MMT-432/144: Repair (short circuit due tantalum capacitor)

For an upcoming HAM radio project I was looking for a 2m/70cm transverter. The coincidental Ebay offer of a Microwave Modules MMT432/144 came just in time. The transverter was offered without further explanation as defective and without accessories or documents. Also the external 15 dB attenuator was not included. The relatively low purchase price in connection with the rarity of this model let me then buy it despite a certain risk to acquire a dead piece that could not be reanimated with reasonable effort.

First of all it has to be said that the installation of the transverter's main board in the Eddystone die cast aluminium box is extremely maintenance and repair inconvenient: The power amp module, the subboard of the frequency switch and all connections have to be removed or desoldered beforehand, just to get to their rear solder side. I wanted to spare myself this effort.

The initial investigation revealed a solid short circuit in the power distribution. The transverter has in the input beside a reverse polarity protection also an additional overvoltage protection, which responds above 15 volts and is realized with a thyristor of the type MCR106-1. After detailed consideration and study of the circuit diagrams available in the internet I suspected this thyristor as the culprit: Possibly it had a solid conductivity in the PN junction due to a previously suffered considerable overvoltage, which would be quite possible during operation without additional fuse in the power supply line. No other components on the board could be made out in this regard. The reverse polarity protection diode was flawless.

 

 

The replacement of the thyristor - the old one was cut off at its legs above the PCB and then the new one was soldered to it - did not change the initial situation. Very annoying, because now the main board had to be removed with a lot of effort to find the cause.

 

 

Underneath the circuit board a 22-µF/16-volt tantalum bead was found, which was responsible for the short circuit in the supply circuit! This surprise - and my extreme annoyance about this sneaky installation - the manufacturers of the transverter had definitely succeeded.

 

 

Replacing this tantalum capacitor with a conventional electrolytic of higher voltage capability eliminated the fault. Also the second tatalum of the same type and value above the circuit board was exchanged preventively. After laborious reassembly and subsequent realignment, this transverter, which is now over 45 years old, is now working properly again.

The 15 dB attenuator was realized with one 150 ohm resistor and two 82 ohm resistors - all three non-inductive five watt types - in a small Teko project box as PI circuit. This attenuator in front of the input of the transverter is mandatory for drive levels above 500 mW.

 

 

Conclusion: It is not always easy to reach the goal. Always count on constructive inadequacies or inaccessibility! After such a long time, tantalum electrolytic capacitors always harbor a certain susceptibility to failure and also a safety risk, especially if the voltage rating is low. In the case of damage they tend to a solid short circuit, which possibly causes further damage in the respective circuit. For this reason, it is also imperative to provide a separate fuse in the power supply connection line for devices that do not have an internal fuse, as in this case. If this is missing, catastrophic failures can occur. For the MMT-432/144 and similar devices a 2.5 A fast blow fuse is sufficient.

To thank the Author because you find the post helpful or well done.

That's quite different to mine, which was an early model (only one part of band).
I used mine with an FT101ZD mkII for a while and then a modified CB with entire 10m band added, with, PTT, RX & TX connectors for this transverter and the 2m version. Then in January 2006 I got an FT817ND and the two transvertors went into a drawer.

To thank the Author because you find the post helpful or well done.