Signal Generator Distortion

8/22/2005 11:26:31 PMRich, W3HWJ

I have a vintage HeathKit SG-8 Signal Generator. Replaced the electrolytic and changed the selenium rectifier for silicon diode. In the low frequency ranges, the sine wave is non-symmetrical; negative portion is lower amplitude and flatter than positive. My guess is bias resistors must have changed value, or that Heath's original design had this flaw. Any suggestions?
Rich

8/23/2005 12:37:53 AMNorm Leal

Hi Rich

Resistor value drift can cause non symmetrical waveform. Here is the schematic. Resistors often go up in value over time.

http://web.archive.org/web/20000831195612/www.continental.freeserve.co.uk/heath/sg-8.gif

Norm

:I have a vintage HeathKit SG-8 Signal Generator. Replaced the electrolytic and changed the selenium rectifier for silicon diode. In the low frequency ranges, the sine wave is non-symmetrical; negative portion is lower amplitude and flatter than positive. My guess is bias resistors must have changed value, or that Heath's original design had this flaw. Any suggestions?
:Rich

8/23/2005 12:28:59 PMRich, W3HWJ

Thanks, Norm. I did read on another website that the cheap SGs of the 50s were compromise designs that used freq. multiplication and the "as-designed" waveforms were not optimal. I have the schematic and will check out the voltages and resistor values. It also refuses to oscillate in the 30 to 40 MHz (sorry, megacycle) range. This unit was designed before Hertz got adopted! Any merit in using a higher-mu triode like a 12AX7?
Rich

:Hi Rich
:
: Resistor value drift can cause non symmetrical waveform. Here is the schematic. Resistors often go up in value over time.
:
:http://web.archive.org/web/20000831195612/www.continental.freeserve.co.uk/heath/sg-8.gif
:
:Norm
:
::I have a vintage HeathKit SG-8 Signal Generator. Replaced the electrolytic and changed the selenium rectifier for silicon diode. In the low frequency ranges, the sine wave is non-symmetrical; negative portion is lower amplitude and flatter than positive. My guess is bias resistors must have changed value, or that Heath's original design had this flaw. Any suggestions?
::Rich

8/24/2005 12:14:46 PMNorm Leal

Hi Rich

Waveforms won't be perfect with your circuit.

Higher frequencies will be first to drop out with when components drift.

12AX7 has more gain but doesn't handle current of a 12AU7. Might try 12AT7. This tube is used as an FM oscillator.

Norm
:Thanks, Norm. I did read on another website that the cheap SGs of the 50s were compromise designs that used freq. multiplication and the "as-designed" waveforms were not optimal. I have the schematic and will check out the voltages and resistor values. It also refuses to oscillate in the 30 to 40 MHz (sorry, megacycle) range. This unit was designed before Hertz got adopted! Any merit in using a higher-mu triode like a 12AX7?
:Rich
:
:
::Hi Rich
::
:: Resistor value drift can cause non symmetrical waveform. Here is the schematic. Resistors often go up in value over time.
::
::http://web.archive.org/web/20000831195612/www.continental.freeserve.co.uk/heath/sg-8.gif
::
::Norm
::
:::I have a vintage HeathKit SG-8 Signal Generator. Replaced the electrolytic and changed the selenium rectifier for silicon diode. In the low frequency ranges, the sine wave is non-symmetrical; negative portion is lower amplitude and flatter than positive. My guess is bias resistors must have changed value, or that Heath's original design had this flaw. Any suggestions?
:::Rich