I took a quick look inside the Sangean U3 and the LB100 Lunchbox mainly to answer the question about the AM antennas. I discovered some interesting things about how these radios are engineered and built. Both radios are well designed and use high quality components. While not up to the standards of the old Panasonic radios for build quality, it's probably safe to say that this is about as good as it gets for today's radios. Both radios seem to be designed to be worked on and repaired. The large number of connectors allow easy dis-assembly and access to circuit boards.
I've included an inside view of the U3's large speaker and the single coil AM antenna which measures about 5 inches long. The speaker is 8 ohms and is rated at 15 Watts, which is twice the rated 7 Watt output. It appears to be a high quality speaker. I want to do a more complete dis-assembly of the U3 later on and see what ICs it uses so they can be compared to what the Lunchbox has.
The LB100 also has a high quality speaker. I couldn't tell the ratings since they covered the magnet with foam. One half of the radio has the display and related control circuitry on one circuit board. The other board appears to be the power and audio output board. It also has the AM/FM radio circuitry and the 3 inch AM antenna mounted on it. Note the massive heatsink on the TDA7266L 5 Watt Mono Bridge Amplifier IC. The other IC is a UTC 4580 Dual Op Amp for the tone control. The unmarked IC on the reverse side of this board is probably the combined AM/FM function chip which is probably a proprietary Sangean-designed circuit.
The AM performance of the Lunchbox is not as good as the U3 mainly because of its smaller ferrite bar antenna. The FM performance is almost as good as the U3 because they both have very similar designs and probably use the same antenna and ICs. I'll try to confirm that when I take the U3 apart completely. I hope this answers at least some of the questions a lot of you have had about these two radios. More to come later.
Update:
There's a glaring error in the article I wanted to correct. (I blame being in a hurry, having a senior moment, my cat ate the schematic, etc.) I stated that the dual op-amp chip was for the tone control. The LB100 obviously has no tone control. It does need at least one stage of amplification before the output transistor so my guess is that's what it's for. The spec sheet for the chip says that's what they are commonly used for and I just ran with that. I like to be as accurate as I can, so if anybody sees any other errors like this, please call me on it.
How easy would it be to replace the LB-100's 3" ferrite with a longer one? Is there space in the cabinet and on the board for a longer one? In the photo I looks like it might fit one a half inch longer. Do you think that would make a difference?
Posted by: Mark | February 24, 2017 at 09:25 AM