Midland 13-898B Low SSB Power Out

[Justin B.]

I continue going through this neglected radio and I think I have got most of the gremlins exorcised except SSB power. Receive seems fine so I left the synthesizer alignments alone and I went through the transmitter alignments and have about 2.5W dead-key and 5-5.5 W modulated. I have set the SSB bias and carrier null but can only get a little less than 2W out on both USB and LSB.

The only adjustments that did not go perfectly is the USB and LSB carrier freq adjust. They are both off and adjusting the associated trimmers doesn't seem to have any effect on the frequency.

Just for grins I replaced ALL of the electrolytics on the audio and main boards, cleaned all trim-pots before adjusting, etc. Any ideas of what else to look at would be appreciated.

73,

 

[nomadradio]

I replaced ALL of the electrolytics on the audio and main boards

Good start. Eliminates a whole list of possible faults. There are a handful of slug-tuned coils/transformers that get peaked to align that radio. Most of them that are in the transmit chain are 10 mm square with a thin threaded ferrite slug inside. Many of them have a built-in capacitor that sets the frequency it will tune. When that capacitor fails it becomes an open circuit, but not one you can test. The resonant frequency of that coil not jumps upwards a few MHz. Increasing the coil's inductance will only partially make up for the lost capacitance in the circuit. It will cause the apparent peak position of the slug to be dead even with the rim of the hole in the can. This appears to be a resonant peak, but it isn't. The maximum inductance of that part is reached with the slug at this "flush" position. The dead capacitor causes the slug to be adjusted for more inductance to reach resonance. But it won't. It will just get "close" to being resonant at the end of its functional travel. Turning the slug clockwise reduces inductance, and the transmit signal falls off. Turning it counter clockwise makes the slug rise above the rim and also reduces inductance. Acts like it's a peak but it isn't.

Sometimes there's a cheat to fix this without replacing the 'can'. But not always. Your problem may lie elsewhere, but this is a hazard for old solid-state radios and easy to check.

73