I have a Midland 13-893 AM/SSB 23 channel crystal-synthesized radio that I'm having an issue with and I'm a bit stumped. It has a 4watt carrier on AM, but talking into the MIC and applying audio barely modulates the output, maybe gets 0.5 watts over the carrier with a loud whistle. In SSB its a similar story, has about 4watts max output when using the MIC. (Side note, this came from a CB shop that closed in the mid-late 1980s and the this radio was in the box of units with issues, so this radio has sat for probably 30 years)
To start, I went through and aligned the synthesizers, one of them had a very weak signal and the oscillator wasn't receiving full power, which lead me to my first discovered problem, a protection diode on one of the power rails was split open, so at some point somebody has flipped the power..... I replaced the diode and now all the oscillators are working properly. A few of the mixing crystals had drifted low over time and I had to swap out a few of the padding capacitors to get them in range of their adjustments. After that mess it now receives on AM/SSB. The receive doesn't seem to be as strong as it should be, the Photofact says to inject a 0.5uV signal for one adjustment but I had to bump that to 10uV to get output through the receiver. But I have no idea how calibrated my old Anritsu MG3601A RF signal generator is or if the output is still working that low......
So next, to try and track down the low output issue, I started at the microphone and followed the audio trail through both the AM modulation path and SSB modulation path. Everything seems O.K. all the way through, the signal was increasing through the AF amplifers. But I'm quite new to this so I don't have a great feel for the level the signal should be at in each point. I did hook up an 8 ohm speaker to the PA jack and the PA function drives a speaker and the audio doesn't sound garbled or off.
Other things I have tried is adjusting VR15, the RF power adjustment, and it doesn't make a noticeable difference in output on AM, but I can get about 8-9 watts in SSB whistling into the MIC with the MIC gain at maximum and VR15 at max travel.
Looking at the demodulated output and RF on an oscilloscope (through a home-made sampler) there doesn't look to be any distortion.
So that gets me to where I'm at now, I'm stumped. Any ideas??
Thank you
CB tricks has the schematic here
To start, I went through and aligned the synthesizers, one of them had a very weak signal and the oscillator wasn't receiving full power, which lead me to my first discovered problem, a protection diode on one of the power rails was split open, so at some point somebody has flipped the power..... I replaced the diode and now all the oscillators are working properly. A few of the mixing crystals had drifted low over time and I had to swap out a few of the padding capacitors to get them in range of their adjustments. After that mess it now receives on AM/SSB. The receive doesn't seem to be as strong as it should be, the Photofact says to inject a 0.5uV signal for one adjustment but I had to bump that to 10uV to get output through the receiver. But I have no idea how calibrated my old Anritsu MG3601A RF signal generator is or if the output is still working that low......
So next, to try and track down the low output issue, I started at the microphone and followed the audio trail through both the AM modulation path and SSB modulation path. Everything seems O.K. all the way through, the signal was increasing through the AF amplifers. But I'm quite new to this so I don't have a great feel for the level the signal should be at in each point. I did hook up an 8 ohm speaker to the PA jack and the PA function drives a speaker and the audio doesn't sound garbled or off.
Other things I have tried is adjusting VR15, the RF power adjustment, and it doesn't make a noticeable difference in output on AM, but I can get about 8-9 watts in SSB whistling into the MIC with the MIC gain at maximum and VR15 at max travel.
Looking at the demodulated output and RF on an oscilloscope (through a home-made sampler) there doesn't look to be any distortion.
So that gets me to where I'm at now, I'm stumped. Any ideas??
Thank you
CB tricks has the schematic here