Transmitter of the Electronica SS701 6METER (50MHz)
One of the hardest parts of the conversion of this transceiver was the conversion of the poweramplifier.
It wanted to oscillate very badly due to the layout of the transceiver.
The measures taken to prevent unwanted oscillation are good grounding of the case to the PCB at a lot of places.
And the use solder-wick to connect the housing of the 10mm filtercoils to each other.
Use of 47nf SMD capacitors to decouple the powersupply also helped .
In the lowpass filter , all the capacitors are SMD and placed on the bottom.
SMD capacitors have less inductance and this is important in these amplifiers with a lot of gain.
I had to replace the JFET(FET6)) by a BF900 dualgate mosfet because i blew up the JFET, and could not find a compatible one.
I recalculated the original bandfilter from 27MHz to 50MHz , bit this did not function as i wanted to.
So this filter is changed completely by the use of LT-Spice.
The Mixer is also changed from a dual-gate mosfet to a NE602 which is a balanced mixer.
The benefit of this mixer is that the ocillator is already dampened by 30 or 40dB at the output.
When using the dualgatemosfet , the bandfilter has to attenuate the oscillator by 70dB and the wanted 50 MHz by only a few dB's.
It is not easy to make such a filter.
By using a balanced mixer , the filter now can be much more relaxed.
I have not measured what came out of the original filter at 27Mhz, but i don't expect nice fugures.
In the later design of these transceivers the mixer is replaced by a balanced mixer.
Blockdiagram of the transmitter part:
Replaced RF-power transistor:
The original transistor is also tried at 50MHz and is able to make about 8 watt , but with a much lower gain.
An additional stage between the mosfet and 2SC1973 was nessessary.
The RD16HHF1 started oscillating very wild when i turned the coil L31 on the PCB. I thought the shortest pin of the inductor must be connected to the drain of the FET.
The cause of the oscillation was a 1uH coil which was originally in the base of the 2SC1945. The coils are placed next to each other and coupled. Apearently they knew in production and that is why the draininductor was placed so strange.
It could be solver by removing the 1uH inductor L44 and placed a 100 ohm resistor instead of it.
The amplifier is now stable. No oscillation anymore.
Be carefull when ordering the RD16HHF1 on ebay. There are a lot of fake transistors, and the most nasty of this is that the drain and source are changed so you have a short on your powersupply.
Some pictures of the inside of the converted transmitter part:
Schematic diagram of the transmitterpart:
