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Reviews Categories | Transmitters: Commercial/Military/Marine adaptable to ham use | Midland 70-340 Help


Reviews Summary for Midland 70-340
Midland 70-340 Reviews: 1 Average rating: 5.0/5 MSRP: $(missing—add MSRP)
Description: Very versatile VHF land-mobile radio easily adapted for ham use
Product is not in production.
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AC5XP Rating: 5/5 Jan 8, 2007 11:09 Send this review to a friend
You won't get on 2 meter for a lower price!  Time owned: more than 12 months
Midland LMR VHF radio 70-340
By Loek d'Hont AC5XP

The LMR 70-340 is a mobile VHF radio manufactured by Midland Japan. The design is from Hitachi. It was targeted at professional and commercial users.
The LMR70-340 is a 40-watt, dash-mounted version of a larger family of radios like the 70-380 (80 watt) and the 70-385 (110W). Trunk mounted versions using a remote head also were made (70-4xx models) but they are basically all the same radios with the exception of the different sized PA modules.
“A” models cover the 2-meter HAM band straight out of the box, the "B" models require a modification, albeit minor (as the "B" models start from 148 MHz and up)
These radios stem from the early eighties and were manufactured all the way into the early nineties. They stem from an era where professional communications relied on point-to-point (or point-to-repeater) communications using "brute force" RF power combined with dedicated frequency assignments, instead of the trunked communications through lots of connected base stations (where a host controller assigns available channels through one or more control channels), the latter which is in vogue today.

This old method of dedicated point-to-point channels assigned to a particular user I think is actually better than today's trunking. True, trunking allows much more efficient use of available spectrum, but the problem with trunking is that connection certainty is not 100%, the user's ability to connect to one another is expressed in terms of "probability" with these kind of systems. This is OK on a quiet, normal day when the local constable wants to leisurely report to his precinct that he just gave you a ticket for speeding, but things go wrong in a serious emergency where just about every available user tries to use his- or her radio. Think 9-11 and the associated communication failures with it, and you know what I mean here. As a matter of fact, many emergency and public services are seriously rethinking trunked-radio today as a result of 9-11.

But back to the Midland 70-340. If one lifts the hood and bottom, one will see the radio is built very well. Glass-epoxy boards throughout, and everything located neatly and easy to access. Wiring cables use quality connectors relying on individual wiring (not the dreaded ribbon cables) and boards are easily replaced if needed.
As a matter of fact, the quality of build is much better than the ham radios of the era.

The design is straightforward. The receiver starts with two J-FETs , one being the RF amplifier preceded by two adjustable high-Q resonators . The same resonators (3 x) also follow it before the J-FET mixer is reached. The mixer brings things down to 21.4 MHz into a quality crystal filter. From there a fixed LO crystal is used to mix down again to 455 kHz , which is handled by the MC3357P, a very popular IC from Motorola (popular in the eighties that is) . This chip handles all the analog FM processing (limiting , squelch, demod and so on). It is assisted in this task by two ceramic 455 kHz filters, as well as a quadrature tank for the demodulator.

The first (variable) LO is generated by a synthesizer. The synthesizer uses the concept of the swallow-counter in combination with a dual-modulus prescaler, so no mixing is used.
This concept is a novel design that stems from the Seventies, it allows the use of a prescaler in the synthesizer loop while maintaining the step resolution equal to the reference frequency on the phase detector. If one would use a fixed prescaler instead, this would not work, for instance a divide-by-four fixed prescaler in combination with a 5 kHz reference frequency on the phase detector would result in 20 kHz step resolution at best. But as said, the dual-modulus concept does not have this drawback while still allowing for high VCO frequencies without the need for mixers. If you want to learn more about this method you could (as an example) study the MC145158 datasheet from Motorola which explains it well.

The synthesizer is controlled by a CPU which relies on a 2716 EPROM for channel programming. 80 channels can be programmed this way (some models allow only 40 models I have found though, for whatever reason). The EPROM is on its own plug-in board which has another logic chip on it, for what seems to be for the purpose of minimizing the number of pins needed for the board.
The module allows for channel programming (simplex or semi-duplex with any shift) and PL-tone programming for both TX and RX. The latter assumes the PL-tone board is present, but I have found that this is the case for most models you buy from the several surplus sources. Minimum programmable step-size is 5 kHz.

The manufacturer’s idea was that the original EPROM module can be removed from the radio without soldering after which it can be UV-erased and programmed again by a dedicated programmer (the Midland 70-1000). Problem is of course that these programmers are difficult to obtain.
A program circulates on the web (MRP70.EXE) that allows you to input desired channels and PL tones for a host of Midland radio families (including the here discussed 70-340) and generate a binary EPROM source file as a result. This file can be used to program a 2716 EPROM (which is the one used on the channel module discussed here) using a standard EPROM programmer. So the only thing you would need to do is carefully desolder the EPROM from the plug-in module, program it on the standard programmer and place it back on the module board. It is probably a good idea to use an IC socket in case you want to change things again later on. If you do, make sure you get a very low profile socket, otherwise the module does not fit well in the radio anymore due to the increased EPROM chip height that the socket causes.

There is a company that sells compatible channel modules for this family of Midland radios. The module relies on FLASH memory, using an ATMEL low-cost processor. The module can be programmed through your PC’s printer port, in EPP mode, without the need for UV erasing. The module actually allows for 4 selectable banks of 80 channels each. (No, I have no ties with this company). I bought one; it works quite well but I believe it to be overpriced. Using the method of stand-alone EPROM programming works just as well, so if you have an EPROM programmer (or know someone who does) this is the way to go.

Back to the rest of the circuit description. As mentioned above, most radios are already equipped with the PL-tone module, this module works quite well and can be individually programmed (through the EPROM module) for each channel, even allowing for different PL tones for TX and RX. There is a button on the front panel of the radio called "Monitor" that allows you to turn off the PL squelch if needed.

The transmitter exciter uses a second synthesizer loop which contains the FM modulator.
The exciter is somewhat unusual and works as follows. The first receive LO (made by the RX synthesizer as described above) is mixed with a separate VCO used for the transmitter. This is then divided down by a fixed divider and passed off to an MC4044 stand-alone phase detector. The reference frequency for this phase detector comes from the same reference crystal that is used for the receiver synthesizer, albeit divided down somewhat to make things more palatable for the MC4044. A phase modulator modulates this reference signal before it enters this transmitter PLL loop through the MC4044. The relatively high reference frequency in the loop allows for the transmit PLL to be very fast, so that it can follow the phase modulated reference signal. The transmitter PLL tracks the receiver 1:1, albeit with an unavoidable offset error which is compensated for by the CPU controller (so the user does not have to keep this offset in mind when programming channels).

The nice thing about this used method is that the modulation deviation is always the same regardless of what the synthesizer frequencies are. With direct VCO modulation (as an alternative) this is not the case due to the non-linearity of the varactor diode, and also PL tone modulation is difficult in that scenario due to the low frequencies associated with PL tones (meaning the PLL has a tendency to "loop those out" in such an alternative scenario)

The rest of the transmitter is pretty traditional; things are beefed-up to respectable RF power levels using hefty Mitsubishi power transistors. The PA is housed in its own shielded module for all radio models, but easily accessible. The 110W version uses two of these transistors in parallel (not sure if it is in push-pull though)

One thing that is worth mentioning is that the radios are not wide-band in design; for instance the B model is specified for 148 MHz to 173 MHz but you cannot program the channels more than about 4 MHz apart. For the receiver, best is not to exceed 2.5 MHz I have found, but the transmitter is about 4 MHz wide, so a bit more.
To align the radio after channel programming, (you need to first program the desired channels), select the channel closest to the center of your programmed channels, then adjust the RX synthesizer VCO to be centered, and then peak all the helical resonators in the receiver RF stage, as well as the two LO VHF tanks. Next step is to peak all the variable capacitors used in the PA and driver. Then you are done!

To summarize, you won’t find a more versatile VHF radio capable of operating on the 2-meter band for such a low cost (some models go for as little as $10), so I can highly recommend these.
One word of caution: The models that the sellers depict on the several internet auctions are the prime of the pack; the ones that they will send you will not look as nice. Some actually look very beat-up after many years of service in utility vehicles. This means you will have some cleaning to do on your purchase. The good news is that they usually look like new from the inside, although there are exceptions on this rule.

Who ever said that hobby of ham radio is so much more expense today than what it was in the old days is completely wrong. It is all what you make of it!

Loek "Luke" d'Hont AC5XP
 


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