The AM Forum

Howdy Gents,

I am restoring an old 23 Channel Lafayette Telsat 23 (early 70's era) CB. If I can get it running, I am thinking about putting it up on 10 meters.

Here is the issues...

4 Watt Carrier is good, but when the transmitter is modulated, I can not reach 100%. The Negative peak hits 100%, the positive peaks are considerably less than 100% and then they start to Flat top.

I am trying to decide if I am running out of Modulator headroom, or if the power amplifier section is lacking in the power department.

I have modulated it with a 1kHz signal through the mic jack, then also through the external input on the back of the transmitter, the results are the same.

I already replaced the power section filter caps,
The Thermo-resister is in spec (34 ohms cold), the Voltage is near Spec (16Vdc)... so I decided to ask the Gurus... any ideas?

Have you looked at the audio signal at the output of the modulator??  You're running out of headroom on half of the sine wave.  Look at the audio from the aux input to the output.  Early audio stages probably not the problem since you say the results were the same with signal injected at the aux input.

What's the 113.9khz as shown on the scope in the second pic??

Fred

Fred,
Thanks for the reply. I have not looked at the output of the modulator output yet, but will check there next. This circuit board is not marked at all, and the "High Quality" Sam's Photofact pictures leave a bit to be desired...

I agree that the issue is in the modulation section itself and not the Mic Preamp area... I actually think one of the 2 - 2SB337 Germanium Transistors (Output Modulators) may be heading South... I have yet to find a modern, silicon replacement...



The is normally the Frequency Counter, as to why it is picking up 113.9 kHz on a  27.005 MHz carrier modulated with a 1kHz Sinewave, I have no idea...

It does look like head room problems but have you looked at the phase of the microphone?  It might be a phasing problem.  Another issue might be a mismatch between the modulator and the class C load.

I'm leaning toward a mismatch problem.  Just 'cause it was commercially built doesn't mean it was designed right

Al

I would definitely look at the modulator stage when its into its positive peak clipping condition.   Compare each base to emitter waveform of the push pull pair.  Remember to float your scope since the negative probe will be referenced to a positive voltage due to the PNP topology.  Then look at each of the collectors and see how they look.  Since your using a sine wave generator no need to worry about audio phasing however when talking into the mic, make sure you phase the audio so you have more positive peaks than negative ones.  First try to get it working with the signal generator though. 

Not sure if the FCC had any requirement for limiting the modulation percentage on those rigs.  If so, there is a chance that it was designed to not produce 100% modulation.  Then again, as we all know, if your going to limit the audio percentage its mainly necessary to do that in the negative direction since you don't want to cut off the carrier.   

Good luck with the project!

Joe, GMS 

I bet if you put a sine wave into any CB it will look like that or worse.
It might look ok with voice, or maybe not, 10% distortion was the optimistic point aimed for...
I think they were all short of audio power and the mod transformer was well undersize for the job.

I am not sure all ham transmitters would do 100% both ways with a sine wave....

Oh, and I do not think transistors go soft, they short or open, resistors change value, but I doubt transistors do...

You won't.

You will have to find a replacement silicon transistor and re-bias the circuit.

If you can find a schematic that has voltages listed, you can determine circuit currents and biases for the germanium transistors and then convert to silicon device biases.

Look up the old Germanium spec on frequency, current, and power, and then calculate the new biasing components.

Actually, I have yet to come across a CB that will not do a clean 100% Modulation.... until this one...

 The biggest limiting factor of the modulation transformers is Bass fidelity... Most modern Transformer modulated radios start to distort the waveform around 200Hz...


That maybe true, I only have 2 Amateur transmitters/transceivers that will do AM... A Knight T-60... which will not do 100%, and a ICOM IC-718, which uses that weird Downward Modulation... and doesn't get used on AM


I am starting to agree... I figured those old Gi transistor may have started to get leaky, but when I O'Scoped the Collectors, they are both pushing 25V P-P and 180 out of phase with each other...

Hello Joe,

Thanks for the information and assistance. The Sam's Photo Fact states to set the modulation to 80%...

I have also adjusted the 'Range Boost" R6 and see very little change, which makes me think it isn't driving hard enough to being the feedback into play. Even when I drive it into over-modulation the Range Boost doesn't do anything... but then again, looking at the schematic, if I am inputing the signal through the external Modulation Jack, it won't.... as the feedback goes all the way to the Mic Pre-amp first stage transistor Base...

What you want to look for with the scope is the audio quality before it gets combined with the RF carrier since you know that does not look great!   If a bias voltage shifted in any of the audio stages you could very easily get the type of waveform that your seeing.  So I would start at the primary of the modulation transformer and if that looks bad then just keep going back stage by stage until you find the problem.  With a scope you should be able to zero in on it very quickly. 

Let us know how you make out with it. 

73,
Joe, GMS     

From what I remember from doing service on this model and looking at the Lafayette schematic, if you set the modulation to 100%, you will be in distortion mode and even worse if you turn the "range boost" on and modulate it with a microphone. Set the modulation to roughly 80%. After 40 years, the transistors may be starting to deteriorate. I would also check every resistor and capacitor. In the Lafayette original schematic, the mod transistors were labeled 2SBF1, 2SDF1, 2SBF1A, and two 2SBF5. Eventually, those codes rolled into the codes you see on the SAM's schematic.

The receiver section has one IC (ICF-1) which was a known failure device. The receiver also has a "mechanical filter" which will distort receive audio on strong signals.

This model was not one of Lafayette's better performers.

I think the performance is abut right for that. Those were not the best quality and the positive side of the modulation is always harder or more demanding or so it seems.

I am starting to think you guys are correct.... this is working "as designed"... I rebuilt the bias section and gained... nothing...  :o

I am fighting the urge to make it work correctly now... or trying to understand why it is doing what it is doing...

Obviously you can redesign the entire modulator with better transistors, mod. transformer, etc. if you feel the need. Some of the later Lafayette models also had better audio and better final and driver transistors. On 10 meters, where the happy day is just working someone on AM, whether you're at 80% or 90% modulation, is probably not a great concern on the receiving end.

Scott,

It "may" be interesting to find where the problem is.  You may choose not to fix it and that's OK but it would be nice to see whats wrong with the design assuming its a design issue.  I encourage you not to change components and shotgun it hoping you will find the issue, but rather with your scope probe look at the audio stages and see where things go astray.  The audio should be at least symmetrical since your audio source is a sine wave.  You may not be able to modulate fully, but a sine wave should modulate the transmitter equally in both directions.  What you have shown is that your able to modulate in the negative direction fully but the symmetry is such that becomes asymmetrical at some point in the audio chain.  This asymmetrical issue is likely what is keeping it from modulating in the positive direction like its doing correctly in the negative direction. 

Yes, in the end on the CB band it may not matter but gaining some addition knowledge by going through it may be rewarding to you. 

Joe, W3GMS         

Joe,

Thanks for the reply...

I have been probing though the audio chain and I haven't found the smoking gun stage ...yet...

I'll take another look at it tonight/tomorrow. I am interested as to the why it is doing what it is doing, or where the lacking in design is, Heck, I wouldn't mind making it work the right way... I have $5 into this radio, it is asking to modified into submission.

I would like to put it up on 10M, on the rare chance I hear someone out there. 10M is about the only Ham band I can run on at the moment due antenna restrictions... and VHF/UHF around here is pretty lacking... I like HF...

Glad to hear that you have not thrown the towel in on it yet  :)! 

Let us know what you find.

Have fun with it.

73,
Joe, GMS

If you can reverse the connections on mod xfmr primary, you may see the positive peaks hitting at 100% while the negative peaks may at 80%.  This may be difficult to do because of the printed circuit board, but may be a good test. 

If you're able to do this and you see no change, then the problem is in the RF final.  May be something in the design limiting the positive peaks to 80%

Fred

Maybe RF base drive and biasing?
On a tube rig, poor positive modulation can be caused by screen problems, grid drive and bias problems.
Maybe the RF section can not put out enough additional power under modulation.
At 100% modulation, the RF output device will get 24 volts I would guess, and maybe the circuits or parts will not pass that power.
Or something is biased off center.
If you crank the audio up, does the positive modulation increase or stick at the same level?

I am not 100% convinced that the problem is all in the audio/modulation section... but I am starting there is working my way to the antenna jack... ;)


Originally, when I cranked up the audio, both of the positive peaks flat top at the same time, and the Negative peak hits or exceeds 100% shortly after... More to follow

So I started probing the audio section:

Baseline Modulation is first photo, then the collectors of Q12 and Q13, the Output Modulation transistors... that don't look right...  :o

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)

Fred, do you mean basically switch the connections to to the Primary from the collectors on the transistors?

That would be actually easy to try as this uses T0-3 style transistors and they are wired to the PC mounted transformer...

I'm going to withdraw my phasing problem suggestion although it may be an issue with voice audio (as Joe pointed out, we are dealing with a sine wave source here) and stick with the mismatch suggestion.  I think what you have was designed into the circuit - namely, a better match from the modulator to the class C load might yield better positive peak results.. 

Al

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

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  
  

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

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.

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

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

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

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...

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

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

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       

Is the RF driver stage modulated?

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

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...

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.

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

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