IC-R8500 Repair

Was glad to help a good friend, VK6UM Larry, with the repair of an IC-R8500 recently. After some digging around it was clear this was a later version than the schematics I could find online applied to. The fault was the blinking display, a characteristic of a PLL not locking up. 

A few minutes with my ICQ7A used as a sniffer confirmed VCO_A had no output. A few voltage measurements and it appeared the special Icom chip, IC3, was not enabling the power to the VCO. Knowing that chip was likely to be unobtainable, and for thoroughness, I checked the main micro was sending instructions to the synthesizer board. 

I was able to confirm both shift registers, 4094's, were receiving pulses. But the shift register feeding the special Icom chip had no signals on the output pins that drive pins labelled "CON0" through "CON3". It seemed a reasonable assumption that these pins mapped to the output pins enabling the VCO's. So a replacement shift register was obtained.

Upon installation there was no change. A closer examination showed the data pulse was too low in amplitude to be considered a logic high. It transpired, after a fair amount of thought and measurement, that the data line was being loaded by a 130 ohm resistance that should not have been there. The fault was traced to a capacitor array on the main board having broken down somehow. Once removed from the PCB no amount of cleaning the defective part could remove this unwanted resistance, though it did change in value. 

A replacement part could not be found. So in desperation something was fabricated in-situ with 1206 22pF caps and fine wire. Partial success. The radio is now working within narrow segments of it's wideband coverage.