Kenwood TM-732E with no display

The Kenwood TM-732 was introduced onto the market between1992 and 1994. It can receive on 118-174 and 410-470 MHz. It transmits on 144-148 and 430-450 MHz with a maximum of 50W, which makes it very attractive to HAM and MARS operators around the world.

This particular unit came in with the main display not working. The Power switch also had no effect, however, it could transmit as normal. In the bottom left hand corner of the display the channel number was visible. As the radio received and transmitted, the OM was keen to see if we could save it. So the set was opened up and the service manual consulted.

As the power switch was not working and the power LED was not lighting up, this was the first spot to begin with the analysis and HW debugging.

SW1 is the power switch. The first thing I noticed was that when I probed the contact of the switch there was no voltage available. Tracing the circuit back, I saw that voltage was on C7 but not on C8. As C7 is on the top side of the board and C8 on the bottom, the issue looked like a Via from top to bottom on the circuit board was gone. Closer inspection of C8 showed that the electrolytic was indeed leaking and had most likely eat through the trace.

A new C8 was procured and the via was replaced by a wire from C7 to C8.

Once C8 was replaced, the radio was back working again. Job done!

HomeMatic HM-CC-RT-DN Thermostat

 The HomeMatic range of home automation products are excellent value for money. I have had them in service in my home for 10 years. Recently, I came home to a very, very warm living room. My HomeMatic thermostat was showing an F1 error message. This means that the valve must be sticking, making temperature adjustments impossible. 

Now, sticking valves are a common thing in radiators. After a summer of no movement they tend to stick at the beginning of winter. Removing the temperature adjustment knob and banging the valve head with a hammer is usually all that is needed to free things up.

So I removed the Thermostat and freed up the valve, however, I was still getting the same F1 error message. Time to take the HomeMatic thermostat to the bench and see what's up.

Attaching it to my lab power supply showed that the device was drawing 300 mA whilst doing nothing. That is way too much for one of these extremely low power devices. So the case was opened to see if the gear-box was sticking and causing the electric motor to draw excessive current.

The mechanism was taken apart and inspected. It turned freely and without any issue. So a deeper inspection of the electric drive components on the circuit board was necessary.

The board was pulled and instantly a component fell out into my hand.

This component was at one time soldered to the RFK1 board behind it. So it was resoldered into place and the unit powered up again, however, this time the display on the thermostat showed a "trd" error message. Upon reading the documentation, I saw that this means the transciever module was not working.

I am guessing that the component on the RFK1 board contained loading coils for the antenna. Once they fell off, the SWR went high and burned out the RF circuits. 

No repair for this one.