Pye/Philips  M(X)290-series  
General  Information

Identifying M(X)290-series Sets

With a M(X)290-series set in your hands, the first step is to look for an ID plate on the rear panel carrying details such as the serial number and operating frequency. If the set has no ID plate, or the plate is blank, be very wary - it will be necessary to dismantle the set to identify it, and even then it may be difficult to get a positive ID. The best advice for first timers is to steer clear of such sets.

If the set has an ID plate engraved with information, you're over the first hurdle! To find out if the set is suitable for conversion, you must be able to interpret this information.

The first thing to look for is the model type; this is usually given on the ID plate under 'Cat. No'. There are seven basic sets in the M(X)290-series, the M293, M294, M296, MX293, MX294, MX295 and MX296. Each of these sets come in a range of guises, with various options and front panel controls to suit particular uses. The first thing to note is that sets with the 'MX' prefix are synthesised, others are crystal-controlled. The number after the M or MX tells you what type of set it is - 293 (VHF, AM), 294 (VHF, FM), 295 (Band II, FM) or 296 (UHF, FM).

Next, look at the section of the ID plate marked 'Code No.' or 'Options Code'. This usually contains an alphanumeric code which holds information about various aspects of the set's specification including channel spacing, frequency band, channel capacity and transmit power.

The first letter of the code indicates the channel spacing, normally 'V' (25kHz spacing) or 'S' (12.5kHz spacing). The channel spacing specification affects several things within the set, the most obvious of which are the transmitter deviation setting (easily adjusted) and the bandwidth of the main IF filter in the receiver. Most of the sets you come across will be 12.5kHz specification.

12.5kHz spacing is fine for the 4m and 2m amateur bands, where 12.5kHz operation is now the norm, but might cause problems on the 70cm band, where 25kHz is still standard. The problem manifests itself in distorted audio when listening to well-deviated signals. However it is a fairly simple matter to remove the IF filter of a 12.5kHz UHF set and replace it with a 25kHz version.

There is a third, rarely seen, channel spacing code indicated by 'R' (20kHz spacing). This is only found on synthesised sets. If you wish to do a conversion to the 6m band, an 'R' specification E0 band set will be a positive advantage; otherwise such a set will need extra modifications, over and above the normal conversion, to work satisfactorily on the other amateur bands.

The second and third digits of the 'Cat No.'/'Options Code' constitute the 2-digit band code, indicating the operating frequency band of the set. The band code takes the form of a letter ('A', 'B', 'E', 'K', 'T' or 'U'), followed by a number ('0', '1', '2' or '4') or, in rare cases, the letter 'W'. In some cases there are two band codes together; the first indicating the transmit band, the second the receive band. Look at the next section (Conversion Possibilities) to see how to decode the band code.

The last digit of the 'Cat No.' is a number indicating the transmit power, either '1' (25 watts), '2' (15 watts), '3' (10 watts), '4' (6 watts) or '5' (1 watt).

M290-series (crystal-controlled) sets come with 1 to 6 channels. If you have a M-290 set with less than six channels, it is a fairly simple matter to fit the missing crystal sockets and tuning coils etc onto the pcb (all the tracks and holes are there), and bring it up to the full six channels.

The situation with MX290-series (synthesised) sets is more complicated. There are 16-channel sets with an edgewise rotary channel select switch (the most common variant), 40/80-channel sets with a rotary channel select switch and a LED channel display, sets with a 16-way channel selection switch and an additional LCD display, sets with a keypad and LED display, and even trunked sets with no channel selection switch on the front panel.

The basic 16-channel MX290-series set is easily converted to give 32 channels and similarly, 40-channel sets are easily converted to give 80 channels. With a bit more work you can convert a 16-channel set or a single channel trunked set to give 64 or more channels.

M(X)290-series Conversion Possibilities

Assuming you have been able to identify the model type and the band code, you are now in a position to decide on whether or not a M(X)290-series set can be converted to FM operation on the amateur band of your choice. In the table below we have graded the difficulty of conversion as follows: D1 (easy), D2 (complicated), D3 (only suitable for Stephen Hawking):

Model Type	Band Code	Mode	Frequency (MHz)	Conversion Possibilities
M293	E0	AM	68-88	4m (D2), 6m (D2)
MX293	E0	AM	68-88	4m (D2), 6m (D3)
M293, MX293	M1	AM	105-108	4m (D2)
M293, MX293	M2	AM	138-141	2m (D2)
M293, MX293	A0	AM	148-174	2m (D2)
M294, MX294	E0 or EW	FM	68-88	4m (D1), 6m (D2)
M294, MX294	B0	FM	132-156	2m (D1)
M294, MX294	A0 or AW	FM	148-174	2m (D1)
MX295	K1	FM	174-208	2m (D2)
MX295	K2	FM	192-225	2m (D2)
M296, MX296	T1	FM	405-440	70cm (D1)
M296, MX296	T4	FM	425-450	70cm (D1)
M296, MX296	U0	FM	440-470	70cm (D1)

Test Equipment Requirements

We recommend the Pye/Philips M(X)290-series because for the conversions we have rated as D1 (easy) there is no need to replace coils and other components on the pcb. In most cases the conversion can be performed by replacing crystals (crystal-controlled sets) or the synthesiser memory chip or PROM (synthesised sets), and then retuning the transmitter and receiver RF stages.

For these conversions there is no requirement for sophisticated test equipment - a multimeter (or a 0-10V reading voltmeter) and a set of RF trimming tools are the only essential items.

A RF power meter and a RF signal generator are useful aids to retuning the transmitter and receiver respectively, but they are by no means essential. If you don't have access to a RF power meter, a VSWR meter with a forward power indicator or a home-constructed RF probe connected to your multimeter will serve the same purpose. If you are short of a signal generator, you can either build a low-power oscillator to act as a signal source, or you can use a consistent off-air signal source eg a local repeater.

A frequency counter can help you to get the crystal or synthesiser frequency exactly right, but if you lack one, you can use another ham set operating on the same band to provide a reference instead.

Conversions we have graded as D2 (difficult), generally require the replacement of pcb components and more complex retuning and realignment procedures.

Conversions we have graded as D3 require considerably more RF and constructional ability and are only really suitable for seasoned PMR converters.

What's Involved in a Conversion

If you have acquired a M(X)290-series set suitable for conversion to the band of your choice, you may find that it has one of a number of selective signalling options fitted onto the basic 16-channel chassis. Generally speaking, these are best removed and consigned to your junk box.

Next, for both M(X)290-series & PF-series sets, you need to obtain crystals (crystal-controlled sets) or a PROM/EPROM (synthesised sets) for the frequencies you require. Once you have fitted these, the conversion simply consists of retuning the receiver and transmitter RF stages to get them operating efficiently on the new band.

It is very important to use a proper non-metallic trimming tool for adjusting RF coils and trimmers, as otherwise it is all to easy to damage fragile ferrite cores which may then be impossible to remove without destroying the coil former.

Sticking tuning cores can be the cause of much aggravation to PMR converters, and if you are unlucky enough to find one if your set, you must try to avoid applying excessive force (though only experience will teach you what this means in practical terms). Rather than forcing the core, apply a steady pressure for several seconds to allow time for the congealed grease within the former to loosen. If there is even a small amount of movement at this stage, it shows that the core is not stuck fast and, given patience, it should be possible to remove it. However, if a ferrite core appears to be stuck fast or if the top of the core has been damaged by the attempt to move it, it may be possible to remove the lower lid from the set and adjust the core from underneath. If the core still refuses to budge, apply a small amount of olive oil and leave it for a while, preferably a few hours, in a warm location. If this treatment fails, you can try heating the coil former with a hot air blower.

Occasionally you will get a core which will stubbornly refuse to move whatever treatment you apply to it - in such cases you may have to drill it out, though as often as not this damages the coil former in the process. This is where a well-stocked junk box becomes useful. If you can find a similar coil former, you should be able to unsolder the coil windings from the unservicable former and transfer them to the new one.

Finding More Information

Go back to the PMR Conversion Index for more model-specific information about Pye/Philips M(X)290-series and PF-series conversions.