8 Transistor AM Radio teardown - Page 1
Forum section: electronics / general

author: sonnytiger
author_rank: Frequent Contributor
timestamp: January 23, 2014, 11:24:18 am
content: Not terribly interesting unfortunately, if anyone has any information on this guy that would be cool, and if someone could explain superheterodyne mixing to me, that would be awesome as well.Thanks for watching!

author: zapta
author_rank: Super Contributor
timestamp: February 02, 2014, 09:07:08 am
content: 8 transistors was on the high end side. The common ones had 6 transistors.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: February 02, 2014, 11:15:41 am
content: Quote from: sonnytiger on January 23, 2014, 11:24:18 amAn RF mixer (which is contained in a superhet receiver) is just a stage that takes two input signals (of different frequencies, we'll call them f1 & f2) and produces two main output signals whose frequencies are related to the input signals.One output is at a frequency that is the sum of f1 and f2.The other is at a frequency that is the difference of f1 and f2.In most cases only one output frequency is required so some sort of bandpass filtering is required to remove the unwanted signal (plus other unwanted mixing products).In a superhet receiver the mixer output frequency is at a fixed intermediate frequency (typically 455kHz).It's fixed because it's much easier to make a sharp RF filter good for a narrow slice around one frequency than one that can be moved around. This filter is normally tuned circuits formed by the IF transformers but can be a crystal or ceramic filter on better receivers.At one input to the mixer is the whole range of signals, including those on the AM broadcast band.Applied to the other input of the mixer is a locally generated signal (from the local oscillator) that is varied in frequency (by adjusting the tuning capacitor). This normally operates 455 kHz above the signal being tuned in.The ability to vary the local oscillator signal frequency is what changes the stations. For example a local oscillator set to 1000 kHz (1 MHz) will have the receiver tuning stations around 545 kHz.The ferrite rod forms part of another tuned circuit that keeps signals on frequencies other than that selected out of the receiver.Very simple superhet receivers are possible with maybe 4 or 5 transistors. In these cases each transistor may do two functions. For instance it is possible to have what is called an 'autodyne' mixer which is also a local oscillator. Whereas an 8 transistor radio may have an RF stage, and possibly extra IF and audio stages for better sensitivity or loudness.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: February 02, 2014, 05:55:13 pm
content: Cool! Thanks a lot for the explanation! Cool stuff.

author: mariush
author_rank: Super Contributor
timestamp: February 02, 2014, 09:19:18 pm
content: You can power that just by connecting two wires at one side of the battery section - you see the wires going from the battery terminals into the radio. It should work with 7-11v, maybe even wider range, may work even with 5v from a usb cable or a phone charger.Such opportunity and you can't even power it on....

author: sonnytiger
author_rank: Frequent Contributor
timestamp: February 02, 2014, 11:52:01 pm
content: I just tested the continuity of the terminals and none of them are connected together so it is doing something different then what we expect.

author: mariush
author_rank: Super Contributor
timestamp: February 03, 2014, 01:25:24 am
content: It's powered by 4.5v , you have 6 batteries but 2 sets of 3 batteries connected in series :bottom : [+ -- - ] [+ -- -] [+ -- -]top : [+ -- - ] [+ -- -] [+ -- -]You can see the label with the batteries at around 1:15 in the video. There's several wires coming from the terminals, which may mean the radio treats the batteries as a split power supply ( - -4.5v - 0 - +4.5v so you may have to use two separate isolated 4.5v-5v power supplies to power it up. But it's still super easy if you think for a couple of minutes and inspect the circuit.