| | JOTTINGS on the PTBM121D4X CB Radio This board is found in a number of multimodes, including the Tristar 747, Hy-Gain V and the Concorde II CB radios. Some variation in the front panel controls and crystals is to be expected. In particular, whilst the two rigs mentioned use 20.555 (HI HI), 20.330 (HI), 20.105 (FCC) or 19.88 (Midn) crystals, other rigs use a frequency doubler circuit with crystals exactly half these values.
One well known problem is poor transmitted audio on UK frequencies caused by lack of tune range. A simple fix is to retune the relevant crystal(s) on the PT0S011AOX (padding with extra capacitor may be needed).
Common faults include the AVR going open circuit (rig lights but no TX/RX) or short circuit (very odd effects like no TX on FM but needle hard over on SSB with accompanying whistle); channel switch goes faulty - the ratchet wheel becomes loose on the spindle. Sometimes it can be degreased and glued with araldite. The Darlington transistor can go o/c resulting in no power output on AM or s/c resulting in no modulation (but FM/SSB O.K.). You must replace with the correct type or use two NPN transistors to form a Darlington pair. (If you don't know what that is, read a few electronics magazines!).
Q39 and Q40 form a TX/RX switching pair. Any fault here can cause odd effects, usually with TX/RX functions mixed up, for example TX light on during RX and only a weak hiss from speaker.
By far the most common faults, unfortunately, are caused by people fiddling with adjustments. Very few adjustments can be made satisfactorily without the aid of the proper equipment. I advise you to follow the Service Manual to the letter, especially when dealing with SSB adjustments. The only one you can do. easily, is the RV5/RV6 carrier null adjustment where you key the mike on SSB and adjust until no reading is obtained on the power meter (with mic gain on zero).
If you turn the AM bias adjuster you will increase the power output slightly on AM. You will, however, reduce the modulation. With a scope, the setting is simple; without a scope you can only adjust it for the loudest reported modulation on TX and cross your fingers. You will probably read about 2/3 the power on AM compared with FM when the adjustment is somewhere near.
If transmitted power is uneven across the frequency bands, adjust the coils as follows: the two marked TXLO for maximum power on the very channel 1 LO band: the two marked TXHI on channel 40 (perhaps 80) on HI band and the remaining TX coils on channel 20 (60) on the Mid band. Repeat the procedure as many times as necessary to achieve an even power spread.
Should you need to adjust the VCO, make a proper square adjustment tool or you will break the screw core.
The RF transistors are simple to remove if you desolder the leads and remove the complete aluminium heatsink with the transistor still attached. | |
