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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 1st Aug 2021, 5:55 pm   #1
G6Tanuki
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Default Mirage B23A linear amp.

I acquired this for a pittance at a car-boot-sale today; it's a 2.5-Watt-input, 30-Watt-output linear using a Motorola MRF240A transistor, with RF-sensing for the TX/RX switching, and a grounded-gate preamp using a U310 FET.

The PA side looks to have been inspired to a good extent by Motorola Application Note AN-0791 from 1979.

These little amplifiers were popular 40 years ago as an add-on for the likes of the Yaesu FT290 and Icom IC202, converting your portable into a relatively-respectably-powered mobile station.

This one worked - but then there was a burning smell! There's a RF choke between the output of the transmitter low-pass-filter and ground. I guess this is intended as a static-discharge path. The choke is about 15 turns of fine wire wound on a phenolic/bakelite former - the whole thing being about the size of a modern 1/4-watt resistor. The former had crumbled, the enamel on the winding had failed and it was acting as a dummy-load across the output. Given that my antenna is DC-short-to-ground I was happy to excise this little choke - and the smoke stopped.

Now with 2.5W input from my Yaesu-Vertex VX150 handie-talkie I get 30W output - most of the time! Occasionally it only produces about 10W but a bit of vigorous keying/unkeying the PTT on the Yaesu gets it going again. I'm thinking tarnished TX/.RX changeover-relays....

Circuit showing the offending choke below.... it's also intriguing for the way the output-filter uses two resonant coax-lengths as both capacitors to tune the Pi-filter and 'stubs' to suck-out harmonics.

That's cunning!
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Old 1st Aug 2021, 6:01 pm   #2
Jon_G4MDC
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Default Re: Mirage B23A linear amp.

Is it biased as Class AB I wonder? There were plenty for FM only - in Class C.
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Old 1st Aug 2021, 6:13 pm   #3
G6Tanuki
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Default Re: Mirage B23A linear amp.

This is indeed biased AB; there's a simple bias-supply using a forward-biased 1N4002 diode and some low-value resistors feeding DC to the MRF240A base: if I force it into transmit-mode in the absence of any actual drive the PA starts drawing around 200mA - there's a "FM/SSB" switch on the front panel but all that does is to add a 'hang' time to the RF sensing changeover in order to avoid the relays trying to follow the average speech-envelope on SSB and chattering themselves to destruction.

[there's a 'SSB Delay' pot accessible through a convenient hole on the side of the case]

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Old 3rd Aug 2021, 11:18 am   #4
Wendymott
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Default Re: Mirage B23A linear amp.

Helloooooo .... I dont suppose you could photo the pcb, particularly the Strip line input and output, and give dimensions .... pretty please..

I have found the manual, but, obviously it doesn't go into detail.....

Failing that I have an Alinco DR150 output IC module.. that I could graft in...and reverse engineer the low pass filters etc.
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Old 3rd Aug 2021, 2:36 pm   #5
G6Tanuki
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Default Re: Mirage B23A linear amp.

Hi Wendy.

I will take the lid off and do some photography for you. I can't currently find my dividers so would it be OK if rather than taking detailed measurements I took the photos with a steel rule alongside? If I take photos from a good distance away but zoomed-in on my DSLR it should minimise any parrallax errors - and if my dividers turn up in the meantime I can take some precise measurements for you.

I rather like the design of the Mirage - it seems a bit better-designed/constructed than what I've seen in Microwave Modules amateur-band linears!
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