HELP..LOW OUTPUT VIKING2..AM..

[ve6pg]

HI FROM TIM..DONT KNOW WANT HAPPENED...SUDDENLY OUTPUT ON AM
IS VERY LOW,PLATE CURRENT DOWN...BUT FULL OUTPUT ON CW..WHEN I TUNE UP IN CW MODE,I HAVE GOOD OUTPUT..WHEN I SWITCH TO AM,THE OUTPUT IS DOWN TO ABOUT 20 WATTS...ANY IDEAS?...THE RIG WAS JUST FINE THIS AFTERNOON,TUNED UP IN NO TIME,A COUPLE OF CONTACTS MADE ON 40,AND 75..I LEFT THE FILAMENTS ON FOR A WHILE,KNOWING I WOULD BE HAVING A QSO IN A COUPLE OF HOURS...THEN WHEN I THREW THE SWITCH...NOTHING!!..PANIC!! I CHECKED EVERYTHING,ALL SEEMED OK,TURNED TO CW,AND FULL OUTPUT...SO BEFORE I DRAG IT OUT OF THE RACK,I WOULD LIKE SOME INPUT...THANKS...73...TIM..

[N8YE]

Is the drive level the same as in cw mode? I think the screen voltage of the final(s) is derived differently between am and cw.  If I remember correctly, in cw , the screen voltage is derived from a slider on the big power resistor. In am mode , the screen voltage is derived from a dual parallel resistor . Perhaps something is amiss there. I'd hate to think the mod transformer opened up to a higher resistance..
 73...Steve...N8YE

[ve6pg]

WELL STEVE,THE DRIVE CONTROL IS AT MAXIMUM..WHEN I SWITCH TO AM,I AM GETTING POWER OUT,APPROX 40 WATTS NOW...THE PLATE CURRENT DROPS TO ABOUT 175MILS MAX,AND THERE IS NO GRID CURRENT READING,OR BUFFER...WITH THE 40 WATTS,IT DOES START TO CREEP UP,TO ABT 60WATTS,AND THE MODULATION IS JUST FINE..NO CHANGE THERE...MY DWG FOR MY VIKING ARE NOT THE BEST,BUT I WILL LOOK INTO THOSE RESISTORS...THESE LITTLE THINGS ARE MEANT TO TRY OUR PATIENCE...73...TIM...SK..

[WA1GFZ]

Check your bias voltages in both modes. V2 is very easy to work on
Check the big power resistor under the chassis.

[WD8BIL]

Not good news.
Sounds like a problem in the mod tranny. The secondary is shorted out in cw with the mode switch. In the phone mode the B puss is directed thru the mod tranny.

Good luck.

[Jack-KA3ZLR-]

I would start with the Resistor here, and work my way back like the others are saying, mine went bad, a couple of windings just disintagrated Check all connections wiggle the pubes, do all your basic checks first then run for the mod tranny...

[W8ER]

Tim,

With with the symptoms that you outline, I would not suspect the mod transformer. It sounds like you have a drive problem and that would be the place to start. You need to get reliable and proper buffer and final grid current readings.  

The first thing would be to check and or replace the tubes and see what effect that has. After that, I would go to the power supplies and make that the first checks with a voltmeter. Next, I would check voltages in the early RF stages and, as was suggested earlier, look at the bias voltage on the finals. Something should be apparent by now.

Generally all of the power resistors should be checked and while you are in there, look very carefully at the power supplies. Replacing filters and maybe even solid stating the rectifiers would be a good move. Be careful though. Making repairs is the first order, making modifications should not be attempted until it's back working properly.

If you need a good schematic, I think I either have one or can get one for you. BAMA is a good internet source for boatanchor documentation. A search on Google will turn up that site for you.

My email address can be found by looking at my profile here on the AM Forum. I would be glad to help if you need further assistance.

--Larry W8ER

[N8YE]

Check out you meter shunt resistors and meter switch contacts. You might be getting false/erratic readings on grid drive,driver plate current and osc plate current.
...Steve...N8YE

[WD8BIL]

BUT FULL OUTPUT ON CW..WHEN I TUNE UP IN CW MODE,I HAVE GOOD OUTPUT..WHEN I SWITCH TO AM,THE OUTPUT IS DOWN TO ABOUT 20 WATTS...

Full output on CW means drive is OK. Nothing there changes when switched from CW to Phone.

The only thing happening with the CW/Phone switch is the keying line is opened and the mod tranny is un-bypassed.

[ve6pg]

OK..LOTS OF POSSIBILITIES...BTW,OUTPUT DOES CREEP UP TO ABOUT 50WATTS,AFTER A WHILE...I WILL LOOK INTO THIS BIG RESISTOR UNDER THE CHASSIS,AS WELL AS A CAP,C8,WHICH I BELIEVE IS A .5
  WHEN I'M IN QSO,MY AUDIO IS "OK FINE",BUT NO METER SHOWING ON GRID OR OSC..STRANGE...MOD JUMPS AROUND NICELY,PLATE IS ABT 175MILS,AND BUFFER IS GOOD...I HAVE BEEN ASKING OUR GROUP ON 3725AM ABOUT THIS,AND THEY SAY THE AUDIO IS NO DIFFERENT...I CHANGED SOME PUBES,AND NOTICED NO DIFFERENCE IN CW OR PHONE...THE MYSTERY DEEPENS....TIM...SK..

[WA1GFZ]

Look at all the circuits around the mode switch and ohm out the modulation transformer.

[WA1GFZ]

another thought. Maybe it is your high voltage supply getting sucked down
when the modulator is running. Check the filter caps and rectifier tubes.
Maybe a 5u4 is going soft and can't pass the extra current required by the mudulator.  The stock high voltage caps may have gone dry.
You can get a lot more C with modern caps in a smaller case size.

[WD8BIL]

Maybe it is your high voltage supply getting sucked down
when the modulator is running.

That's a great thought Frank !! I notice this happens to a much lessor degree on my Viker 2.
Solid stating the rectumfinders helped a lot but it is still a bit noticable.

[WA1GFZ]

Well think about it. Whe in AM the load on the high voltage supply is  almost double to make 50 watts of audio.

[WD8BIL]

Any progress Tim ?

[ve6pg]

WELL,NO...THE RIG IS STILL ACTING WEIRD,IT NOW IT HAS ABOUT 70 WATTS OUT,AND NOW,THE PLATE CURRENT STAYS AT ABOUT 200MILS,IT DOESNT CHANGE,EVEN WHEN I TURN DOWN THE DRIVE...VERY WEIRD...HAVENT HAD THE TIME TO GET IN THERE JUST YET,BUT I AM GETTING ALL KINDS OF INFO FROM ALL YOU GUYS,WHICH IS A GREAT HELP...KINDA OF DISAPPOINTING,AS I JUST SOLD A VIKING2 TO A FRIEND,AND THEN THIS HAPPENS...I TOOK THE 807'S OUT,AND TRANSMITTED(WITH OUT AUDIO),THERE WAS NO DIFFERENCE..THE SEARCH CONTINUES...TIM....SK..

[WA1GFZ]

How about adding 250 or 500 pf across your loading cap to see if anything changes. Maybe the loading caps are bad and the rig is over loaded.
Check screen  and grid bias voltage on the final tubes. Excessive bias would reduce plate current and cause the current to remain stable as drive is reduced. Low screen voltage would reduce plate current and output power.

[W2VW]

Shouldn't more load capacitance = less loading? How about an on the edge screen bypass capacitor pulling the screen Voltage down?

[WA1GFZ]

Dave,
True if all the other caps are good. Say the caps are open then adding C will increase output power. yup screen cap could be leaking but voltage would be low.

[W2VW]

OK. How about the screen cap is on the edge and starts heating up and leaking when using AM due to the higher Voltage of the modulaton. You pull the rig out of the case and stick it on the bench with one hand in pocket and the screen Volts measure OK.

[W2VW]

OK. How about the screen cap is on the edge and starts heating up and leaking when using AM due to the higher Voltage of the modulaton. You pull the rig out of the case and stick it on the bench with one hand in pocket and the screen Volts measure OK.

[WA1GFZ]

I don't remember if a stock V2 has a mudulated screen. Doesn't it come off the big slider shared with the 807 screens? There are 2 sliders are there not?

[W2VW]

Dunno. I ass umed that that is how they did it.

[W8ER]

Guys,

Since the guy came here asking for help, wouldn't it be nice if he would report back what he has done before we build him a new rig?  :?

--Larry W8ER

[WA1GFZ]

Yea,
We don't want him to think we are nuts just yet.