Circuit Board Layout : PTBM121D4X
Circuit Board Layout : PTBM048AOX
CB Radio History from its early days
CB Radio Repairers in the UK
This board is found in a number of multimode SSB radios including the hy-gain 5, Concord 2 and so on although some variation in the front panel controls
and crystals is to be expected. In particular, whilst the two rigs mentioned use 20.555 (double high), 20.330 (High), 20.105 (Mid) or 19.880 (Double Low)
other rigs may use a frequency doubler circuit and have crystals exactly half these values.

One of the main problems with this board is poor transmitted audio on the UK frequencies caused by lack of tune range. A simple fix is to re-tune the
relevent crystal trimmer on the PTOS11AOX which may also include a little extra padding with a small value capacitor.  If you fit an eprom board you could
"switch in" a capacitor to bring down the frequency from the 27.605MHz (Ch 16 High, FCC) to 27..60125MHz or something near (Ch 1, UK40) and wire this
to the spare side of the switch. This can also be done without the eprom board but will still show Ch 16 instead of Ch 1. At least you would be on frequency
and be able to chat reasonably unobtrusively to other breakers who are on the UK band.

A very common fault includes the AVR going open circuit (Rig lights but theres no tx or rx) or goes short circuit causing strange effects like no tx on FM but
needle hard over on SSB with a loud whistling on the carrier.

The channel switch can go faulty by the ratchet wheel becoming loose on its spindle. Sometimes this can be degreased with a switch cleaner, dried then
glued back with araldite or some similar type of adhesive.

The Darlington transistor can go open circuit causing no power on AM or short circuit resulting in no modulation on AM  but FM and SSB unaffected. You
must replace this with the correct type or use two NPN transistors to form a "Darlington" pair.

Q39 and Q40 from a TX/RX switching pair. Any fault here can cause odd effects like the TX/RX functions mixed up, like TX lights on during RX and only a
weak hiss from the speaker.

It will come as no surprise to many of you that the most common faults are caused by inexperienced people fiddling with adjustments. Very few
adjustments can be made satisfactory without the use of proper equipment especially with settings for SSB. It is a necessity to follow the manufacturers
service manual to the letter. The only SSB adjustment which can easily be done is the RV5/RV6 carrier null adjustment where you key up the mic on SSB
and adjust until no reading is shown on the signal meter. You need to have your mic set at zero for this or make a simple TX mic plug especially for this

If you turn the AM bias adjuster it will show as an increase of power but your modulation depth might go down.  With a scope the setting is simple, without,
you can only adjust it for the loudest reported modulation on TX. It will be somewhere near when the AM power is approximately 2/3rds to that of FM.

If your transmitted power is uneven across the bands, adjust the coils as follows. The TX can Lowest on the Pic below adjusted for best TX power on the
lowest channel you have and the TX can just up a little for maximum TX power on the highest channel you have. Then set it midway between its frequency
range and adjust the other two TX cans which are together a little higher up than the other two for maximum power. You may have to adjust again slightly
until you achieve the best power spread you can over all the channels.
If you need to adjust the VCO grubscrew manufacture a properly shaped plastic tool that will fit exactly into the grubscrew otherwise it will snap and will
need replacing. If it does snap I find the best way to remove the grubscrew is to completely unsolder and remove the VCO block and unscrew it from the
other side. If you replace it you need to get the right part otherwise your VCO coverage could be seriously reduced. You can buy a plastic radio trimming kit,
which is quite cheap, which has a correct size/shaped blade for just this job. Do not, under any circumstance, use metal precision screwdrivers.

If you need to replace the output transistors I find the best way is to unscrew the heatsink from the chassis, then unsolder the Transistor legs and remove
the whole part. That way you can easily unscrew the plastic insulating screw - remove and replace the transistor itself (making sure the mica washer
doesn't drop out) and re-screw back onto the heatsink. Then you can refit the heatsink in its entirety back onto the main board and re-solder the transistor
legs. Its much easier than trying to remove the transistor in situ.

Please see the diagram below for component positions
CB Radio Repairers in the UK
CB Radio History from its early days