Here's a random collection of MT500 information that I've found useful. It's been really hard to figure some of this stuff out - even from the official Mozarella documentation, which is internally inconsistent...

You can find a very nice copy of the MT500 Parts Catalog here (PDF), and Eric WB6FLY has been scanning all the manuals he can find and saving them here.

There are two basic designations, Basic and Universal. I think the difference is that the Universal models have a side connector for a speaker/mic and antenna, while the Basic only has the top-deck jacks for speaker/earpiece and antenna.

There are four versions, broken into two case sizes and styles:
Omni Slim-line

That is, the Omnis are thicker than the Slim-line, and they both come in short and long cases. The thickness comes from the plastic tray that holds the antenna. Not clear why the length difference mattered to anyone; as far as I can tell, it just adds empty space in the case.

In addition, they could be had in 2W (H23) and 5W (H33) models. The Slim-line only came in 2W. You can tell a 5W by opening the back cover (four captive screws) and find a tall silver/chrome box near the PTT switch; this is the higher-powered output amplifier - there isn't room for this in the Slim-line body.
Xmit current drain (15Vdc)420mA980mA440mA1000mA
Rcv current drain (15Vdc)70mA70mA
Standby current drain (15Vdc)11mA11mA

It doesn't get easier. There are three VHF frequency ranges: 136-150.8 MHz, 150.8-162, and 162-174. There are internal board model numbers for each of these, and the frequency range is encoded in the overall model number (in theory):
2ndpower output2 - 2 watts
3 - 4 watts UHF / 5 watts VHF / 6 watts LB
3rdband1 - 25-54 MHz
3 - 136-174 MHz
4 - 403-470 MHz
6thmodelB - basic
U - universal (see omni/slim-line...)
7thsquelch1 - carrier
3 - tone PL
9thnumber of channels (1,2,4,5,6,8)

Where did UHF and LB come from? I have no idea. One of the tables just introduces it out of nowhere! I only have documentation for the VHF model.

The units use 'crystal elements' (also known as ICOMs in some circles, I don't know why) to set the operating frequency for each channel. These are a complete temperature compensated 3d overtone oscillator. For transmit, they always operate at 1/3 the final frequency; no confusion there. For receive, the IF is 17.9MHz, and depending on the range, you need either a 2x or 3x crystal element, because the board has either a doubler or a tripler in the injection path. This, despite the schematic, which always shows a tripler in the receiver injection path - the parts list shows a different receiver injection part U8: 5105177D43 for Low, 5105177D13 for Med/High. There is some implication that only the BBU board exists in a version that has the '1' or 'L' range (136-150.8), using a 2x CE.

There are different CEs for 2x use and 3x use, but it's not obvious from the parts list. Moto lists three part numbers:
KXN1075AReceiver, use as required depending on model
KXN1083ATransmitter, 2W, use as required depending on model
KXN1042ATransmitter, 5W, use as required depending on model
It's completely unclear why there are different CEs for different tranmitter power. According to International Crystal the 1083A and 1042A end up being the same part. (But Ray AG7S writes to tell me that the two parts have different tolerances: 10PPM for the KNX1083A and 5 PPM for the KNX1042A. The are identical as basic frequency label would indicate. It may be that the 1042A (closer tolerance) is required for the higher power transmitters (5W vice 2W for the other lower power unit). This is pointed out in the Theory/Maintenance Manual 68P81012C55-B, which I've never had the pleasure of seeing!

The IC site also shows that the receive CE exists in two versions, even though Motorola only applied one part number - apparently you were meant to order by frequency only, and Motorola would do the rest.

The crystals themselves are in a HC-49/X package. Most of the crystal manufacturers will make these crystals, re-crystal your existing elements, or sell you 'new' elements to your specs. West Crystal seems to be the most clueful of the bunch, and has good prices, but even they weren't able/willing to change the range of a 3X CE to handle a 2X frequency.

Ed KD8KZ amused himself for the better part of a weekend, taking apart a mid-band (162MHz) KXN1075A to figure out how it works. He reports that there are several discrete components and a thick-film hybrid part. He traced out the circuit, measured many of the components, and drew up this schematic (PDF). He then took apart a low-band (144MHz) unit to compare them, and managed to convert the mid-band unit to work with a suitable low-band crystal:

1) Remove crystal and C5.

2) Replace crystal with appropriate Fc = (F-17.9)/2 unit.

3) Remove windings on L2. Replace with 14 turns of #30
magnet wire (slightly larger than what's on there originally).

4) Tune L1 and L2 for peak at Fc.

5) Put in radio and tweakalize for best performance.

For the xmit CEs, Change C6 to 33pF. I used surface mount (1206) caps;
the board is meant to take a radial lead part with very fine spacing,
but the 1206 fits fine.

Ed is considering going into business converting CEs, since the mid-band ones are easy to find on ebay, but the low-band ones seem scarce. Either that, or we've joked about building a synthesized CE that would be user-programmable, possibly splicing into the channel switch connector and supplying up to 6 frequencies from one unit. If you think either of these ideas is interesting, drop me line... (also note that Bob KA4LMW at The Portable Clinic has both 8-channel and frequency synthesizer modules for the MT500.)


This is the tune-up procedure according to the Service Manual, slightly edited for brevity and laziness.

Receiver alignment

Step	Adjust	For		Measure	Note
1	L6,L7	max volts dc	M2	--
2	L1,L2,	max volts rms	M1	Inject carrier freq signal to antenna.
	L3,L4,L5			Adjust signal gen for indication on
					-30dBm scale of voltmeter. Adjust
					L1-L5 for peak at M1. Maintain
					voltmeter on -30dBm scale by reducing
					signal generator output.
3	CE1	Marked freq	M1	Reduce sig gen to minimum output.
					Inject 17.9MHz +/- 100Hz into
					crystal filter FL1 to produce at least
					10dB rise at M1. Count the low i-f
					frequency at M1. Record within 10Hz.
					Must be 35kHz +/- 1.5kHz.
					Turn off 17.9. Adjust generator to
					exact carrier freq and output for
					-40dbM at M1. Adjust CE1 to produce
					the previously recorded i-f freq
					within 50 Hz.
 -- alternate method --
3A	CE1	Marked freq	M1	Inject 17.9 into FL1. Adjust sig
					gen to read -40dBm at M1.
					Connect output of ac voltmeter to
					oscope. Set timebase to 5ms/div
					and gain to get about 3 div.
					Set sig gen to exact carrier freq.
					Increase output until scope shows
					an AM signal (the i-f resultant). 
					Adjust to zero beat.
 -- alternate method --
3B	CE1	Marked freq	M1	Instead of oscope, listen to audio
					output. Increase sig generator until
					an audio tone is heard (the resultant).
					Adjust to zero beat.
4	Repeat step 3* for the remaining channel elements

L1 is at the lower right of the board with a hole for the speaker magnet (from the solder side). L2, L3 and L4 are directly above it in a line. L6, L7 and L5 then go from left to right, forming a T.

M2 is just above and to the right of the adjustment hole for L7.

M1 is above the speaker hole, just above three traces that cross horizontally. These traces connect (at their left end) into an array of 2x5 pads; counting left to right, M1 is the third pad.

Transmitter alignment

Step	Adjust	For		Measure	Note
1	L101,	max - volts dc	M101	Repeat at least once to ensure 
	L103				a maximum has been obtained. Typical
					value is -1.0VDC
2	L104	max power out	antenna	Repeat steps 1 & 2. L104 is the 
					trimmer on top of U12.
3	L107	specified power	antenna	If CE103 is used, remove it to
					gain access to L107.
					For 5W radios, set L107 for 5W.
					For 2W radios, set L107 for 2.1W,
					then adjust U12 in direction of
					minimum current for 2W.
					Reinstall CE103 when complete.
4	CE101	marked freq	antenna	Freq counter through 30dB attenuator.
5	repeat step 4 for other channel elements (be sure to use the switch!)
6	PL deviation 
7	R211	+-5kHz dev	antenna	connect audio oscillator to mic
					input; set at 50mV rms at 1000 Hz.
8	repeat step 7 for other channel elements.

L104, L103 and L101 are at the left lower corner of the board, from bottom to top.

M101 is a large pad that connects left of L104 and L103. There should be a small arrow trace pointing at the pad.


The common antenna is a rubber-ducky, helically wound, with a protective cap. They are coded for frequency range by a dot on the threaded portion ... if they haven't been trimmed.
color dotdescription
Yellow136-142 MHz
The top deck has a 3/32" mini phone jack for an alternative antenna. The jack is threaded 6-32, but I haven't been able to find a suitable adapter with threads. I found something close - mini phone jack, BNC female, larger threads (8-32) - and re-cut the threads. This made the adapter very fragile and prone to break off (in the radio). I gave up on that approach and started making adapters with short lengths of RG-174; this takes the stress of a large antenna cable off of the mini jack.
Note that there's a rubber grommet around the recess for the mini jack, and if it's misaligned and your plug has a large diameter, it won't go in all the way ... leading to much hilarity and confusion.


I modified a couple of radios with power jacks in the frame, mounted near the volume control. You can see the connections I used. This works OK, but I have not yet found a satisfactory jack that is a panel mount rather than a PC mount (which needs to be epoxied to the frame). This one, which is a normal phone plug, has +12 on the tip, so it shorts to ground when you pull it (or it wiggles free). Not good.

Packet radio

My main use for these radios is APRS, and to that end I have attached a DB9-M connector which brings out signals in the "KPC standard": See the photo earlier on the page:
1TX audioorange
5RX audiogreen
The RX audio is taken from the high side of the volume control, so it's not affected by the knob setting. Pull PTT to ground to activate.

Back to my amateur radio page.

Last updated Oct 05 2011 by cak