Re: Sick SG504

John Bennett

The high side buffer amplifier (Q70) is the most likely suspect. This is a TO128 NPN transistor, Tek part number 151-0474-01. There are several suitable replacements from the cable industry, but the cost effective BFR94A works fine. Be sure you are ordering the TO128 (studded) package, as the NXP variant is an SOT23.
Also check to see if Q70 is getting drive at its base. If not, there is a small carbon composition resistor inside the high frequency cavity oscillator that sometimes cooks itself to death. I forget it's value, but if you open up the HF cavity (the one to the rear) this is easy to check and replace (with care).
Far less likely to fail are the HF series and shunt diodes, CR80, and CR155, CR158, & CR160, respectively.
After repair, the unit will need a recal, particularly the HF harmonic suppression and leveling step (you will need a spectrum analyzer).
John Bennett

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