Viking Valiant - trouble in paradise

[N2LK]

Hey Gang-
Thought I had this unit about finished but in last 2 days I have now poped (2) 22 Ohm 1/2 watt plate resistors feeding V16, one of the 6146 mod tubes. The main 8A AC fuse also has blown these 2 times with this resistor POP!

Just rewired that this am, was all fine, power out  and just blew it now again. AC line voltage was at 122Vac...set the Screen Grid voltage to -70vdc...

This also only happens on the 80 meter band so far, not on higher bands, I dont use 160M

The other day while operating I heard a hum starting in the rig that seems to be in the area of the T2 or T3 transfomers. It lasted say 10 seconds and quieted back down...all operated ok so just passed it off...

Now I am blowing these resistors on this one mod tube...

I did just the other day tightend the transformer covers, the nuts were not completely tight, perhaps I have done something doing that??

I also cleaned up the "Special Cap" in the loading section, seems fine but perhaps for 80M cap part its just not doing the job?

Any suggestions is greatly appreciated.
73
John-N2LK

[N1BCG]

What's your modulator current with no mod and on peaks? I toasted the same plate resistor when the bias supply failed.

[N2LK]

Good question Clark....not really sure...whenever I have tried to adjust the static modulation current the lowest static reading I can get by adjusting the pot is 70ma range, cant move it down to 60mA....

When I blew it today, a few minutes ago was on 3.9Mhz, in AM mode..grid at 8ma, tuning the plate to 330ma as manual says and the resistor blew itself to smitherrines.....meter was reading plate in that 300ma range when it blew. I must admit its quite an exciting but nerve racking experience...this into a 50 ohm dummy load too....

I have used the rig many times on cw, did try AM a week ago, seemed ok listening to myself on my receiver but I never was able to adjust the modulation pot to the 60ma static level.

Not sure I really answered your question huh??

[N1BCG]

The same bias supply is used for both the RF and AF power tubes. If you can't get low enough resting current on the modulators then the RF tubes may be high also. Check the filter caps. Hum and low bias suggests a cap going bad.

[N2LK]

Thats a good idea...maybe the electrolytics I swapped out have gone bad.
Will check the resistances for the transformers also in the manual that has that chart.

Question - high voltage wire to use..  is that something I could get at say a Big Box store? I have tons of
14 gauge stranded, 600V rated but dont know if that would be acceptable if I have to rewire?

Thanks for the help, keep ya posted.

[N1BCG]

You should be able to use similar wire to what's already in there. The bias supply is less than -200 Volts. The key is to measure the bias on the RF grids (at the C73 side of L7) and set the modulators for around 60 mA resting current.

I run my Valiant at 300mA for AM and CW. Much less stress, particularly given the higher line voltages these days compared to back in the 50s.

[DMOD]

I would make sure the Clamp adjustment yields less than 180V on the Finals' screens.

Secondly measure the resistances between each Mod tube plate cap and the CT HV feed to the Mod transformer. They should almost be equal.


Phil -  AC0OB

[w3jn]

Parasitic/ultrasonic oscillation.  Try swapping the plate caps on the 6146 mudulators from tube to tube, might stop whatever feedback path is there.

Otherwise your best bet to track this down is a scope.

[N2LK]

I have checked the plate resistances as recommended, all ok...I swapped the plate caps also.

Question - Where is R22??? 
a 5K adjustable resistor....I have looked thru the schematic twice and cant find it..

Will keep hunting...maybe bad print on my schematic copy.
73

[N2LK]

I have a late production Valiant and R22 must be the newer R62 5K pot on side of chassis....just covering all the bases...

[N2LK]

Here is something new, for some reason I am only now reading -1.4vdc on pin 5 of the V17 modulator tube...even when I pulled it out the pin is still reading that low voltage,,,,was up in the -54vdc range till now.

5K pot R61 seesm working its range on my meter resistance wise, I put in a replacement LV rectifier tube and no change.

I also have 2 replacement Mod tubes coming today but wanted to see any ideas why this voltage has dropped all of a sudden.

Radio loads up in cw mode fine, but I just cant get GET Mod Bias down to the 60ma resting figure when I switch to AM mode and plate at 330ma..into a dummy load...

Will keep working at it, any thoughts always and continued appreciated.
73

[N1BCG]

The voltage at pin 5 on both mod tubes should be the same, so either something is dragging the voltage down or a connection opened.

There are 100 Ohm resistors (R46,R47) in series with each mod tube grid. If the resistors test ok and you don't get proper bias voltage (-46) then a secondary winding on the driver transformer (T4) may be open.

[N2LK]

Thanks Clark, radio works fine in cw mode.....to paranoid right now to flip to AM to set that Mod Bias just yet, its set all the way CCW for now.
Will check out T4 now.

Question- I replaced the paper 15mf-450V electrolytics (C93A,B C96A,B) with new 22mf instead....would that have a major impact on the modulation section performance?
73

[KA2DZT]

Thanks Clark, radio works fine in cw mode.....to paranoid right now to flip to AM to set that Mod Bias just yet, its set all the way CCW for now.
Will check out T4 now.

Question- I replaced the paper 15mf-450V electrolytics (C93A,B C96A,B) with new 22mf instead....would that have a major impact on the modulation section performance?
73

No, to answer your question.

[N1BCG]

C93A,B are in the bias supply. C96A,B are in the LVB+ (300V) supply. The input cap on my bias supply went bad, effectively making it a choke input and cutting the bias voltage. Replacing those four caps with newer ones as you did can only help.

The modulator bias issue is likely a 100 Ohm resistor or the driver secondary.

[WD5JKO]

Are the new bias caps installed with the + side grounded? If so, then that is correct.

Jim
Wd5JKO

[N5DMC]

John, How's the Valiant coming along?

Doug

[N2LK]

Very, Very Very Well, thank you 

I tore out all the wafer switches 3 weeks ago and soaked in vinegar/salt solution...cleaned up nicer that what I had done to that point. I rewired all of them back in via construction manual, schematic and my photos.

I discovered I had mis-wired 2 of the main PA tank coil taps in reverse!! Duhhhhhhhhhhhhhhhhhhhhh

Changed out the 40M and 80M padding caps so they are 2KV rated now

Now tunes up on 80 and 160 just fine...set Mod Bias...all is good.

Made my first ever AM contact this morning and chatted for an hour with 2 guys, good reports. One recorded my audio and played it back to me, I was astounded...sounded fine.

I feel like a Novice again!!
Thanks all, hope to speak with you on this beast soon.
73
John-N2LK

[N1BCG]

Congrats, John. You've earned a well working Valiant. Send me an email when you'll be on sometime. It would be great to hear the product of all your effort.