SA2CLC radio stuff

 I got a trailer full of transmitter delivered in september. A transmitter built to order by one of the "Big guns" back in the 1950's, SM5RM. It was probably built by SM5LI around 1951, who at the time worked at Standard Radio in Bromma. Standard Radio was a swedish manufacturer of radio equipment, known for the military CT-450, RA200, SSA400, and amongst others the CR300 series of receivers. The build quality is exceptional. Everything is heavy duty. High quality components from the SRT production line is used, and all of the chassis parts are beatifully crafted, panels are engraved etc. Not that I needed another project, but hey, you can't pass 400lbs when it's being delivered to your doorstep. Transmitter before it was taken apart for shipping The transmitter sports 2x813 in push-pull in the power amplifier. Modulators are also 813's. There is a 2M transmitter in the rack as well as an antenna tuner, fully metered etc. The power supply contains a big oil fi...

  A directional coupler for the amplifier. In this post we take a look at the directional coupler I built for the amplifier. I wanted a built-in coupler to measure power and have SWR protection. I built a tandem match coupler. Why, you might ask? Because I’ve built one before with good results, and after struggling with a Bruene coupler design(which I could never get to work properly), I decided to stick with what I know works. Schematic representation of a tandem match. The transformers are wound on ferrite toroids where the primary is one pass of coax thru the center. Note that the turns ratio needs to be the same for the transformers, even though the cores may differ. Theory in brief (with disclaimer) Expect no in-depth math here. This is a simplified explanation and more of a picture series on how I built mine, along with some thoughts on how to improve it. A tandem match coupler works by sampling both current and voltage in the transmission line using transformers: T...

Part 4 – Power Supplies At first glance, power supplies may seem straightforward: transformers, rectifiers, filter capacitors and you’re done. In this project, that’s true for some of the low-voltage rails, but the bias and especially the screen supply needed considerably more thought. High Voltage Supply The high-voltage (HV) supply is built around a 3 kVA UI-core transformer , followed by a full-wave rectifier, an oil-filled capacitor, and bleeder resistors. Since Swedish homes typically have 3-phase service, I wired the two 230 V primaries in series across 400 V . This helps balance the load between phases and reduces light flicker during key-down. The secondary winding has multiple taps; I selected a tap that provides about 3.2 kV DC under load. The two smaller transformers are 400-230 V transformers. I run them from 230 V, with their secondaries connected in series to get 800 VAC, rectified and smoothed to 1100V. The board visible is the HV detect, HV multiplier and meter shunt...

Finished board, waiting to be populated with filter components  In part 3 we have a look at the input circuit used. Sine this is a grid driven linear, we have to feed rf exitacion to the control grid. If we look at the schematic, we can see the load - or swamping resistors - which basically sets the input impedance the transmitter will see, sans stray capacitances and inductances. The resistors sits in series with the bias supply, and are groun ochded at RF by a ceramic disk capacitor. Input circuit shown, My build is heavily influenced by EX8A. Also load resistors (2 k) shown. I ended up 1 k load resistors. There are designs using a broadband transformer for this kind of input circuits, but I wanted to wet my feet in filter making and tuning. On my 2x813 linear, I used an LDG Z100 autotuner, which works fine, but there is always the possibility to tune to a harmonic, so for this, I settled for a design using conventional fixed and switched PI filters. Everything is switched from a...

  Custom N-Adapter for the Amplifier Build Yesterday during the amplifier build, I needed a coax adapter – an N male to N female – but a bit longer than usual, to connect the amplifier output from the directional coupler to the T/R board. It turned out I hadn’t paid attention to the minimum bend radius of the coax, so I had to get creative. At first, I planned to just order an adapter, but I couldn’t find exactly what I needed. Instead, I decided to modify some existing connectors. Interestingly, the backshell thread on many clamp-type N connectors is the same as the retaining ring thread – 5/8"-24 TPI – meaning you can actually thread a female connector into the back of a male RG213 clamp connector. Unfortunately, that combination would have been too long. I own an early 1940’s Blomqvist lathe, a close copy of the famous South Bend 9". It’s fully equipped with all the gears for cutting threads, so my solution was to set up for 24 TPI and cut the thread directly onto the...