UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Building the 'Minimod' AM modulator-questions!

David G4EBT · 23rd Oct 2018, 11:56 am

Howard built his Mini-Mod on the PCB that I designed back in 2015 (based on the same layout as that of Ian's original design). I recently modified the PCB - still based on the 2015 layout - but leaving more copper on the board. I only did that to reduce the etching time, and hence, the risk of undercutting narrow tracks in the etching process, not that the 2015 layout suffered from that.

Regarding drift/frequency stability, I've just built a Mini-Mod based on the updated PCB and like Howard's, there is minimal drift on mine. At switch on, the frequency was 998.900 kHz, and after three hours, it was 999.283 kHz, so in that time it had only drifted 383 Hz. I dare say that any valved radio with which the Mini-Mod was used would have drifted more than that! (I used polystyrene caps for C1 & C13, as before).

I'm not sure where I sourced my BF256s from, as I've had them for years, but as with the previous Mini-Mod that I made, the pinouts didn't conform to the data. Initially the transistor wouldn't oscillate but on changing the drain and source connections, it worked fine, with a nice clean sine wave about 10 V p-p. I know that Howard got his BF256 from Spectrum, and it world fine straight off.

A current model Peak Atlas DCA55 transistor tester would only identify the Gate, an earlier DCA50 thought a BF256 was a diode, and a Chinese cheapo multi-tester correctly identified that it was an N Channel J-Fet, but would obligingly call any lead anything, according to which test clip went where.

I made a waterslide transfer to 'tart up' the front panel, and a little comb-jointed box in which to house the Mini Mod. It works fine with 45cms of wire as an antenna.

First pic in the 2015 PCB.
Second pic the 2018 PCB.
Third pic outlines the issues I had with the BF256 connections.
Fourth is the front panel transfer.
Last is the boxed up Mini-Mod, much like my earlier one.

Sincere thanks to Ian, J4JQT, for all the enjoyment and intellectual stimulation that I've had from building this great little gizmo, helping to keep homebrew alive!

23rd Oct 2018, 11:56 am	#63
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,760	Re: Building the 'Minimod' AM modulator-questions! Howard built his Mini-Mod on the PCB that I designed back in 2015 (based on the same layout as that of Ian's original design). I recently modified the PCB - still based on the 2015 layout - but leaving more copper on the board. I only did that to reduce the etching time, and hence, the risk of undercutting narrow tracks in the etching process, not that the 2015 layout suffered from that. Regarding drift/frequency stability, I've just built a Mini-Mod based on the updated PCB and like Howard's, there is minimal drift on mine. At switch on, the frequency was 998.900 kHz, and after three hours, it was 999.283 kHz, so in that time it had only drifted 383 Hz. I dare say that any valved radio with which the Mini-Mod was used would have drifted more than that! (I used polystyrene caps for C1 & C13, as before). I'm not sure where I sourced my BF256s from, as I've had them for years, but as with the previous Mini-Mod that I made, the pinouts didn't conform to the data. Initially the transistor wouldn't oscillate but on changing the drain and source connections, it worked fine, with a nice clean sine wave about 10 V p-p. I know that Howard got his BF256 from Spectrum, and it world fine straight off. A current model Peak Atlas DCA55 transistor tester would only identify the Gate, an earlier DCA50 thought a BF256 was a diode, and a Chinese cheapo multi-tester correctly identified that it was an N Channel J-Fet, but would obligingly call any lead anything, according to which test clip went where. I made a waterslide transfer to 'tart up' the front panel, and a little comb-jointed box in which to house the Mini Mod. It works fine with 45cms of wire as an antenna. First pic in the 2015 PCB. Second pic the 2018 PCB. Third pic outlines the issues I had with the BF256 connections. Fourth is the front panel transfer. Last is the boxed up Mini-Mod, much like my earlier one. Sincere thanks to Ian, J4JQT, for all the enjoyment and intellectual stimulation that I've had from building this great little gizmo, helping to keep homebrew alive! Attached Thumbnails __________________ David. BVWS Member. G-QRP Club member 1339.