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RSR FG-32 3Mhz Function Generator

Discussion in 'Troubleshooting and Repair' started by Kopter6, Aug 8, 2013.

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  1. Kopter6

    Kopter6

    4
    0
    May 13, 2010
    I just acquired a RSR FG-32 3Mhz Function Generator.

    I hooked it up to a scope and noticed when I was looking at a 1khz sine wave it looked fine but as I slowly turned up the amplitude the bottom part of the sine wave started to flatten at around 6v p-p.
    The unit has an output to 20v p-p.

    I don't have a schematic. Do any of the users here have any suggestions on how to go about fixing it.

    Any help will be greatly appreciated.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,500
    2,840
    Jan 21, 2010
    It would be useful if you can post images.

    My first question would be "is the sine wave being clipped, or is something else happening?" I would look at other waveforms too.
     
  3. Kopter6

    Kopter6

    4
    0
    May 13, 2010
    It seems it's only happening with the sine wave selected.
     
  4. elebish

    elebish

    177
    12
    Aug 16, 2013
    You might want to check the "dc" offset of the gen output. Couple the scope vert input to gnd mode, set trace on the scope to the center graticle, now select dc coupling to scope and vert attenuator at 1 volt per division. Crank up the gen output and see if the sinewave is centered with equal amplitude of sine wave above and below the center graticle at 6 v p/p. If not equal, adjust internal dc offset on generator to obtain balance on crt. Most high end generators have the dc offset as a user adjustment on the front panel. If no luck, check the gen power supplies. There should be a neg and pos voltage used to supply the op amps. Make sure the neg and pos voltages are same. Example: -12 and +12 volts with respect to common. These voltages are usually regulated with TO 220 case style regulators, pin 1 in/pin 2 gnd/pin 3 out. If that checks ok then one of the op amps or a coupling component could be bad. Ed.
     
  5. Kopter6

    Kopter6

    4
    0
    May 13, 2010
    Well, I opened the case up and looked at the pcb. There were about 4 to 5 burned resistors right near amplitude output BNC connector.

    I'll replace them and see what happens.
     
  6. elebish

    elebish

    177
    12
    Aug 16, 2013
    Generator offset problem

    Be careful, something caused those resistors to burn. Check power supply voltages as I suggested earlier and especially the PS diodes and filter caps. More than likely, something overloaded the PS, like a shorted op amp which would be consistant with your dc offset problem. Isolation of the PS is a good start but you will need a schematic, unless you are a veteran at this. Ed.
     
  7. Kopter6

    Kopter6

    4
    0
    May 13, 2010
    Good point. I'll give it look see.
     
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