I have incorporated the above mods into my gear. There is no question in my mind that there are visible improvements to be gained by the transmitter mods. Not just getting rid of the poor l.f. response, but improving the contrast range, and really losing that 'fm' video look and giving a picture as good as commercial off-air pictures, especially if a proper black level clamp (BLC) is included. I believe that the mods to the transmitter are essential, even if you have not done the rx mods, but the capacitor and the resistor (simple pre-emphasis) feeding the drive level preset should be removed and a link inserted instead. This helps by improving the syncs and reducing the hf peaking (assuming proper pre-emphasis is being used e;sewhere).
The receiver mods. If fitted, the mod resistor onto the pin of the TA8804 inside the can should be removed and the filter components really need to be changed to the standard CCIR de-emphasis circuit which will mean an extra amplifying stage with a BLC to compensate for the attenuation, to ensure compatability with other ATVers. I have been working on a small board (see below) that will fit on the top of the tuner can and replace the video circuits entirely .
On the transmitter module the 1.5uF capacitor is best managed by bridging the existing cap with a 0.47uF 0805 SMD (i.e. same size as the 1uF original). The pre-emphasis components to the right of the preset should be shorted out. I have found that some frame droop can be caused by increasing the 1uF, and I feel that it is best left as 1uF for most signals. The 10nF is easier to fit if laid on top of the 35pF capacitor.
If after doing the tx mods you get no sound, check that the tiny 39k resistor is not displaced and that there is not a bridge between it and the small cap to its side after soldering in the .01mfd cap. A pulsing picture (on and off of frequency with some modulation) probably will be a solder bridge from the 220k resistor to a through connection just to its side (left side in photo above).
One (new) receiver was rather 'speckly' and appeared slightly low gain, the quadrature coil (4 turns green enamel wire, just visible below the A in signal strength meter picture) needed a nudge(less than 0.5mm to the right on the right hand half of coil, this centred the signal nicely.
Once again thanks are due to VE6ATV and please ensure you visit his website for full details and any further updates.
If you have any hints or further mods let me know and I will pass them on for others to see.
ADD-ON UPGRADE BOARD FOR COMTEC 23cm TRANSMITTERS. click picture for details
The course chosen was the Black Level Clamp with agc option of the MAX7450. R1,R2,R3 attenuate by about 4dB. This gives a good range for most 75 ohm 1v inputs. But a 500 ohm pot could be used instead.
Use the 1k preset inside the Comtech can to set max deviation. Sadly,no supplier is currently stocking the MAX7450. Contact Maxim UK for samples/ordering details. Click on circuit for full pdf.Take pin 7 to ground to change from +6dB gain to 0dB. If you prefer not to have a negative supply, there is the MAX 7452.There will have to be a coupling capacitor >470mfd instead of R5.Connect the pad to ground instead of -5v. Pins 5 and 7 must be either +5 or ground not o/c . The pad under the ic must be connected to -ve to get an output.
TX LOW SOUND CARRIER MOD
Place a 10nF capacitor with a 4k7 in series across the 47k resistor (in red ring).
Do the same if required for the second sound channel across resistor to the left of the red circle.
SMD 0805 components will just fit in the space.If the 6.5Mhz is not required lift one leg of the left hand 1k to stop the carrier being used.It will save a little bandwidth and power. Picture: G1MFG
In response to requests from a number of Amateurs, I have modified the above circuit
to add a simple clamp and amplifier to replace the unobtainable MAX7450 .
If you need a smd layout , click on this diagram for a pdf. .
Any npn medium power transistor will do for Q2. The output Darlington will give 2v pk video out across 150R. Set R1 for consistent results over a range of signal strengths.