Treetop Circuits

SB-390 Hints

This is a compendium of hints, caveats, and observations, mostly from owners of the SB-390. I'll update it periodically.

1. In two radios, the audio was weak and the AGC didn't work. The problem was traced to poor contact in the IF output jack. Apparently, a half-century of disuse or overuse had left a resistant film of oxide, etc. in the center contact.

The easy way to check this is to go to AM mode, and tune in a station. The signal at the IF input on the board should be between 40 and 150 millivolts RMS. If it isn't, suspect the connector first; there's also some circuitry inside the radio which could be the culprit.

A shot of DeOxit into the jack is a good idea in any event.

2. Make sure that you install the short standoffs on the correct terminals. If you make a mistake here, it's impossible to see once the board is in place, and nothing will work.

3. As noted in the manual, the noise limiter can't be expected to work properly in SSB. But it's sometimes useful to use SSB mode on an AM signal -- just tune to bring the carrier to zero-beat with the BFO. This is a poor man's version of synchronous detection (not true synchronous detection because it isn't phase-locked, but maybe that's nitpicking). It's been observed that turning on the limiter while tuning makes it easier to find zero beat. No idea why -- but it works.

4. Mechanical filters provide a huge advantage in a crowded band. But there's a downside. The frequency of the BFO relative to the filter response is critical. For good audio, the BFO should be just off the shoulder of the filter response. If it's too far down on the slope, the lower voice frequencies will disappear. If it's too far the other way, and the other station is transmitting a lot of audio below 300 Hz or so, or even a bit of the opposite sideband, tuning becomes very critical.

In KWM-2's and S-lines, this can happen because the filter and/or the BFO crystals change with age. With a variable BFO, we can compensate. To establish a starting point, tune in a carrier (the calibrator signal will work). Tune the receiver until the S-meter just starts to drop off. Then tune the BFO through zero beat, and about 300 Hz beyond.

5. In two cases, a weak BFO tube caused the adapter to fail to switch modes.

6. In one case, the owner disconnected the BFO coupling capacitor, C535, but did not remove it. Under very noisy conditions in AM mode, the resulting high-amplitude spikes at the final IF stage coupled to the BFO circuitry via stray capacitance, causing the adapter to switch momentarily into SSB mode. Removing the capacitor fixed the problem.