Page last updated 11th Dec 2018
The AX-190 receiver pictured above was first marketed in Australia by the Tandy Electronics stores in the early 1970's. I guess the 'A' signifies Amateur. The photo is of an Allied receiver but the Realistic was identical 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 was unaffordable for me at the time but I have finally acquired one! It's not that they are scarce, or a high end receiver, but I've always liked the look of them and the purpose of buying this one is to get familiar with it circuitry and see if I can make any improvements apart from giving it a new home and a good clean-up.
First job was to replace the hand full of electrolytic capacitors on the receiver's I.F. board even though they checked out OK on my meter. Then I aligned the 455 KHz amplifier stages with a spectrum analyser and tracking generator. The I.F. 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 comprising the gain and shape of the passband. The 3dB bandwidth is around 4.8 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 457.7 KHz to achieve good opposite sideband suppression.
I plan to do a complete realignment of the receiver and repaint the front panel trim ring as well as the case in colours as close to original as possible.
I will investigate the possibility of increasing the 456.5 KHz crystal frequency to get it closer to where it needs 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 457.7KHz crystal and this confirmed my suspicions about the crystal needing to be higher in frequency. I'll also consider using a 'synthetic crystal' to do the job of the BFO crystals if their their frequencies can't be pulled.
I tried fitting a trimmer capacitor to adjust the frequency of the BFO crystals but whilst this worked the frequencies could not be moved very far. I have obtained a Prog-Rock kit from QRP Labs that I will build and program with suitable LSB & USB frequencies that are compatible with the I.F. filter bandwidth.
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
I bought this old receiver recently and I have in mind some modifications for it
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From studying the circuit diagram it appears that the RF stages are tuned by a separate pre-selector control to provide a sharper front end tuning response. The ganged variable capacitor in the VFO compartment is also part of the tunable 1st I.F. amplifier. As the VFO is tuned over the 500 KHz range so too is the 2nd I.F. I need to check the alignment of this.