Update: My Viking 1 power supply cap replacement

[w1vtp]

Here's how my power supply cap replacement came out.  This V1 uses the KS3K modification.  A bit different that a Timtron mod but we'll see how it goes.  I already know it puts out RF.  I did fire up the unit after I did the PS mods and no smoke and the voltages were about where they should be.  I have not fired up the unit in the AM mode but it should work.

I will include a before pic and after pics with closeups of the 807 screen area and the LV / bias area.  I am pleased to get rid of the hanging electrolytic garden syndrome and actually have secure mounting points for all the caps. At one point, when the bias caps were unstrapped, it looked like I was gutting the poor thing with the caps hanging out like, well you know if you have ever field dressed a deer.   I guess the next thing to do is to touch it off and see if it makes music.

I still have to make a PTT circuit that will be compatible with the 32V1 transmitter as that's where it's destined to go.  Oh yes, after I get all this going I have to get one of my two 122 VFOs to work with the transmitter. Wish me luck.  So far no burns so my shaky hands are doing pretty good.

This post will show the before pic and the schematic for the KS3K mod

Al

[w1vtp]

Here are the "after" pics. 1) Overall view, 2) 807 screen detail, & 3) LV / Bias PS detail

[N0WEK]

Nice clean work!

It's amazing how much space you gain with modern caps.

[W1AEX]

Yes indeed, new electrolytics can open up a lot space under there.

Al, When I re-did my Viker I found that several of the original resistors in the area pictured below were quite well-baked. One of them crumbled as I was removing it. I ended up replacing them all and that resolved the random jumps in OSC and grid current that annoyed me for a few weeks. Might be worth doing before you wrap it all up. Then again, yours might run fine for the next 50 years!

Looks like a nice clean Viker One!

Rob W1AEX

[KA2DZT]

When some of the SK oldtimers come back in their next lifetime, they will be buying back their old rigs.  First time they open one up, they'll shout where the hell are my filter caps??

[w1vtp]

When some of the SK oldtimers come back in their next lifetime, they will be buying back their old rigs.  First time they open one up, they'll shout where the hell are my filter caps??

Fred

I have them in a body bag (zip lock).  I'll give the guy tha V1 along with the body bag

 

[w1vtp]

Yes indeed, new electrolytics can open up a lot space under there.

Al, When I re-did my Viker I found that several of the original resistors in the area pictured below were quite well-baked. One of them crumbled as I was removing it. I ended up replacing them all and that resolved the random jumps in OSC and grid current that annoyed me for a few weeks. Might be worth doing before you wrap it all up. Then again, yours might run fine for the next 50 years!

Looks like a nice clean Viker One!

Rob W1AEX

Yup, Rob, these resistors are infamous for crapping out.  The one with the green leads is the meter shunt for the osc and it works just fine so I'll leave it in for the time being

Al

[N3GTE]

I've got a Viking 1 that has a 829 in the final. Was talking with a friend and he was nice enough to give a new 4D32 to convert it back. I used to run it stock back years ago. Once I was talking to Pete the mm he said that it had very piercing audio. It's on my todo list. BTW any of you guy talked with Ed KS3K lately?? He change his call to K4CWM years ago. Real knowledgeable and very nice guy!
Terry

[w1vtp]

I've got a Viking 1 that has a 829 in the final. Was talking with a friend and he was nice enough to give a new 4D32 to convert it back. I used to run it stock back years ago. Once I was talking to Pete the mm he said that it had very piercing audio. It's on my todo list. BTW any of you guy talked with Ed KS3K lately?? He change his call to K4CWM years ago. Real knowledgeable and very nice guy!
Terry

That KS3K call has quite a checkered past, doesn't it?  He's all but pulled the disappearing act.  I do wonder why he added the 220 mfd cap at the cathode of the 6C4 to ground.  Seems to me that negates the neg.. inv... feedback

Ed has nothing on his bio and no email.

Al

[N3GTE]

Ed did like to keep a low profile.Haven't seen him since the early '90's maybe. It's been a long time. There was a group of us that would drive down to MD (abt 2hr ride 1 way)to visit the MARC club meetings. We will discuss all manor of radio. Always had good sage advise for fixing up these old AM tx's. Don't know anything about a checkered past.
Somewhere he wrote up a modification for the Viking I /II that use a 6EW6 for the 1st stage rather than the 6AH6. 6EW6 has a bit more gain. I don't remember if it used a 6C4. it may have had use a 12AU7 with two sections connected in parallel. You probably know this but if you are using the stock driver transformer it has very poor frequency response. Replacing it will improve the audio greatly.
Keep us posted on your great work!
Terry N3GTE

[w1vtp]

Ed did like to keep a low profile.Haven't seen him since the early '90's maybe. It's been a long time.

<snip>

You probably know this but if you are using the stock driver transformer it has very poor frequency response. Replacing it will improve the audio greatly.
Keep us posted on your great work!
Terry N3GTE

Thanks for the info, Terry.  Now I'm putting out the call for the Hammond transformer part number for that driver transformer.  Might as well do that or maybe there is another modification in the wings but haven't heard anything concrete from a friend of mine about that.

PTT circuit is next

Al

[N3GTE]

Don't know what you are thinking for the ptt circuit but I have some relays that are like the ones used in the later Johnson tx's. The coil is 10K and are used in the plate circuit.
Terry

[W3GMS]

I've got a Viking 1 that has a 829 in the final. Was talking with a friend and he was nice enough to give a new 4D32 to convert it back. I used to run it stock back years ago. Once I was talking to Pete the mm he said that it had very piercing audio. It's on my todo list. BTW any of you guy talked with Ed KS3K lately?? He change his call to K4CWM years ago. Real knowledgeable and very nice guy!
Terry

That KS3K call has quite a checkered past, doesn't it?  He's all but pulled the disappearing act.  I do wonder why he added the 220 mfd cap at the cathode of the 6C4 to ground.  Seems to me that negates the neg.. inv... feedback

Ed has nothing on his bio and no email.

Al

Al,
That appears to be a mistake in the schematic.  I have not run the calculations, but I can't believe any audio is appearing across that 220 MFD capacitor.  

Joe, GMS

[w1vtp]

That KS3K call has quite a checkered past, doesn't it?  He's all but pulled the disappearing act.  I do wonder why he added the 220 mfd cap at the cathode of the 6C4 to ground.  Seems to me that negates the neg.. inv... feedback

Ed has nothing on his bio and no email.

Al

Al,
That appears to be a mistake in the schematic.  I have not run the calculations, but I can't believe any audio is appearing across that 220 MFD capacitor.  

Joe, GMS

Yeah, Joe.  If that cap is really in there, I'm for taking it out and seeing if it plays better.  I wonder if anyone else has use this audio mod on their V1 or V2's.  I'd be interested in any input.

Al

[W3GMS]

Been missing you on the AWA AM net OM.  Can't wait to hear the Viking and I am sure the cap needs to go.  You may have to adjust the amount of fb with the series resistor in the FB circuit.  Also sometimes a bit of RF bypassing is necessary depending on how much RF sneaks pass the plate RF choke. 

Have fun.....

Joe, GMS
 

[WD8BIL]

Al.......if you still have it open check the bias voltage divider resistors. It might be a good idea to change them out also. I went to 1 waters for added security.

[AB2EZ]

Al
Joe

I looked at the schematic by KS3K for Viking 1 and 2 audio modifications on the AM Window Web site:

http://www.amwindow.org/tech/htm/vikinga.htm

The value of the 6C4 cathode bypass capacitor is probably supposed to be 0.22uF (-j241 ohms at 3kHz). Its purpose would be to roll off the gain around the loop, to less than unity, at higher audio frequencies... before reaching audio frequencies where there is too much extra phase shift introduced around the feedback loop (causing instability problems). It may not be a good idea to remove this capacitor if the value is 0.22uF.

It might, however, be a good idea to reduce its value to something like 0.1uF, in order to extend the flat portion of the frequency response of the modulator to higher audio frequencies, and to extend the linearity improvements of the feedback to higher audio frequencies.

One must make this change cautiously because of the possibility of introducing instability due to excessive phase shift in the open loop amplification chain at those higher audio frequencies.

Quantitatively:

The impedance at audio frequencies, looking into the cathode of the 6C4 (with the larger feedback loop associated with the 500k ohm resistor, RFB, not connected to the cathode) is approximately: 1/the transconductance of the 6C4 = (approximately) 450 ohms, in parallel with the 1.5k ohm physical cathode resistor. Therefore the impedance looking into the cathode of the 6C4 = (approximately) 350 ohms.

The audio frequency feedback ratio, which is: the input impedance looking into the cathode of the 6C4 / 500 k ohms (i.e. RFB), is not significantly affected by a 0.22uF bypass capacitor until the frequency is 2067 Hz... which is the frequency where the impedance of the bypass capacitor is -j350 ohms.

Below that audio frequency, the feedback ratio is 350 ohms / 500000 ohms = 1/1429.
Above that audio frequency, the feedback ratio is approximately (1/1429) x (2067 Hz / f).

Let's assume that the open loop voltage gain from the cathode of the 6C4 to the output of the mod transformer is around: 15 (for the 6C4 stage) x 100 (for the 807 stage) x 1.5 (for the modulation transformer step-up) = 2225.

Then, at audio frequencies below 2067Hz, the gain around the loop is 2225/1429 = 1.56.  

Furthermore, instability (positive feedback) is avoided if the open loop phase shift is not too large for frequencies below f= 3200 Hz. This is the audio frequency at which the gain around the feedback loop drops to 2225 x (1/1429) x (2067 Hz / 3200 Hz) = 1

Stu

That KS3K call has quite a checkered past, doesn't it?  He's all but pulled the disappearing act.  I do wonder why he added the 220 mfd cap at the cathode of the 6C4 to ground.  Seems to me that negates the neg.. inv... feedback

Ed has nothing on his bio and no email.

Al

Al,
That appears to be a mistake in the schematic.  I have not run the calculations, but I can't believe any audio is appearing across that 220 MFD capacitor.  

Joe, GMS

Yeah, Joe.  If that cap is really in there, I'm for taking it out and seeing if it plays better.  I wonder if anyone else has use this audio mod on their V1 or V2's.  I'd be interested in any input.

Al

[WD8BIL]

I have the other Viking audio mod in the Viking Bud. At the cathode of the 6C4 is a 1K resistor with a 10uf bypass.
Seems to give plenty of hi frequency response. Last checked a few years ago IIRC the sidebands were down in the -30's dBc at 5Khz so it's not too wide.

Can I get a witness???

http://www.amwindow.org/tech/htm/viking2.htm

[AB2EZ]

Yes... but the negative feedback (if you have any in this design) is eliminated by the 10uF capacitor... unless you are injecting it somewhere other than at the cathode of the 6C4.

Stu

[WD8BIL]

Hi Stu......... yes, I deleted the bypass on the cathode of the second 12AX7 stage and ran the NFB from the output of the 807s all the way back to there.
Seems to work and sound as I wanted.

[AB2EZ]

Bud

Okay... that sounds like a good way to do it.

Best regards
Stu