 The Valiant is a transmitter that has many land mines. The padding caps for the PA plate tuning can be replaced by three 100Pf 5Kv ceramic RF rated doorknob caps or a 300Pf chassis mount mica transmitting capacitor. This cap has mounting feet in the base of the unit. Due to the it's size, the cap can be mounted to the top of the chassis. Lead length not critical due to it's use on 160&80 meters only. The fixed loading caps are another weak link. Replace the variable loading cap with a three section broadcast band air variable capacitor. You may have to swap places with the coarse and fine loading controls. I have two valiant transmitters . In one of them I used a 4 gang broadcast band cap. There was sufficient capacity to operate down to 40 meters without switching in any padding capacity. In many respects the Valiant is a very poorly engineered transmitter. The user MUST take the time to re-engineer it . Look on the AM window for my Valiant mods . They cover most of the bases without much drilling and blasting.
Good luck,Tim WA1HnyLR

[WQ9E]

Tim,

I have been tempted to install a multi-section variable in mine similar to what you did and Heathkit also did when they turned the DX-100 into a DX-100B.  I operate into well matched antennas so I could get away with very close spacing for the variable loading cap.  But so far I haven't had any failures with the fixed loading to inspire me to redo that section.  If you can shoehorn in a large enough variable it would certainly take care of the loading switch and turnstyle cap weaknesses.

[WB5IRI]

All combinations of the coarse loading caps work on other bands, which leads me to believe one or more of the padding capacitors in C39 must have failed. I'll investigate that tomorrow.

Doug

[W3RSW]

   

   Re: Valiant go boom -- again
« Reply #2 on: Yesterday at 09:10:49 AM »
Quote
Welcome to my world.....you are now a member of the Valiant PING club....the sound of the plate current slamming the RH side of the meter movement. Good luck.

You guys need to put a padded peg or fuzzy stop about 1/4 in. Off scale.  

Hope you find a quick solution. Know you'll do it.

[ka4koe]

As others have noted, the band and coarse loading switches are a constant source of issues. My recurrent plate current zooming issue, combined with zero power out, was traced down last night (actually had time) to a contact on the band switch that merely needed retensioning. The clue was no power out. The skee-matic is your friend.