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Old 12th Feb 2016, 11:33 pm   #1
Phil G4SPZ
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Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
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Default Comark FET tester type 185 (1968)

In the words of Brian R Pateman: “Good grief Phil, you do come up with some obscure kit!” and this is the latest example to emerge in good health from the operating table. I bought this for £5 at the last NVCF specifically because I had previously struggled to test some FETs using a multi-meter. The unit looked complete, and when I got it home I discovered that it was originally designed to be powered by three PP4 9-volt batteries. As these have been unobtainable for decades, the meter went on the shelf awaiting a bit of time and inspiration, which eventually arrived last weekend.

I measured the battery compartment, only to find that it was just slightly too small for three modern PP3s to fit in sideways without removing the paxolin contact strips, so I dismantled the meter and was somewhat dismayed to discover three Mallory TR-115R 6.75 volt mercury batteries lurking inside, in a fairly advanced state of decomposition. Fortunately the corrosion had not extended beyond the spring contacts into the wiring or internal circuitry, and a quick clean-up was all that was required. I did a quick check and was relieved to confirm that the moving coil meter was working.

I had no service or operating data, and my internet searches plus an appeal on the Forum have to date drawn a blank, so I lashed up some alternative batteries. The three PP3s to replace the PP4s were easy. As for the internal mercury batteries, investigation revealed that these were wired in series and, due to their relatively low capacity and long life, probably provided the Vgs voltage supply, controlled by the right-hand calibrated dial, at virtually zero current as the gate of the FET under test is normally reverse-biased. These batteries were probably expected to have a life of several years in normal use, and were perhaps not considered to be user-replaceable as the whole meter has to be dismantled to get access to them. I did consider making some replica batteries using alkaline or lithium button cells, but to be honest I felt that maintaining originality was going to be impossible anyway, largely due to the difficulty in making replica cylindrical PP4 batteries. PP4s are still allegedly available, though at crazy prices, and are currently listed as ‘out-of-stock’ at the only known supplier. So I opted to see if I could somehow power the meter using readily-available PP3 batteries.

The open-circuit voltage of three 6.75 volt mercury batteries would be a shade over 20 volts. I measured the voltage delivered by two new zinc-carbon PP3s and found it to be just under 20 volts, so I thought that would be close enough. With the five batteries lashed into circuit and a sacrificial 2N3819 in the test socket, I switched on. The meter worked and gave sensible readings within the range of the test transistor’s parameters, although several of the slide switches were intermittent, which I rectified with a dose of Servisol. I then checked the calibration of the two voltage controls, which give variable Vds and Vgs, and found them to be completely accurate according to my DMM.

With the original battery contact strips removed, the space for new batteries now extended width-wise into the main body of the meter. I found that I could just about get five PP3s inside, although this was somewhat dependent on the brand of battery. As usual, I found that ‘Sony’ branded pound-shop PP3s were a few thou’ smaller than most others, and I have used these successfully to construct several replica HT batteries in the past without problems. I wired up five battery snap connectors and, with some juggling, got the new batteries fitted.

The meter’s front panel and case were pretty dirty, but cleaned up very easily using baby wipes and a duster. It now appears to be in as-new condition, and I don’t suppose it has ever seen much use.

Since completing the renovation, I have been trying to work out how to drive this meter. On the face of it, it seems fairly logical. Starting by switching the meter ‘on’ by rotating the Vds control clockwise, turn the range switch to ‘BAT TEST’ and make sure that the needle reads above the marked ‘bat min’ position - this test only checks the main Vds batteries - and then switch to ‘CAL FSD’ and adjust the adjacent preset to give FSD on the meter. This latter action compensates for the voltage of the Vgs battery supply, originally the three mercury batteries. The tester can reverse the polarity of the applied Vds and Vgs potentials so enabling n-channel, p-channel, depletion mode and enhancement mode FETs to be tested. Select the appropriate polarities, plug in the FET to be tested, set the Vgs dial to zero, set the range control to 30mA, switch to Ids and advance the Vds dial to the value stated on the transistor data sheet. The meter will indicate the drain-to-source current at zero Vgs; rotate the range control to get a meaningful meter reading. Then increase the Vgs control and watch the drain current change. Data sheets usually state a range of Ids values for stipulated Vds and Vgs potentials, enabling the device under test to be evaluated. You could plot the results graphically if desired. FET device parameters do seem to have a wide ‘spread’. Increasing the gate voltage further enables the pinch-off voltage to be measured and compared with the datasheet.

The gm measurement procedure is still unclear, as gm varies to some extent with Ids. The best way I’ve found so far is to set the FET under test into conduction at around 1mA on the 3mA range, switch to gm and press the gm button, when the meter will indicate gm in mA/V on the 0-3 scale. If the reading exceeds FSD, switch to the 10mA range and repeat the test. Again, gm values have a wide spread for a given device, typically from 2 to 6 mA/V, or 2000-6000 micromhos or microSiemens as often quoted in datasheets. As long as you are getting readings somewhere within this range, it’s fairly safe to conclude that the FET under test is a good one.

I am still on the lookout for an operating manual or any service data for this meter. It was apparently released in 1968, as The Wireless World reported in July of that year. My example carries the serial number 4550, so it’s interesting to speculate at what point in the production run it was made, and I do wonder how many were built and where they are now. There is so little information ‘out there’ that it’s tempting to conclude that I own the sole surviving working example, but I expect that you know different...

Phil
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Last edited by Phil G4SPZ; 12th Feb 2016 at 11:36 pm. Reason: Hyperlink added
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