caesparza 4/18/2018 11:45 AM
Problem with signal generator
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 4/18/2018 12:47 PM
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. [ATTACH]48531[/ATTACH]
caesparza 4/18/2018 1:15 PM
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 4/18/2018 1:44 PM
[QUOTE=caesparza;493313] Any other suggestion?[/QUOTE] Check the thermistor (TH101) isn't open circuit. I think it should measure a few k ohms.
caesparza 4/18/2018 2:38 PM
It reads 13.57K. It's the little bulb?
Enzo 4/18/2018 3:30 PM
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 4/18/2018 3:56 PM
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 4/18/2018 4:03 PM
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 4/18/2018 4:26 PM
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 4/18/2018 4:44 PM
I connected the generator to an amp and the output is a ticking sound, not a continuos tone.
dstrat 4/18/2018 5:08 PM
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 4/18/2018 5:10 PM
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 4/18/2018 5:18 PM
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 4/18/2018 5:21 PM
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 4/18/2018 5:25 PM
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.
dstrat 4/18/2018 5:28 PM
well says 5v rms if set on high that is quite a lot of signal try the lower setting.
caesparza 4/18/2018 5:48 PM
Yes I have the fine adjustment very low, I'm aiming for 150mV. Now I'm measuring 46.6 on the mV scale on the DMM. R109 says 3.3K on the schematic, I don't find any 3.3K resistor on the PCB. I'm trying to follow the schematic but I don't see the resistor.
dstrat 4/18/2018 5:59 PM
well not sure where but if you find Q101 you should find the R109. so you have a clean output with 46mv? sounds like its probably ok voltage wise, if you got that output. 150 mv might overdrive a tube amp, guess depends on the amp. it also occurs to me that someone possibly changed some components in there.
caesparza 4/18/2018 6:02 PM
What I don't get is why the sound is so low, I mean I'm in the x1K scale, the wave should be really high in pitch, and it sounds like a medium distorted note. Like an 80's synth note sustained (is the only description that comes to my mind).
dstrat 4/18/2018 6:07 PM
try x10 should kick up the frequency. I wonder how old that unit is you have?
caesparza 4/18/2018 6:19 PM
The solder points on the PCB look unmolested, pretty neat indeed. The 3.3K is changed for a 5.6K on the PCB. What kind of voltage should I measure there on R109? Can you explain "voltage above the both sides of R109, then at the source, drain of the FET." a little bit more? Thanks again. CE
dstrat 4/18/2018 6:26 PM
sure, the manual says the power supply should deliver 28 vdc to the unit. depending on how the fet, transistors are biased you would likely see about half the 28v more or less at the source drain, or collectors and emitters but not always. it would hard for me to guess what they should be beyond that. its more about what its got and then do the math and see if its in the ballpark. Edit: there are people here that could tell you more or exactly what to expect, beyond my skill level.
caesparza 4/18/2018 6:38 PM
Probably 1980 from what I have been able to find. I think I'll have to read about this FET devices, I know nothing about them.
dstrat 4/18/2018 6:43 PM
that age I would expect the caps needed checking at least, also the voltage set with that trimmer will likely change how the fet is biased and the voltages. I mainly do tube gear so solid state stuff I leave alone, I know enough to be dangerous :)