Oscilloscope reads at some circuit points, not others

[dbctronic]

I'm waiting for a reply on the Pico Tech site. Many reads, no replies.

I'm working with a TL082 based Wein bridge oscillator. Suddenly, the 2204A scope shows some circuit points as live, others dead. It likes the oscillator output (370kHz, 4.5V peak). After a few dead results elsewhere, I tried looking at the output through resistors. At 1k, it sees half amplitude output. Above about 5K, suddenly nothing.

I tried rigging another TL082 as a unity gain buffer. Signal shows at its input but not at the output. Next, I followed the oscillator with a 1N4148 diode feeding a 0.47uF cap / 10k resistor combo. Multimeter shows expected 3.68V output on cap. Scope shows nothing. Tried hooking diode to the output of the buffer. Same result--multimeter reading good, no scope output at all. Swapping A and B scope probes shows that the oscillator output continues to be good while the signal on the other side of half a dozen multimeter-verified resistors is nada.

Am I overlooking something? How can a scope probe see signal through 1k but not 5K? How not through a diode, which has an on state impedance far below 1K?

Thanks in advance for plowing through this!

[Neomys Sapiens]

It would be helpful if you would provide more information about the settings on your scope. Like coupling, trigger level, mode, horizontal and vertical factors...

[StillTrying]

"no scope output at all"

Does that mean just a straight line trace or no trace at all.

When you're probing unknowns it's better to have the trigger set on AUTO rather than NORMal, because there'll always be some sort of visible trace.

How are the +V -V & GND supplies for the TL082 produced? Sounds like it's stuck at the rails.

[gcewing]

Another way of getting no trace at all is using DC coupling and having a DC offset that's off-scale on your volts/div setting.

[dbctronic]

Thanks for the replies. A Pico Tech board person said that I probably had a defective unit. I have since determined that this is the case, and just today, I see that Channel A is now totally dead using three probes that all work on Channel B, no matter what settings I try, and Channel B now drops apparent signal voltage from several volts to several mV when resistance in series with the probe exceeds 350 ohms, not 4K as before. A rectifier and multimeter verify that the actual signal voltage does not drop out this way.

Time to go scope shopping!! I like the 2204A, which I bought 5 years ago. Any opinions on whether it's still a good purchase at US$159 for basic beginner use? It seems to do about everything I'm likely to need, mostly monitoring a single signal at a time and spectrum analysis.