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Vintage Cobra 138XLR AM/SSB 40 Ch CB with 858 PLL With 30-day return - 1 year free service policy Here is an excellent 40 channel Vintage Cobra 138XLR mobile, made by Dynascan, made in Japan. The circuit board in this 138 is identical to the early version President Washington base stations, and has the 858 PLL, 2SC1973/2SC1306/2SC1307 transmit strip. This unit has been professionally repaired, refurbished, and modified. (I have 30 years active CB/Amateur Radio experience and 18 years Two Way Radio, component level, warranty bench repair service experience.) Replaced driver transistor, 2SC1306. The bad transistor had parasitic oscillations, resulting in fluctuating quiescent current in ssb mode, found while adjusting driver bias. Channel selector switch contacts where cleaned, polished, and treated with ProGold G5 contact enhancer. When dirty, they cause the LED read-out to flicker or go out, and/or, you may hear scratching/popping sounds as you change channels. All old heat sink compound has been cleaned off and removed. New heat sink compound has been applied to all components requiring heat sinking. DC pass transistors TR36, TR25, and TR24, have been replaced with TIP31A's. The slide-on connectors were removed and the wires soldered and covered with heat shrink tubing. A +/-10 kHz shift control was added to the face plate. This is required to access the "A" channels (radio control toys). I use these channels for model rocketry beacon hunting. The dimmer control now controls AM tx power output. The Tone control now controls AM/ALC. This allows the operator to adjust ALC to 100% modulation when operating at low power. With out this mod, when using low power, the transmitter will overmodulate. Or, the operator can use out-board audio processing to a fuller extent. Please note: Transmitting on non-fcc allocated frequencies is not permitted. TX power calculations derived from a calibrated oscilloscope shows SSB is 27 watts peak envelope power (SSB power limiter was disabled). AM dead key is 2 to 7.5 watts peak power dead key, AM at full modulation/power is 15 watts peak envelope power. All Local oscillators have been adjusted and set to frequency using zero beat method, referenced to WWV at 20 mHz. Adjustments were made after the radio's operating temperature stabilized. Usually powered on over night at room temperature and adjusted in the morning. The PA circuit has been rewired. This modification removes the annoying "pop" or "ping" sound transmitted during ssb key-up. So far I have worked on seven of these radios, all exhibit the same problem. RX is excellent, tuned for even sensitivity from channel 1 to channel 40. The 23.5 mHz circuit for the noise blanker was tuned. The noise blanker works very nicely now. The noise most affected by the blanker circuit is the grinding sound arcing insulators make at high voltages, ie power lines. The noise most affected by the ANL circuit is the popping sound spark plugs emit. Unfortunately, the 138XLR suffers from a lack of alternator whine filtering. If this is an issue, aftermarket power line filters are available. Recapped, ALL aluminum electrolytic capacitors have been replaced, voltage ratings increased. This radio is very clean. The knobs and covers have been washed. I installed stick-on rubber feet, nice for base station set-up. I think someone painted the covers brown. Not a bad job, but black paint shows in some places. The unit does show usual wear. Electronically, the unit is in 100% top operating condition. You won't need to find a technician to ensure reliability. (Vintage cobra 138XLR am/ssb 40 ch variable power/alc was posted and is owned by: Queen Wright) |
q-wright@chicagopartsnetwork.com (Queen Wright) for more information.
