Hi chaps,
I recently acquired an old Hewlett Packard 209A signal generator being chucked out at work. 4Hz-2MHz in 6 ranges, separate variable sine and squarewave outputs, sync in/out. The unit works perfectly except for the sinewave output level control, which doesn't give a consistent output if the control is adjusted. If I turn the level up to get 1VRMS it gradually starts dropping down to a few hundred mV, so I have to wind the output back up, and then it starts dropping again...It takes several minutes to get a fixed level out of the unit because I have to constantly re-adjust the output until it settles down.
I've tracked the problem down to the the pot itself (actually a dual gang pot wired as a T attenuator - see attached snap of the 209A service manual). I've removed the pot from the unit and can confirm that the resistance is not constant after the pot has been rotated, the rear element being the most unstable of the two. It's an Allen Bradley dual unit, marked "2100-0447 Type J". Looks fully sealed, so I can't get into it to try spraying it with contact cleaner. The parts list in the HP service manual says it's a "var dual tandem 20 - 0.3dB 600ohm +/-20%" part. Measuring the two elements I get 10K on the front and 30K on the back, although given that the resistance is unstable I don't know if these readings are accurate.
The pot obviously wants replacing. Question is, can I use an off-the-shelf unit to substitute into the circuit and still get roughly the same performance as a variable T attenuator? If so, what values and tapers will work?
Cheers!
I recently acquired an old Hewlett Packard 209A signal generator being chucked out at work. 4Hz-2MHz in 6 ranges, separate variable sine and squarewave outputs, sync in/out. The unit works perfectly except for the sinewave output level control, which doesn't give a consistent output if the control is adjusted. If I turn the level up to get 1VRMS it gradually starts dropping down to a few hundred mV, so I have to wind the output back up, and then it starts dropping again...It takes several minutes to get a fixed level out of the unit because I have to constantly re-adjust the output until it settles down.
I've tracked the problem down to the the pot itself (actually a dual gang pot wired as a T attenuator - see attached snap of the 209A service manual). I've removed the pot from the unit and can confirm that the resistance is not constant after the pot has been rotated, the rear element being the most unstable of the two. It's an Allen Bradley dual unit, marked "2100-0447 Type J". Looks fully sealed, so I can't get into it to try spraying it with contact cleaner. The parts list in the HP service manual says it's a "var dual tandem 20 - 0.3dB 600ohm +/-20%" part. Measuring the two elements I get 10K on the front and 30K on the back, although given that the resistance is unstable I don't know if these readings are accurate.
The pot obviously wants replacing. Question is, can I use an off-the-shelf unit to substitute into the circuit and still get roughly the same performance as a variable T attenuator? If so, what values and tapers will work?
Cheers!
