Upgraded FRG7

[rubicon]

I was recently given a very rusty and unloved Yaesu FRG7, too rough to repair, but too good to throw away.
So, with the help of a disk grinder, the main chassis was repurposed to make a pseudo copy of the Pogson Deltahet receiver , published in a 1963 Australian electronics magazine.
The design replicates the Wadley loop design to such a degree that all the original Yaesu controls and dials function identically to the factory original, despite using 15 vacuum tubes instead of silicon. Even the calibration marks coincide.
My iteration does depart substantially from the Pogson design in order to use available period components. Valve lineup uses 6AU6 sharp cut-off pentodes for the loop amplifiers, audio output and mixers, 6BA6 remote cut-offs for the AGC controlled RF and IF amplifiers, ECH81 triode/hexode for the third mixer and product detector. Finally a 6BL8 triode/pentode becomes the audio preamp, and second mixer. The first IF at 40 megahertz and loop at 37.5 duplicates the RACAL RA17 frequencies, the Yaesu used 55 / 52.5 megs, although the actual frequencies used can vary.

[Wendymott]

WOW !!!!... What a project. I have fixed a few "Frog 7"s. Lots of unused space. GOLD star to you Rubicon.

[deliverance]

Very impressive a FRG7 on steroids.

[Ian - G4JQT]

Very impressive. I wonder how the various RF specs compare to a good original FRG7?

I bought new FRG7 in about 1980 after being very disappointed with the newfangled Kenwood R1000. Good on paper but hopeless with a decent antenna, and the front panel 20/40/60 dB attenuator gave the game away!

The FRG7, despite being a much older design, was much better - having a preselector that really did do its job - mostly!

[rubicon]

Once I get the last few bugaboos ironed out, I will do some tests, although it will outperform the FRG7 by virtue of the higher voltages of valves automatically giving a higher dynamic range, the second mixer is still exposed to the 1 meg wide first IF filtering.
I have noticed this version is very much quieter with background noise than the true FRG7.
One major suprise was the complete lack of birdies and spurs despite having no sheilding, unusual for a multi conversion design.
This one took 2 months to complete, due to having to custom wind each coil, and having only the 1950s iron dust cores/slugs to play with.... these being largely ineffective at the higher frequencies, and of unknown quality.... donated by a few random old TV chassis.
The one shortcut was to use the original FRG7 harmonic generator, nomatter what I tried, a valved version had way too much output, fortunately the low impedance output of the transistorised HG is a perfect match for cathode injection of the third mixer.

[rubicon]

Originally Posted by Ian - G4JQT

Very impressive. I wonder how the various RF specs compare to a good original FRG7?

I bought new FRG7 in about 1980 after being very disappointed with the newfangled Kenwood R1000. Good on paper but hopeless with a decent antenna, and the front panel 20/40/60 dB attenuator gave the game away!

The FRG7, despite being a much older design, was much better - having a preselector that really did do its job - mostly!

Preliminary tests show an MDS of -108dBm, On AM 30 % mod -91dBm for a 12 dB SINAD. Not great but plenty sufficient for HF. Because I keep fiddling with it, this will likely change, umm, probably downward.....
I dont have an unmodified FRG7 to compare.