Confusing 13cm Transmitter Information!


*** *** November 1 / 2003

A number of e-mails have been received since publishing the Platinum mods last month. A few with Comtech transmitters, from sources other than G1MFG, reported the measured output power of their units to be typically 25 mW which agrees with what Gile's says he measured. I believe I have an answer for at least some of the confusion.

From here where G1MFG spec's the 13cm transmitter output as +9dBm (8mW), the fact that my modules output ≈8mW, and from the Comtech 'FS24TSIM' stickers inside my modules, I believe that my transmitter modules from G1MFG are FS2400TSIM 8 mW modules.

All of the transmitter modules, including mine, are identified on the outside with Comtech stickers as model FM2400TSIM which output typically 25 mW. From Comtech documentation, the only physical difference between the 8 mW FS24TSIM and 25 mW FM2400TSIM modules appears to be that the FS24TSIM has an extra signal pin, identified as 'LEVEL SW', located between the 2 audio input pins. This location lines up with a circuit board trace which supplies power to the final output transistor and which is connected to the 12 volt power trace with a 0 ohm resistor. By removing the 0 ohm resistor and installing an extra pin on FS24TSIM modules, output power can be varied by varying the voltage supplied to this pin. It is evident on my modules that a pin had been located, but was removed, from this location. It appears that someone at Comtech may have modified and identified some 8mW FS24TSIM modules as FM2400TSIM (with exterior labels at least) without realizing, perhaps, that the 2 transmitters had different output levels. If this is true, then the only question remaining is why G1MFG spec's the transmitters with FM2400TSIM 25mW modules as being 8 mW.

I do not know if this is a temporary situation with 8 mW modules being supplied or what other suppliers may be shipping now or in the future. If you want to check the model or what the output power of your transmitter may be, but do not have a suitable power meter, perhaps try checking the smaller identification sticker inside the transmitter module for what it indicates. The stickers in my modules were on the PLL I.C.

 Note: All links to have been removed on this page as the web site no longer exists.

Confusing G1MFG Information!

G1MFG spec's the 13cm transmitters as +9dBm (8mW) and goes on to say "we measured two randomly-chosen samples at 26mW on 2nd August!". The year is not mentioned, however I wish I had ordered my transmitters that year. 26mW is 300% out-of-spec, but I doubt anyone would complain!

There is information about increasing the transmitter output to 30mW on the G1MFG mods page by removing a resistor. 8 mW to 30 mW, or a 375% increase, by simply removing one resistor is a bit much to believe! Reminds me of the ad's for gas line filter like devices promising 100+ miles / gallon.

The TVHAM (American) web site transmitter technical notes mention increasing the 13cm transmitter standard output of 20 mW to 30-35mW by removing one resistor. 20mW to 30mW, a 50% increase, by simply removing one resistor is again, hard to believe.

You will not see a 50 to 375% power increase by removing a 270 ohm resistor from a 75 ohm output. The increase will be even less into a 50 ohm load. The increase was so little that I never even bothered to record it.

So what is the real 13cm transmitter output? 8 mW, 20 mW or 26 mW?

The 4 transmitters I received were Comtech #FS2400TSIM which measured from 8 to 10 mW. Comtech spec's this transmitter module output as 9 dBm (6min/13max) or 8 mW (4min/20max).

I think you would see Comtech quickly add a new model # to their product line
(with appropriate higher cost) if the 8mW transmitter output was really 20-26mW!

According to spec's, the 20 mW mentioned in the technical notes is possible but I wouldn't expect to see that, except for a very small percentage of units and then only for a small range of frequencies near the frequency they are factory tuned at (likely near the center of the tuning range).

Interesting. . .   Comtech does have an identical transmitter with an output of

14 dBm (13min/16max) or 25 mW (20min/40max).

This confusing information must be old information from when the 25mW transmitter was supplied.
If so, then it is time some web pages were updated / corrected!


Confusing Gold Mod 3(a) Information!

Gold Mod 3(a) to improve picture quality says to short out L2, but on the G1MFG 13cm receiver schematic (that is closer to the video circuit of a 23cm, than a 13cm, receiver) shows shorting out L1. None of my receivers, had L1 or L2 shorted out. Shorting out either inductor removes half of the audio sub-carrier filtering so could only make things worse.

Additional information about L1 & L2 in the de-emphasis article says: L1 & L2 which are marked as being 6.8uH – are in fact 4.7uH. This must have been a short term production problem or else an inductance meter somewhere needs service, as all of the 6.8 uH inductors I checked were 6.8 uH.

I have to wonder how much of the information found, came about to correct problems caused by incorrect 4.7uH inductors, and how much of this information remains to cause problems for receivers with 6.8 uH inductors being incorrectly modified.

In the de-emphasis article, the schematic at top of page 2 differs from other information I found.
R5 (4.7K) does not exist (at least on current receivers) and some component values are different.

Why stop now? While nit-picking I may as well add that (one of) the Gold receiver instruction sheets says "there is a wire link on the underside of the Gold receiver PCB which" . . . etc., etc., etc.,  Again, this must be some more old info as I have never seen any links!
Above I added "one of" to the instruction sheet reference because I now have numerous instruction sheets. If you don't like the ones at you can find other versions on the TVHAM web site.

It is interesting (actually - makes no sense) that my 13cm & 23cm receiver video and sound sub-carrier filter circuits are nearly identical and for the same frequencies, but use different capacitor values (i.e. different filter characteristics). The 13cm receiver has better video quality than the 23cm receiver and it was no surprise to see the 23cm video quality improve after changing 23cm receiver components to match 13cm receiver values.

It is interesting that 13cm receiver (with fair video) components are close to the values shown on the G1MFG 13cm receiver schematic, but the schematic is more accurate for the 23cm receiver (with poor video) video circuit which uses different component values than shown on the schematic.

It seems that every place information for a mod is found, that there are differences in the circuit and component values which seldom match what is in the actual circuit. Most never realize the component differences as how many are crazy enough to remove and measure components, or have the equipment to measure component values. I only did so because I was looking for solutions to problems. I have to conclude that there have been many receiver circuit versions supplied.

Enough gripping!  Back to the mods . . . .