Kenwood TS 940 Problems and Troubleshooting Guide
If you open up your Kenwood TS 940, turn it on, find it dead and not working at all. The first suspect would be on the power supply of the radio. But fixing the circuit of radio is not so easy as it appears to be. When you look at the radio, all you can see is a complex of wires, and there is no apparent way to fix it. The power supply point is not given separately at the opening of the external body of the radio. Rather, it has been fixed inside really abruptly.
Changing the AVR board
So, we need to change the AVR board and some transistors at the heat sync end. For this purpose, we would primarily remove the DC fuse protection from its previous location. It is quite interesting to know that the internal design of this radio is very similar to common Ham radios.
The most common problem with the AVR board is that the voltage required by every point of connection is overused. The radio can operate at a very low range of voltages, but an excess voltage point is given. If you calculate the total number of voltages required by certain points, you can see that there is a waste of 40% of energy.
Now, as we have tested all the components and plugs that were using excess power, we would remove them with new low voltage operating plugs. It solves your problem of low voltages. By using a solder, you can fix your new plugin place. It can be of about 5 volts each replaced by all those heavy plugs.
Changing the diode
The Zener diode is grounded for the elimination of power supplies and semiconductors. When the power supply is turned off of the semiconductors, make the voltage move upward to the capacitors present in the regulator circuit. Generally, a 33 volt Zener diode is used, which limits the movement of voltage to about 31 volts.
So we can say that close every the purpose of the diode is to limit the current passing through it. There is no fuse installed in it, and only a device that regulates voltage is placed so that the limit on voltage is made coming from the output transistors.
So, if you adjust any of your transistors with a voltage higher than 30 volts, the Zener diode would make a circuit short, saving itself where it is destroying all the semiconductors and the power supply points. To solve this common problem, we would replace the Zener diode with one of higher voltage, mostly about one watt.
The resistors are automatically needed to be changed to operate with your relevant Zener diode. It would change the overall response with the voltages and currents even if it exceeds over 30 volts now with the new powerful transistors.
If you change the location of the fuse, it will turn off the power supply when you reach a voltage of about 30 volts. So it would help if you made sure that the new voltage of about 28 volts is stable and would not turn off your power supply. It would allow you to work with lower voltage supplies.
Changing the heat sync
Now we need to change the heat sync, which is generally present at the back of the radio. For this, the replacement of transistors is required with the transistors of high rating. The fuse also needs to be relocated. After then the semiconductor attached with heat sync needs to be replaced. Now your whole heat sync system is replaced, you have achieved a step further to improve your radio's working.
Changing the frequency pattern
The most common problem that most of the Kenwood TS 940 users face is that the frequency is displayed in dots on the screen. The most predictable reason behind it is that it is due to the lock loops being displaced from their position. One way to solve this issue is by removing all the solder present in the parts of lock loops.
After you have removed the excess solder from the lock loops, the next step is to replace the locks in position and applying the solder again. In the majority of the cases, the problem is solved by adopting this method, but if your problem persists, it means that you have to go through the AVR board of your radio.
There are generally six lock loops in the AVR board. Four of them are directly present in the AVR board where has two of them are present in the CAR board. Before trying to replace them, you need to perform a voltage check on those locks.
On the loop lock board, the five-pin connector should be about 5 volts, and the three-pin connector should be about 15 volts. The fixed-voltage range for each lock loop is about 4.5 volt when it is in the lock, and if it is above the lock, then the voltage is about zero volts.
To detect the exact problem, whether, with the AVR board or CAR board of the Kenwood 940, we need to use a voltmeter to check the four pins of connectors on the lock loop board. If these loops are not fitted on the lock, and the voltmeter shows the current of zero volts, then it is correct.
Now we need to check the second connector. Suppose the voltmeter shows that the voltage is still about 0 volts. It makes the confirmation that the problem is with your loop lock board. We will now proceed to the checking of various loop locks if they are showing a voltage of zero volts for the lock, which is not fixed with the loop. Checking each of the outputs will give you the exact idea that how many loops are in the working order.
To aid the process of checking voltages, you can also use the circuit diagram to get the exact idea of which range of voltages is correct. You can consider replacing the whole AVR board with a new one, or you can replace and re-solder all the locks of the loop lock board again.
Using an oscilloscope for the measurement of voltage is a good option. It would exactly pinpoint the component that has an issue and needs replacement. Most of the problems in Kenwood TS 940 appear due to the presence of low-quality components used in it and bad soldering done with those components.
Changing the external capacitor of the device
Sometimes your radio is not working properly due to a small-sized capacitor being installed in it. So replacing the old capacitor with a new bigger one and making slight adjustments in the connections would get your radio back to work in no time.
So try replacing your capacitor with a new external capacitor of about 28 pF having resistors of 56 ohms at both ends. Use good quality soldering with it, and your radio would be back to normal working in no time.
So, these are some common issues faced by the Kenwood TS 940 users and their solutions. Mostly the issues are because of bad soldering done in it and cheap quality components used or due to malfunctioning of any of the components of the device.