567
Item nr.
Largest Matchbox radio
| Production | England/The Netherlands, 1950 & 2022. |
|---|---|
| Bands | MW. |
| Semi- conductors | TA7642, BC547. |
| Cabinet | Wood. Size 26x24x14cm. |
| Power | Batt 1.5V (1xD) @ 1.2mA. |
| Documents | Schema. |
For this little garden entertainment piece, I first got myself an old speaker unit from the NVHR Swap Meet. The Plessey speaker may have been produced for several years, but I found at least one reference to it with a visible date stamp of July 1947, so it is about 75 years old. My electrical design resembles the well know Matchbox Radio (with ZN414 IC) and it's improved version, the Kid's Radio, but with some further additions. 
First, my ferrite rod antenna (from a scrapped Philips D2130) had a small extra coil, that I used to include an antenna jack (not very useful). The BC337 transistor was replaced by the BC547, which gives a bit more gain. To avoid clipping and overloading, the biassing current was increased by fitting R3 of 47k (instead of the usual 100k). Also, the coupling capacitor C4 was increased to 2.2uF. To avoid instability, the supply to the TA7642 was decoupled with capacitor C5 (of 10uF).
The ferrite coil measured 630uH and the tuning cap (two sections) 180pF, so the lowest tunable frequency could be around 480kHz. With minimal capacity, I could not tune Paradijs (at 1467kHz) well, so I decided to use only the smaller capacitor section. Later, when Paradijs was discontinued, I wired the larger capacitor section to hear Caroline (648kHz). The speaker box contained a multi position switch. With this switch, the radio can be turned off, or the speaker is connected directly or through an output transformer (secondary 2 Ohms, Primary 36 to 1000 Ohms).
The described mods hardly made the sound better. So I later reverted R4 to 820R, the transistor to BC337, and R3 to 120k. The current draw is now around 1.2mA.
There are two LPAM stations in my region, Radio Paradijs and Jinx Radio. Both are reproduced in my garden, Jinx is even so strong that some distortion is heard. Volume can be controlled by rotating the radio. In the extremely soft weather of the Fall of 2022, I enjoyed the radio in my garden well into November.
| Obtained | 6/2022 from NVHR Swap Market. |
|---|---|
| Condition | 8. |
| Value (est.) | 6,3€. |
I fitted all electrical parts on a perspex sheet, which became the back of the radio. You can use the radio with headphones by simply connecting them, it will be switched on automatically then. 
For easy pickup and handling, there is an opening near the top behind which I glued a small wood bar.
The voltage on the agc capacitor varies from 1.06 to 1.00 Volts. The current into the TA7642 varies from 250 to 300µA.