Collins 651S-1 receiver repair
Repair of a Collins 651S-1 receiver. The 65-95 MHz VCO module was bad, so the builder found a similar (but not identical) replacement module from a different model receiver and added an op-amp based circuit to simulate the missing DC tracking signal output. By over-damping the PLL loop filter the loop locks and receives properly; the loop is stable and responsive from 17 to 29.9999 MHz, but not much below that — the repair is nearly complete.
Shared over time
Each entry reflects what was shared on that date. Details and invitations may have changed since then.
Collins 651S-1 receiver repair
Repair of a Collins 651S-1 receiver. The 65-95 MHz VCO module was bad, so the builder found a similar (but not identical) replacement module from a different model receiver and added an op-amp based circuit to simulate the missing DC tracking signal output. By over-damping the PLL loop filter the loop locks and receives properly; the loop is stable and responsive from 17 to 29.9999 MHz, but not much below that — the repair is nearly complete.
Posted by @mikewarot · Builder identity supported: @mikewarot · Original comment ↗
From the discussion
Technical details of the VCO module replacement and PLL stabilization: an over-damped PLL loop filter lets the loop lock, a substitute module from another model receiver was fitted, and an op-amp circuit simulates its missing DC tracking output; reception is stable from 17 to 29.9999 MHz though not below that.
“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.”
@mikewarot · Nov 2, 2026 · Original reply ↗