So.. I have a portable radio that broke. My little brother put the batteries in backwards. There was a bad smell and the back was hot. I took them out and It worked. Sort of. There is no sound. I only tested in FM mode. Any idea how to fix this? Radio is Tecsun brand, model PL-606.
 

Which batteries it used? Alkaline or Zinc-Carbon? I've never know that if you put batteries backward, this destroys the entire product.

You would need to disassemble it and look into the electronics.

Most likely the audio output amplifier will be toast, as that uses to run direftly from the battery voltage. Plus can not tolerate any resistance or so in series with the supply, so nothing to protect it.
The rest will be behind some regulators, so there just the regulator ICs may be damaged. Plus it is likely these will be separated by some resistors or filters, so rather high chance the regulators will survive.
Fortunatelly both the amplifier, as well as regulators are rather common components.

And I don't think there would be anything damaged in the main signal path, where the main proprietary parts are.


Edit: I just looked into PL310  schematic, which seem to correspond to the same description as published in PL606 manual (mainly using the same SI4734 DSP radio IC) where practically the whole radio is behind a transistor power switch, only the processor is supplied all the time

So I see rather high chance for it to be fixable...

Don't touch it unless you're experienced in soldering on circuit boards, or you risk making it even more broken. Desoldering is harder than soldering, if you aren't careful you can lift traces.

Tecsuns aren't really cheap, so you might want to take it to someone who actually has the experience.

It fixed itself when I disassembled and reassembled it.Don't flame me I just thought it was a cheap no-name radio.

The DSP is in fact today the cheapest way to make any radio, the rather good RF performance (Sensitivity, selectivity, RF dynamic range, cross modulation, no problems like misaligned or unstable RF, maybe the feel of analog tuning control is just odd, as even when the control is supposed to be "analog", in reality it is translated to digital control anyway) even of the cheapest models comes as a bonus.

In practice, things are not so rosy with cheap DSP radios. All of them lack proper tunable pre-selector filter banks, and so, resulting dynamic range is poor and there is a multitude of parasitic reception channels. Having just a rudimentary input filter means there is a significant reception at heterodyne harmonics, where strong FM ant TV stations reside, resulting in annoying interference all over the band.

As with everything, it depends on brand and tech used. I despise the cheaper, random brand DSP radios. Those are utter crap. But in the ham radio world, good DSP radios exist.

The tecsun is likely to still be a digitally tuned analog radio, but no way of knowing without seeing the ICs it uses. Not that it matters if you already have it.

yeah, if you compare $5 radio with $500 set, yes, $5 DSP radio will likely not outperform a $500 analog communication receiver set.

What I'm talking about is analog tuner design vs a DSP in a comparable price category.
What I was talking about is when e.g. you want to produce a $5..$15 purse radios. With analog design you end up barely fitting into the $15. Using DSP, you can easily push the price down to $10 while maintaining the profits (this is your motivation) and still get way better performer in nearly all aspects.

And same can be said even when you are aiming for some higher price range higher spec market: With DSP you get mainly significant manufacturing cost savings for the same quality category.

As with everything, it depends on brand and tech used. I despise the cheaper, random brand DSP radios. Those are utter crap. But in the ham radio world, good DSP radios exist.

The tecsun is likely to still be a digitally tuned analog radio, but no way of knowing without seeing the ICs it uses. Not that it matters if you already have it.

Later receivers of that style all use SiXXXX or their Chinese clone radio-on-a-chip ICs. They are all low-IF/direct conversion design, where input signal after a little bit of amplification goes to a pair of CMOS switch mixers feed by 0/90 degrees PLL synthesized heterodyne. Resulting I/Q quadrature pair of intermediate frequencies gets a bit of lowpass filtering and is fed into two audio-style high bit sigma-delta ADCs. Rest is DSP.

Works nicely for $50 radio, but like already said before, the design is handicapped by lack of any proper preselector bandpass filters.

Preselectors (LNA stage plus a filter between LNA and mixer) are used mainly to suppress amplifier/mixer noise from the mirror image frequency. Quadrature mixing suppresses mirror image by itself, so the preselector filter does nothing for the noise.
For near blockers, wich are the main concern onbroadcast radios (strong transmitters in the same band, so picked up by the antenna), the preselector is useless, it is lacking selectivity, sothe only thing remaining is the input linearity.
And that uses to be way better with integrated CMOS.
The cheap analog radios suffer from tracking issues, so the preselector is by far not as efficient (on FM) and antenna loses a lot of selectivity (on AM).
With quadrature mixers and double conversion to zero IF (first is in analog, down to about 200kHz 1st IF, the second conversion down to zero IF is then in the digital, so the channel selectivity is turned into simple low pass filter) the preselector is replaced by the image suppression of the mixers and zero IF filtering. For MW antenna, the DSP allows the thing to not rely on any tracking whatsoever and instead tune the loopstick exactly each time the station is tuned in, so the antenna could be designed ashigher Q, so more sensitive, with less cross modulation from surrounding stations (higher Q means even a small frequency difference yields lower levels).

Example: We have two local stations at 103.0 and 103.4 MHz. But a weak distand station at 103.2 MHz is also present reasonably above noise. Did not met any analog radio able to pick up the 103.2 here, even with disabled AFC. But practically all most common DSP ICs have no issue at all, once they allow AFC feature.

Similar, receiving rather weak 963kHz station beside 954kHz local was possible on an analog tuner only after replacing the original IF filter with a 6-element ladder one (nothing tobe found on any commercial broadcast receiver), yer picked up by all DSP ones without any issue.

And the DSP I meant the ICs used in the purse radios in the $10..$20 range (using mainly AKC695x, KT0915, RDA5708).
Price point comparable analog ones use to use mainly CXA1691 (and predecessors), TA2003 and similar AM/FM chips, with a loopstick for MW, single preselector tank for FM, SFU455 for AM and SFE10.7 for FM. You never got anything better in this price range.

Generally, the more you limit overall unintended signals energy at the mixer input, the better the receiver works. That way, a weak preselector with a single LC tank or even a ceramic bandpass filter over a reception band is better than nothing at all. And better receivers have multi-stage tunable filtes of course.

CMOS mixer used in today's radios on a chip has reasonably good dynamic range for cheap, but this advantage is mostly negated by a negligence to any pre-filtering in inexpensive receivers.



 



 

If the mixing products do not fal linto the IF band rangeanddoes not drive anything into nonlinearity so it creates intermodulation products, it won't matter. But the harmonics, as well as the nonlinearity from strong disturbers may flip some extra noise/disturbers intothe IF bandpass, so they must be kept below an inband noise level.
 
There isalways a bandpass on the input, even on the cheap DSPs on MW and FM, on MW it is the antenna itself (which is way better than on analog tuners, because of always perfect tuning alignment vs 20..30kHz mismatch on the cheap analog), on FM it is the grounding coil for the antenna and internal AC couplings to block low frequencies and the internal on chip low pass RC filter, plus the mixer activity timing (designed to suppress the oscillator 3'rd harmonic mixing).
It is paper wise by far not as "good filtering" as on an ideal analog implementation, but the better dynamic performance of the CMOS inputs makes it less sensitive on it and it does not suffer from the misalignment.

The problem with analog radios of the cheap price range is the misalignment alone passes better the disturbing signals than the wanted one. Often it is just incorrect alignment, but the alignment is one of the most expensive step in radio manufacturing, so it just cannot be perfect on a cheap product. Argument that more precise alignment would solve many problems is not relevant, because then the production cost would mean we are talking about a more expensive price product, which then competes with a DSP featuring e.g. 3 or 4 element bandpass on the FM antenna input, on top of what is within the IC.

Plus the common simple ceramic IF filters used in cheap analog tuners have very significant parasitic pass bands, allowing completely wrong signals to pass into the not that linear (not designed to get anything else but the wanted signal) IF amplifier, creating mess there. The simple low pass on thd low 1'st/zero 2'nd IF and the high linearity of the IF processing of the DSP eliminates this problem inherently without any extra penny spent on it. Yes, you may use additional LC stages to stop the parasitic passbands, but thenyou have more components and a few extra alignment points, so again not the $15 category anymore.

@Medved, do you have some practical experience with radio or just putting some bare theory?

In practice, radio reception is a constant fight with a myriad of interference frequencies. We call it RF smog. There is *always* a non-linearity at input stages, mixer, IF amp and ADCs. When hit by unintended frequencies not removed by a pre-selector filter, multiple spurs forms, even in better designs. That can be thought as rising of the noise background, and losing the ability to receive a weak signal. Also there are other problems, like a reception on heterodyne harmonics and limited spectral purity of heterodyne signal itself, today usually formed by a PLL circuit in the vicinity of  noisy DSP structures nearby.

And btw, better radios-on-chip try to tune magnetic antenna rod too, even adaptively.