Viking II No Oscillator Current

[W7POW]

Hi everyone,

One of my Viking IIs quit working and lost oscillator current.  I checked the voltages on the + side of C10, C11 and there is NO B+ VOLTAGE at C10.  Looks like I lost my LV B+.   I measured resistance across the LV choke L2 and it appears to have opened.  Any ideas what component could have failed to draw enough current to damage L2?  C10 checked ok and I can see nothing obvious.  Thanks for any ideas.

Chris.

[KA2DZT]

First thing to do is check for a short on the LV B+.  Double check the choke, it's rare that it would go bad.  If you're sure it's open you may be able to fix it.  Open the choke (not sure what style choke it is),  it may be a open solder connection where the leads connect to the coil.

Also, the choke itself may have shorted to ground, check it for a short to the frame

Fred

[WD8BIL]

Tack a 5 ohm 10watt resistor across the choke for a minute. If the osc current comes back it is the choke.
Replace or repair.

[KM1H]

If theres a short a 5 Ohm resistor wont limit much current, use about 1K at least 2W and let it be sacraficed if necessary. Better than blowing the 5U4 from overcurrent.

Carl

[KA2DZT]

If theres a short a 5 Ohm resistor wont limit much current, use about 1K at least 2W and let it be sacraficed if necessary. Better than blowing the 5U4 from overcurrent.

Carl

You're right Carl, 500-1K at least.

Budly doesn't want to use up too many of his ohms.

Fred

[W7POW]

OK, thanks guys.  I have been real busy today and have had no chance to get out to the shack to investigate further.  Hopefully tonite.  If not I have all day Friday and will get to it then.  I will keep you all posted.

[KA3ZLR]

HI,

Check the 6AL5 Maybe..?

73
Jack
KA3ZLR

[WD8BIL]

Nah, the 5 ohm resistor will work just fine to prove the fault. I've done it a hundred times. 5 seconds and he'll know.

[Opcom]

A cheap way to limit power without risking burning up resistors is a bank of 3-5 ceramic light sockets in parallel, and a box of 25W-100W lamps. This contraption can be put in series with the equipment under test and will limit the current while letting enough through to do some testing. As more current is needed, just screw in more and more lamps. I guess it is not very scientific but it works.

[KM1H]

Nah, the 5 ohm resistor will work just fine to prove the fault. I've done it a hundred times. 5 seconds and he'll know.

And in half that time the 5U4 could have shorted and taken out the transformer. No need to take foolish risks with someone elses rig    
One resistor is a hell of a lot easier than a light bulb kluge, Im all for KISS.

[KA3ZLR]

Hi,

Ya know, Prolly wouldn't hurt to Solid state them Rectifiers, while yer in there.


73
Jack
KA3ZLR

[KM1H]

Why?

The iron isnt exactly robust, especially the modulator, and if you monkey with the voltages a lot of resistors will need changing unless you want to cook screens, etc. Use a 5R4 in place of the 5U4 as its better able to handle current peaks and the slightly more voltage drop is good with a high line voltage.

There is no need to hammy hambone old rigs for a few more watts, use a linear instead.

[KA3ZLR]

I wasn't looking to hot rod anything up only to better regulate the system I like Solid state
rectifiers did mine didn't kill it.

I mean't no disrespect and I won't comment further.


KA3ZLR

[KM1H]

Ive nothing against SS when there is a real benefit, you wont catch me running 866's, 872's 3B28,s, etc and the voltage difference is very little.

I also scrapped all the seleniums in my Vik II CDC.

[W7POW]

Thanks for the replies everyone.  Sorry it took me so long to get back to the Viking II.  All is good for now...after verifying that the choke was open and no shorts were to be found on the B+, I scrounged a choke out of an old Dumont scope with close to the same DC resistance (the original viking is 280 ohms, the replacement choke measures 350 ohms.)  The new choke is of unknown current rating  and is a little smaller than the original...I will monitor how warm the choke gets and do some testing to make sure it will be fine in the transmitter until I can find an original Viking LV choke or maybe I can just leave it and replace the filter caps with a higher voltage and value.

Chris.

[WA1GFZ]

Going SS allowed me to go to choke input filter on LV and put the 5 volt windings in series with primary dropping the heater voltage from 6.8 to 6.3. Now the LV transformer runs almost cold rather than almost saturated and hot.