Midland 13-898B has LSB Only

[Justin B.]

I'm a sucker for vintage radios but they often come with "issues." I received a "new" FleaBay 898B today that the listing said will not power on, I figured it wouldn't be a big job. Got that part fixed which was the fuse on the PS board probably popped because electrical tape on connections to replaced bulbs had fallen off so probably shorted to chassis. Not sure why factories used solder-in fuses (maybe saved a nickle) so I swapped in a fuse holder. All of the bulbs were dead so I replaced them with 12V model train bulbs and continued on to the next issue.

Issue numbner 2 is that I seem to only have LSB, no AM or USB. Have good power output and modulation on LSB but zippo on USB or AM. Looking at the schematic I see that one set of four crystals, along with associated oscillator, covers LSB while another set of four, and oscillator, take care of AM and USB. That narrows it down pretty fine so tomorrow I'll get out the scope and start poking around. Maybe the bandswitch is still gunky or maybe the oscillator transistor Q102 or isolation diode D107?

73,

 

[nomadradio]

If you have a ham or shortwave receiver on hand, you can tune in one of the four AM/USB crystals and 'eavesdrop' on it to see if it's running. If you can hear the carrier from one of that set of four crystals, the gremlin is elsewhere.

73

 

[Justin B.]

Never thought of that. I'll have to dig out my old DX-440, it's the only "digital" SW receiver I have - all the rest are analog.

Thanks for the tip!

73,

 

[Justin B.]

Took my trusty old DX-440 out to the shop and performed the checks you suggested. On LSB I got 5 bars on the signal strength and on USB I barely got 1. So, the signal was present but extremely weak compared to LSB. Firing up the scope and poking around the LSB and USB oscillators I found the LSB oscillator "oscillating" but no waveform out of the USB osc. The voltages on Q106 look reasonable so I'm thinking that X115 might be dead. I'm heading back out to the shop to hook my signal generator, with proper 11.275 MHz freq, to the high-side of that crystal and see if the radio wakes up. I have used this method to test/isolate bad crystals in other radios - some old Lafayette tube-types come to mind...

More to come...

73,

 

[Justin B.]

After removing the sig gen from crystal I tweaked the trimmer for that crystal and it started working! I've had h\Heathkit HET Osc do that, too.

I think I jynxed myself wondering about what else the "tech" that twisted the wires together and taped on the replacement lamps might have jacked around with. When starting the alignments I found two crystals I couldn't get to come it - X102 and X103. When comparing the crystals to the schematic they were different! I subbed the sig gen onto X103 and they started working correctly. I'm wondering if some bonehead might have swapped these crystals in trying to get some "special" frequencies. I haven't did the math to see where the incorrect crystals would put the freq, maybe tomorrow.

Also, I found that if I run off 12V I can adjust the carrier balance for a reasonable null but still just a bit above spec. If I run off line voltage I have a hum/buzz and output is full. Maybe the buzz is getting into the mic circuit and causing the constant 12 W out. Have to look at that, too.

Took out the clock and found the motor frozen just like my other, was hoping to get a good clock out of the deal but no joy.

So, it looks like the hunt is on for a 7.7933 and 7.8100 MHz crystal.

73,