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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 27th Jul 2020, 10:43 pm   #1
space_charged
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Default Analogue Comark thermocouple thermometers

I have come by a couple of analogue K type thermocouple Comark thermometers. I remember using such things them when I was a research student at Uni in the 70s.
The ones I have are models 1602 and 1604.

If anyone has any service information on either or both of these I'd love a copy. The 1602 which reads from 0-100 on the meter scale, is dead on accurate. It reads from -120 to +1000C in several ranges.

The 1604, which has a scale of 0-10 and reads from -60 to +500C, is way off. On ice it reads +5C.

There are pots inside, presumably for calibration. I may be able to deduce what they do (base and slope?) by noting their current settings then adjusting them to see what they do.

Any information would be appreciated.
Charles
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Old 27th Jul 2020, 11:11 pm   #2
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Default Re: Analogue Comark thermocouple thermometers

Not sure if this is any help - circuit of a Model 1905.
I've owned mine for some time, and it did fail at one stage due to a particular resistor going, which was a "known issue"

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Attached Files
File Type: pdf Comark 1905.PDF (376.5 KB, 26 views)
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Old 28th Jul 2020, 12:38 am   #3
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Default Re: Analogue Comark thermocouple thermometers

Thanks for the pdf Bazz4CQJ, but sadly there is not much similarity with the innards of either thermometer. They are quite different from eachother too, the 1602 and the 1604.

Like all Comark kit of that era, they are well made and have lovely big meters. I use digital stuff all the time, but I have a very soft spot for moving coil based test equipment. When ever I can, I use either my AVO 8 MK II or one of my Taylor multimeters
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Old 28th Jul 2020, 1:24 am   #4
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Default Re: Analogue Comark thermocouple thermometers

The 1604 has one large internal pot, sealed with a dash of white paint. It looks like its a multi-turn pot, so I put an additional mark on it to identify the original setting. The white paint on it instantly flew off when I turned the pot a bit. With the thermocouple on ice in a dewar, I observed the effect on the reading when I adjusted the pot. Sure enough, the needle could easily be set to zero, corresponding to the known temperature in melting ice.

Once I remove the probe from the ice and let it dry off and warm up, the reading is much more reasonable. next I'll check it at 100C in boiling water. If its reasonable I'll be well satisfied. If not, I presume one of the other sealed pots (there are two) are the "slope".
K-type thermocouples are more or less linear from 0 upwards. Careful adjusting of that and re-adjusting to 0 on ice should give me a reasonable result. I suspect the second "slope" pot may be for values below the internal reference (less then 0).
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Old 28th Jul 2020, 8:23 am   #5
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Default Re: Analogue Comark thermocouple thermometers

possibly CJC calibration
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Old 28th Jul 2020, 3:28 pm   #6
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Default Re: Analogue Comark thermocouple thermometers

Yes most probably it adjusts the cold junction reference voltage. I'll do some more tests today, also take some photos of it.

The case of the 1604 was damaged, cracked in one corner. I repaired that using a piece of angled aluminum bonded in with epoxy resin. Both the aluminum and the plastic case were roughened before bonding. The case has a handle at the top that pulls out and the plastic bushes that it slides in were missing. I made a couple of bushes out of brass on my lathe and just bonded them in with epoxy.

The meter had some dirt on the scale, so I took it apart to clean it. In doing that I found one of the supports had fractured off so that too had to be bonded back. Its looking quite nice now.
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Old 5th Aug 2020, 3:38 pm   #7
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Default Re: Analogue Comark thermocouple thermometers

Comark 1600 series Analogue thermocouple thermometers.
A most useful range. I use one for checking transistor case temperatures and soldering iron, and lend it to my wife for jam and marmelade making.
Late 1960's to early 1970's.
Made up of modules based upon an extraordinarily versatile chopper based millivolt amplifier. Then a whole range of modules for diffrent temperature ranges and thermocouple materials. This makes the manuals very scrappy and you will have to read and select what applies for your model. The 1604 model has ranges covering -60C to 170C in 10degree spans. The other models cover 100degree spans and full range.
I use my 1601 covering 100degree the most useful, as my other model 1642 fussy to use.
I attach a catalogue details showing what the various model variants are. Not the best copy, but it was coloured paper and made thirty years ago.
Also details of the thermocouples.
And finally the manuals giving circuit and board layouts for the DC chopper amplifier, the DC power supply and the calibration resistors to obtain the ranges. And the various add-on for temperature trips and AC mains power. supply etc. The calibration procedures are also given, though the descriptions of purpose of the adjustment is a bit confusing as it varies with each variant of the basic modules.
The cold couple thermistor is the STC G14, glass bead, 2mW/C, with 10k ohm at 20C. Not over critical (compared with the STC A14 glass tube used in so many AF oscillators of that period). It seems suitable alternatives are available from Vishay / Philips, GE (USA) and Murata.
The thermocouples usually seen are K type, Ni-Cr/Ni-Al, which are pretty linear with temperature.
The meter is a very nice taught band 500ua with a fine 5" scale.
I have never seen any information upon these meters elsewhere, so I hope these attachments will be helpful, even though they do not cover all the variants. They will need studying to find what applies to your variant.

I also posted details of the Comark 1905 insulation tester back in Aug 2009.
Bazz4CQJ has posted it again several times more recently in response to enquiries from others, thus saving me the task of re-posting it.
I had someone selling a rare manual which I had provided at a handsome price without acknowledgement. Not quite the spirit of this Forum, so now I water mark everything.
I see KevinFScott was asking about the chopper amplifier in 2011, and never came back when I offered the information then. If he is still around, this posting will provide the circuits.
This the same amplifier was used in the Comark 1305 multimeter and probably the FET Tester, both raised in on this Forum in the past.
wme_bill
Attached Files
File Type: pdf Comark_1600-wm.pdf (1,006.1 KB, 10 views)
File Type: pdf Comark_1601-wm.pdf (1.19 MB, 3 views)
File Type: pdf Comark_1602-wm.pdf (1.08 MB, 2 views)
File Type: pdf Comark_1603-wm.pdf (707.8 KB, 3 views)
File Type: pdf Comark Cat & thermocouples-1969-70.pdf (3.09 MB, 7 views)
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