Tech TE-65 Vacuum Tube Voltmeter (VTVM)
Written by Bryce Ringwood   
The tech TE 65 is a 1970s Japanese valve voltmeter. The circuit diagram is similar to the Nadler VTVM described in the article about multimetersTech TE65 VTVM front view
 
In the 1970s, Japan had had almost 10 years of its economic miracle, but quality was not always as good as comparable items made in the Western world. It was the era of the "Japanese Transistor Radio" and strange Japanese-English instruction manuals. Nevertheless, Japanese goods were cheaper than their Western counterparts, and industries in the West (shipbuilding, for instance) suffered to the point of extinction in many cases. By 1990, the miracle was almost over.
 
One thing the Japanese excelled at, was moving coil meters. This was a big challenge to my friend Joe, who was a representative for Evershed and Vignoles. He would show me how well balanced and robust his meters were compared to a KEW meter from Japan. He would hold the meter vertically and rock it from side to side at an angle of 45 degrees. The KEW needle would move slightly either side of zero. (if you looked very hard!).
 
It was Joe who thought a VTVM would be a good project for me. I bought a meter movement, but the VTVM ended up as the 'S' meter for a home brew communications receiver. The circuit also used a double triode in a brige circuit.
(Eddystone, Collins and a few others used VTVM circuits in their 'S' meters. They are nasty and need balancing the whole time.)
 
The TE-65 would have been an inexpensive instrument intended for keen hobbyists and presumably, small service centres. Before I purchased the instrument, I used Joe's test. The needle obediently sat on zero. I was totally unprepared for what happened when I got it home.
 
The idea behind a VTVM is that it has a huge input resistance, so that high impedance circuits can be measured with a minimum of error. The TE-65 (and Nadler, as well as many others) have an input resistance of 11 MegOhms. This is good enough for troubleshooting most radio and audio amplifier circuits. The high impedance allows you to test capacitors for leakage - especially audio coupling capacitors. You can also test very high resistances of 100s of MegOhms, and so on.
 
To be realistic - a 20 000 Ohm/Volt meter on its 500 volt range will present an impedance of 20 000 * 500 Ohms = 10 MegOhm = nearly the same as a VTVM without all the hassle of mains leads and so on. Nevertheless, if you work on vacuum tube circuits, you will sooner or later come up against the limitations of digital multimeters and standard analogue multimeters.
 
The TE 65 circuit consists of a selector switch for mode followed by a decade switch for range selection. - basically two potentiometers, one for voltage, and one for resistance range. A 6AL5 diode is used to rectify AC for AC measurements. It is arranged as a peak detector, rectifying both positive and negative parts of the cycle, storing the peak-to-peak result on a 0.02 µ capacitor. The AC range has been calibrated so that AC RMS values are coincident with DC volts. A 12AU7 is used as a DC amplifier with the meter in the cathode circuit. A 1.5 volt battery is used to provide current through the resistor under test on the Ohms range.
 
The first thing I did was to take the instrument apart to make sure nothing was obviously wrong before applying power. As expected - the battery was loose because the battery holder had cracked. What wasn't expected was that the battery was a Japanese type that had probably been installed in the instrument for over 45 years.(surely not?) It had some slight leakage and measured 1.4 volts. I temporarily repaired the battery holder with a hot air pencil and replaced the battery with a new one. Internally, the instrument looked to be in good unmodified condition. I reassembled it - but before switching  on, I saw the meter was completely stuck. Nevertheless, I courageously switched on, but the meter stuck in all sorts of odd positions - as if someone had thrown sand in the movement. I removed the meter and removed the clear plastic front cover. The movement was now completely free, so I next attempted to replace the cover. As I was about to do so, the movement froze with the cover about an inch from the needle. It looked as if static electricity was jamming the works, so I wiped the cover with a damp cloth -whereupon everything worked again. Obviously the instrument doesn't like our Highveld Winters. Maybe this is why the item is in such good condition - nobody could use it. I imagine breathing on the cover will make it usable. Whilst this meter behaves in a rather extreme manner - it is worth noting that static build up can affect the reading of nearly all analogue meters. (Static build up can also send digital parts straight to silicon heaven.) Computer anti-static spray for screens seems to fix the problem.
 
Calibration is probably best done by testing the voltage of a used 1.5 volt battery using a known good digital voltmeter, and then checking the voltage reading on the DVM with the same battery is within a few percent  (about 3%) on the 1.5 volt range. If you really do need to adjust the internal potentiometers - be aware they can be quite fiddly and tempremantal. (Use a used battery, so that the voltage is slightly less than 1.5 volts).
 
I calibrated the instrument using a used 1.5 volt and a used 9v battery. The 9v battery is the more important, since the instrument is supposed to be at its most accurate mid-scale. The AC range was calibrated against a 22 volt transformer (mid scale on the 50V range). The other thing that had to be done was to ensure that the AC-zero and the DC zero coincide. Unlike the Nadler, the DC- range can't be adjusted. Unfortunately, all the adjustments on this instrument interact, so its a case of getting the AC and DC balance to coincide, checking the DC rand, then the AC range, then the balance again and so on. 
 
After a while, it became clear that something wasn't quite right. The DC scale was around 3% out between the centre and end. The zero changed between ranges (a sure fault) and the input impedance measured around 5M instead of 11M. After performing an AC measurement, the DC zero balance was affected. I wish I could say that I found the fault. I merely inspected the wiring and moved some of the parts while I was carrying out measurements to find the fault. Because of the high values of some of the resistors - I had to use another VTVM to service this one. I suspect the case of the 0.005 capacitor was shorting to ground - but its only a guess. At any rate, after the tests, the instrument was readjusted and works as expected with one eexception - a slow return to zero on the AC range. 
 
There is a Rider book on VTVMs here. The book on VTVMs by Rhys Samuel can be found here. The second book is worth downloading even if you have no intention of getting a VTVM, since it has quite a few troubleshooting chapters. In most cases you could use a good quality DMM.
 
Here is a partial schematic of the instrument.  The manual is available from BAMA or Radiomuseum. This circuit diagram is intended for helping to understand how these instruments work, and may be incomplete/inaccurate for servicing purposes.
Partial Schematic of Tech TE 65
The circuit is a little different from its peers, since Tech have cunningly decided to eliminate 3 resistors in the Ohms measuring circuit. To assist with understanding what's going on, I have coloured some of the wiring. Also note the input to the 6AL5. Enjoy puzzling it all out! (The function switch is shown in the "Off" position). There are 7 voltage ranges - 1.5V, 5V, 15V, 50V, 150V, 500V and 1,5kV. (AC RMS and DC)There are also 7 resistance ranges - 1k,10k,100k,1000k,10M,100M,1000M. Finally - why is there a 9E7 resistor, rather than 10 Ohms in the resistance measuring circuit and what are those funny resistors doing in the AC measuring circuit? (answers via email, please!)
 
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