Transister Modulator Assistance Needed

[W3GMS]

So I mis-typed above and decided to go in an start rebuilding the Output-modulation bias section as required...

My first part to pull and check was R63 (2200 ohms). It read 2800 ohms... so I installed a 2.2k and that made a couple changes.

The Emitter voltages are still about 15v on both Q12/Q13 transistors.


Now the modulation and collectors look like this after the R63 change:



My next targets are R64 (47 ohm) and R65 (Thermister  33 ohms cold)

Scott,
The top photo of showing the sine wave vs. the modulated waveform leaves me with a question.  Where was the probe that shows nearly a perfect sine wave?  Also do you have any scope pictures of early in the audio chain?
The other pictures in that same set of three shows some highly distorted waveforms so I am a bit confused as to where you measure the clean one in the top photo.  
Thanks,
Joe, GMS  
  

[W3GMS]

Another interesting probe point would be the primary winding of the driver xmfr.   If its clean at that point then you can forget about anything earlier in the audio chain. 

The secondary of the modulation transformer is another good probe point. 

In the end, the final amplifier has to be capable of producing 16 watts of PEP output power for 100% modulation assuming the carrier output without modulation is 4W. IF the final runs out of headroom you will never be able to have clean positive peaks at the 100% level.   I am getting ahead of myself but just looking down the path as you discover details about this particular transmitter.         

Joe, GMS

[Pete, WA2CWA]

The final output transistor, 2SCF8, was typically rated for about 3.5 watts out. You can squeeze more out of it, but audio quality (given what it is) will suffer.

[N2DTS]

I thought the clean looking waveforms were the reference (input)....

[W5EFR]

So I mis-typed above and decided to go in an start rebuilding the Output-modulation bias section as required...

My first part to pull and check was R63 (2200 ohms). It read 2800 ohms... so I installed a 2.2k and that made a couple changes.

The Emitter voltages are still about 15v on both Q12/Q13 transistors.


Now the modulation and collectors look like this after the R63 change:



My next targets are R64 (47 ohm) and R65 (Thermister  33 ohms cold)

Scott,
The top photo of showing the sine wave vs. the modulated waveform leaves me with a question.  Where was the probe that shows nearly a perfect sine wave?  Also do you have any scope pictures of early in the audio chain?
The other pictures in that same set of three shows some highly distorted waveforms so I am a bit confused as to where you measure the clean one in the top photo.  
Thanks,
Joe, GMS  
  

Joe, that is Tee'd straight off the Signal Generator and into channel 2 of the O'Scope, the other end is feeding the external modulation input on the Rig

[W5EFR]

Yes, switch the leads from the xfmr to the collectors.  This will put the flat topping on the negative peaks,  if the problem is in the audio chain.  OTH if you end up with the same conditions as you have now then the problem is in the RF output.  Most likely something in the design to prevent positive peaks running over 80%.

Another thing to look at is the power supply B+.  Put a scope on the B+ and see what it looks like at full modulation.  It may be sagging on positive peaks.

Fred

Copy all.. I'll try that wire swap on the transformer tonight

Also, I'll check the B+, last time I checked it, it was sagging to 12.3V under modulation... I'm thinking about removing the 2 diodes in the rectifier, lifting the center tap from the secondary on the power transformer and dropping a full bridge in the place to try and to get the B+ up...

I already replaced the multi-section single can 100uf/30v power filter caps with 1000uf/100V... due to 250mVAC of ripple on the power

[KA2DZT]

The schematic shows a B+ of over 16 volts.  12 volts seems low.

[W5EFR]

The schematic shows a B+ of over 16 volts.  12 volts seems low.

Agreed..

Removed the Full Wave Rectifier last night and dropped in a Bridge Rectifier...

Got 16v on one side, but 6 on the other... before the Bridge Recto got a tad hot...

I have to figure out this retarded power plug jumper wiring ...

I also swapped the Collector feeds to the Modulation Transformer... no Change...

[Pete, WA2CWA]

You might want to disable or disconnect on-board power supply and use a bench supply to do the testing.

[KA2DZT]

The schematic shows a B+ of over 16 volts.  12 volts seems low.

Agreed..

Removed the Full Wave Rectifier last night and dropped in a Bridge Rectifier...

Got 16v on one side, but 6 on the other... before the Bridge Recto got a tad hot...

I have to figure out this retarded power plug jumper wiring ...

I also swapped the Collector feeds to the Modulation Transformer... no Change...

No change,  seems the problem is in the RF or more likely something with the power supply.  As Pete suggested, use a solid bench supply if you have one.

Fred

[W3GMS]

Scott,

After you get the P.S. issue resolve take a look at the collector of Q9,10 and 11 and see what the quality of the sine wave looks like.  Make sure your not over driving the first stage when you take those measurements.  Ideally you should have a nice clean sine wave prior to the driver transformer since after that, things don't look very good.  If you see clipping of the negative or the positive peak of the AF waveform while driving the input with normal mike level you then can zero in on whats wrong in that particular stage.  Another test to do is to disconnect the secondary of modulation transformer and put a resistive load on it to simulate what that the final amplifier impedance is.  You can get a pretty good approximation by knowing the steady state collector voltage divided by the steady state DC current feeding the final amplifier transistor.  Lastly the range boost circuit is well filtered but when the pot is adjusted to zero ohms the first rectifier and cap act as a peak detector.  That then determines the bias voltage on the first stage.  So you end up with the voltage being a result of how much feedback the range boost circuit offers.  It may prove interesting to lift that and just apply a fixed bias voltage to the first stage for testing purposes.

Having fun yet?   

Joe, GMS       

[W2VW]

Is the RF driver stage modulated?

[W3GMS]

Hi Dave,

He did not post the entire schematic but I was under the impression that its just a conventional Class C final modulating the collector voltage going to the final transistor.  It should modulate cleanly if done right but who knows.  The final might not be able to support the extra voltage during the positive peaks of the audio voltage.  I still want to see the earliest point in the audio chain where the distortion is occurring and with some more probe points that should tell the story.

Joe, GMS     

[W2VW]

Hi Dave,

He did not post the entire schematic but I was under the impression that its just a conventional Class C final modulating the collector voltage going to the final transistor.  It should modulate cleanly if done right but who knows.  The final might not be able to support the extra voltage during the positive peaks of the audio voltage.  I still want to see the earliest point in the audio chain where the distortion is occurring and with some more probe points that should tell the story.

Joe, GMS     

Hello Joe!

One transistor stage modulated class C is not going to hit 100% positive unless the modulator waveform is purposely distorted to accentuate positive peaks.

Could just be the design. Dunno.

73,
Dave

[W5EFR]

Howdy gents...

yes, I can drop the built in supply and power it off a bench supply. That will rule out the built in supply or not...

I didn't get a chance to work on it this evening..

However, here is the entire schematic..

as you can see, it is a multi-supply 11 pin power connector that is confusing as hell...

[W2VW]

They partially the RF driver which is good. The thing should be able to make 100% positive.

Take a real good look at the driver and final resistors and voltages plus alignment.

If the little 3 legged beasties are leaky all bets are off.

[AB2EZ]

If R6 is set to a low value, and if C16 is leaky or has much less than its nominal capacitance... then the range boost circuit would also act as a positive peak limitor (limiting the positive peaks of the modulated voltage applied to the RF output transistor). This problem could also arise if R6 is set to a low value, and the diode has excessive reverse saturation current.

To check this, look at the voltage between the top of the 4.7 uF capacitor (the capacitor immediately to the left of the diode), and ground (with your oscilloscope), while the transmitter is on, and modulation is applied. This voltage should be constant (no significant modulation), and should have a value of between 1x and 2x the B+ (for 100% modulation with a 1 kHz sine wave), depending upon the setting of potentiometer R6.

This effect, if it exists, would be worse at lower modulation frequencies.

This effect can also distort the waveforms at the output of the modulator transistors, by placing a non-linear load on the output of the modulator.

Stu

Scott,

After you get the P.S. issue resolve take a look at the collector of Q9,10 and 11 and see what the quality of the sine wave looks like.  Make sure your not over driving the first stage when you take those measurements.  Ideally you should have a nice clean sine wave prior to the driver transformer since after that, things don't look very good.  If you see clipping of the negative or the positive peak of the AF waveform while driving the input with normal mike level you then can zero in on whats wrong in that particular stage.  Another test to do is to disconnect the secondary of modulation transformer and put a resistive load on it to simulate what that the final amplifier impedance is.  You can get a pretty good approximation by knowing the steady state collector voltage divided by the steady state DC current feeding the final amplifier transistor.  Lastly the range boost circuit is well filtered but when the pot is adjusted to zero ohms the first rectifier and cap act as a peak detector.  That then determines the bias voltage on the first stage.  So you end up with the voltage being a result of how much feedback the range boost circuit offers.  It may prove interesting to lift that and just apply a fixed bias voltage to the first stage for testing purposes.

Having fun yet?  

Joe, GMS        

[Opcom]

Can you test the modulator itself with no RF? Apply a resistor across the secondary and work on the amp that way, it may be more productive at eliminating suspicions. The resistor could be a simple 25 Ohm or 50 Ohm 10W or 20W WW pot. Adjust it taking voltage readings and resistance readings (or current readings) asnd see where the modulator is sweetest and most powerful. Then see if that matches the class C stage's operating voltage and current.

By testing the modulator separately, all RF crap is omitted.

[W3GMS]

Can you test the modulator itself with no RF? Apply a resistor across the secondary and work on the amp that way, it may be more productive at eliminating suspicions. The resistor could be a simple 25 Ohm or 50 Ohm 10W or 20W WW pot. Adjust it taking voltage readings and resistance readings (or current readings) asnd see where the modulator is sweetest and most powerful. Then see if that matches the class C stage's operating voltage and current.

By testing the modulator separately, all RF crap is omitted.

I had suggested that earlier but no results reported as of yet.  I think Scott has been busy with other stuff.

From my earlier post:

After you get the P.S. issue resolve take a look at the collector of Q9,10 and 11 and see what the quality of the sine wave looks like.  Make sure your not over driving the first stage when you take those measurements.  Ideally you should have a nice clean sine wave prior to the driver transformer since after that, things don't look very good.  If you see clipping of the negative or the positive peak of the AF waveform while driving the input with normal mike level you then can zero in on whats wrong in that particular stage.  Another test to do is to disconnect the secondary of modulation transformer and put a resistive load on it to simulate what that the final amplifier impedance is. You can get a pretty good approximation by knowing the steady state collector voltage divided by the steady state DC current feeding the final amplifier transistor.  Lastly the range boost circuit is well filtered but when the pot is adjusted to zero ohms the first rectifier and cap act as a peak detector.  That then determines the bias voltage on the first stage.  So you end up with the voltage being a result of how much feedback the range boost circuit offers.  It may prove interesting to lift that and just apply a fixed bias voltage to the first stage for testing purposes.

Having fun yet?   Wink