MTG Hoymitz DG80 Frequency Meter

 

 

 I do enjoy a good mystery, especially radio orientated. See the Morton Cheyney for example

This recent acquisition (with a pretty weird name reminiscent of an IKEA product) began to get interesting once I'd looked unsuccessfully for some on-line data, because I drew a complete blank. I then happened to look at the label on the back though and discovered the manufacturer MTG (UK) had a head office in Bournemouth.. less than 10 miles away.

When was the frequency counter made? The easiest way to check was to look on the integrated circuits (see below) and I found most of the 74 series chips were dated 1979 with the later ones 1980. Also there's that black boxy thing with the red label which has a date in August 1980. Those dates are reassuring because this pre-dates firmware.. meaning that any replacement parts should be plentiful and cheap. The only parts that might tend to be tricky are things for RF amplification and frequency division. In those days capacitors were long lasting apart from the odd tantalum type.
 

 

 

Although nearly all the parts seem to be bog standard things this frequency standard module (also carrying the "MTG" label... actually "MTG Frequency Standards") looks custom, although maybe the contents of the module are freely available. Is that crystal 5MHz or a third? Possibly the design idea was to use 1666.6KHz but then swapped to the fundamental to simplify matters?

The equipment looks like a basic or initial model because the input carries the label "CHANNEL A", suggesting a dual channel version may have been planned.

Below is a picture of the front end parts showing MECL chips mounted on a ground-plane style circuit board.

The MC10216P is an amplifier chip with a spec of around 2nSec. The MC10138L is a divider (2/5/10) and the MC10125L converts MECL voltages to standard TTL. This was fairly common practice in the1970s and 1980s as high speed TTL wasn't yet available. Those MECL chips are selling for around £2 to £3 so if duff, replacements will be available.

On the right you can see seven rather neat inter-board connections.
 

 

  So... who was MTG (UK)?? A search of Companies House reveals such a name in the right period and engaged in activities covered by the SIC 82990 or "Other business support service activities n.e.c.".. take your pick from the activities shown below!

It seems the company was dissolved around 2017 but had been renamed circa 1995 and appeared to be operating as a travel agent.

Around the late 1970s and 1980s some new UK firms were in competition with Japanese companies that were slowly capturing the electronics market in the business sector below that ruled by US companies. Because prices in the latter were sky high, Japanese firms were capturing the area of lower priced equipment but still maintaining really high prices. This enabled UK competition to supply decent products. Eventually though these were forced out of business leaving far eastern products to regain control and of course up their prices by organising a scheme much like earlier outlawed retail price maintenance. "Stick to our published pricelists or we won't supply you".

Did MTG bite the dust?

 

 

 

 

 I decided to carry out a couple of checks then if all was well plug it into a mains supply.

My capacitor meter told me that all the larger types in the power supply area were as good as new, at least in terms of capacity and ESR, so I plugged it in and turned it on. The on/off switch wasn't too happy but both Standby and ON seemed to work. The first picture shows "Display test mode" from holding down the button and the second "Self Test" which I reckon is the display of 1MHz (=1.000001MHz) derived from the crystal module.

 

 

 

 I connected my TinySA set to provide 200KHz at -7dBm and found the counter failed to display this so I need to connect a second more convenient signal source and figure out where the problem lies.

As a matter of interest I checked one of the case securing screws and found it was 4BA so it would seem UK manufacture is likely, then a look at the parts revealed several were labelled "RS" so it was definitely made in the UK.

As it was coffee time I decided to look further for MTG but first looked for Hoymitz which is a very rare name. I found a 17th century baptism record in Austria then found some records in the UK Newspaper Archive and the London Gazette.

In 1974 until 1982 a company called Hoymitz Electronics was operating in Middlesborough selling electronic typewriters but apparently got into financial trouble and commenced winding up procedures around 1977 then finally deleting their Hoymitz Electronics Ltd business name in 1982. A year later Hoymitz Ltd was also deleted as a business name. Moving on to MTG (UK) Ltd I found that this company (registered in Glasgow) was also wound up but in 2017-2020 but by then they were in the travel business. A combination of MTG and Hoymitz isn't visible neither is "Hoymitz Instruments". From what little information I found it looks like the owners of both MTG and Hoymitz got together to either manufacture or import electronic equipments but the business failed to penetrate their market and just stopped trading leaving nothing in their wake. Perhaps the model name DG80 refers to the release date of 1980?

 

 

 

 Here are a few of the records I found two of which were in local newspapers in 1974 and the other in the London Gazette. A picture of 9 Albert Terrace is shown above. There are no Hoymitz's listed in on-line records and maybe the letters MTG are initials?

 
 

 

 

 The above picture shows the relationship between MTG and Hoymitz and below a picture of the two circuit boards forming the base of the counter. Top right you can just about see the RF input fuse.

 

 

 I powered up the counter and plugged in my recently repaired Rohde & Schwarz signal generator set to 1MHz.

The counter display didn't change from "0" so I looked underneath the counter to check the BNC input wiring with a scope. A nice sine wave appeared at the BNC connector. Adjacent to the connector was a small 250mA fuse which I found to be open circuit. Replacing this put the sine wave at both ends of the fuse but still "0" on the display. Maybe there's some damage caused by whatever blew the fuse? I also wondered if a dry solder joint could be present so pressed on the larger of the pair of circuit boards. The display suddenly read 1.000.000 but the precise location of the dry joint wasn't obvious so I called a stop to testing and decided to examine the circuit board tracks under a magnifier.

A circuit diagram for the SG80 will not be available so I looked for a counter of roughly the same vintage and below I've copied the Phillips PM6611 (80MHz) and PM6613 (250MHz) input circuits.

 

 

 Looking at the input circuitry of the DG80 its design is relatively simple, using a MECL MC10216P triple line receiver as a Schmitt trigger feeding an MC10138 counter and an MC10125 MECL to TTL converter.

A pot and a front panel control will set the parameters for the Schmitt trigger and numerous diodes etc (including the input fuse) should protect the counter from an excessive input voltage. This type of problem might occur when inadvertently setting a device under test to "transmit".
 

 

 

 As you can see above the counter worked fine on the bench. I found the sensitivity control was back-to-front with "Sensitivity" fully anti-clockwise which hadn't helped when I'd briefly tested it before. As yet, the reason for the frequency difference is unclear as I did not check the R & S generator after I'd got it working. I then proceeded to check its performance by twiddling the input frequency and its level with the results shown below. Sensitivity was set to maximum but I noticed slightly extra stability could be tuned using the sensitivity control. I didn't use the 50 ohm termination button.

 

FREQUENCY

 INPUT

FREQUENCY

 INPUT

 100KHz

 -38dBm

 120MHz

 -19dBm

 200KHz

 -38dBm

 130MHz

 -16dBm

 300KHz

-39dBm
 

 140MHz

 -14dBm

 400KHz

 -39dBm

 150MHz

 -12dBm

 500KHz

 -39dBm

 165MHz

 -10dBm

 2MHz

 -34dBm

 180MHz

 -9dBm

 4MHz

 -34dBm

 200MHz

 -7dBm

 22MHz

 -30dBm

 210MHz

 -6dBm

 30MHz

 -30dBm

 220MHz

  -5dBm

 50MHz

 -29dBm

 230MHz

 -3dBm

 235MHz

 0dBm

 Looking at the information printed by the coax socket it was somewhat surprising to see the spec. Max input voltage is 100 volts RMS into 1 Mohm or 10 volts at 50 ohms (the latter obviously dependent on the 2 watt rating of the 50 ohm input resistor). The test inputs above ranged from 2.5mV to 225mV. Note that the input has a 20pF capacitance which will degrade the input somewhat at the higher frequencies (40 ohms at 200MHz for example) resulting in higher power levels being required to maintain a given sensitivity setting as the frequency increases.

The information also states that the counter will work from 10Hz to 200MHz. My tests were limited to 100KHz and above, being within the spec of my R & S generator.

I didn't try the effect of the 10dB attenuation or the filter buttons.

Next.. the only fault, and which probably meant the disposal of the counter. The bad on/off switch. This has a very limited travel in the ON state and ofen flips back to OFF. Oddly, on/off switches are a common prblem with quite expensive test equipments. I had problems with both my Rohde & Schwartz plus both my Hewlett Packard signal generators in that respect.

As you can see below it was easy to remove the front panel which is screwed in place but has only 4 wires to detach. The main connections are made via thise blue multi-way connectors.

 

 

 

 The on/off switch solder connections can be seen here. You can also see that the switch has 9 pins. Removing it wasn't easy because it was jammed in.

 Here's the switch. If you know anything about such things you might notice it's a bad choice because it uses solder tags designed for wires rather than being designed for PCB mounting so it's jammed in.

Removing it detached several plated-through holes plus a short length of track which now need to be repaired.

Finding the correct replacement was tricky. The correct switch needs to be a 3-pole on-off-on type plus being a PCB type. Typically STMS303A2.

After lots of looking I found one from RS, but later for some inexplicable reason, when I went back to order one it had disappeared..

In the end I had to order one from Amazon with solder terminals just like the original.
 
 
 

 To be continued

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