Ira, the answer is that YES, it can be converted to receive the broader range of World FM as opposed to Japan FM. Before I comment on how to do it, which Eric has already mentioned, I'll first comment on the circuitry conversion.
Japan tuners and World tuners vary not only in the frequencies received but also the received range. Japan tuners only have a 14mhz bandwidth. World tuners have a 20mhz bandwith. Problems with converting a tuner from 76-90 to 88-108 is several fold:
(1) the tuning dial will always be incorrect. You would be tuning blind
(2) Even with the service manual, virtually all boomboxes (at least the ones we collect) are PARTS OBSOLETE. The swap can be done but some parts will have to be changed, then the entire FM will have to be realigned. If you are replacing a transistor, capacitor or resistor, you can usually cross the original with another. However, the RF tuning coils and transformers usually are listed in the service manual and have a manufacturers part number, not the product value, i.e. uH, without which we wouldn't know which appropriate part to substitute.
(3) The cost to do the conversion and alignment from a tuner guru specialist will not likely be less than the cost to procure a donor box from which you can also extract the tuning dial scale as well.
(4) A service manual is almost a requirement because each manufacturer specifies unique locations to adjust the tuners, and specify the frequency of the signals to inject, etc. Without this...... well, do you just guess? Service manuals for many models, especially those from asia, and generics, well they don't exist.
So the solution is to use what are known as FM Band expanders. These are small inline devices that install between the external antennas and the tuner's antenna-in junction. Since the problem is most notable in vehicles, the devices that are out there have been manufactured specifically with antenna splices with automotive jack/plugs. It makes for easy retrofit. They do, however, require 12v power to function. When off, the normal FM signal reaches the radio. When activated, the device shifts the received signal an amount corresponding to the device's calibrated spec (they are sold with frequency shifts of 10, 12, 14, 16 18 and 20mhz). In other words, the japan tuner shows a received frequency of 80mhz. If you have a 12mhz band expander device, you would add 12mhz to the indicated frequency to arrive at the actual frequency received which would be 92mhz. AM signals are passed normally and are unaffected. Unfortuantely, these devices is not completely without issues. Most notably, since the bandwidth on japanese radios is only 14mhz as opposed to world radio coverage of 20mhz so the covered range is either going to have some overlap or some non-covered frequencies. The problem with non-covered frequencies is obvious. The problem with overlap is that the tuner could fight between 2 frequencies (for example, with a 14mhz shift, a tuner can receive 90 and 104mhz at the same time. 2 Channels fighting for one spot on the dial does not lend to good results.
So. How to install? The devices generally come in 2 different styles; Box and Bullet shaped ones. I have read pros/cons for either so I won't comment except to say that it needs to fit inside the boombox so size and shape should be main consideration for the particular boombox. The antenna cables can be modified to be spliced into the system with no external evidence of the retrofit. I would install a power switch someplace so that the band expander only powers up when needed and not unnecessarily draw power when not needed. I probably won't rely on the boomboxes power switch because that would mean the expander would be powered up even if the tuner is not being used and the extra current draw might cause the switch to wear out faster. If a boombox has enough room inside, a pair of expanders could be implemented and an external switch installed to permit "switching and selecting" the amount of frequency shift to maximize coverage.
Advantage of this system is that the tuner can be left entirely stock including the tuning dial. Only you know that 76mhz is actually 94mhz with an 18mhz shift. It also works whether the tuner is a digital or analog as it makes no circuitry changes. It merely shifts the received frequency to one the tuner is able to handle and works transparently.
I have a few RX-7700's which I am planning to do this retrofit on. Great box to do it on since it's desirable enough to expend the time, effort and money, has lots of room, and plenty of places to install additional switches without looking out of place. Lastly, if I recall correctly, Australian companies are the pioneers for these types of devices. I've been told they're the best and they can sell you custom shifts. I've also seen cheaper chinese ones but those are generally sold with only 1 or 2 varieties of frequency shifts. They all say their's is the best so take that with a grain of salt.
I imagine this is a topic that would be of great interest to many collectors since there are so many Japanese Grails with otherwise "useless" FM out there that is making their way into the US collections. Is there a way to "sticky" this?
IRA, you owe me for my sore fingers. Typing this thing was a pain!
~Norm.
should invest in in one -- just for the experiment 
