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Midland 13-898B Low SSB Power Out
- Thread starter Justin B.
- Start date
C306 is after the diode detector, we should not have AC appearing here.
Are you sure you were not probing C310's cathode? That would look just like the waveform you shared.
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Those waveforms are a mess, but it looks like the audio IC is working if there is 2.5v pk-pk coming out.
The mic signal going in looks like an AM modulated RF envelope, not an audio signal. IC301 pin 2 image shows 500uS/division and that should correspond to a 2kHz wavelength fitting in each division, but all I see is solid yellow indicating a much higher frequency. The various ceramic caps should be getting rid of some of that. Should be.
Its almost like the audio is working the AGC, and that AGC is modulating the RF that is riding in with the mic signal. Can/have you try swapping mics with the other working radio?
When doing things like this, it is better to inject a constant tone of known amplitude and frequency into the mic jack (I know I said yell into it). This way when you probe different places at different times, the waveforms can give a better picture of what's going on. You should also have the option to set the DSP or bandwidth of the scope channel so you don't see the RF stuff, but I am more curious why it is even there.
And I know the analog scope guys will roll their eyes at this, but having the voltage and time displayed on the graticule/division lines makes it easier to see what's going on.
Edit to rearrange paragraphs so that my whining is at the bottom and to also mention that injecting a clean audio tone instead of voice gives you something to trigger the scope on.
The mic signal going in looks like an AM modulated RF envelope, not an audio signal. IC301 pin 2 image shows 500uS/division and that should correspond to a 2kHz wavelength fitting in each division, but all I see is solid yellow indicating a much higher frequency. The various ceramic caps should be getting rid of some of that. Should be.
Its almost like the audio is working the AGC, and that AGC is modulating the RF that is riding in with the mic signal. Can/have you try swapping mics with the other working radio?
When doing things like this, it is better to inject a constant tone of known amplitude and frequency into the mic jack (I know I said yell into it). This way when you probe different places at different times, the waveforms can give a better picture of what's going on. You should also have the option to set the DSP or bandwidth of the scope channel so you don't see the RF stuff, but I am more curious why it is even there.
And I know the analog scope guys will roll their eyes at this, but having the voltage and time displayed on the graticule/division lines makes it easier to see what's going on.
Edit to rearrange paragraphs so that my whining is at the bottom and to also mention that injecting a clean audio tone instead of voice gives you something to trigger the scope on.
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I'll dig out my old Heathkit IG-102, I think it has audio out...
Just for grins I disabled the ALC by removing R305 and I was able to "yell up" right at 3W out! I monitored an AM signal with my talkie and audio received sounded crisp. I'll plug in my uBITX V6 QRP radio tomorrow and dial in a corresponding frequency to listen to the SSB signal.
73,
Just for grins I disabled the ALC by removing R305 and I was able to "yell up" right at 3W out! I monitored an AM signal with my talkie and audio received sounded crisp. I'll plug in my uBITX V6 QRP radio tomorrow and dial in a corresponding frequency to listen to the SSB signal.
73,
Brandon - I'm more confused every day! I pulled out my QRP rig, dialed in the freq for ch16 and the LSB sounded nice and crisp. I also pulled the "good" 898B onto the bench and it had the same 27 MHz component in the mic audio as well so this might be irrelevant to my issue. Will look at it more when my brain rests for a while. I used the same mic on the good radio...
I think we might be looking in the wrong spot. All of these symptoms, IMO, are of a bad final or driver. Consider for a moment your AM dead key. No audio involved and its still low. That makes me think driver, final, or an IF can or filter issue.
I know you have said that the mic area looks different on the scope than on your good radio, but I just can't relate a low dead key with an audio problem. I think its time to probe the collector of the driver and final. in AM TX no modulation.
EDit, i hate to jump ahead of myself, but R357, the 6.2ohm resistor feeding the driver and final, maybe that went bad and all the driver and final current is being limited by the resistors next to it. If its bad, don't just try a new one before trying to figure out what the cause was.
I know you have said that the mic area looks different on the scope than on your good radio, but I just can't relate a low dead key with an audio problem. I think its time to probe the collector of the driver and final. in AM TX no modulation.
EDit, i hate to jump ahead of myself, but R357, the 6.2ohm resistor feeding the driver and final, maybe that went bad and all the driver and final current is being limited by the resistors next to it. If its bad, don't just try a new one before trying to figure out what the cause was.
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I almost keyed up and said that yesterday. Even if the junctions are functional and show "good", abused rf transistors sometimes just continue to lose gain, and power, of course, until they fail. As you were.