Receiver & Transmitter Mods
for G1MFG / Comtech units

My		into the

I started my FM ATV adventure using a standard 23cm receiver. Everything was ok until I tried a Gold receiver. Since, I have spent a significant amount of time investigating problems to make my Gold receivers useable. This page describes my  long DETOUR  from other projects to fix a problem with the Gold receiver that eventually resulted in the  Platinum Modifications  and improvement mods for all of the G1MFG 13cm & 23cm receivers and transmitters. These modifications may also apply to similar Comtech units from other suppliers. This page is to share my modification experience, information and hopefully make a worthwhile contribution to on-line information. On this page

find links to schematics and component data sheets, what initiated my investigations, the video waveforms used to investigate problems and the effect each modification has upon these waveforms, and the  Platinum Modifications .

If you have a Gold receiver, check out the Remove Gold Mods Mod that shows why
each modification should be removed and how to do it.

Please

if you have any information or comments to share.

 Note: The g1mfg.com web site no longer exists so previous links to site have been removed.

Also Note: Much has changed since these pages were written. I don't believe Comtech Rx's & Tx's are any longer available and there's now much better equipment designed for R/C aircraft FPV to use, but the pages are being left for those that still have and use Comtech units to reference. And for those still using these Comtech units, please note that the vertical sync problem is actually a transmitter design problem that's next to, if not impossible, to correct and that the receiver mod's are simply an attempt to try and make the problem less sever.

I know how much work and time is involved in maintaining a web site, so I normally excuse a few errors or disparity.
However, though out this page I have commented on information that I found confusing at g1mfg.com in order to help others make sense of it. Perhaps I am simply slow, too critical or expect too much. If so, I apologize to Giles @ G1MFG.
My comments intend no offense and I hope none is taken.

P.S. It was never my intention to discredit the Gold mods during investigations.
I am simply reporting what was discovered.

Data Sheet, Schematic &
Other Technical Information Links

It was very difficult to find any technical information, other than what is was on the G1MFG modification page(s), on these or any other of the ATV modules manufactured by Comtech, a manufacturer of tuners in Taiwan. Poor quality JPG images of a Comtech 13cm receiver and transmitter schematic are were on this G1MFG page, but as noted, they are not 100% accurate. Other than the voltage regulators, the 13cm & 23cm receivers look alike, however they have slightly different circuitry and many component values differ. For what it is worth, here are better copies of the Comtech Receiver & Transmitter schematics that I made and used before creating my own schematics which are accurate for the receivers & transmitters received in 2003.

Test equipment setup while working
on the receivers and transmitters.

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get addicted to technology. While institutions such as Morningside Recovery deal with extreme addictive behavior, the steps to recovery from technology addiction can be as easy as turning off your computer or cell phone.

The Platinum Modifications
for G1MFG 13cm & 23cm Receivers and Transmitters

These receivers & transmitters may not perform any worse than similar units and the receivers have very good sensitivity, but they can use improvement. The Platinum modifications improve the receiver and transmitter performance as much as possible without adding additional circuitry.

There is also a mod to provide a "S" meter signal from the 23cm tuner, where to find the13cm tuner "S" meter signal and how to add a "S" Meter and Expanded Scale Battery Volt Meter to the receivers. Digging deep into the tuner and transmitter module circuitry and by custom programming the PIC chip, there are some neat things possible that I plan to explore and present here in the future.

Note: These modifications are not for everyone and many will require
a very small tipped soldering iron, tweezers and a good pair of magnifying glasses!

Warning! Perform these modifications at your own risk!

Use the following links to quickly locate individual modifications and other information.

Modifications are quite simple and only require changing a few components, so don't let all of the waveforms and information scare you away from trying these mods. I wanted to document, for my own reference, what I found and I may have included more than necessary of the 400+ recorded waveform images. I hope explanations are clear and thorough enough to properly explain things and answer any of your questions.

Field Rate Multi-Burst Signal shows typical Vertical Sync Period Waveform.

The un-modified 13cm receiver video is very good. The 23cm Basic receiver is worse due to poor low frequency response of the transmitter. The Gold mod that increases the value of C8 in the receiver makes the receiver's output worse, to the point that the receiver becomes unusable due to the vertical hold problem.

Field Rate Square Wave Signal to enhance DC Restoration Problems.

The un-modified 23cm units have trouble handling a signal of this type. Even though the received waveform from Platinum modified receivers is less than ideal, other equipment seems to have no problem dealing with it.

13cm and 23cm Platinum Receiver Waveforms
show that there is little difference between the two.

Reference: Transmitter Schematic

In the transmitter modules, video goes through a parallel C1-R1 (100p // 2.7K) video peaking network before going to VR1, for deviation adjustment, and through C2 to the modulator.

Waveforms show the effect of
C1//R1 Video Peaking Network
on video signal going to the
transmitter modulator.

Received Multi-Burst Waveforms
without and with transmitter peaking.

The C1//R1 video peaking circuit effect is removed in the receiver by a series C-R circuit. This is likely some form of noise reduction pre-emphasis, however between manufacturers there seems to be no standard as to the amount of peaking applied. This presents a problem for ATV groups where equipment from any number of different manufacturers could end up being used.

Does everyone have to figure out how to modify their particular equipment for a flat response before adding standard CIRR pre-emphasis? Or is the group forced to use whatever type of equipment the group initially starts out with and forever locked in to using? Or does everyone plead ignorance and ignore the fact that pre-emphasis varies between equipment and simply live with the consequences?

I would appreciate hearing from anyone who has answers for any of the above questions or how different groups have addressed this problem. I would also appreciate hearing from anyone who has good links to information regarding this topic. Any information received will be shared with everyone here.

Component designations (i.e. R1, C1, L1, etc) refer to components on 13cm Transmitter Module Schematic.
 

Component designations (i.e. R1, C1, L1, etc) refer to components on 23cm Transmitter Module Schematic.
 

This modification improves the low frequency response of the transmitter by changing two capacitors in the transmitter module.

NOTE: This mod will make the Gold receiver vertical hold problem worse, so the Gold receiver mods must be removed. Then keep going and do it right by performing the Platinum receiver mods.

Place cursor over text to show
component identification arrows.

This Modifications Effect

Received Field Rate Waveforms
Before & After Transmitter Modification
(Waveform video taken directly from the
receiver's tuner output pin.)

Close-Up of Waveform Vertical Sync Period

After modification the sync level remains more
consistent. This is half of the solution to the Gold
receiver's vertical lock problem.
(Removing Gold Mod-3b is 2^nd half of the solution)

Increasing the value of the 36 pfd capacitor also increases the PLL response time. This makes it impossible for the PLL to correct large frequency step changes so the PLL filter 1.0u capacitor must be increased in value to reduce the response time. Before increasing the 1.0 ufd capacitor's value, it was interesting to watch received video fluctuate for a second or more while the transmitter's PLL corrected small transmitter frequency changes.

It is nice to have frequency agile transmitters, but PLL circuits in FM video transmitters can be a big problem. Think about it. First, we want a PLL with a fast response time to quickly respond to transmit frequency changes. Even if we don't mind waiting, the response time must be at least fast enough to correct for the largest frequency step change that will be made. Then, when we modulate the VCO with video, we want a PLL with a slow response time that will not correct the VCO frequency changes due to FM modulation. We can not have it both ways so compromises must be made. So if you want a good FM video transmitter, forget about being frequency agile and use a crystal oscillator type transmitter.
 

13cm Tuner

"S" Meter Signal
A signal strength voltage is available
on pin 9 of the receiver's tuner.
(Yellow arrow indicates Pin 9)

This voltage varies from ≈ 0.9V (no signal) to 4.3V (AGC saturated).

See Receiver Sensitivity Table for
"S" meter voltage versus signal strength.

See "S" Meter Mod for details of how to add a "S" meter.

23cm Tuner

The 13cm & 23cm tuners look (and are) very similar.

A signal strength voltage is available on pin 19 of the TA8804F receiver I.C. in the tuner. Unlike the 13cm tuner, a jumper must be installed to make the "S" meter signal available on pin 9 of the tuner.

See Receiver Sensitivity Table for
"S" meter voltage versus signal strength.

This modification is the G1MFG "S" Meter Mod, with some additional information.
Reference: G1MFG - "S" Meter modification

In the tuner, locate the two pads (indicated by yellow arrows) on either side of a compartment 'wall'. There is just enough room under the wall to solder a small wire link between the pads. The signal strength voltage will now be on pin 9 of the tuner (indicated by red arrow).

See "S" Meter Mod for details of how to add a "S" meter.

The G1MFG "S" Meter modification fails to mention that pin 9 of the tuner is connected through a 220 ohm resistor to pin 10 of the SP5055 PLL I.C. This has no effect on the signal strength voltage, but if one ever wanted to make use of the PLL I.O. signal, then it would need to be relocated. Fortunately, it is easy to relocate the PLL signal to the same tuner pin that the 13cm tuner has this signal available on.

1. Remove the 4 MHz crystal.
2. Relocate the 220 ohm resistor indicated in photo A to the position indicated in photo B.
3. Re-install the 4 MHz crystal. The PLL Port 3 I.O. signal is now available on pin 12 of the tuner.

NOTE - The "23cm Transmitter - Improve Video Mod" should also be done!

Refer to 23cm Receiver Tuner Schematic

This mod will extend either the Analog AFT (Automatic Fine Tuning) or Digital AFT signal from the TA8804F I.C. to pin 5 of the 23cm tuner. This signal is not important to have available at this time, but I plan to make use of the AFT signals to provide Automatic Fine Tuning. When sucessful, I will present details in a future article.

Place cursor over text
to show component identification arrows.

• When "Low  in Frequency" the Analog AFT voltage is > Vcc/2 or 2.5 volts.
• When "High in Frequency" the Analog AFT voltage is < Vcc/2 or 2.5 volts.
• When "On Frequency" the Analog AFT voltage is = Vcc/2 or 2.5 volts.
   
• When "Low  in Frequency" the Digital AFT Out2 = 'Low' and Digital AFT Out1 = 'High'.
• When "High in Frequency" the Digital AFT Out2 = 'High' and Digital AFT Out1 = 'Low'.
• When "On Frequency", one or both of the Digital AFT signals may be pulsing and at some point,
  both will be High. The period of the pulse train (on the unit I experimented on) was 16 ms.