K5AM - Collins 75A-4 K5AM       Boatanchor collection      
Collins 75A-4 receiver


Serial number 2484.
Three filters; SSB, AM, CW.

History.
1955-1959. Approximate date of manufacture.
Manufacturer: Collins Radio Company, Cedar Rapids, Iowa.
Original cost: $595.00 . . .  $5100.00 in 2016 dollars.
The two extra mechanical filters, 0.5 kc and 6.0 kc, were optional extras.
1987. Purchased by K5AM.
1987. Complete check-out, lube, and alignment.
1987-2016. Installed on boatanchor bench in shack. Used routinely for on-the-air AM work with boatanchor transmitters.

Modifications, 1987-1997.
Replaced two conductor line cord with safer three conductor cord, with ground.
Replaced C144 with high-quality silver mica, to protect filters. This is a crucial preventative measure, which every A-4 should have.
Replaced power supply filter capacitors.
Replaced all old, notorious "black beauty" capacitors, using high-quality modern components.

Test results; November, 2016.
HP 8640B, HP 3400A(true RMS), HP 5382A, HP 334A.
Collins specifications: for 6 dB SNR at BW 3 kc, 1 V. Mode and other conditions not stated.
Sensitivity, SSB, BW 3.1 kc.
Freq(kc), 6 dB SNR(V), MDS(dBm)
     1850, 0.13, -130
     3800, 0.17, -130
     7100, - , -127
     14200, - , -130
     21200, - , -125
     28400, - , -131
Results: excellent.
Sensitivity, AM, BW 6 kc, modulation 1000 cps, 60%.
Freq(kc), 6 dB SNR(V)
     1900, 0.10
     3900, 0.11
     29100, 0.10
Results: excellent.
Sensitivity, CW, BW 500 cps.
Freq(kc), 6 dB SNR(V), MDS(dBm)
     1850, 0.13, -130
     3550, 0.20, -135
Results: excellent.
Calibration test.
Note that the front panel "zero set" allows adjustment for 5 kc.
     1) VFO linearity. Checked each 100 kc point; all within 1 kc.
     2) HFO accuracy. Checked each band; all within 2 kc.
Results: excellent.
Filter bandwidth test, -6 dB.
Panel switch, filter nominal BW(cps), measured BW(cps).
     A, 500, 472
     B, 3100, 2839
     C, 6000, 6067
Results: excellent.
Filter loss test.
Panel switch, filter nominal BW(kc), receiver gain relative to SSB filter (dB).
     A, 0.5, +1
     B, 3.1, 0
     C, 6.0, +6
Results: excellent.
AM THD test.
Collins specs: less than 10% distortion, conditions not specified.
Test conditions: 0.5 watt AF output into 8 ohms, 3.890 mc, 50 V, BW 6 kc, modulation 1000 cps 60%.
     Measured: 3.4%
Results: excellent.
By-ear test, in speaker. Sounds great, SSB, CW, AM.

Summary. An exceptionally fine specimen of a classic receiver.

Mark, K5AM.

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