...... Welcome to G4IZH.com ......

Here are the modification details for converting a Trio TS-700 two metre multimode transceiver to four metres. The changes are quite involved and do require patience but the rewards are very worthwhile resulting in a 15 watt multimode transceiver covering 70.000 to 70.500 MHz.
I have now converted two of these rigs and the full details are below. NB, the TS-700G and TS-700S have many circuit changes and this will need to be considered before attempting a conversion.
Please do not blame me if you make a hash of the conversion..........

The following units require modification, RX front end, HET,
MIX, FINAL and BPF.

RX Front End unit
a)     Fit a 47pF capacitor across T1
b)     Fit a 27pF capacitor across T11
c)     Fit a 20pF capacitor across T12
d)     Fit a 5pF   capacitor across C2

HET unit  
a)  Remove the crystals X1,X2 and X101 and disconnect and isolate the leads from the band switch to positions 125,126 and RPT.   Fit a 51 to 51.6 MHz overtone crystal (HC25/HC6 etc) in X1 location.
b)     Fit a link across diode D1
c)     Remove 2 turns from coil L4
d)     Fit an 82pF capacitor across T1 sec
e)     Fit an 82pF capacitor across T2 sec
f)      Fit a   75pF capacitor across C62
g)     Fit a   75pF capacitor across C68
h)     Fit a   47pF capacitor across T5
i)      Fit a   39pF capacitor across T9
j)      Fit a   39pF capacitor across T* (Not shown on circuit diagram but is adjacent to T9).
k)      Fit a   20pF capacitor across C6

NB,  The core in L4 is adjusted for linear +/- RIT swing and the core in L2 will adjust the crystal frequency by about 10KHz.

MIX unit
a)     Fit a 39pF capacitor between TP1 and ground
b)     Fit 33pF capacitors across vari cap diodes D2,D3,D4 and D5
c)     Fit a new  "Test Point"  to the anode of D2
d)     Change L1 to 5 turns 3/8" dia 18swg
e)     Change L2 to 2 turns 3/8" dia 18swg
f)      Change L3 to 4 turns 3/8" dia 18swg
g)     Change L4 to 6 turns 3/8" dia 18swg

FINAL unit

a)     Change L1 to 3 turns 3/8" dia 18swg
b)     Change L2 to 4 turns 3/8" dia 18swg
c)     Change L3 to 2 turns 3/8" dia 18swg
d)     Change L4 to 5 turns 3/8" dia 18swg
e)     Change L6 to 5 turns 3/8" dia 18swg
f)      Change capacitors C5 and C6 to 68pF
g)     Fit a 33pF capacitor across TC3

NB, 18swg is
equivalent to 1.2mm.

BPF unit

Due to the restricted space in this unit for housing a four metre band pass filter it was decided to remove the coil and its tuning capacitor and dirctly connect the input to the aerial socket, there is room in the Final unit to fit an additional 3 pole low pass filter if required.
The rf sensor which detects power for the front panel S/Power meter is in this unit and needs to be modified in the absence of the coil, this is simply achieved by connecting a 15pF capacitor in series with a short "pick up" lead wrapped a few times around the lead connecting the aerial socket, the capacitor is connected to the junction of D1/L1.

ALIGNMENT

Most of the measurements were taken using a 60MHz scope with
1 to 1 probe.

HET unit

Connect the scope to TP1 and tune T1 and T2 for maximum, transfer the scope to loc 133 and tune the coils T3,T4,T5 and T9 for maximum output in the receive mode, this should be approx 150mV.

RX front end unit

Set "DRIVE" control to mid point, connect a signal generator
to the aerial input tuned to 70.2 MHz, find the signal on the TS-700 vfo, at this point the vfo will not read 70.2 MHz, this will be aligned later, tune T1,T11 and T12 for maximum received signal.

MIX and FINAL unit

Connect a power meter and dummy load to the aerial socket,
ensure that the "DRIVE" control is at mid point and the mode switch to FM.
Connect the scope via a 20pF capacitor to the new test point
created earlier, tune T1 for maximum signal, remove scope.
Tune the coils T3,T4,T5, and T6 and the trimmers TC1,TC2,TC3
and TC4 on the MIX unit for maximum output using a scope or some other rf sniffer.
In the FINAL unit tune TC1,TC3,TC4,TC5 and TC7 and adjust the
spacing of L6 for maximum output shown on the power meter, this should be about 16 watts.
Switch to AM and adjust VR3 to give 5 watts, see manual.

VFO unit

Finally calibrate the VFO using an accurate signal source.
The parameters are set with careful adjustment of L1/TC1 in
VFO unit, these are accessible from outside the box under metallic foil, DO NOT OPEN the VFO box.
L1 sets the calibration at 70.000 MHz and TC1 sets the
calibration at 70.500 MHz, these are of course interactive and repeated adjustments are required to make the vfo accurate across the range. The dust cores should now be sealed with beeswax to prevent them moving and a final adjustment of the receiver front end tuned circuits can be made, due to the nature of these larger coils they should not normally need to be sealed.
The full service manual can be downloaded from the internet free in order to assist you finding your way around the transceiver.

Additional Mods.....
 
The "horrible" Jones power plug has been removed and replaced by a 3 pin XLR which conveniently fits the same size hole. A reverse protection diode has been fitted and the existing rear panel fuseholder has been pressed into service as the DC line fuse, the mains transformer was removed as 13.8v DC operation only is required.
See the separate sections for correcting the CW operation, ie aligning it with USB and not LSB as it was originally, utilising the original tone burst generator (Not req on 4metres!) to provide CW sidetone.

Below are some pictures after conversion.....