Problem with signal generator

[caesparza]

Hi everyone.

I have got a Leader LAG-26 Signal Generator that I was planning to use to troubleshoot tube amplifiers, when I connected it yesterday I got pulses from it, not a continuos sine wave. Is the generator bad? It has a sine and square waves, but the output just gives pulses.

Any help would be appreciated.

Thanks.
Carlos.

[Dave H]

Is the output a sine wave but it's pulsing in amplitude?
You could try adjusting the feedback trimmer VR102 to see if that stops it bouncing.
Make a note of where it is now so you can put it back if it has no effect.

Leader--LAG-26--service,user--ID5690.pdf

[caesparza]

Output is not a sine wave at all.

i tried adjusting that trimmed with no success. I left it where it was.

Any other suggestion?

Thanks.
CE

[Dave H]

Any other suggestion?

Check the thermistor (TH101) isn't open circuit. I think it should measure a few k ohms.

[caesparza]

It reads 13.57K.

It's the little bulb?

[Enzo]

The sine/square switch is set to what? And what does the output look like on the other setting?

Does it act this way on all the frequency ranges?

[caesparza]

Hi Dave,

First thank you for your kind answers and your time.

The unit behaves the same in all frequencies, I have tried sine and square, but basically sine waves.

The first thing I did was to measure the output voltage to aim for the 150mV AC to test the unit with an amp. But I observed that the voltage reading on the DMM was not steady, it jumped on different values, but I could check that the controls for output and output fine tuning were working, the voltage went up when I increased but the reading kept jumping between certain values.

The frequency control works as well cause you can hear how the pulse gets thiner the higher the frecuency is.

On the scope the wave is not steady, it goes up with a peak, not a curve as in a sine ware, then flat then the peak again.

I'm sorry if the description is not using the proper words, I'm new to this stuff (have worked on tube amps before) and I don't now how this units work.

Hope that information helps.

Thanks again.
CE

[caesparza]

The trim pots on the inside look rusty.

And the big caps 100uf 160V look like burnt were the cable goes in. I don't know if that indicates something. The rest of the unit is really clean.
CE

[dstrat]

I did not look at the schematic yet but if the power supply is not filtering DC it would likely make the oscillator output pulse.
just a guess at this point.

Listen to the output thru an amp it should be a clean signal, if you hear hum, jackpot!

[caesparza]

I connected the generator to an amp and the output is a ticking sound, not a continuos tone.

[dstrat]

ok in that case I'd look at voltages at fet Q101 , transistor Q102 - 103 those are responsible for sine osc .
it could be that its way out of adjustment, hard to say really. do you have a communications receiver?

edit: well it would be helpful for checking all the ranges, but not really a show stopper.

[caesparza]

Hi dtrat, thanks for your answer.

I don't know exactly what do you mean by communications receiver, you mean an amp to connect the generator?

[dstrat]

well its just a radio really but most of them receives 50khz to 30 mhz. but dont worry about that.
I used one once to set up an old tube type signal generator years ago.

But you will need to take voltage readings at q 101 for starters to see if its operating correctly.
voltage above the both sides of R109, then at the source, drain of the FET.

also the e-caps around the three would also be suspects. (C103-C106)

[caesparza]

Ok I decided to move a little beat the trimers inside, VR102 allowed me to get constant output at x1K. It sounds like a distorted wave, not a clean sine wave. The other ones doesn't seem to do anything.

I wonder how this things are calibrated.

[dstrat]

well a precision crystal oscillator usually to calibrate but wouldnt worry about that so much for tube amps.
sound like its just out of adjustment from what you just said.