Viking 2/ v5/ caps/grid current

[N9axl]

If someone has their Viking opened up - I want to know which caps are connected to which pins of v5. I have zero/low grid current -only see 2.5ma with drive set to 10 - and wonder if there is a cap missing. I replaced what was there, but I think one is missing. Or perhaps this is operator error and the drive should be at 10.

From what I have been told and read it seems that this is probably about 60 percent of what should be there. 4ma I think would probably be enough to drive them to close to full power. The warning in the manual says 4ma per tube, but that was in 1953 and it appears that limit was lowered later. I've seen statements that as low as 1.75ma per tube is sufficient.
And these are both 6146 tubes.

[Pete, WA2CWA]

Typically for a pair of 6146's, no more then 6 ma of grid current.

[N9axl]

Thanks. This is much clearer than the copy I have.  What I am trying to figure out is what are possible causes of low grid current e.g. Weak 6146 tube, missing cap, bad resistor.  Where would it be reasonable to start looking?

[KB2WIG]

You need to have  all of yer ducks in a row. Watt id start eith is the osc; use a receivrr  and tune for peak; and ho downthe line with the buffer,etc

Klc

[N8ETQ]

  Yo'

     Could be a lot of things, some simple tests
would be to swap the 6AQ5 driver with the 6AQ5
Clamp tube and see what ya get. The "DRIVE" pot
is a high failure item as well.      simple
voltage checks should tell the tale.

Clean Manual copy here:

http://bama.edebris.com/manuals/johnson/viking2%282%29/


GL

/Dan

[N9axl]

I have been working through from the oscillator to the buffer.  I get plenty of oscillator current. Probably 10-15ma in the vfo position and 7-8 in Crystal.

 I also get plenty of buffer current. I was stumped by this for a while until I replaced my new mica cap with another. Evidently that one was bad. I can actually max out the buffer current at 25ma with the drive set at 10. I'm getting a mid range indication with the drive set at 4.


And yes I have replaced the drive pot, replaced both of the 6aq5 and swapped them around.  I was at one point for a moment seeing 2ma of grid current with the drive set at 10.

I have checked the voltages and they seem in line.  There's just no grid current. 

  I am wondering if I just happen to have some 6aq5s that don't work well at rf frequencies.  Perhaps replacing them with some 6ah6, which was suggested to me and I haven't done it yet, might make a difference. I have made a detailed drawing of V5 and will compare that with the schematic. Perhaps there is something miswired or a cap is missing.

Voltages are more or less what they should be - and this is with the switch in the VFO position.

V5 - pin1. 22.7
Pin 2 0
Pin 3 and 4 6.3ac
Pin 5 327
Pin 6 91.4
Pin 7 22.7

V6
Pin 1.0
Pin 2 0
Pin 3 and 4 6.3ac
Pin 5 345
Pin 6 106
Pin 7 0

[DMOD]

I am wondering if I just happen to have some 6aq5s that don't work well at rf frequencies.  Perhaps replacing them with some 6ah6, which was suggested to me and I haven't done it yet, might make a difference.

The 6AH6 is a voltage amplification tube. The 6AQ5 is a power tube and they are not swappable.

I don't know who suggested that to you but whomever it is is wrong, or you misunderstood them.

As I suggested to you previously and you never answered: Does the bias voltage on the Grid go more negative when tuning the OSc and Buffer?

And I also suggested replacing C25 which is failure prone.

Phil - AC0OB

[N9axl]

Sorry. I thought I had. C25 has been replaced. On the thought that maybe a batch of the silver Micas I got from the same vendor in one order were bad I just replaced it again with ones from a different vendor - 2 25mmfd in parallel. That made no difference.

No the bias voltage, measured at r17, stays at -74 volts.

Now there are two ceramic disk caps next to L7 and L1 on the top I have not replaced. I haven't been able to track down in the schematic what they are yet.

[DMOD]

...Now there are two ceramic disk caps next to L7 and L1 on the top I have not replaced. I haven't been able to track down in the schematic what they are yet...

L21 and L7 and the two caps, C32, C46 have nothing to do with the problem at hand.

As was suggested before, take a magnifying glass and bright light and check the wires going to and from coils L4, L5A, L5B and the FREQ switch. The location of these coils is shown in the manual.

Check closely the FREQ switch contacts.

Since you found some jerry-rigging before, who knows what was changed or mangled.


Phil - AC0OB

[N9axl]

I checked those before and didn't see anything but I will go over them again. Thanks.

[WA2ROC]

I had a similar problem with my Viking II where grid drive was low on 20 meters and up, but OK on 160-80-40.  I started probing with my digital voltmeter and one side of the screen dropping resistor on V6 (R19) read 300 volts or so (LV) and the other side read about 5 volts DC.  There was a tiny crack in the resistor, making it's resistance somewhere near infinity.

Just another possibility....
 

[N9axl]

Thanks. I didn't see a problem with R19 when I checked it two or three weeks ago, but, obviously, as this gets closer to functioning and proper voltages are applied parts can break down.  I will check this again.

[DMOD]

I had a similar problem with my Viking II where grid drive was low on 20 meters and up, but OK on 160-80-40.  I started probing with my digital voltmeter and one side of the screen dropping resistor on V6 (R19) read 300 volts or so (LV) and the other side read about 5 volts DC.  There was a tiny crack in the resistor, making it's resistance somewhere near infinity.

Just another possibility....
 

Recall that  previously this is what I sent you in regards to potential OSC and Buffer-Driver problems:

Quote
In the oscillator section one may find the oscillator cathode current to be too low resulting in low buffer and final grid current. This can be traced to screen grid resistor R19 changing value, so replacement is warranted. A 1 Watt carbon composition resistor or carbon film is necessary since this resistor is part of RF feedback to the crystals. Another problem child is the C29 silver mica which can be replaced with a single 47 pF 1kV ceramic unit. Replacing the medium gain 6AU6 with a higher gain 6AH6 results in more drive at the higher frequencies and I found no need to re-bias the oscillator stage.

Another silver mica that will deteriorate is C25, the buffer-to-final grid coupling capacitor. A single 68 pF 1kv ceramic cap helps here as well. If you are not getting enough final grid current or buffer drive, check R23 for value change. R23 should be replaced with a 47k-51k ½ Watt carbon composition as well.

Another drive related problem can be found with R25. Notice that in stock Vikings, the Drive pot R25 is the only load for the LV power supply when in the AM mode and not transmitting. I added a new 200K resistor in the LV supply to shunt some current away from R25 so that it does not dissipate as much power.  Make sure the replacement pot R25 is a true 5 Watt WW resistor.

However, one should only do testing and initial checkout with the 6AU6 Osc tube.

http://amfone.net/Amforum/index.php?topic=41456.50

[N9axl]

Thanks. I'm out of town for a week or so but I will get back to this.  I've replaced these with no joy, but I will recheck my work -- always possible I have a bad solder joint -- and as I've found even new parts are sometimes not up to snuff. Appreciate all the advice.

[KA2DZT]

Check L6 the grid RFC.  That choke would typically be about 2.5mh.  Make sure the grid tuning cap is peaking up the grid tuning.  Not sure what type cap is used.  Some caps have stops at min and max capacitance.  Others have rotors that will turn a full 360 degs.  The peak tuning should occur within the range of the tuning cap and not at min or max capacitance.

Fred

[N9axl]

I will look but I don't remember seeing a variable cap near L6. There is a ceramic disk cap - C28 which is what you are referring to?

I haven't replaced either this or C45 yet since I didn't see this as related to the issues I am having and they were ceramics - which normally are OK and these had no apparent damage.

[KA2DZT]

Two different things to check.  If the RFC L6 is bad or shorted you will lose the drive signal to the grids.  Second thing,  the grid tuning cap should peak up the drive signal within its tuning range,  meaning not at min or max capacitance.  If this doesn't tune right you need to check the grid tuning coils and switch to make sure the right coil is switched in the circuit for the band selected.

Fred

[DMOD]

Check C22 (Buffer Tune) and make sure the wires leading from C22 to L5 and the 6AQ5 plate pin 5 isn't shorting and is soldered properly.

C22 and L5 form a Parallel resonant circuit that supplies RF to the Grids of the Finals.

Check C25 again and make sure the capacitor leads are going to the right places and soldered well.

Phil - AC0OB

[w4bfs]

after you have checked all this and still have no drive you might consider checking out the buffer stage meter shunt .... iirc this is a 5.1 Ohm resistor shunted across the meter giving a 50mA (?) range .... most of these carbon comp resistors go higher in value with age and you may not be driving the next stage properly even tho the meter sez it is

[N9axl]

Thanks all. I checked everything suggested and am seeing no broken wires,etc. I do see that the switches are still dirty. It seems they still have lots of oxidation on them and are not bright and shiny.

I put some deoxit  on but it perhaps wasn't sufficient or didn't get applied where it needed to be - I had the very small 25ml squeeze tubes which were hard to work with.  I have ordered some additional - this time in the spray can with the extender tube - and will see if that makes any difference getting some additional product down in there.

[Pete, WA2CWA]

I put some deoxit  on but it perhaps wasn't sufficient or didn't get applied where it needed to be - I had the very small 25ml squeeze tubes which were hard to work with.  I have ordered some additional - this time in the spray can with the extender tube - and will see if that makes any difference getting some additional product down in there.

I would not start spraying deoxit on the switches. If the switch wafers are paper-based phenolic, they can absorb the deoxit and actually make the wafer somewhat conductive. I would put some deoxit on a q-tip and just wipe the contacts as best you can and working the switch as you do it. The only thing that has to be cleaned is where the switch contacts wipe the movable contact area. And use the deoxit very sparingly.

[KL7OF]

tube socket(s)

[WD5JKO]

I may have missed it, but I see no mention of which band was tried. The grid drive might be fine on 80m, but low on 10m, etc. Also realize that this transmitter when every part is good might have grid drive issues on the higher bands. As far as tube replacements, many hams have replaced the 6AU6 in the VFO and the Vik 1/2 oscillator section with a 6AH6. I know that Tim WA1HLR found a pin for pin replacement for the 6AQ5 for his Vik 1.5, all in a attempt to get more grid drive on the upper bands. I forgot that tube number, but I recall him telling me that the Drive pot needed a mod to limit the maximum screen voltage.

The stock design with the EFJ 122 VFO, or a crystal should have no trouble on 160-80-40m, so I'd resist doing any mods until those bands function normally.

The band switch is ceramic, and everything around it is pretty robust. If it were me, I'd stuff paper towel around and under the band switch, and spray good old brake cleaner on the contacts. Do so with good ventilation, and power off. I know, I know, many will say NEVER do this.

The 6146 RF finals might have an issue too, especially if they are very old. The Vik I 4D32 final often gets a cathode oxide flake on the control grid, and when hit by the electron stream that flake super heats, and becomes a electron emitter. The result is reverse grid current which subtracts from the grid drive. The effect in the early stages is worst on 10m, and over time starts causing trouble on the lower bands. This issue is less common on the 6146, but can still happen. Have the finals been swapped out?

Jim
Wd5JKO

[N9axl]

I did swap out the finals.  That seemed to have no impact.

This evening, since my deoxit came today, I will do a careful cleaning of the switches and the tube sockets and see what effect that has.

[N9axl]

Well, I have checked all the suggestions I have been given and see no grid current of any kind.

I still think there is an issue on v5 as I am still not sure what pins c21, c26,c24,c27 attach to and would like to confirm that. One of these caps seems to be missing.

I see no issues with shorted wires, etc. I am completely and truly baffled at this point.

All I can think of besides the caps is to replace both 6146 tubes and v5 and v6 and see if that makes any difference.