TV 7 problems on Mutual Conductance

[N4LTA]

Pulling out some equipment to get re-done after the fire. The TV7 was latched up lightly and got some dirty firehose water on the outside bit appears to have little or no damage inside.

The outside is cleaning up of but the mutual conductance  test does not work at all. The meter does not move at all - not a wiggle.  The manual says that the problem may be a bad transformer and thankfully that is not the problem. The power supply seems OK - both the plate and bias supplies. When I set up for a tube test - I can monitor the pins and get a good plate voltage at the socket when the button is pressed. The grid voltage on the socket before the button is pressed and is a negative voltage that is variable when the bias pot is moved  - which seems normal.

so it appears that the plate and bias voltages are working indicating that both rectifier tubes are working. The meter works because I can adjust the line voltage smoothly.

I tried testing a rectifier (5Y3) , and also a 0D3 VR tube and both tested fine. I have used De Oxit on all of the push buttons.

It seems the problem is in the Mutual Conductance  test circuit somewhere in the meter circuit.

Anyone out there with experience know of an obvious place to begin or a common problem with the TV7?

Tracing this thing out will be a bear!

Thanks in advance!

Pat
N4LTA

[W1NB]

I’m not familiar with the TV7 but based on what I see in the schematic and manual I would confirm that the #3 winding of the transformer is not open, check R120, R121 and R122, and check the function of all segments of S110. I know you said the transformer is OK but winding 3 could be opened and you would still have plate, bias and filament voltages.

[N4LTA]

Thanks for the help. Tested winding #3 and it is OK thank goodness. Tested all of the resistors in circuit. All are close, nothing that would cause a zero deflection on the meter.

Cleaned all of the pushbutton switches with DeOxit and verified R122 is connected to S110 S7.

Verified that the bias pot center tap is supplying a variable
bias voltage to the top of the R120 stack, through SS110-7, through S0109A and through S110-6

The bias pin at the tube socket has variable bias , so I think the bias to the tube is OK and the secondary seems to be imposing the Transformer sec #3 AC voltage onto the bias voltage

I gave all of the rotary switches a good cleaning with DeOxit also.

Still no signal at the meter on the Mutual Conductance test.

Still looking!

Pat
N4LTA

[K8DI]

stupid question:

Are you using a good tube to test with? Have you tried a few types/sockets in case the socket is bad? In other words, could it actually be working?

Ed

[W9BHI]

I just finished cleaning, repairing and calibrating four TV-7's and a TV-10.
Get the military manual and do the voltage checks.
If it got wet, I would check the pots and all of the push button switches as well as the rotary switches.
Check the components in the bridge circuit, that is what does the mutual conductance test.
I use a set of known value bogey tubes to calibrate the B, C and D ranges.
If you check the voltages with a DMM, make sure you use an appropriate shunt to simulate a 1000
ohm per volt reading or everything will be off.
If you get to the point where you are stuck, I will repair and cal it for you if you send it to me.
Don W9BHI

[N4LTA]

I am using a good 6C5 tube. There must be something open in the meter circuit in the  mutual conductance test . When the mutual conductance pushbutton is pressed -  Nothing happens - the meter does not move at all .  As I said earlier - it tests  rectifier tubes, diodes and VR tubes just fine.

At this point, I  am not trying to calibrate, just trying to get the circuit to work when doing the mutual conductance test.

The inside looks good no sign of water damage or heat damage.

My guess is somewhere a contact has failed or something has come loose.

I think I will start at the meter with a known good triode set up for test, and follow things back toward the mutual conductance circuit and see where I loose the meter voltage. Not much fun with hundreds of ceramic rotary switch connections.

Setting up the tester for a 6V6 tube with no tube in the socket - I see heater voltage, grid voltage that varies with the bias pot. when I press the mutual conductance test pushbutton, I read plate and screen voltage while the button is pressed. The bias adjustment varies the grid voltage regardless of the test pushbutton position. The meter does not move, regarless of the setting of the range switch with a good tube in the socket.

Pat
N4LTA