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Author Topic: Motorola VHF Amp Help  (Read 2123 times)
AE4SE
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Posts: 7




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« on: June 21, 2012, 02:04:21 PM »

I have a Motorola Vhf NLD 7702A. I need a schematic an info on tuning it to work on 144 MHz. The amp worked OK  on 2 meters but after someone borrowed it, it has stopped working. Amp is quit broad banded as is but should be brought into the proper spec. I figured I would go ahead an do a complete rework fixing what ever went wrong an moving the filters lower into the band. After opening it up for first time, I am surprised there are no part numbers to look up.  Need all the help I can go on this one.

Amp worked fine but now will not transmit. I donnt hear the relay kick in or anything. I have the correct power but I am not sure where to start without schematic. If you have experience with this amp or know who could help please contact me. Thanks Gene


73 AE4SE
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KB1GTX
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Posts: 461




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« Reply #1 on: June 21, 2012, 06:03:37 PM »

With no relay,, did you check to see if there's power on the board?
Is the fuse blown?,, if so, was it wired backwards and is the reverse diode toasted.
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AE4SE
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Posts: 7




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« Reply #2 on: June 22, 2012, 01:42:22 AM »

Checked for a fuse an I could find nothing simular to a fuse of any shape or size I have ever seen. I did not do much of anything else tonight. There is power at some points on board. The relay is Comm Instruments, which rarely ever go bad. Its sealed in a metal can an I have never known one to go bad. Still it could be the problem. Its hard to know what is what the caps are not standardized. Numbers that do not correspond to caps, possible Motorola numbers like Heathkit used.  There is a small transistor beside the relay I suspect maybe trouble some, most likely used to power relay. I will do more this weekend but without a way to look up these numbers replacing parts could be a headache. I am really going to need a schematic. If I do find the culprit part I have no idea what to replace it. Thanks for help so far it is appreciated.....
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NA4IT
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Posts: 885


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« Reply #3 on: June 22, 2012, 04:20:24 AM »

You might poke around on this website (http://www.communication-concepts.com/). Most of the amps and kits they have are Moto designed. I have found their amps to be very similar to the Motorola mobile amps like you have.

Here is a hint... if you bypass the relay, and put a fan on top of the amp, they make a pretty good repeater amp.
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W9GB
Member

Posts: 2626




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« Reply #4 on: June 22, 2012, 06:33:23 AM »

Curtis -

For Motorola Land Mobile Radio (LMR) amplifiers, you have TWO Roads for repairs.

1. Post question at BatLabs or Repeater Builders Tech Info Page (RBTIP), the current or former Motorla dealers may have those service manuals (schematic, part numbers, board layouts) scanned.
The Service Manual will provide the data you seek.

2.  Helge Granberg's (K7ES/OH2ZE) Application Notes.  Helge wrote the books for Motorola's Solid-State RF devices since late 1960s, untl his death in 1996.  The Motoola VHF amps, such as the one you referenced, closely follow those notes and designs.
Based on your electronics knowledge and experience -- this may be enough.

w9gb
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