Heathkit V-7 VTVM Repair

The V-7 VTVM was only made in 1956 and the V-7A was manufactured from 1957 to 1961. This VTVM was one of the first Heathkit products to use a printed circuit board. I got this VTVM for almost nothing but all the parts seem to be there except for the shielded probe. I have a later V-7a that I can use for parts if this one turns out to need them. I decided on restoring the older unit because it was in better condition.

This circuit is fairly typical of the Vacuum Tube Voltmeter designs of the mid-1950's. It has an isolation transformer whose secondary provides the 6 VAC for the filaments and approximately 130 VAC for the plate supply or B+. There are two tubes, a 6AL5 twin diode, and a 12AU7 twin triode. The twin triode has a filament wiring arrangement so that it can be run on 6 volts. The 130 VAC is fed through a selenium rectifier and the resultant half-wave rectified DC voltage is applied across an electrolytic capacitor to provide a B+ of 70 volts relative to chassis ground but the actual capacitor has around 160 volts across it. The chassis ground is at approximately half-way between the positive and negative rails allowing for a negative voltage of -70 volts to be applied through a balancing resistor network to the cathodes of the tubes.

The 12AU7 is wired in a configuration known as a "balanced differential amplifier". The twin triodes are connected so that their anodes are tied together and fed directly with 70 volts DC. One triode is configured with its grid tied to ground through a 10 megohm resistor so that a constant current flows through it and the same voltage is always seen at the top of its cathode resistor. The second triode is wired with a 3.3 megohm resistor at its grid so that a DC voltage proportional to whatever is being measured is applied to this grid. The meter movement is connected between the tops of the two triode cathode resistors. If the voltage is the same measured at the top of both cathode resistors, the meter movement will measure zero due to there being no current flow between them. If there is a voltage differential between them, the meter movement will show a deflection indicative of the size of the DC voltage on the grid.

The two rows of resistors in the schematic are the multipliers for the voltmeter on the bottom left and to the right of that, are the resistors for the ohmmeter as can be seen with the battery being located on the bottom. The two diodes of the 6AU5 tube provide a full wave rectified signal when an AC voltage is to be measured. The V-7 was designed to have an internal 1.5 volt dry cell to power the ohmmeter portion of the meter.

The circuit was all complete when I took it apart, with no missing components. The line cord was still intact. I did a quick check of the filter capacitor with a capacitance meter and it showed a value that corresponded with what was stamped on it. I checked the selenium rectifier with an ohmmeter and it seemed to be OK. I double checked the line cord with an ohmmeter to make sure there were no broken connections or a shorted transformer. Once I decided everything was safe, I plugged the unit in and turned it on. The tube filaments lit up and I checked the voltage on the electrolytic capactor, it was 70 volts DC. I also checked the voltage across the filter capacitor for a high AC component and it was much lower than suspected. A fraction of a volt.

I put the V-7 meter in the lowest range and touched the positive DC input terminal with a screwdriver and there was no deflection. Thinking that the 12AU7 might be bad, I checked it on a tube tester. Both tubes tested strong with no shorts. I put them back in the circuit and figuring that they might not be getting B+ voltage I checked the anode terminals for 70 volts. The anodes were getting their B+ so what could be the cause of the problem? I figured I better check for cold solder joints and broken board connections but would need to take the board out .

I separated the circuit board from the chassis and the battery holder. The battery holder is attached to the front chassis of the meter by two difficult to access nuts. The circuit board is sandwiched between this battery holder and the chassis. Its attached to the chassis by a small nut and a metal bracket. There are two large brass nuts that connect the circuit board to the back of the meter movement. The two connectors that connect the meter circuit to the meter also attach under these brass nuts.

Once I had the circuit board out so I could examine the copper traces and the solder connections, I checked the continuity with an ohmmeter. There were some breaks and cold solder connections in various parts of the board. As a precaution, I re-soldered all the connections adding new solder to them.

I reconnected the circuit board to the chassis and mounted the spade connectors for the meter movement under the brass nuts. I put the battery holder back attaching it also to the chassis with two nuts. Checking and rechecking to see that nothing was out of place, I plugged the VTVM into the wall socket, after a couple of minutes I was able to see the meter move to the right and using the zeroing knob put it to zero on the scale. Putting the range switch onto the smallest scale I touched the input terminal and saw a movement. I connected alligator terminals to the two input terminals and connected it across a nine volt battery I got an approximate reading considering a proper probe with a high impedance resistor wasn't being used. I connected a 32 volt AC source to the AC terminals and got a fairly accurate reading. The voltage section seems to be working OK. The only thing that needs to be done is constructing a high impedance probe to get accurate readings. Once this is completed, I will install a battery into the VTVM and check out the ohmmeter.

My particular VTVM had a filter capacitor that seemed to be OK and might have been been replaced at some time over the years. To be on the safe side, the capacitor should be replaced with a new one near the same value 15 microfarads and at least 200 volts working voltage. The selenium rectifier can be seen in the above picture as a black box in the extreme top left of the picture next to the filter capacitor. Some restorers automatically replace any selenium rectifier that they find, but my policy is to keep it if it's still working. If a selenium rectifier is replaced with a silicon device it must be realized that the selenium rectifier has a much higher voltage drop than a silicon rectifier. The 70 volts that this VTVM was designed to work with would rise to about 90 volts which could cause the meter to give improper readings. A dropping resistor would need to be put in series with the silicon diode and the value and wattage calculated to give a voltage drop of approximately 20 volts. In the late 1950's to early 1960's, it was routine for TV repairmen to replace the large and bulky selenium rectifiers that were found in 1950's TV's to replace them with much smaller silicon diodes with a thermistor in series with them.

As I had re-soldered the connections on the bottom of the circuit board, I decided to also resolder the connections to the rotary switches and balance and zeroing potentiometers on the front panel. There seemed to be some problem with the switch connections so I sprayed in some contact spray and "exercised" the rotary switches by moving them through their travel about 20 or more times. After this I let the contacts air dry overnight and exercised them again once everything was dry.

Parts needed

1) 1/4 inch phono jack

2) Two female "panel mount' banana jacks (red and black).

3) Two short lengths of black and white hookup wire. (3 inches)

4) Small plastic project box (Hammond 1551G) or equivalent

5) One 1 megohm resistor 1/2 watt.

All these parts can be obtained at Radio Shack.

I came up with the idea of making an adapter for this meter so that generic meter leads could be used for all the functions, AC and DC voltage, plus resistance. The original DC voltage probe that came with this meter consisted of a phono plug connected to a shielded cable with a probe on the end housing a 1 megohm resistor inside.

Once all the parts are obtained, the box should be drilled to a size just slightly smaller than the outside diameter of the black plastic cover of the plug. Remove the metal part of the plug and put aside. Make sure the part with the inside thread is the one that's sticking out. Insert the other end into the black plastic box as shown in the picture. If it doesn't slip in easy, ream the hole larger with a reamer or a little sandpaper. Once inside, secure it with some hot melt glue. Take the box and drill two small holes on the other side for the red and black banana jack/binding posts. Drill holes and install as shown in the picture above. Solder the wires as shown in the picture, black to the outside and white to the inside. Install the metal part of jack inside the black plastic housing. Solder the black wire to the black binding post and solder a 1 megohm resistor between the white wire and red binding post. Put wires and resistor neatly inside the box and install top box cover. Your adapter is now complete.

Take the back of the meter off and install the adapter in the front phono jack. Obtain a digital meter that reads accurately and use this as your reference. Obtain a fresh 1.5-volt battery and a 9-volt battery to be used in the calibration process. Let the meter heat up for about 30 minutes and plug two generic meter leads into the adapter. Put the voltage range control on the 15-volt setting. Zero the meter with the DC control on the front panel. First, take a reading of the 9-volt battery with the digital meter and then compare it to the reading you see on the VTVM. If it's within 3 percent it should be OK. Take the 1.5-volt battery and measure the exact voltage with the digital meter and put the VTVM on the 1.5-volt scale. Look at the reading, if it's within 3 percent it should be OK. The AC section can be calibrated the same way with a function or signal generator and a 10K resistor. Set the signal generator to a low frequency like 100 Hz and make sure it's putting out a pure sine wave. Connect the output of the signal generator across a 10 K resistor. Measure as high voltage a as you can get out of it and compare the voltage between the digital meter and the VTVM on the appropriate scale. Use a lower voltage like 1.5 volts RMS and see if it's accurate. In my meter, the DC voltages were very close but the AC voltages were out by a little amount. On the circuit board are calibrating potentiometers. They are clearly marked for AC or DC calibration.

The ohmmeter needs a 1.5 -volt battery in order to work. It's installed with a standard "C" cell with the negative terminal touching the spring and the positive tip touching the screw inside the holder. It would be a good idea to clean the screw head with a pencil eraser and the surface where the negative part of the battery touches the spring. Once the battery is in place, turn the instrument on and wait ten minutes for it to heat up. insert test probe leads in the common and AC/Ohms jacks. Short the test probes together and adjust the zeroing adjustment for 0 ohms on the scale and take them apart and adjust the right hand "ohms adjust" dial for an infinite reading. If the meter will zero but not allow you to set it for infinity, you either have a poor battery or a bad connection either between the battery and screw or spring or in the wiring. There is also the possibility of resistors that have changed their value, but that's the last thing to check. In my case, the "ohms" adjust control didn't allow the meter to go up to infinity. The problem ended up being a bad battery connection.

In my book sold on Amazon, "Getting the most from your multimeter" by mr electro, I get into the history of the multimeter and VTVM and how to use them and the modern digital meter. The V-7 is featured and it's explained how the VTVM still has a useful place on the modern workbench.