Motorola AN762 berbasis Broad Band HF Linear Amplifier dengan sepasang 2SC2290This amplifier is 1=1 copy of the AN762, but comes with T/R control system, and relays, SWR protection system and switched LPFs for two "bands" (in this case 10 & 14 MHz and 18 to 29 MHz). Penguat ini adalah 1 = 1 salinan AN762, tetapi datang dengan T / R sistem kontrol, dan relay, SWR sistem perlindungan dan menyalakan LPFs untuk dua "band" (dalam kasus ini 10 & 14 MHz dan 18-29 MHz). Mmatched pair 2SC2290MP was used instead of MRF454 due to price and availability. The amplifier worked when built with Amidon ferrites and 2SC2290 transistors per Granberg's application note, on all bands up to about 10 MHz, but higher up, the initial Pout was low - just 20 W on 28 MHz. To cure this the output toroidal transformer's (T3) primary parallel Mica capacitor bundle needed reduction, both 470 pF Micas were removed and replaced by one around 330 pF for best power on 28 MHz. Also slight changes in the T2 feedback loop's parallel caps increased the output power. As a last resort, the amp aimed to be driven by Icom IC-703 10 W radio, called for a 3 dB attenuator to be inserted at amp input. If you look carefully, you see the AN762 has holes and PCB traces for the pi-input attenuator, but as there would have been too little space for the resistors, I added a patch on the PCB, so the RF drive from the input T/R relay makes a longer route and the original attenuator space remained vacant. The attenuator was eventually bypassed with 820 pF capacitor to increase drive power on higher bands. The whole unit's bypass losses (also RX) were 0.5 dB on 14 MHz and 1 dB on 28 MHz.. The cabling and how you do it, affects, but also the ordinary PCB- type relays induce some losses.
IC-703 provides PTT output, which goes low during transmit. IC-703 menyediakan output PTT, yang pergi rendah selama transmisi. The necessary T/R relay drivers were built on separate PCB not shown here. That PCB also includes the SWR protection system's final end, from the sensitivity trimmer onwards. SCR triggers the T/R system when the SWR is too high and switches the T/R relays on bypass and +Vcc feed relay open. The reflected power sensor with a toroid, is better placed at the T3 secondary before the LPF switching relay RL3, if the LPF is selected manually and can be on wrong band by mistake. There is no extra space reserved for the SWR detector, but I used a small 2- sided PCB over the RL3 relay. The RF from T3 secondary gets looped via the detector and back to the amp PCB via short jumper. If the LPF switching is automatic, then the SWR detector can be installed on the coax that runs from T/R output relay to ANT connector. When the SWR protection (SCR) triggers, you need to switch off momentarily the amp from the ON/OFF switch to reset, or switch DC power off and on.
The LPFs uses 500 V Mica capacitors and Amidon toroid coils on both 5- element low pass pi-filters. The coils are made with typical toroid materials, the T50-0 may run little warm, so I would afterwards opt for the next larger toroid ring. The LPFs used here, were selected by the antenna in use; a 20-15-10 m tribander. This way and by using 2- pole relays, the design only has two LPF switching relays instead of 4 or 10. If a need arises to work on low HF bands, such as 7, 3.5 or 1.9 MHz, LPFs for those can be built on separate enclosures on antenna feeder cables. That way the LPFs are always "selected automatically" right, provided you switched on the right antenna and if you did not, the SWR protection will trigger. This scheme works, unless you have a 5- band trap-antenna, or something that is fed with just a single feeder on all bands.
Note: Ferrites I used in AN762: T1: Amidon BN 61-202, T2: Amidon T 68-6 2pcs, T3: Amidon FT 50-61 14pcs, L3 and L4: Amidon T 68-6 one each, L1 and L2: VK-200 RFCs, Q3: MJE3055 (TO-127), D1: BD135. (C5 and C6 capacitance changed for best output on 28 MHz )
Use 1 mm copper wire or rivets on PCB feed-throughs and see that you get them all there. You may add some on the input and output ends of the PCB here and there, to make the ground planes work. Use adequate heat sink for proper cooling and preferably a copper bar plate as heat spreader under the transistors. Pay attention the PCB's stand-off's holes and those of the 2SC2290 flanges are positioned precisely. Test the bias circuit without the RF transistors first - you should get from about 0.7 volts up to little over 1 volt. When the unit is fully wired and PCB is secured on the heat sink, use some thermal conducting paste and bolt the 2SC2290s on, do not over tighten and use as thin layer of thermal paste as necessary. Last: solder the 2SC2290 leads on the PCB - avoid mechanical strain on the RF transistors. With current limited PSU, adjust the bias current to 200 mA and use reuced RF drive power when doing the initial tests and tweaking the amp. The unit I built, draws 21 A on low bands with 160 W output and little less on 14 MHz, but only about 11 A on 28 MHz where the achieved output in the end was about 90W.