The ‘Retro-Helper’ – a full-transceive add-on for the Retro-75


The ‘stock’ Retro-75 was designed with a crystal-controlled transmit section. This was done to keep the cost low and the chances of builder success as high as possible. The new ‘Retro-Helper’ extends the capability of the Retro-75 by converting the transmitter  to full VFO control. The transmitted frequency now matches the receiver tuning for maximum operating flexibility.






                            The Retro-Helper measures 2.20” x 1.40”.                                                    Ready to install in the Retro-75





                                       Installed in the Retro-75                                                                  So that’s what those extra holes were for!



The Retro-Helper is designed to cover the tuning bandspread of the Retro-75 receiver (45-50 kHz). As designed, it complies with FCC and international spectral purity requirements to an upper frequency of 3900 kHz. As tested, Retro-75 spurious content over 3850-3900 kHz was a maximum -48 dBc and 2nd harmonic content was -46 dBc.  Operation above 3900 kHz results in degraded spurious performance as the LO approaches 2 MHz.  Operation at frequencies lower in the 80M/75M band are not a problem. 


  If broader frequency coverage is desired, the builder should note any falloff in transmitter power at the ends of the tuning range.  If this is an issue, reducing the values of resistors R3 and R6 broadens the Retro-Helper tuned peak response.   [ As tested with the ‘stock’ tuning range, transmitter output power was flat within 1 dB.]


Click here for Retro-Helper Instructions



The Retro-Helper is in stock and available now. See our website  ‘pricing/ ordering’ page for further details.


Dave Benson, K1SWL  

March 18, 2011