The National RX-7200. So beautiful. So sweet sounding. So powerful. And yet it has a few shortcomings that limits it's appeal to some boombox enthusiasts. (1), it's internal power supply is set for 100V. As only Japan uses a 100v mains system, this means that the voltage is incorrect for virtually everywhere else. (2) it has a 9khz increment tuning step for AM band; while this may be OK for Japan and at least some parts of Europe, it is a nuisance for North America and everywhere else where a 10khz AM tuning step is used. Finally (3), it has a very limited 76-90mhz tuning range. While #1 is easily corrected by switching out the power transformer with the correct voltage, #2 is not adjustable since it appears that the control IC is designed in that manner. However, it does appear that the MN6045 processor is capable of either a 76-90 or 88-108 mhz tuning band coverage so this apparently can be changed.

Author note: The author of this article has not personally tried or tested this upgrade procedure to verify if it works. Therefore, we suggest you take care and proceed at your own risk. Additionally, the process involves tuning the band coverage range by selectively tuning a low band station and a higher band station // repeat procedure to fine tune the band coverage. This is in itself not unusual and common in most tuning band coverage adjustment procedures. However, it also requires you to know the station frequency you are using to make the tune and also restricts you to those stations. In general, the most common factory specified tuning band coverage procedures involves the use of a RF signal generator to generate the signals that would be used as a reference point to tune to. In other words, it may involve tuning the low end at 88mhz. However, if you are relying on a public broadcast signal as reference to make the adjustment, perhaps the first available strong station is 92.3 mhz. While I do believe this may work, it is not ideal. A RF signal generator would be able to generate any frequency signal on standard AM/FM bands. On the other hand, RF signal generators are not cheap so unless there is a need to do more such future adjustments, such an acquisition would be impractical for most people. One more note: The procedure requires you to unspool and trim a couple of coils. You should know that this is a one-way street that can't be undone. The moment you tinker with those coils, (or any of the tuning caps, etc), it's not something that you can undo by walking back the process a step at a time so make sure it's what you want to do before you begin, and make sure you are 100% familiar before you start. It's easy to cut off a coil. It's much more difficult to restore the coil after it's been cut.

Conversion Step by Step below:

Original source posted by Vladimir (Vlad33), a member of the Shizaudio.ru forum
Translation provided courtesy of Alex (forum member Mystic Traveler)

Stage I –Chip tweak

Switching the chip to euro range – this is “leg” 32 (REGN = region).
Open the box from below, unsolder capacitor (pic 1), remove lid.

Connect chip “leg” 32 (REGN) with “+” of chip current supply – that’s the “leg’ 1 (Vcc).
You can simply connect legs 1 and 32 with a link, any wire will do, but it’s easier to connect
by a short wire leg 32 with a capacitor on pic 2, there we have “+” of supply (Vcc).

Next move: in order to activate the second digit after the decimal point on the display we need to mount 3 resistors. Those resistors are shown on Pics 3 and 4.

They can be mounted simply from below of the circuit board without unsoldering and removing it
From the box.
Solder resistors to the contact pads shown on Pic 4.
Their ratings should be between 47K and 10K, select rating so that the last digit brightness was the same
As that of other digits.
Well, we are now done with the chip region switch, place the lid back, solder ground capacitor (Pic1), connect all connectors and test how it’s working (Pic 5).

During the test radio will function only on one channel, approx. 90 MHz, that’s OK, no worries,
We are not there yet.

Stage II – Tuner adjustment.

The first step is to retune heterodyne. Unsolder coil L203 (Pic 6), carefully unwind the coil wire leaving only 4.5 (FOUR and a HALF) turns. Cut off unwound wire, clean out wire, tin it, solder the coil back.
Replace the “sensitivity” capacitor (Pic 7 – in blue), this capacitor is mounted in parallel with adjusting one (CT201), I replaced it with one of 3 pF but it’s possible that other capacity would be needed.
Then unsolder “sensitivity” coil L201, VERY carefully, without damaging it since it has no frame (Pic 8).
Carefully unwind one turn of the coil wire, cut off unwound wire, clean out wire, solder the coil back.

Since the radio is to be retuned by varicaps here goes initial voltages for the Japanese range,
I had the below voltages:
76 Mhz – 1.03 V;
89.9 MHz – 9.41 V.
Thus, we need to fit in the above interval.

Important: the low frequency limit is adjusted by aligning heterodyne with coil,
Whilst the upper frequency limit – by capacitor (Pic 6, in blue) – this is the main rule of heterodyne and “sensitivity” contours’ adjustment.
Put all connectors, those of tuner and chip, back into place. Power the unit up.
We need a voltage-multimeter, connect it as shown on Pic 6 – red arrow Uh (3d “leg’ of the CP204 connector, white wire) - this the tuning voltage.
Set the chip to 87.50 MHz, get the tuning voltage – we need to stay close to 1 V.
Turn the core of heterodyne coil L203 (in blue on Pic 6), if we are within 1.0 – 1.5 V then that’s good.
Now by tuning buttons set the frequency to the upper range end - 108 MHz.
Check the tuning voltage - we need 9.5 – 10 V, we need to turn the adjusting capacitor of heterodyne CT202
(use a non-metal or isolated screwdriver!), turn slowly to get the tuning voltage of 9.5 – 10 V.

After that reset the chip frequency (simply turn National off and back on) and then again measure the tuning voltage on 87.5 MHz, again set 1 - 1.5 V with the core of heterodyne coil L203,
Move to 108 MHz, set there 9.5 – 10V with the capacitor CT202.
Repeat the above procedure 3-5 times trying to achieve voltage level as close as possible to 1 v when at the low end and 9.5 – 10 V at the upper range end.
If you can’t reach 1 – 1.4 V by turning the core of heterodyne coil L203 then unwind one more turn from the coil L203 (only 3.5 wire turns will be left) and try to adjust everything again as described above several times.
Ideal situation is when you have 1 V at the low end and 10 V at the upper range end.
But 1.4 – 10 V should be also fine as I have it now and it works.
Now check how radio works – you should hear all stations in your region.

Stage III – Tuner sensitivity adjustment.

This procedure is similar to that for heterodyne adjustment.
The same rule applies:
Adjustment at the low range end is done with the coil L201, at the upper end – with capacitor CT201,
Also perform it several times, not less than 3.
Connect a short piece of wire (about 10 cm) to antennae input – CP201 connector, contact 1 or 2.
Try to catch the first radio station at the low range end – by stretching or squeezing the coil L201 (with a plastic tool) achieve the best reception possible.
I live in a big city of Moscow thus I was having kind of an issue while adjusting sensitivity – all stations here have strong signals and while working with L201 all stations were being received equally clear, with very good quality of reception.

Now retune radio to the upper end – near 108 MHz, pick the closest station.

By adjusting CT201 capacitor (with non-metal screwdriver) achieve the best reception quality (Stereo, no distortions) in the middle of CT201 capacity. If you can’t then there may be a need to replace a capacitor on Pic 7. In stock there is the 12 pF capacitor there. By testing and probing I gradually reached 3 pF and in this case the CT201 capacity was set to middle.

Also please try to adjust the contour on Pic 9, in course of time it might have drifted.
Identify end positions of the core where Stereo is gone, then simply set it in the middle position.

There you have it, mates!
Good luck with tweaking your great Nationals RX-7200!