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PCMA002F board
The Amstrad 900-901 CB radio is a "middle of the road" quality CB radio. It benefits from
having a Scratch resist casing, switchable roger bleep (901) and  LED "Fairy" lights instead
of a signal meter. A very popular rig of the early 80's and still has a lot of admirers today.  
The board is used in a multitude of Cb's, a couple looking exactly the same but a different
colour and name. Less obvious but the same board is the Mustang and Transcom series.
No Lights on RX-TX but working ok otherwise - Replace the IC BA656 located at the front of PCB
RX and TX lights stuck on - No RX but slight Hiss, Try replacing D20
Shorts out power supply - 1N4001 protection diode ok - Try replacing the Audio IC LA4422
No lights/completely dead - Look for burnt out track on the CB/PA switch PCB
Lights up but no TX/RX - Check D3 (6v1 Zener) near PLL IC
No RX or very low and distorted - Replace FM IF Detector IC 3(LA1230)
Low O/P power - Replace O/P transistor with original, 2SC2166. Or  Q7 (2SC2086)
Stuck on TX - Replace Q22 (2SA608) located in front of the 455khz filter
No TX and TX light does not light - But RX ok, Replace Q18 (2SA608) located to the left of the Audio IC
Very low RX - But TX ok, Try replacing D11 which can go short circuit
RF Gain mod - Cut out resistor on the back of the RF gain Pot to increase its effectiveness
Roger Bleep mod - To alter the duration of the bleep change the value of C116 (10uf 16v) On rigs without a roger bleep switch, wire one in series with one
leg of this capacitor
VCO - Can be used in a cybernet 134AOX board, likewise the reverse applies
Improve mod - By adding 100nf across C29 and C31, adjust RV3 to suit
No RX or TX - Could be caused by the small copper foil burnt out on the mic socket PCB from the common 0v pin
The square FET - (Field Effect Transistor) K107E1K can be replaced by a 2N301 or 3SK45. Fit so that the metal tag is to the left as you look from above
with the fascia towards you
Crystal filter - Replace C71 (2pf) with the crystal filter. As in the Cybernet 134 board
Squelch - Make this snappier by changing C99 (1uf) to 100nf. C99 is hidden beneath the cableform towards the front of the PCMA002F board. These rigs
use an inductor to adjust the 10.240MHz oscillator. If the inductor becomes damaged replace with cirkit part 119ANA5874HM. The value of C18 (33pf) may
need to be changed
Audio distortion - On high volume. Replace C107 (1000uf) next to the audio IC (damaged by cross connection or 24v)
O/P heatsink - Remove output transistor and solder up the gaps in the chassis to improve heat sinking and reliability. The big 3r3 resistor (green) can be
removed and replaced with a wire link to increase O/P power but then an extra heatsink would be required otherwise the O/P transistor would overheat
and blow
Fidelity 2001 - No channel display, try D400 at the front end of the PCB or D3
VCO Broadband - Replace 27uf capacitor (rear of VCO can) for a 33uf
Bad RX - Can be caused by can T9 being damaged by long mounting thumbscrews (use proper fixings)
Inverted Mid Block - Nearly always causes noticeable "hum" after "mid blocking"on quiet received signals. Likewise sometimes on TX mod. Careful
routing of UK/MID switching wires can make a difference although the main cause of this is the Inverter "hex buffers" used to convert the LED from
negative to positive to power the 7132 PLL IC not having all its input pins that arent used grounded
HOW TO MID BLOCK AN AMSTRAD
THE HARVARD LAYOUT IS A LITTLE DIFFERENT.. CLICK HERE...
CB Radio History from its early days
CB Radio Repairers in the UK
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