HOW TO FIT A MID BLOCK (SIMPLIFIED)
If this is your first time at fitting a mid block chip I suggest you try it out on a
cheap second rig rather than practising on your pristine Rotel 240.
All information is theoretical and I do not condone the adaption of CB radios to
work beyond their legal type approval. The information below is for informational
First of all you need a PLL IC 7132 chip (average cost £7) and possibly some ribbon cable (coloured is easier and best) but normal wire can be used, it just
makes it a touch harder. A point to note is that the radio you are "mid" blocking must have a positive display. The Rotel RVC 220 and 230 have a negative
display. The way round this if you are handy with a soldering iron and have a de-soldering pump (you should have one of these anyway, they are invaluable
and cheap) is to remove the LED display and change it for a Positive type. Then change the neg/pos wires around (yellow/black) to the channel selector
board and proceed as follows..
HERE WE GO..
1/ Cut track to isolate pin 14 of the PLL 7136/7 (LD)
2/ Cut track to isolate pin 15 of the PLL 7136/7 (PD)
3/ Run some wire from track side of pin 15 to the middle connector of a 2 pole switch (this is the PLL common wire) then solder another piece of wire the
same length from pin 15 of the PLL to one end of the same switch.
4/ Take the PLL IC 7132 and prepare it as in the diagram to the right.
PLEASE NOTE that the PLL IC is viewed from the Underside with
the indent towards the top.
5/ Solder the ribbon cable 1 to 8 on to the 7132 leaving about a 5"
6/ Wire link as shown onto 7132
7/ Solder a piece of wire approx. 8" long to the point marked * on
the 7132 diagram to the right. The PLL is now ready to be soldered
to the main PCB.
8/ Solder pins 12, 13, 18, 19, 20 of the 7132 to the corresponding
pins on the 7136/7 or in the spare holes next to the 7136/7
9/ Solder the other end of the * wire to the remaining position on
the "Mid/UK" switch.
10/ Solder the flying ends of the ribbon cable (Or whatever you used)
to the positions marked 1 to 8 on the channel selector PCB as shown
in the diagram below taking care not to accidentally short any connections.
|Click any picture for
a close up with
The MID block is now fitted and will need tuning in
Switch the CB on and put it on MID block channel 1. On a close by rig transmit a dead carrier on the same channel. Does the midded rig pick it up? if yes so
far so good, if no then adjust VCO very slightly using a plastic trimmer tool while still transmitting on the other rig. You will hear a "motorboating" type noise
getting slower the closer it gets to being tuned in until it picks up the other rig. Now turn both rigs to channel 40 UK and make sure "Midded" rig still receives
on it. If it doesn't then re-adjust VCO slightly again and re-check mid 1 afterwards. If it wont tune in at both ends you need to "broadband" the VCO by changing
the 33pf cap (behind VCO in wax) for a 22pf cap and try the above again.
Now put both rigs on channel 1 MID and transmit from "midded" rig. If not on channel slightly adjust the little red trimmer (CT1) to bring it on the correct
channel. Likewise check channel 40 UK the same way. Once satisfied you need to check the balance of power over the two bands. This needs to be done
with a 50 ohm dummy load as with an antenna it might give an inaccurate reading according to the SWR. The balance of power can be attained by adjusting
T4, T3 and T2. NB. you will probably have to re-set RV5 for LOW power.
Decide on a channel, say 30 mid. Put the antenna back into the "midded" radio and either listen for a signal or get a friend to throw a dead carrier who is a few
miles away. While watching the signal meter slightly adjust T7, T8, T9, T10 and T11 to get the best "Peak" on the meter. Now TX on your other rig on the same
channel and adjust the RV (silver horseshoe) behind the signal meter so it sets toward the end of the 30plus on the meter.
Now you have a choice. Many people just trim channel 20 UK to 27.7900 by adjusting the other red trimmer (CT2) which puts the frequency a little low on UK
and a little high on Mid (hardly noticeable) or drop an 18uf or 22uf or 33uf cap (Depending on the radio) across one leg of the x-tal and earth switchable by the
mid/UK switch using the other pole. 22uf or 33uf switched on with MID. Please see pic for positioning of this cap and the wiring of it on the switch diagram.
The frequency on both blocks can be set near perfect with this. There are other ways but thats how I do it.
NO UK40 after adaption? If this happens you will need to separate the two pin 19's on the 7132/7137 with a small value capacitor, say a 10uf or similar.
UK OK BUT ONLY ON CH19 ON MID after adaption? Could be because the display is a negative bias one (Like the Rotel 220/230's) See first note at top of
A comment about mid blocking a Binatone 5 Star or even a Rotel RVC 240 or York JCB 863. I know there are lots of switches you could use but I prefer to keep
them and do something a little different. I fit piggy backed mid block switches on the 'Dimmer' (Or display on 5 Star) control. Why? because you have to turn the
dimmer anti-clockwise to brighten the display, so if you use an on/off type pot with 3 legs then you can use the 'off' position to click to the 'mid' block. The CB
loses its channels on mid if you turn the dimmer down due to the LC7132 PLL voltage to work coming from the display itself so this prevents that and you
keep the original functions of the radio. I think it is the best option, and if you are lucky enough to get an on/off volume control with 6 connectors you can also
switch in a small value cap to bring it bang on frequency on both bands rather than setting it in between (See pics below)
This VR control was taken from a scrap Colt 295 CB, many of the older AM Midland radios have
the same or similar VR's too. You can use the similar ones with only 3 connectors but as above,
you can only set the UK FM freq. low and Mid block freq. high to find a reasonable balance.
This is a slightly different way of fitting the Mid block. Same 'piggy back' style but this time it is fitted trackside of the board when there is space to do it. I find it
easier but on some radio's there isn't a big enough gap. Ideal on the large Cybernet 134's. The LC7132 PLL is held to the board with sponge tape which also
helps prevent any accidental shorting on the board.