Viking II Hum Issue

[w9bea]

Hi All:

Working on my Viking II today and after a long hiatus, I find that it has a loud hum on the TX signal both on AM and CW.  Something happened while it sat on the shelf.

I am suspecting the filter caps, but before I run out on a tangent with soldering iron in hand, I thought I'd ask the list about this.  A sanity check, if you will.....

There are 2 multi sectioned caps and one oil.

I looked at C9 the Oil cap first.  no oil leaking, no metal bulging, good.  With Plate Switch on I am measuring something like 755 VDC and 3 volts AC (my fluke will measure both AC and DC and display them on the same LCD).

I am guessing this is okay, so i moved forward.

There are C10/11  and C12/13  It appears that the 10/11 is 450 volts and the 12/13 is rated at 150 WVDC.

Measuring on C10/11 first (a pain to get to the connections due to the insulation on the wire) I am measuring this:

Red lead to ground measurement... +286VDC and 20 VAC
I couldnt grab the green lead as the connection is buried real well.  I will take another stab at it if you think I should.

On C12/13, I measure the following:

The black lead going to the to pin 7 on the 6AL5 -58VDC and 26 VAC

The blue lead going to the pin 6 of the 6AL5 -53VDC and 12 VAC.

I am guessing that this represents some serious AC leaking, correct?

Are there any other caps that I ought to look at changing out in the radio right away?  I am looking at the wax covered caps and thinking this might be a job worth doing, but I sort of wanted to hold off on swapping those until I get the hum off of my CW note and AM carrier first, getting the radio back on the air and them Ill swap the wax ones out later, unless you think it needs doing right now....? 

There is a large wax cap buried under the oil cap that goes from one of the modulator tubes to ground.  I'll have to move the oil cap to get to it.  Should I escalate replacing this one?  I am not sure what part number or value it is, but it looks awfully big.

Any input/ideas on if my assumption that the filters are bad would be appreciated.

73--Wally W9BEA

[N0WEK]

I'd replace both the electrolytics in any case and leave the oil cap alone. Hopefully that'll get the hum out of things and then reevaluate from there.

[Ralph W3GL]

Oil caps age well however, REPLACE the Electrolytic caps!

And while you are in the area, any other electrolytic's that may
be under the chassis along with the paper caps in the audio...
 
Use your Fluke to check the resistors in there as they age as well.
It's not unusual to find resisters well out of their 20% tolerance range

[KM1H]

Once you replace the electrolytics and paper caps check resistor tolerance in the audio section; while not hum related it might improve the audio.

If it is a kit rig tighten up every ground connection and use external tooth SS star washers where missing.kits used screws and nuts, factory built was mostly riveted but still check everything since a lot of assembly work was farmed out to bored housewives and they used screws.

My oil filled showed no leakage or bulge but when removed the paint on the side facing the chassis was scorched. I had some fairly new 100uF 450V Sprague ATOM's and 100K 3W MOX resistors so used them. Regulation is excellent and AC ripple is way down.

Carl

[w9bea]

Hi All:

I replaced the 4 Electrolytic Sections with new caps from Mouser.  Transmitter seems to be working okay now.  I got on 40 Meters and heard W3GMS, WA3VJB and a few others but I couldnt break through the QRN.  I did work VE6PG though and had a nice QSO with him.

The only issue is that the PTT mod that got put into the radio came apart somehow while in the middle of the QSO.  I click my microphone PTT, I hear a faint relay clicking but I need to throw the Plate B+ switch to go on the air.  Darn!  I'll need to dig into that a little later I guess.


Oh well, at least we are on 40 now! 

73--Wally W9BEA

[w1vtp]

Oil caps age well however, REPLACE the Electrolytic caps!

<sip>

Amen!  I mentioned in another post about the vile white cloud that came from one of my EICOs 'lytic caps during testing.  I can't imagine the chemical makeup of that cloud. I beat a hasty retreat and turned on the exhaust fan I have in my work area.  Change the lytic caps even tho they seem OK.

Carl:  interesting your comments.  I'll be sure to check my oil cap on the V1

Al

[w5gw]

Having owned 5 or 6 Viking IIs over the years I just do a global replacement of all electrolytics.  Never had any issues with the oil impregnated paper caps, but for sure they can fail, from my experience working on weather radar sets eons ago.  Also, the carbon resistors tend to increase in value so testing each one and replacing with correct value and wattage is my second step in gettin these beauties back on the air.  

One thing to note, there is a ill founded logic that if an electrolytic was initally say a 15 uFd at 450 VDC it is better to replace with one at say 50 uFd at 450 VDC.  This logic can have sometimes have unintended consequences.

E.g., Someone had put 50 uFd caps into a Viking II's LV supply that I worked on recently.  I had a terrible chirp   on CW until I replaced these with their correct values of 15 uFd.  The same goes for resistors - there is an ill founded logic that if it called for a 1/2 Watt, there is nothing wrong with replacing with a 5 Watt or even higher wattage (hey these new MOS resistors are much smaller right!).  That can lead to expensive components failing, such as a transformer (e.g., if a component shorts ahead of the resistor it is better for the resistor to fail rather than have excessive current build up and present itself back to the power source).  

So in general, I always try to observe component values and capacities.  

[KM1H]

The 8uF oil cap in my 1961 built V-II CDC had scorch marks on the side facing the chassis and quickly went into the trash. A pair of 100uF 450V ATOM's I had on hand plus 100K 3W MOX resistors replaced it and gives a much better regulation of the B+ as well as almost no ripple.

The LV filters were replaced with the next higher value along with some extra filtering in the audio drivers as there was too much remaining ripple for my liking.

Carl

[w5gw]

The 8uF oil cap in my 1961 built V-II CDC had scorch marks on the side facing the chassis and quickly went into the trash. A pair of 100uF 450V ATOM's I had on hand plus 100K 3W MOX resistors replaced it and gives a much better regulation of the B+ as well as almost no ripple.

The LV filters were replaced with the next higher value along with some extra filtering in the audio drivers as there was too much remaining ripple for my liking.

Carl

Carl,

Again, this is my experience, I'm sure others will have different opinions and results.

Increasing capacity in the HV from 8 to 50 uFd probably won't contribute to any significant chirp.  But the drop of the LV voltage after key down increases in time such that going from 15 to 50 uFd created a significant chirp on CW.  You actually get better voltage stabilization with the 15 uFd LV caps.  Chirp was about the same whether using a crystal or a VFO.  If you don't care about CW then it probably doesn't matter all that much.

As far as adding filtering in the audio stages, I use an audio spectrum analyzer, o'scope, and a two tone test generator to evaluate these stages.  The stock filtering seems more than adequate.

73

Gary

[KM1H]

Actually 16uF is the next standard value but I went to 22 since I had several on hand. There is no chirp on any band as I just confirmed however the rig is used for AM only so far.

With high gain pentode audio I didnt care to tolerate any ripple on the DC and small electrolytics were also on hand and small in size. Just call me fussy. I think I'll start Phase 2 of the audio mods soon as Ive had enough OTA reports to compare between the rebuilt to stock V-I and the V-II Phase 1 to know Im heading in the right direction.

Carl

[KZ5A]

I'm a little vague on how increasing the value of a filter cap introduces "chirp" into a TX.   

My suspicion has always been that many component values in production radios are selected on the basis of being the least expensive useable component.  Filter caps are particularly suspect in this regard since "C" value and cost vary together.   There is also physical size aspect since modern electrolytics are much smaller and some caps were probably selected as the largest value that would "fit".

So I always roll up filter C values significantly when replacing old ones and so far have never seen anything but improved supply performance as a result.    The only down side I am aware of is increased surge current at startup, particularly if also going from tube to solid-state rectifiers.   In cases where the increased surge is an issue I'll add a thermistor or relay based soft-start module.

Anyway the "chirp" thing is intriguing since I can't picture how that would happen, which usually means I'm missing something.

73 Jack KZ5A   

[KM1H]

Likely raises the output voltage too much for the 6AU6 circuit to handle and the crystal current goes way up, a good place for a zener rather than juggle values.

Carl