tech-instr: Ohms range flaw

I noticed that the Ohms range was erratic. Many or most of the schematics don't show it, but the 1.5V nominal D Cell (mono cell) is actually only connected when the left switch is at ohms.

Also the battery will run down quickly if there is a short on the probe leads on the low ohms range and on Ohms.

The resistance in series with the battery to test lead is low on low ranges:

9.7
90 + 9.7
900 + 90 + 9.7
9K + 900 + 90 + 9.7

Etc.

So if a voltage is on the test input then there can be considerable input current if the left switch is at Ohms (battery connected).

Naturally the 9.7, 90 and 900 resistors were a bit cooked. The 9.7 seems to have been 12 Ohms, it read 4 Ohms. Perhaps it had a parallel resistor. The 90 and 900 were both pairs of resistors.

So on the lowest range the battery supplies between 110mA and 160mA depend on if near exhausted or new. Obviously the other ranges progressively use about 1/10th each time.

As the valves take 600mA and the 6.3V winding is earthed at one side, it might be feasible to use a 1.25V to 1.6V regulator  based on an LM317 (minimum is 1.2V approx.). However that would be easily damaged on the low resistance ranges by an in circuit voltage.

Perhaps the safer approach is simply to add a 250mA fuse to protect the resistors and battery on the low resistance ranges. An LM317 could have a reverse rectifier across it, but unlike a battery which can sink current long enough for the fuse to blow, there is nothing to sink current from the external source to blow the fuse.

 

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Part of the chassis was corroded with a leaking cell when I got this meter. The D cell is not easy to replace, because six screws need removed.

As mentioned, a simple series regulator is not robust as there is only 9.7 Ω between it and the test input on Ohms lowest range.  I considered a suitable yellow or amber LED, but the current is nominally 140 mA on zero ohms on lowest range.

After some experimenting to get enough current on the lowest ohms range (the higher ones are simple) I added:

1. Dual centre cathode Schottky rectifier to the AC 6.3V with anodes joined.
2. 1000 μF capacitor for smoothing and higher average voltage. A 330 μF wasn't enough.
3. 22 Ohms series resistor
4. 2 x 1N5404 in series as a shunt to give about 1.4V (a bit less on lowest range).
5. A second 1000 μF capacitor across the shunt (35V).

 Later I will add a 250mA fuse. That might blow before the 2 x 1N5404 burn out and the 97 Ω cooks, because they are rated at 3A. I found previously that the 1N4001 has a lower voltage drop than the 1N4004 or 1N5404 at the same forward current. The 1000V parts are so slow that they work as cheap PIN diodes and don't need snubber caps on a bridge.

Other observations

You should wait for warm up and only adjust Zero on the DC + Volts with input shorted. The resistance of probe, switches and slight drop of supply volts etc would make the Zero wrong on lowest resistance range. The DC - needs its own Zero setting. The higher resistance ranges should give the same Zero with probe shorted.

The best Ohms Adjust position (probe open circuit) is about the very end of top DC Volts scale, but only if the Zero adjust done on the DC + Volts setting. It should be the same on all resistance ranges as the shunt regulator has no external load. Essentially its just a series resistor in series with the meter as are the internal AC adjust (most left looking from rear) and DC adjust (most right looking from rear).

The AC balance preset (middle rear) simply adds a small DC offset via a 10K variable series resistor to 0V. A 30 or 33 kΩ connects that to HT and a 40 MΩ (82 MΩ on Eico) connects that point to the buffer amp input. The 6AL5 / EB91 peak detector has an 5 MΩ (18 MΩ on Eico) output. Obviously the thre paper caps on the peak detector and the AC input need replaced.

The Zero adjust is varying the -HT between the pair of ECC82/12AU7 cathode resistors.

The  DC - is achieved simply by reversing the connections to the meter and the three series resistors (AC, DC gain presets and front panel Ohms).

The Eico schematic is more complete but seems to have errors. The TC-65 manual schematic is partial.

Part of the reason for DC - nis that the input isn't isolated. The probe common is connected to the case and chassis. Using an earthed 3 core mains cable is needed for safety.

Conclusion

With AC from 30Hz to at least 10 MHz and useful readings to VHF/UHF, 11 Megaohms input resistance and up to 1500 V (many cheap 750V DVM are 1M Ohm and about 300V max), easy to obtain valves this is still a useful test instrument. The lack of a battery hatch, mains operation and missing or poor instructions means there may be a dead leaky cell inside.

The simple modification adds about 150 mA load to the mains transformer that already supplies 600 mA at 6.3V AC. After over an hour on the lowest Ohms scale the transformer is still cool.

The lowest range reads a 75 Ω resistor correctly if open circuit Ohms Adj is set to end of scale at 15V mark. The higher Ohms ranges measure correctly set to the end of scale at 50V mark. I'm not too sure why. However even a cheapest DVM is better for resistors from about 1 Ohm to 1 Megaohm. More expensive, but still cheap have a wider range.

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Additionally schematics show a pilot or on lamp fed from the 6.3 V AC. My example of the Tech TE-65 has a neon. The series resistor just hangs off the 110 V AC secondary tag. Hence my shunt regulated 150 mA max load and nominal 1.4 V supply is unlikely to be overloading the 6.3 V AC winding.

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With careful adjustment of mechanical zero (equipment off) and zero on DC+ the DC- is also at the same zero. Then if DC zero is correct the middle AC balance is adjusted for zero. The AC gain (left most preset) was checked with a 2V 50 Hz squarewave and was only a fraction high. I'll check it again later with the signal generator part of my spectrum analyser (19kHz to 1.5GHz) with a 50 Ω termination and T-piece after I've tidied the wiring inside disturbed by resistor replacements, adding the regulator and a panel fuse.

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