I purchased this little radio as a replacement for a malfunctioning SRF-59, hoping to upgrade to a digital tuning version of the SRF-59. Sadly, the M37W is NOT a digital version of the SRF-59. The SRF-59 is a radical departure from conventional superhet radio design, using a proprietary CXA1129 IC. The M37W is a conventional superhet design, and out of the box it is much less sensitive and selective than the SRF-59. However, this receiver has a devoted following, and since I did not return the radio for a refund, I thought I would get inside and document it, and how it can be improved to be the best it can be. Incidentally, I was able to repair the SRF-59 - the AM oscillator had gone out of adjustment.
This view shows the rear of the radio. Once the battery is removed, and the belt clip removed, there are three screws holding the back on the radio. It appeared to me that there might be a fourth screw, but apparently there wasn't on my radio.
With the screws removed, the two halves of the case can be separated. This gives us a view of the top PC board.
The plastic elastomer can be pulled out of the front case. This might be useful if you want to test the radio without completely re-assembling the radio.
To gain access to the bottom board, it is necessary to unsolder the red wire in the lower left hand corner of the top board. Although not technically necessary, it will be easier to remove the top board if the battery clips on the right are unsoldered.
Now we can get our first view of the bottom board, which is the basic superhet receiver. The ferrite bar, fairly large for this class of receiver, is visible in the bottom of the photo. Don't worry - the ferrite bar is not housed on the case bottom. It is connected to the PC board with its own plastic housing. Two screws hold the PC board to the bottom of the case.
To release the bottom board, pry the case away from the jack until the board pups up.
The board may be a bit difficult to maneuver out, but it is free. Once the board has been removed, we have a view of the back of the radio board.
The rear of the board is dominated by an RF shield. Unsolder the shield in four locations.
This gives a view of the bottom side of the bottom board.
Those of you who know me might think I will next be after the ceramic filters. You are correct! CF1 is the FM ceramic filter, CF2 is the AM ceramic filter, and CF3 is the ceramic discriminator for FM detection. Fortunately for me, whoever did the circuit board layout made things very easy - the holes for the AM and FM ceramic filters are large, allowing easy removal.
I have a large stock of 150 kHz FM ceramic filters. My AM ceramic filter is actually a three element stagger tuned +/- 6 kHz filter, which doesn't give really high selectivity - but that is a good thing for reasons I will discuss later. My AM ceramic filter requires a little preparation.
The filters can then be added.
I eagerly anticipated what might happen, and I was not disappointed. First, AM. I expected better selectivity, but I also got a big increase in sensitivity! AM sensitivity is now almost up to SRF-59 standards. The news isn't all good, though. AM is presently treated as a poor stepchild of FM - and that translates here to the tuning voltage steps generated by the digital tuning system. Although all frequencies are tunable, all are not perfectly centered on frequency. This means that in some cases, weak first adjacents cannot be heard even when the stronger station is nulled.
FM performance is not nearly as good as the SRF-59, mainly because the radio is prone to overload. Selectivity is much better, and if I move the headphone wire to eliminate overload, a rim shot 80 miles away comes in on a second adjacent frequency to a local - something that was impossible before. It also came in well in stereo, although not static free as it does on an SRF-59. A less critical situation with two weak first adjacents showed no crosstalk at all, it was severe before the modification, with the station slightly stronger crosstalking onto the weaker signal. Were it not for the overload problem, this would be a much better FM radio than it is.