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Using a Bosch KF161 for APRS
Bosch KF161
   Currently the Museum has a large number of crystal operated Bosch KF161 mobile transceivers available. As these units have no front panel controls, they are very useful for APRS purposes. The rigs come with the appropriate crystal (144.800 MHz) and are fully aligned for the 2m amateur band.

We are able to offer this rig directly from the Museum for the incredible price of 16 EURO only!

Below are some hints and tips that might be useful when applying the KF161 for APRS use. Thanks to PE1HZG for his textual contribution and PE1NDO for the clear images.
 
Tinytrack
If you want to use the KF161 for APRS, you need to add a suitable radio modem. A small modem is available as a DIY kit from the Tinytrack website (see below). This is a small interface, build around a PIC microcontroller, that is linked to the rig, the GPS receiver and (optionally) a PC. On the website, you'll find the full circuit diagram, the PCB layout and the HEX file for the PIC controller.
 
Insufficient audio level from Tinytrack
A lot of people have complained that the audio level from the Tinytrack PCB is too low, resulting in unsufficient span in the HF output. A simple solution is described on the Tinytrack website. Replace R6 (220k) by a 100k resistor.
 
Squelch doesn't work
It has been brought to our attention, that the squelch circuit of some KF161 units doesn't seem to work. This is caused by the fact that the KF161 units we are using here, don't have any front panel controls. Instead they were used with a remote control unit. Some links may therefore be omitted from the rig, preventing the squelch signal from the IF stage to reach the AF stage. The solution is rather simple and requires only two wire links. One existing wire link needs to be moved into another position and another link has to be added. The pictures below give a good impression on how to do this. Click the images to enlarge.
 
Before
  
After
   In the first image you'll see 6 holes to the left of the PCB. In the standard version of the KF161, this is the position for the squelch switch. To the right of the PCB, 12 holes indicate the position where the on/off switch normally resides. In this rig, the on/off switch is replaced by 4 wire links. One link should be added to the position of the squelch switch (6 holes on the left). One of the 4 wire links on the right needs to be changed. Please note that the new wire links are shown rather large in the pictures. This is done for clarity only. It is far better to mount the wires as close to the PCB as possible.
 
Pre-emphasis and De-emphasis
The KF161 units have been operated in a network with phase modulation (PM). For Packet Radio application however, frequency modulation (FM) is required. PM uses the so called pre-emphasis technique, that amplifies the higher audio frequencies more than the lower ones. In the receiver this effect is reversed with a de-emphasis circuit. As this effect is unwanted for FM application, we need to modify the rig somewhat. Fortunately, this is rather simple and only involves changing and adding a few wire links.
 
Before
  
After
   Modification of the transmitter is rather simple as we only have to add a single wire link to the modulation amplifier (NF SENDEN). The soldering side of the PCB contains one track with an obvious cut in it. Next to the cut is the letter F. 'Repair' this track by soldering a small wire onto it.
 
Before
  
Clean
  
After
   Something similar needs to happen in the receiver. This time we'll have to change an existing wire link on the receiver PCB (NF EMPFANG). Locate the letters P and F on the soldering side of the board. Remove the wire link marked P and connect a wire link at F. That's all.
 
Frequency
Some rigs seems to suffer from a frequency offset of a couple of kHz. In some cases this offset is so large, that you can't send or receive Packet Radio properly. This is generally caused by the aging of the crystal. Adjustment is rather easy and involves the use of a frequency counter. Please look inside the rig from the top. A square hole is visible, through which the PCB of the HAUPTOSCLILLATOR can be accessed. This PCB contains 10 sockets for crystals, only one of which is occupied. Next to the crystal, you'll find a reed relay and an adjustable coil. Now adjust the core of the coil. PLEASE NOTE: not to tough the core with a metal object, such as a screwdriver. The core is likely to break! Use a suitable tool instead, made of ceramic, plastic or wood. Alternatively, you may want to create one yourself from a small piece of PCB material.

It is recommended to re-align the frequency again after a couple of months. This is neccessary because the crystal is likely to age a bit more. This ageing effect is most noticable during the first few months.
 
HF Power amplifier - output level
The HF output amplifiers have already been adjusted so, under normal circumstances, it shouldn't be neccessary to re-align them. If you need to re-align the output amplifier for whatever reason, please note that it is very important to first put all adjustable components into the state indicated on the green shields (template). Next align the circuit from front to back. If you don't follow this procedure, the output amplifier is likely to start oscillating.

PLEASE NOTE: The output amplifiers have been designed for 6 Watt output. Although it is possible to obtain higher output levels, this is not recommended as it will certainly lead to the death of the amplifier due to overheating. Once aligned, you can use R26 (on the power supply PCB) to reduce the output to 6 Watt.
 


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