Realistic SX-190 Receiver
Page last updated 9th July 2019
The Realistic SX-190 receiver pictured above was marketed in Australia by the Tandy Electronics stores in the early 1970's. The photo above is of my receiver but they were marketed elsewhere as the Allied SX-190 receiver were identical this one as far as I am aware. I purchased my receiver in late 2018 when it is about 46 years old. I had only just left school at the end of 1972 so this radio, along with my AX-190, was unaffordable for me at the time but I finally decided to acquire one as I have not had the desire to do so up until recently. It's not that they are scarce yet, or a sought after receiver, but I've always liked the look of them and the purpose of buying this one is to get familiar with its workings and enjoy using it.

After using this radio for a few weeks and all appears to work well, I decided to replace the all of the electrolytic capacitors in the receiver even though they mostly checked out OK on my meter. Then I aligned the 455 KHz amplifier stages with a spectrum analyser and tracking generator. The I.F. stage uses two 455KHz mechanical filters and the passband shape looks quite symmetrical although it is not quite centred on 455KHz and I could not tune it there without compromising the gain and shape of the passband. Further tests found the 3dB bandwidth is around 4.8 KHz and centred at 455.700 KHz so the two BFO crystals 454.5 KHz and 456.5 KHz are not ideal as they are only 3 KHz apart which is 1.8 KHz less than the filter width. The 456.5 KHz crystal needs to be around 459.7 KHz to achieve good opposite sideband suppression.

I plan to do a complete realignment and repaint the front panel trim ring as well as the case in colours as close to original as possible.
455KHz 2nd I.F. Amplifier Passband Sweep
I will investigate the possibility of 'pulling' the crystal frequencies to get them closer to where they need to be. The background noise and tonal characteristics are very different when switching between LSB and USB. I used a function generator to simulate a BFO frequency and  this confirmed my suspicions about the crystals needing to be higher & lower in frequency. I'll also consider using a 'synthetic crystals' to do the job of the BFO crystals if I can't pull their frequencies.
To the left is the sweep of the 455 KHz 2nd I.F. stages after alignment. I'm quite pleased with its symmetrical shape and flat top.
As can be seen the centre of the passband is above 455 KHz
To the right is the same sweep of the passband but with the marker table displayed at the bottom

This receiver was acquired in June of 2019 for a modest price. It was advertised on one of the local buy, sell and swap sites.
AX-190 Receiver Chassis Top
AX-190 Receiver Chassis Top

I.F. board is onthe left and R.F. board is on the right and the power
supply/audio board is situated centre bottom of the photo. The VFO is top centre.

Click here for a close-up view
AX-190 Receiver Chassis Bottom
AX-190 Receiver Chassis Bottom

So far I've only found two modifications installed from it's past. I still
need to understand why they were done and whether or not they were
worthwile doing in the first place.

Click here for a close-up view
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From studying the circuit diagram it appears that the RF stages are tuned by a pre-selector to provide a sharper front end tuning response and the ganged variable capacitor in the VFO compartment also tunes the 1st I.F. amplifier stages as the receiver is tuned across any 500 KHz segment of each band.
Prog Rock & Low Pass Filter mounted on VFO cover
Prog Rock fitted to Receiver
Prog Rock wired in

Below are some photos showing how I have fitted a QRP Labs ProgRock kit and 600 metre low pass filter kit to the bottom cover of the VFO. It is being used as a substitute for the two original BFO crystals. As mentioned above, the primary reason for doing this is to better position the BFO frequencies in relation to the mechanical I.F. filter skirts.


The band switch selects two banks in the ProgRock corresponding to the two programmed BFO frequencies which are 451.700 and 459.700 KHz. These frequencies place the BFO approx. 20dB down the filter skirt. I also used spare contacts on the band switch to apply power to the ProgRock only when in LSB or USB modes. DC power to the prog-rock is removed when in standby or AM modes. 


A 20dB resistive attenuator was inserted between the output of the low pass filter and the original BFO oscillator transistor so as not to overdrive the product detector as well as maintain the original peak tp peak Voltage as measured.

BFO Modification/Replacement