Carrier Shift, FMing, SSbing... Warble.. on AM Transmitters.

[ke7trp]

This morning we had an interesting QSO on 40 am.   Every once in a while someone comes along and complains that I am FMing.  We then tested this by:

turning Receiver to LSB.  Then zero beating the other persons carrier.. Then setting it just so you hear the tone start.  As the operator speaks, the carrier is shifted and you can hear it.  Some are slight.. Some have alot of shift.

I cant hear my own Globe Champion.. However,  They tell me I have a HUGE amount of shift. So bad, that they cannot tune me in on SSB. It moves to much.  On AM, I am 7 to 8 KC wide with Fantastic clear audio.. 

The Champs VFO turns off when you unkey and it has to come back on each time you transmit. This means you start low and roll up to the FREQ you want. 

What is acceptable?

Is this a normal thing with an old Class C rig of this design?

  If AM is Clear and nobody has any complaints on my 1954 Globe, Should I try to fix it?   Its only when a guy has a SYNC detector or trys to listen to me on a side band during bad QRM does he notice this.

Clark

[DMOD]

Since I don't have aschematic of the GC, do you mean the filaments are switched on and off or just the B+?

Do you have separate VFO's for TRX and REC?

Is there a simple way to keep B+ on and load the VFO during receive?

Does the GC have provisions for an external VFO. Maybe a solidstate PLL version would help.

Phil - AC0OB

[ke7trp]

Fils stay on.. But I was told the VFO is switched off and not oscilating.  This can be seen when you key up.. They sweep up 2 to 3 KC, This causes a woosh noise on the band.  They all do it. 

From what I understand from talking to my elmer.  Champs had this problem in the 50s.  In the 60s people came up with all kinds of ways to fix it.  He told me it was poor regulation causing voltage drop as you speak. 

He said this is not so much of a problem. Its that now, People have MUCh better Recievers that are showing this slight FM'ing and they are bringing it to your attention. 

[Steve - WB3HUZ]

The FMing not normal but not uncommon. I had the same problem with my Chump. Just about any older VFO will slide onto frequency when you key the rig, so don't worry too much about that unless the slide is very large. If you haven't already, consider replacing all the caps in the VFO with good NPOs. This usually fixes most drift and FMing problems. You may also want to put a new 6AU6 in the VFO.

The FMing will likely be a little more problematic on 40 meters since the VFO is on the same frequ as the rest of the TX (no multiplication). So, if it persists, look at the bypass caps and feedthoughs, if there are any. The VFO needs to be well isolated from the rest of the TX to avoid FMing.

This morning we had an interesting QSO on 40 am.   Every once in a while someone comes along and complains that I am FMing.  We then tested this by:

turning Receiver to LSB.  Then zero beating the other persons carrier.. Then setting it just so you hear the tone start.  As the operator speaks, the carrier is shifted and you can hear it.  Some are slight.. Some have alot of shift.

I cant hear my own Globe Champion.. However,  They tell me I have a HUGE amount of shift. So bad, that they cannot tune me in on SSB. It moves to much.  On AM, I am 7 to 8 KC wide with Fantastic clear audio.. 

The Champs VFO turns off when you unkey and it has to come back on each time you transmit. This means you start low and roll up to the FREQ you want. 

What is acceptable?

Is this a normal thing with an old Class C rig of this design?

  If AM is Clear and nobody has any complaints on my 1954 Globe, Should I try to fix it?   Its only when a guy has a SYNC detector or trys to listen to me on a side band during bad QRM does he notice this.

Clark

[K1JJ]

Clark,

Load the rig into a dummy load and get out your scope and VOM. Put a digital freq counter on the output to know exactly what the drift is. Key the rig on and off and look carefully at the DC voltages on the VFO and buffer as they settle in.  Make sure it's not a voltage regulation problem - cuz that's an easy one to fix. You can drop in a zenor diode/resistor regulator circuit  or two at the right spots and cure it sometimes.  

Also make sure your buffer is remaining stable in current and voltages too, cuz it can easily pull the sensitive VFO tube osc.

As Steve said, look into NPO caps too if the drift is temperature/ keying warmup based.  Also look at ways to keep the VFO on all the time but still breaking the signal path so you don't hear it in the receiver.  The keying VFO is the obvious reason it is settling in every time on key-up.

T

[ke7trp]

THanks for the tips.  Can you guys download the schematic I posted and have a look at the VFO regulator. I dont see any electrolitics there. I thought about adding a cap in. That might help things out.  The VFo voltage is rising and falling when I modulate the rig. I think this is the source of all my trouble. 

My Elmer rememebered that in the 60s' there where all kinds of articles on ways to fix this in the champ and other rigs. I am curious to see those articles if anyone has them! 

I need a "what would you do" type of post with suggested on this circiut. I am sure I can fix this thing.. If I can get that voltage to not sag when I modulate it, the problem will be resolved.

Thanks for the tips!

C

[Jim, W5JO]

There are several reasons for this problem, low grid drive, poor voltage regulation, etc.  Another thing is proper loading of the final.  If you over or under couple you will have the problem pronounced.  If you note the plate current bouncing downward as you speak, it may mean you are over driving the final with audio.

Be absolutely sure you are properly loaded into your antenna and only to 300 ma. and not less than 275.  Then don't drive the Mod meter past 200 ma. at all.  Follow the manual directions to set the grid current.  Change the VR tube supplying the VFO just to see if things will change.  If everything is just right you will see the meter in the plate position just barely wiggle.  That is when things are correct provided your Champ is good electrically.

The HV is switched off when you release PTT.  If you want this to change, there will be major wiring alterations.  Do check to be sure your VFO, buffer and driver tubes are good.

[ke7trp]

THanks alot for the tips Jim. I load to 290 ma.  Flat, Into a dummy or antenna. Right at the dip. I run 12 ma.. It says 10 to 15 ma Grid. I run it in the middle.  If I turn it up to 15 I get alot of peak power increase but no real carrier increase.  The mod meter swings to 200 only on peaks. I use a scope to set audio.  I do get some downward swing on the plate meter.. I lowered audio way way down on my test and its still FMing.

C

[Jim, W5JO]

Then you may have some modifications that are eating your lunch.  My 300A plate current just flickers at 300 ma. with 10 ma. drive and modulation peaks of 200 ma.  Check to be sure all the circuits are correct.  Are the couplates still there? 

[N2DTS]

I cant read the diagram, is the vfo voltage regulated?
By a VR tube?
Does it light up?
What is the current through it, the little ones want 30 ma, the bigger ones do 40 ma.
You could put a small electrolytic after the regulator tube to help.

If its not regulated, it should be!

I dont think you want the vfo active on receive, since that will be what you will hear.

Maybe solid stating the low voltage would help with the regulation.
 
Brett

[ke7trp]

Rig has new electrolytics.
Solid state plug ins.. Maybe go to 3b28s or 866s? This gave me a big power increase so I left it.
Couplates in place but bypassed for better audio.

Checked all tubes except the VFO tube 6au6 and the 0a2. They are INSIDE the VFO which is SOLDERED ON!  What a pain in the scrote.  Going to have to get a Trigger iron to get that box off. My weller desk top iron wont budge those huge globs of 50 year old solder. This leads me to believe they have never been checked in all these years! 

I am going to try a cap after the OA2 VFO regulator.. 22 uf at 250 volt should be good right? There are NO caps in there now.. Just 150 volts out of the oa2. I bet a Cap will fix this.

This rig has worked great for 2 or 3 years.. ALways FM'd.. Its the way they where.. I just want to upgrade it so it does not do that anymore. To many Modern RXers out there listening to me pick up on it. 

I have Valiants that are working. I can toss one inline and play it on 160/40 while this work is going on.  I did an hour of cleanup with a paintbrush.  Going to oil the fan, Clean switches now and wait for a response on Cap size after the regulator.  Friend is bringin me 100 watt Trigger iron over.

C

[ke7trp]

Before I take a torch to this old solder on the Copper BOX..  I want to make sure.. ARE the TUBES in the copper box or are they inside the Metal box? 

To remove the copper box, I am going to have to torch the tabs and remove the solder.  To remove the entire assembly will be a real bear.. Tons of wires, knobs, panel, gear Drives ect ect...

Please tell me its just under the copper box!!

C

[Jim, W5JO]

The 6AU6 oscillator tube is beneath that box.  If you have adequate grid drive on 10 meters, I wouldn't worry about it.  15 Ma. or so tuned up and running on 10.

[K5UJ]

I have a WRL 755 vfo FWIW and it also has the copper cover.  Here's a photo of it with the copper shield off.  Inside is the big air var. cap, two smaller trimmer caps, two inductors, a few small components and a 6AU6.   I figure Leo probably reused as many circuits as possible so....

73

Rob
K5UJ

[ke7trp]

Ok... On the VFO plug there is supped to be 250 volts there.  I have 286 volts.. Probably because its not on a variac right now and I have nearly 130 volts at the wall.

When I modulate 100%, This drops 5 to 6 volts.

When I added a 47 UF cap to Ground,  I get 2 to 3 volts Drop.  Much better on the FMing..  I dont have larger caps.. Should I buy a 100UF cap and try that?

C

[N2DTS]

While I was listening last weekend, I heard someone FM'ing and also jumping in freq, not a lot, jumping like 100 Hz.
The only time you notice this stuff is with the sync detector.

Still, its nice to have a somewhat stable signal.

Brett

[ke7trp]

Yeah..   Dave, PBJ noticed it with his Sync detector.   

I am going to try 100UF on the VFO line.  That should get me down to 1 to 2 volts Drop max.  Also. The main supply has a 6 UF cap.  Thinking of adding two 22 UF caps at 500 volt across the oil can.  This will net me 18 UF total.  That should help even more.  Anyone have any objections? 

I got all the tubes tested, the fan oiled, a major cleaning and Deoxit on the switches and controls.  Glad I took it apart. 

C

[Ed/KB1HYS]

100 Hz jumping around at 7Mhz?  That's a pretty small error. 

How do you define "somewhat stable"? 

[DMOD]

A couple of other ideas from looking at the schematic:

Anything that changes bias on the screen grid (tube pin 6) or control grid (tube pin 1) can cause the VFO tube to act as a reactance modulator.

I noticed that the VFO control grid, 6AU6 pin 1 is biased by the keying circuit, a function of Pin 1 of the 12AU7 and R6. You might want to check voltages there as well to look for any voltage swings. (Ref. pin 3 of the 4 pin connector).

I know of some hams who have replaced the gas regulators (0A2) with a 150 volt or so zener. You might have to change R4 to slightly different value if you use a zener.

C7 is where I would up the capacitance (350VDC electrolytic) if you have room.

Phil - AC0OB

[w3jn]

You do NOT want to put an electrolytic after the voltage regulator.  This will make a nice relaxation oscillator and make things a LOT worse 

One caveat about the capacitors - if you're replacing them with NP0s make sure you figure out which one is the temp compensating cap -that should be a N750, not a NP0.  It's usually a 10 -20 pF or so cap in parallel with the coil.

[ke7trp]

I am placing the Caps BEFORE the regulator on the VFO plug under the chassis.  That should be just fine right?

C

[w3jn]

Yep, no problem there, so long as it's before the dropping resistor to the VR tube.

THere's all manner of things that can cause this.  I think you're gonna hafta bite the bullet and pull the copper cover.  WHile you're in there, clean the variable cap bearings and tube sockets with a bit of DeOxit on a q-tip.  Don't be tempted to douche it down with DeOxit - a dab will do ya fine.  Make sure any bandswitches associated with the VFO are clean.  Shove new toobs in there - that's the most likely source of your trouble.  If it persists, replace any bypass caps with ceramic discs.  Postage stamp micas can be replaced with NP0 ceramics, or those dipped silver micas. 

[w1vtp]

Regulating the VFO is really the answer.  Especially the screen in the osc ckt.  Anyone with a Flex can define your carrier shift as you improve you can get fairly accurate readings over the air.  Another sign of more subtly FM AM transmitter is that one sideband is higher then the other.

GL but the only real answer is to regulate the VFO osc.  That supply voltage shouldn't change at all.  That's why the Eico 722 doesn't have that problem.  It has its own PS

Al

[K5UJ]

What about those little glass tube components?  I think they are small h.v. capacitors?   I've wondered about them.  They're the little glass tubes with wire on each end and some kind of fluid dielectric?  I think they were used for h.v. 60 years ago because they were small but I'm not sure about that.  There are a bunch of those in my 755 VFO so I figure they are probably in the 300A VFO also.  

Rob

[The Slab Bacon]

You do NOT want to put an electrolytic after the voltage regulator.  This will make a nice relaxation oscillator and make things a LOT worse 

Johnny,
           I was thinkin about that, but you beat me to posting it. I did that some years ago and it really went to town! ! Depending on the size of the cap, you could actually watch the VR tube pulse on and off.