CONVERSION OF FM WESTMINSTER FROM "P" BAND TO "E" BAND (79-101MHz to 68-88MHz) ============================================================================== Section 1. Receiver a. General notes. All the coils that need modification are housed in the large cans. These cans should be carefully removed by pulling vertically up and gently rocking side to side along a line between the PCB clips. As the can starts to rise it may suddenly free, so at this stage be careful not to allow a sudden movement to the side as this may crack the coil former by pushing the coil sideways. All the following changes can be done without removing the PCBs, however, if the access is difficult then disconnect all wires and remove PCBs before starting conversion. b. The oscillator multiplier unit. Remove the 15pF capacitors C2,C8,C9 across L1,L3 & L4. Keep them for later. Replace C2,C8,C9 with 22pF silver mica Lemco types if possible, or quality ceramic plate, NOT the cheapo ceramic de-coupling types. When this is complete replace the cans in the correct places (note the doorways to allow the coupling capacitor to pass through), refit the PCB (if removed), fit a crystal in the RX oscillator board, power up and tune L1 for max. reading at TP1 then tune L3 & L4 for max. reading at TP2, repeat the tuning for a max. at TP2, the current should be 30-60uA. c. The RF front end. The ideal way would be to replace all the coils with the correct part No's, but as the parts are not usually available now, a simple change of capacitor value on each coil seems to do the job with very little loss of performance over that achieved with the correct parts. Here we can interchange values to achieve the new tuning range so minimising our outlay on extra capacitors. Remove C5 12pF across L3 replace it with one of the 15pFs from the Osc. unit. Remove C8 10pF across T1 replace it with the 12pF from L3. Remove C15 12pF across L4 replace it with another of the 15pF from Osc unit. Remove C20 10pF across T2 replace it with the 12pF from L4. Finally connect a 2.2pF tubular ceramic across L1 (not across C3). Replace all cans (and the PCB if removed) once again noting the correct position of the doorways in the cans. Tuning is best accomplished with a signal generator but failing this make an injection loop of 1 turn of PVC insulated wire connected to a length of coax, plug this into a 4 meter antenna and using a local station as a signal source, slide this loop over L4 (remove can again for this method), tune T2 for maximum signal at TP1 on the 455KHz IF unit, or if tuning by quieting, for minimum noise on a silent carrier, remove loop, replace can on L4. Remove can on L3 and loosely couple the loop to L3 tuning T1 and L4 for best results, at this stage the RX should be getting quite sensitive and should only need a low signal to quieten the noise. If this is the case then all is well, so replace all the cans, connect an antenna and work through all the coils reducing the input signal so that some noise is still heard. The final tweak of L1 should be done with the antenna connected to the rig and receiving a weak signal. The optimum noise performance does not always coincide with the maximum signal position when tuning L1. End of Part 1. Part 2 covers transmitter modification CONVERSION OF FM WESTMINSTER FROM "P" BAND TO "E" BAND (79-101MHz to 68-88MHz) ============================================================================== Section 2. Transmitter a. General notes. The TX modifications are covered in two sections, the first deals with the modulator driver board and consists of capacitor changes only. Fortunately, in many cases, the values of caps required in the earlier stages can be found in the later stages. All that needs to be done is to remove undamaged, those to be re-used. The coils used have the same part No's for both bands so no changes are needed here. b. Modulator driver unit. Starting from the modulator coils C2 & C9 across L1 & L2 may not need changing but we can check this shortly. Likewise C12 & C15 across L3 & L4 may just be OK as they are (some sets do need changing). Remove the 12 Volt positive lead from the PA unit to disable it during the tune up. With a crystal of 2.925-2.937MHz (carrier/24) in the TX oscillator board, key up the TX and tune L1 & L2 for max. reading on TP1. These cores should peak in two different places either side of the actual winding but if no distinct peak is found and the core is in the centre of the coils then the caps C2 & C9 need to be raised to 120pF. If these coils peak OK then move the test meter to TP2 and tune L3 & L4 for a max. reading. These should peak with the cores towards the bottom of the former but if the peaks are indistinct then C12 & C15 will need to be increased to 150pF if they are 120pF. Now we start the mean and crafty bit, C17 & C20 need to be 39pF not 27pF across L5 & L6. If we look at C23 & C25 across L7 & L8 these are 39pF and need to be 47pF, so remove C17 & C20 and keep them, remove C23 & C25 and put them across L5 & L6, put a pair of new 47pF silver mica's across L7 & L8. Next job is to remove C27 & C30 across L9 & L10 and replace them with the 27pF caps removed from L5 & L6, lastly remove C32 & C35 from L11 & L12 as these have to be 27pF which need to be acquired. Continue the tuning of L5 & L6 by measuring on TP3. Follow up with L7 & L8 measuring on TP4 then L9 & L10 measuring on TP5 and finally move the test meter to TP1 on the TX power amplifier board and tune L11 & L12 for maximum. Those of you who are familiar with valve transmitters will remember the distinct dip in anode current when bringing the anode tuning onto frequency. This technique can be used when tuning the multiplier stages on the modulator driver unit, just leave the test meter on the test point after you have tuned the previous pair of coils for a peak, then tune the next coil for a dip. This will help get the right point, then transfer the test meter to the next test point and tune the second coil of the pair, returning to the first for a final tweak to a peak. c. PA unit. The part No's seem to be the same for both "E" and "P" bands so all that is required is to re-tune, I have done four conversions so far, each giving at least 15 watts out on 70MHz. d. Tx filter. No changes required, seems to be same for both bands. Finally, a very useful tool to assist tuning up the multiplier is a 25-50uA meter connected to a probe loop consisting of one turn of PVC insulated wire, a VHF germanium diode (GEX66 etc) and a 1000pF ceramic capacitor. 25-50 uA +---------+ + gex66 MC l \ l------------+--l<-------> Loop Meter l \ l 1n= l \ l------------+-----------> l_________l - Just lower the loop over the coil on test and twitch the core, you can use this technique to get an idea of the sort of level of RF expected on each stage. It doesn't take long to get the "feel" of it, most stages, except the phase modulator coils, will move the needle hard over if all is OK. I wrote these notes when some of my locals aquired some cheap "P" band Wessies and needed some assistance in "getting them down" onto 70MHz. I have since discovered that these were some of a very large batch of recently disposed of Public Service radio equipment. There must be more out there? Well, that's it, see you on four! Mike, G8AMG @ GB7AAA PORTED AND REALIGNED FROM PC TO AMIGA FORMAT BY BOB G0LBQ - N6XDI.