Bauer 707 has very suprious output when modulated

[kc4mne]

My Bauer 707 that I just converted to 75M has very spurious output. When I transmit I am all over the band!!

Here are two spectrum analyzer screen shots, with the analyzer set to hold peaks . As you can see the dispersion is set at 100 khz per division.
 
One is the carrier only which is clean, one audio is intoduced the mess begins !!
 
Any ideas how to cure or why this is happening?

KC4MNE

[KA2DZT]

Thought you had the rig working.

It may be related to your original problem.  Seems it is something with the modulation,  possible oscillation or ringing.  W7TFO originally thought it was a bad mod xfmr.

I'm sure you'll get more answers on this one.

Fred

[WQ9E]

What are you using to provide the excitation frequency? Given the total dispersion I would check for FM of the frequency control element (external VFO I presume).

[W2VW]

Nasty.

It would be interesting to see what happens with increased grid drive.

[flintstone mop]

A previous post was that he changed something in the Pi-network in the final, that fixed it.
But looks like problem still there under modulation. hmm
75M gets dicey in some rigs

OK Test!!! How did the TX run on it's original freq?? And a move to 160M,,,did ok there?? Or was this a mighty leap to 75M?

[DMOD]

Did you make any changes to L7,8, or L9, their taps, or the tuning caps?

At first glance, it looks to me like you have some assymtrical filtering in the output network.

Phil

[kc4mne]

I think it has to do with neutralization. That is what was causing my prior issue. My grid drive varies from the standby position (4-400's with no plate / screen+ ) to tx position (4-400's on). Problably varing with modulation as well.  Without the neutralization cap in place it is even worse.  Could be mi Pi output also, but a non modulated carrier is spur free.

[KB5MD]

I would try disconnecting the driver at the blocking cap and running the exciter directly to the 4-400's with the vacuum variable shown in the diagram disconnected also.  That is the way I have my RCA and it has 4-400's also.

[KC2ZFA]

The neutralization connection seems all wrong...as is it provides feedback from
the plate of the PA to the plate of the driver. Can this be right ?

[KA3EKH]

Your analyzer is set to 100 KHz per division, going to assume you have the analyzer set to 10 Db per division and looking at the modulated pattern that giving you spurious down ten to twenty Db one hundred to two hundred KHz out from the carrier and that’s hard to do unless you got some really nasty parasitic oscillations or some other evil in your PA tank.
Are you cretin your not hitting the input of you spectrum analyzer too hard and causing its front end to splatter? I would not know how to make a transmitter that wideband and powerful over that large a frequency so that’s why I am questioning your test setup. Don’t mean to be offensive but the display you have doesn't look possible, or at least anything like I have seen. Is the pattern you're getting with voice or a fixed tone? What percentage of modulation is it?
What does your overall efficiency of the PA look like? I would assume that if there are serious issues in the PA it will result in poor efficiency. Hate to distract from the neutralization discussion knowing that is the collect wisdom from the great minds.
RF

[DMOD]

Was there a neutralization problem on say 1MHZ before conversion?

I see no neutralization circuity (unless I missed it somehow) in the original schematic.

In your schematic the 5-100puff is not neutralizing but simply taking the final's RF to ground, depending on the caps Xc and its setting.

I suspect you have some weird resonance due to the cap lead inductance and the cap.

So did you replace the 6CA7 with a 6146?


Phil - AC0OB

[KE6DF]

I still seems like it could be some kind of ultrasonic oscillation in the modulator -- that only shows up under load.

[KA2DZT]

Driver stage doesn't look right.  You should have the grid coil on the grids of the PA then the RFC then any bias or grid leak resistors (or both) then the meter to ground.  The neutralizing cap gets connected the same way you have it.  You made need to reduce the value of the by-pass cap at the cold end of the grid coil (where the neutralizing FB voltage is applied) or you will by-pass all the FB voltage to ground.

Your grid circuit may or may not be the cause of you main problem.

Fred

[KA2DZT]

The neutralization connection seems all wrong...as is it provides feedback from
the plate of the PA to the plate of the driver. Can this be right ?

Peter

The plate circuit of the driver should be the same phase as the grid of the PA.  But I think the whole driver and grid circuit could be improved from the way he has it set up.

Fred

[KA2DZT]

In your schematic the 5-100puff is not neutralizing but simply taking the final's RF to ground, depending on the caps Xc and its setting.


Phil - AC0OB

I agree,  the neutralizing cap is way too large (5-100pf).  Add in the 1000pf by-pass cap at the grid coil and what you have is a second plate tuning cap.  The neut cap thru the plate blocking cap is now in parallel with the PA plate tuning cap.

Your neut cap needs to be much less and the by-pass cap something less than 1000pf.

The neut circuit is a voltage divider off the plate of the PA.  You have a voltage divider but the overall impedance is way too low.

Fred

[w1vtp]

Looks like an oscillator to me.  Effin I was going to neut.. a 4-400 (and I wouldn't, at least for the lower bands).  I would put a balanced tank in the grid circuit and pick off the neut.. cap at the bottom of that tank.  I'd use a much smaller neut... cap - maybe even a gimic cap made out of two buss wires on standoffs.

Where the pick off is now - no telling where the phase is especially as one tunes the driver for a dip.  I'd isolate the driver tank and put in a grid tank that's balanced, fed in the center with the bias.  Then settings wouldn't be as squirrely as I think this one will be.

Take a look at a National MB40 and see how that was designed for a hint on now to do things in the grid.  You really do not need to be as fancy as this but it gives the right idea. Heck, I'd even be tempted to reverse the MB40 and feed the output with a link coupling from the driver tank.  In fact, I believe that Bob W2ICQ  uses this approach all the time with his high power finals. Look at fig 5 on page 4.  You can get rid of the bottom tube as you are not using push pull.  Just be sure to pick the bottom of the grid tank to give you the right phase when hooking up the neut... cap.  This will give you the right setup to do your neut... thing - although I don't think you need it at 75 meters.

Al

PS:  Just looked at page 4 again and fig 3 fits your application

[DMOD]

Are we talking about the Bauer 707 schematic on BAMA? I don't see a 6146 anywhere?

[ke7trp]

http://www.arizona-am.net/PHOENIX/W8QBG/Bauer/index.html

[W7TFO]

Good advice.  Go and do what George did.  It works FB.

73DG

[flintstone mop]

How are things going with the Bauer??

The last post by Clark is a great link for "multi-banding " your  box.
BUT before you get deeper in Zchit how did this transmitter work on its assigned BC freq.??

The 6146 was recommended to give more ooomph drive on 40M. But watch how much you drive the 6146....2.5Ma??

And for neutralizing..........a Tetrode does not need neutralizing.......Or is foot in mouth happening to me??
The schematics from the 'mods link' do not show any neutralizing circuit for the RF P.A.....there is a tuned circuit for the grid of the 6146 Driver.

I did a 'bull-in-a-china-shop' method of driving a McMartin BA-1K with an antenna tuner connected where the driver used to go and feed the antenna tuner with a CW input from a transceiver..........need about 20 watts of juice.
Worked FB OM. The count-down method used in the original exciter was bad engineering and added complications. Soooo, I provided raw RF power to the grids of the 4-400's mentioned above. TX was good all the way to 40M. And 130% Pos peaks easily. The restricted audio response of the McMartin, with that tiny oil filled mod transformer, caused me to pass it on to someone else. Typical short cut 1980's transmitter. Pretty 'furniture' but NO BALLS for audio.
http://www.youtube.com/watch?v=bU5sqjfBomw
Fred

[Steve - K4HX]

I like this one better, especially when the maul is keyed!

http://www.youtube.com/watch?v=grgLXWdd9GA&feature=related

[flintstone mop]

Good one Steve
I sent Tim a DVD before shipment to demo Ray. I had it in a dummy playing a Little Richard oldie with the nice bass and that audio driver with 4 -2A3's pushing the 810 modulators looked so beautiful on the 'scope. There was more on the DVD than what Tim uploaded to YouTube.
Even loaded to 300watts out, the Raytheon was not afraid to modulate the carrier.
RIP ....Tim

[kc4mne]

I removed the neutralization cap and the spurs went away!

[KA2DZT]

I removed the neutralization cap and the spurs went away!

Great,  glad you're making progress

[flintstone mop]

Now it is time to make the Bauer sing for its supper...........we'll be waiting for an announcement of the on-air test.
Congrats......should be a good transmitter.

Fred