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
|
|