Collins 651S-1 receiver repair
Repair of a vintage Collins 651S-1 radio receiver with a failed 65-95 MHz VCO module. Over-damping the PLL loop filter achieves lock, a similar VCO module from a different receiver model is substituted, and an op-amp based circuit simulates the missing DC tracking signal output. The loop is now stable and responsive when receiving from 17 to 29.9999 MHz, with work continuing to extend coverage below that range.
What it addresses
The receiver's 65-95 MHz VCO module is bad, and the replacement lacks one of the two DC tracking signal outputs, so the PLL would not lock and receive properly.
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Collins 651S-1 receiver repair
Repair of a vintage Collins 651S-1 radio receiver with a failed 65-95 MHz VCO module. Over-damping the PLL loop filter achieves lock, a similar VCO module from a different receiver model is substituted, and an op-amp based circuit simulates the missing DC tracking signal output. The loop is now stable and responsive when receiving from 17 to 29.9999 MHz, with work continuing to extend coverage below that range.
Problem: The receiver's 65-95 MHz VCO module is bad, and the replacement lacks one of the two DC tracking signal outputs, so the PLL would not lock and receive properly.
Posted by @mikewarot · Original comment ↗
“Repair of a Collins 651S-1 receiver. The 65-95 mhz vco module is bad, but if you over damp the pll loop filter it does eventually lock and receive properly. I found a similar but not identical replacement from a different model receiver. It lacks one of the two DC tracking signal outputs but I added an op amp based circuit to simulate the missing signal.”