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SG504: Difference between revisions
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==Videos== | ==Videos== | ||
[https://www.youtube.com/watch?v=U8Gma4-jVx4 SG504 Repair and Frustraton from a stubborn unit] by Zenwizard Studios @ YouTube | [https://www.youtube.com/watch?v=U8Gma4-jVx4 SG504 Repair and Frustraton from a stubborn unit] by Zenwizard Studios @ YouTube | ||
==Service Notes== | |||
*Lo Band Harmonic Distortion (Usually 3 dB or Higher) - Make sure pin diodes on R.F. Board are as close as possible to the board. | |||
*Signal Output Good and Level Light is Off (at 650-695 MHz) - Suspect J39 is shorted. | |||
*Harmonic Distortion: | |||
**1. Make sure leads on 070 and Q120 are straight and short. | |||
**2. Be sure C126 and C128 are centered on etched capacitors. | |||
**3. Suspect chip caps which are heat sensitive and tend to crack. | |||
*Reference Oscillator Out Of Specifications (50 kHz or 6 MHz) - Suspect T200 or T205. | |||
* Adjusting for Harmonics - Excessive Harmonic levels at Low End of Low Band can be helped by adjusting C125. Adjusting C125 would lower the harmonic level but it would reduce the upper frequency on the Low Band at which time unlevelling would occur. | |||
*Adjustment Effects - Spreading L125 decreases upper levelling frequency and decreases harmonics at low end. | |||
**Spreading L126 has approx. equal effects - Increased harmonic level and increased upper levelling frequency. (Opposite effect of L125). | |||
**Spreading L128 has same effect as L126 but affects upper levelling frequency more. | |||
*Solution - Adjust C125 to set 2nd Harmonic for -26dB, compress L125, spread L126 approx. 1/16 ", spread L128 approx. 1/8". Reset C125 to begin unlevelling at first arrow past stop at upper end of dial. The worst the 2nd Harmonic should be now approx. -28dB . | |||
**L48 should always be completely compressed. | |||
**L45 may help harmonics by really playing with it. | |||
==Pictures== | ==Pictures== | ||