Bauer conversion 3.805 poor audio performance

[kc4mne]

Well When I modulate my carrier it sound horrible. Attached are scope screen shots of a carrier modulated with a 1000 hz tone. Obviously something is wrong because the vertical lines that look like a comb should not be there. What is causing that??

This conversion is driving me nuts!

[KA2DZT]

Some sort of oscillation is my guess.

I'm also thinking it looks like two signals beating together.

You're right, shouldn't be there.

Fred

[DMOD]

Possibly RF feeding into the audio path?

If either C-39 and C-40 were shorted you might have cross-coupled oscillator or flip-flop operation going on at a different frequency.

What's the frequency of the two waveforms?

From here, it looks like a lower frequency signal of higher amplitude mixed in with your 1kHz audio. Could it be 60Hz not getting completely filtered out at the 4-400 grid-bias pot (wiper of R-27)?

Phil - AC0OB

[KA2DZT]

One thing I'm seeing is that the anomaly only seems to be present on the positive half cycle of the audio signal.  Maybe a feedback problem in the modulator.  Or a high audio frequency oscillation that occurs only on half the audio waveform.  As in only one of the modulator tubes along with its separate driver tube.

Fred

[W7TFO]

It looks like ringing to me, might check the mod transformer for a short.

73DG

[k4kyv]

I had exactly the same problem years ago with the 810 modulator in my HF-300 rig.  The modulator is taking off into parasitic self-oscillation on peaks.  I cured it by inserting a 100-ohm 5-watt non-inductive resistor at the plate cap of each modulator tube.

It may be triggered by rf, since it appears to occur only on positive peaks. That happened in my case, too.

My attention to the problem was called when I began to get reports of severe splatter up to 30 kc/s each side of the carrier.

I recall an article in a 1930s magazine recommending neutralising a push-pull modulator just the way you would neutralise a push-pull rf final.  Never tried that.

[W2ZE]

Don is right on the money with this one. many b'cast rigs were designed with the mod tubes next to the RF tubes. This is ok down in the broadcast band, but when you start getting higher up in freq. this problem can start rearing its ugly head. Also check the neutralization of the RF amp to make sure its not self oscillating.

[flintstone mop]

Yup I think Don got it.
The BC rigs might tolerate 160M ok ,and many don't mind 75M. Usually 40M is a bad thing in a BC rig. The long cabling inside the box wasn't designed for short wave use.
You got a helluva transmitter OM.........

Fred

[k4kyv]

The long cabling inside the box wasn't designed for short wave use.

And some of those wire leads in the rf section are long. The BTA1-R that I parted out had some components in the PA output network strung together with wires about 2' or more long.

But what takes the cake is my BC1-T.  The rf output from the crystal oscillator module was routed to the grids of the 807 rf drivers using a single unshielded wire embedded in the wiring harness along with filament, audio and DC wiring. It was flaky on 160m even though the book says the transmitter is supposed to be good up to something like 2200 kc/s. I replaced that unshielded wire with a piece of scrap RG-58 or 59, and that tamed it down.  807s tend to squirrelly even under the best of conditions.

[KE6DF]

I had exactly the same problem years ago with the 810 modulator in my HF-300 rig.  The modulator is taking off into parasitic self-oscillation on peaks.  I cured it by inserting a 100-ohm 5-watt non-inductive resistor at the plate cap of each modulator tube.

Just tonight I ran across a good discussion of this problem in the 1937 Frank Jones handbook. He described the parasitic oscillation on peaks and mentioned curing it with a series resistor in the plate lead -- as Don suggested.

[KB5MD]

While you're at it, make up some parasitic suppressors for the RF plate leads.  I had problems with my BC transmitter producing parasitics and overheating the tank coil.

Just a little extra insurance.

[KL7OF]

While you're at it, make up some parasitic suppressors for the RF plate leads.  I had problems with my BC transmitter producing parasitics and overheating the tank coil.

Just a little extra insurance.

My BC1G has Parasitic suppressors (2W 47 ohm w/4 turns of #12) on each plate lead

[kc4mne]

Well, I put 100 ohm non inductive resistors in series with the plate connection to each modulator tube and built coil / resistor suppresors for the rf side. The problem is still there. I don't believe I have a problem on the audio side. On 160M the scope picture is perfect. I built two parallel inductor / variable cap networks one goes between the buffer tube and the driver and the second between the driver and the grids of the two finals. The one between the buffer tube and driver I have the inductor and the cap in parallel, both of them wired in series between the plate of the buffer tube and the grid of the driver. From what I can make out off a picture in the internet of a similar conversion, it appears that the var cap is being placed between the grid of the driver and ground and the inductor is placed in series with the buffer plate and driver grid. What is the correct way?  Thanks KC4MNE.

[KA2DZT]

How does the RF waveform look without modulation??  Could you let us know on what frequency you are having the problem, might help.  You say on 160M everything looks good.  Have you tried to use some different audio freqs to run your tests, maybe 200-400hz and maybe 3K-4K to see if anything changes.  How does the waveform look with less modulation, maybe 50%??

I would also suggest that you take a close look at your test set-up including cables to be sure there isn't a problem there.  Also, try another scope and a different audio generator.

How are you sampling the RF??  Maybe move the scope away from the xmtr and try to sample your signal off a receive antenna to see if it's the same.

Are you using a dummy load on the xmtr or testing into an antenna (not likely)??

It wouldn't be the first time I've seen very strange problems being caused by the test equipment.

Fred

[k4kyv]

If you have a good bench scope, try looking at the waveform at the grids of the PA tubes and modulator tubes. If it's a parasitic in the rf final or modulator, that might tell you something.

[kc4mne]

Think I got IT  Changed my tuning networks to look like the one's in the Collins 20V series schematics.

[kc4mne]

Rig is working    Thanks for helping

[KA2DZT]

Well are you going to fill us in as to what you did to solve the problem??

Glad you fixed it.

Fred

[kc4mne]

Problem was not related to the "audio" side.

The official technical answer would be that I did not have enough Grid Drive to have the 4-400 rf amp in class C, plus an incorrectly configured coil/v cap network between buffer and driver tube.

KC4MNE

[ke7trp]

I think that is why George w8qBG changed out that Driver tube on his. Its rock solid on all bands.  Glad you got it working!  Hope to work you some day!

C