A Modification of the MICOR UHF Mobile Low Level Amp for dependability.

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Back to Home The UHF MICOR® Low Level Amplifier
A modification for dependability
Original concept and engineering by Jeff DePolo WN3A
Documented by Kevin Custer W3KKC

Concept:
When running the Motorola MICOR UHF mobile as a repeater or link radio it is a good idea to perform modifications that will allow more reliable operation.

Description:
The MICOR Mobile Low Level Amplifier (LLA) is prone to failure if the unit is converted to higher transmitter duty cycle operation as in repeaters or links. Modification to the heatsinking of the low level amplifier module and reducing the B+ voltage feeding it will result in greater dependability. The controlled stage, driver and PA (when used) seldom give any trouble, but the low level amps account for a majority of the failures. Below is a summary of the shortcomings to the LLA:

They aren't heatsinked well enough. They normally make about 1.6 watts out. There just isn't enough heat sinking capability in that small bracket that serves as the heatsink for the "final" transistor. The other transistors are not heatsinked at all.
The surface-mount construction leaves a lot to be desired. These things were made in the early 70's predominantly, before Motorola really learned the art of surface-mount construction.

Benefit:
Better heatsinking and reduced dissipation will result in longer life from the low level amplifier stage.

Modification:
Reducing the B+ voltage feeding the LLA is one of the best solutions for improving the life of it. Knowing that most MICORs will work just fine with a LLA output in the 1.0 watt range, and often many work down to as low as 0.5 watt, dropping the voltage on the collectors of the last two transistors in the LLA will reduce the power and keep it cooler. I simply disconnected the red wire coming into the LLA, next to the RCA jack and ran it to the output of the +9.6 volt regulator. It usually is connected to the metering board where it gets +13.8 from the purple wire coming into the board from over by the pass transistor in the power control circuit. I picked off +9.6 from an unused pin on the underside of the system board, near the big choke. Steal it from anywhere as long as it's not coming through a thin trace that might act like a fuse.

With the LLA running from 9.6 volts its output starts out at about 1.4 watts and drops to 1.3 watts when fully warmed up. The dissipation/efficiency is greatly improved and it runs a whole lot cooler. Spectral purity is fine and dandy.

Improvement of the heatsinking of the LLA will result by adding a washer or similar metallic piece to help transfer heat from the unit to the large heatsink. This step may not be necessary but certainly should be considered when building a repeater from a UHF MICOR mobile, especially if higher duty cycles are expected.

Take a big fender washer and file it down so that it is perfectly flat. Most washers will be a bit concave because they are manufactured by stamping them out of sheet steel -- in our use it has to be very flat. Dig through your junk box until you find some that are flat and very thin (sorry, don't have a measurement to give you). Anyway, coat both sides of the washer with thermal compound and insert it between the bottom of the LLA and the MICOR heat sink. This will help dissipate some of the LLA's heat to the MICOR's beefy heatsink. Be careful; if the washer is too thick, you'll put a lot of stress on the LLA when you snug down the third mounting screw (the one that goes into the MICOR heatsink). You can probably come up with something that will work a little better than a washer if you look around.

Operational Concerns:
A question about the 9.6 volt regulator may be raised concerning its ability to handle the increased heat dissipation over time. Normally not much current is drawn from the 9.6 volt supply. After leaving the MICOR keyed overnight the heatsink on the 9.6 volt pass transistor gets pretty warm, but not so warm that one should be concerned. Operational tests over some time have shown it will handle the current and in itself not become a source of trouble. In most cases, the 9.6 volt regulator is not taxed because you are likely not running all of the things that the regulator was designed to run.

IN SOME RADIOS it has been found that some resistor values may be different and the 9.6 regulator *may* fall out of regulation. The text below explains this problem.

Dave Novotny WA6IFI wrote:

Hi Kevin,
I found the problem with the troublesome MICOR UHF mobile. There were TWO incorrect resistors installed (vs the schematic). R901 was 5.6K (6.0K measured) vs 1.5K in the book. R902 was 1.2K vs 560 ohm. I installed the correct resistors and the voltage regulator is solid, even with the LLA being powered from the 9.6V rail.

The symptoms were:
1. Transmitter off frequency
2. Receiver works OK until transmitter keyed. When keyed, Rx signal goes away, which makes sense since the drop in the 9.6v supply causes the oscillator to go off freq.

Thanks,
Dave

If you are concerned, verify the resistors above or use a 9 or 10 volt fixed regulator (LM-7809 or LM-7810) either of which are good for 1 amp. Just heatsink it to the radio chassis somewhere, and use proper bypassing techniques. A LM-317T variable regulator can also be used but requires a few additional components.

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This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.