Sentinel 160BL Restoration
Portable Radio From About 1940
This was an Ugly Duckling.
Some research turns up that this is a nice example of a very simple Reflex radio. The first Tube, 1A7G, uses the Oscillator Coil and Tuning Capacitor to tune the signal. This is fed to the 1N5G which amplifies the IF signal. Note that the Volume Control wiper sends the incoming IF back to the input of the 1N5G (via the 1st IF Transformer). Eventually exiting the 2nd IF Transformer to the Diode Plate of the 1N6G where the AM is Detected and Amplified for the Speaker. ALSO AVC is provide via the Audio Transformer, which via the Grid of the 1N6G controls volume.
Together, with the three tubes (and the 1N6G is basically 1A5G with an added diode), the current draw is only 150 ma for filament and just under 7 ma at 90 volts plate.
Apparently this little radio was marketed as a Sentinel, and also as an "Aria" branded radio, which was a department store brand, as "Aria" was sold in Allied Stores in New York (ref: from 'radioremembered.org', "ALLIED STORES (private brand Aria), 1440 Broadway, New York, N. Y.").
Completed So Far
- Inspected and cleaned.
- Some of the wicker-covering was peeling; refreshing some glue repaired this.
- Cabinet blocking inside was loose - glued to reinforce.
- Missing Handle - Replaced with a not-quite-Repro quality (but functional) leather strap.
- Replaced resistors and capacitors as needed, testing for significant out-of-tolerance.
- Aligned, per the “Rider's Perpetual, Volume 11 (1940 and before)”.
In order to begin test and repair, I built a 90V / 1.5V battery eliminator for this "Battery Only" portable. Power requirements, per the schematic, are:
- 150 ma filament voltage at about 1.5 VDC
- 6.8 ma HV at 90 VDC
Option 1 - Mains Powered Battery Eliminator
This option uses an MC34063A switching type converter to boost a 12V gel battery to 90 volts. Only about 7 ma are needed, so it's an ideal option, provided that
Option 2 - DC-DC Boost Converter
This option uses an MC34063A switching type converter to boost a 12V gel battery to 90 volts. Only about 7 ma are needed, so it's an ideal option, provided that switching noise can be kept to a minimum. Currently this works but produces RFI along the AM band. Tuning into a station minimizes the noise, as the AVC acts to increase the audible noise between stations. I'm currently experimenting with L3, a coupled inductance to reduce this switching noise. This 90V supply is a variant of Hans Borngräber inverter.
This design produces significant RFI noise when output is set upwards of 90 volts, so the Trimmer R4, is adjusted so that output is only about 81 volts. Q1 can be most any MOSFET, an IRF830 is recommended. I used an IRF640 and put a couple of ohms in resistance below the Source in order to match the IRF830 "On" resistance.
C2 can be altered up or down a bit - 2.2nf works, 4.7nf lowers efficiency so much the circuit generates only around 50 volts. L1 can be adjusted, as well as L2, however the bulk of the RFI noise comes from Q1 itself, which becomes almost a transmitting antenna of RFI.
- Cut a small block for Upper-Right Inner Cabinet. This may have acted as a closure.
- Cut a small sliver to replace a broken piece for the Loop-Antenna / Back frame. This forms a leg that 'keeps' the back in two small holes the cabinet bottom.