My GR 1001A 400 Hz internal modulation wont work. 6SN7 tests OK, external mod is fine. Check of
resistors shows all are reasonable.

Its a Wien Bridge oscillator, and those can be cranky at 400 Hz , especially with triodes. About 20 years ago,
I changed the two oil capacitors 0.015ufd, for two orange drops for the same reason.

Is there a service note from GR, or has anyone a good tweak? I dont want to take the whole circuit apart, or build
a mock up just to see if it will work with what 6SN7s I've got.

You might try shunting one or both caps with a decade box and adding a bit of capacitance. Tweaking a resistor or two might work also.

I think you will find that a resistor is off value or a component has opened up. That circuit should not be that finicky.

Nothing has opened up, because the stage also is the modulation amplifier for the 6L6 and I
was modulating it with the HP200B yesterday at 15 kHz to see if the sidebands were good. I think
I will tweak those two caps as supposed to be 0.02 uF but they tested as 0.015.

This is the bench where work is being done on this unit. The meter is a Fluke 87 .

I had some problems that appeared to be caused by very slight leakage of C-42 and possibly C-46 in the audio stages. Replacing both with 1600 volt rated mylar caps solved the problem. The voltages present are nowhere near that level, but its a high impedance circuit and I figured that the 1600 volt caps should have the lowest leakage of any that I could obtain locally.

An earlier attempt with 600 volt rated caps worked for awhile, then failed.

--Chuck

C42 and C46 were the ones I changed 20 years ago, and I did sub them, but still no go. R65 is supposed to
be 107 K +/- 1% , but it has gone up to 112.1. I still need to drag those 3 1% silver micas over to the other bench
and see if something has happened to them.

That toggle switch with the wires welded to the panel may need help.
If it is the external-internal switch, it may be the problem.
Good hunting,
Pat

The problem with the modulator is solved now. It was a bad connection in the chain of resistors
that returns pin 1 of the 6SN7 to ground. Because this grid was floating, with an old tube, it
would oscillate, sometimes, but poorly. I also checked all the precision parts in the circuit
under voltage stress. The were all ok voltage wise, and although out of spec from the 1 %,
decided to leave them in. A proper restoration would need stripping both terminals board in the modulator.
This would destroy the glass Sylvania 1N34s.

Next to do is dive into the oscillator case and see if a new 6C4 will allow full carrier set to work at 50 mHz.

The manual (on line) has full details on using the 1001A with test loops to measure the sensitivity of
radios. That would be great to see if all good transistor radios work the same way.

This is the modulator.

I had your symptom on my 1001-A, and traced it to leakage in the terminal board. One of the 6SN7 grids is a very high-impedance point in the oscillator circuit. I restored oscillation by disconnecting the parts and wires from that terminal on the terminal board, and rewiring "in the air".

Dave Wise

I had to hit the books to see which bridge was used and I'm trying to decide if it Wein or Schering.

The total DC resistance to ground for pin1 (grid) is 2.2 meg, 680k, 560k, 560k, and 100 ohm.

One wonders what snivet caused them add the 100 ohm into a 400 Hz circuit.