51S-1 HF Receiver – Blocksome

By Rod Blocksome, K0DAS

A number of years ago I conducted surveys of Collins ham equipment on the CCA reflector. The project was designed to shed light on the many questions collectors had about production quantities, anomalies, scarcity, etc. I published the results on the reflector and, in the years since, I have continued to gather additional data from other sources. This article is an update – along with some additional information and insight added.

I would like to acknowledge the project engineer who led the design of the 51S-1 receiver. It was none other than Ed Andrade, W0DAN – The same fellow who earlier brought forth the Collins 51J-4 and the KWM-2! All three equipments were in production far longer than the average Collins HF equipment – a testimony to the solid design and outstanding performance achieved by Ed and his engineering team. Jerry Vonderheide, W0NGL, later created the 51S-1A – a version powered from 28 Vdc for aircraft operation.

Several years back, an engineer from Ed’s 51S-1 team retired and asked me if I would be interested in the prototype 51S-1 receiver. I accepted of course and spent some time examining the unit, taking photographs, and even contemplating restoring it to operating condition. But after further thought, I decided it was really a historical artifact and shouldn’t be tampered with. So I did some minor cleaning and then put it on display in the Rockwell Collins Museum. Figure 1 shows the front of the radio. One can see several items that are much different than the production versions we are used to seeing. For example it has a noise blanker although the module was missing from the chassis. The main tuning knob is machined from solid brass and painted black. Arlo Meyer, W0LBK, told me they wanted the feel of a weighted knob for tuning but it was going to turn an extensive gear train.


Figure 1 – 51S-1 Receiver Prototype

In addition to the main tuning knob, a list of other changes includes:

The “Zero Set” knob is round like the S/Line instead of the small “bar” type used in production.

The Collins logo is located above the phones jack.

The noise blanker knob is located where the production logo was later placed.

The “Rejection Tuning” scale has expanded tic marks.

The meter switch is labeled “RF, Line 1, Line 2” (The prototype had two line audio outputs).

Under the hood things are similar but different in a few areas as shown in Figure 2 where the prototype on the left is placed beside a 1963 winged emblem production model for comparison.

The prototype cabinet is a modified KWM-2 cabinet (obvious from the rear).

The rear panel connections are different.

It has a five fluted Megacycle knob (Early production 51S-1 were the same and then it was changed to four flutes).

There is a tuner shield with holes for alignment and a tube stage on the left.

There are two lamps on the PTO dial.


Figure 2 – 51S-1 Receivers – Prototype on the Left

The first production of the 51S-1 occurred in August 1961 when three units were completed just before the end of fiscal year 1961.  But things started to roll in 1962 and continued for 20 years.  By the time production ended, some 12,136 51S-1 receivers came forth from the Collins factories by my count & estimates.  Tables 1 and 2 provide details resulting from my research into the production history of this fine radio.

Table 1 – Production Quantities of 51S-1 Receivers

Fiscal Year 1 Production


51S-1 51S-1A 51S-1B/BF 51S-1F/AF Total
1961 CR/Anamosa 3 0 0 0 3
1962 CR/Anamosa 262 0 0 86 348
1963 CR/Anamosa 410 9 0 192 611
1964 CR/Anamosa 486 14 1 283 784
1965 CR/Anamosa 306 39 90 350 785
1966 CR/Anamosa 430 133 90 289 942
1967 CR/Anamosa 758 38 177 378 1351
1968 CR/Anamosa 535 4 148 324 1011
1969 CR/Anamosa 515 0 103 260 878
1970 CR/Anamosa 557 13 9 132 711
1971 CR/Anamosa 543 12 12 196 763
1972 CR/Anamosa 569 0 21 137 727
1973 CR/Anamosa 315 3 4 115 437
1974 CR/Anamosa 621 7 0 237 865
1975 CR/Anamosa 350
1976 Richardson, TX 550
1977 Toronto, ON 100
1978 Toronto, ON 170
1979 El Paso, TX 250
1980 Salt Lake City, UT 200 2
1981 Salt Lake City, UT 200 2
1982 Toronto, ON 100 2
TOTAL 12,136

Note 1 – The Collins fiscal year ran from 1 Sep. to 31 Aug. until 1974 when it was changed to run from 1 Oct to 30 Sep. Therefore FY1974 was 13 months long (1 Sep. 1973 to 31 Sep 1974).

Note 2 – Very little data has been found for FY80 – 82. These are best estimates.


In general the serial numbers were not purposely scrambled on the 51S-1; however, there are strong indications in the data that around 1975 some 124 51S-1’s were assigned early serial numbers that were likely used for other types (e.g. 51S-1F).  The reason is unknown.  Other than this instance, the serial numbers are sequential without regard for specific type number until 1975 when different serial number blocks were used for unique 51S-1 types.  Production ran to about 1982, but no data has been found for units produced after 1979, so I’m estimating 500 units for 1980 and beyond.

On most receivers you will see an “MCN” followed by several digits.  This is the Manufacturing Control Number and is used for configuration and revision control as the equipment progressed through the manufacturing process.  Only after a manufactured unit has passed final test and quality control inspection is a serial number affixed to the radio.  The system of assigning and even marking MCN numbers on the unit changed over time.  There is not a lot of MCN data available to draw firm conclusions but it appears as though the MCN numbers were only unique for a given top level part number.  Then starting in 1975, someone decided to simply use the MCN number as the serial number.  At that point the MCN was probably unique for all flavors of 51S-1 receivers.

A caution in using the MCN to analyze serial number and production quantities:  It was common practice to do “Spec Builds”.  In other words – produce radios that did not have an immediate customer order.  These units would go into finished goods inventory until sold.  Frequently a customer order later would come in for a few radios of a certain “flavor” but finished goods contained only radios of a different “flavor”.  Often the solution was to pull the finished goods radios and run them back through the factory to be reworked into the flavor ordered by the customer.  In this situation the radio would receive a new serial number, new top level part number, and perhaps a new model type, but the original MCN would remain.

The 70K-7 PTO also carried the Collins Logo and an MCN.  It is interesting to note that the PTO changed to the Collins Round Emblem (or “meatball” as it was affectionately called) in 1961 when all the other Collins Products made the change – except for the amateur equipment and the 51S-1 receivers.  They continued using the winged emblem up until the fall of 1969.  A lot of equipment was in process on the factory floor when the edict came down that “nothing goes out the door with the old logo”.  To avoid scrapping a lot of front panels, the engineers came up with the idea of placing two tiny grey screws on either side of the meatball to plug the holes used for the former winged emblem.  Thus the so-called “RE-Transition” was born.

The MCN on the PTO is another analysis guide to determine receiver quantities.  But remember spare PTO’s were built and shipped to customers with their own maintenance shops.  Also an early serial number receiver will occasionally have a PTO with a late MCN – usually the result of PTO replacement somewhere along the line.

Table 2 – Serial Number Model for Production History

Fiscal Year Logo Emblem Serial No. Range Production


1961 Winged 1 – 3 CR/Anamosa
1962 Winged 4 – 352 CR/Anamosa
1963 Winged 353 – 964 CR/Anamosa
1964 Winged 965 – 1749 CR/Anamosa
1965 Winged 1750 – 2535 CR/Anamosa
1966 Winged 2536 – 3478 CR/Anamosa
1967 Winged 3479 – 4830 CR/Anamosa
1968 Winged 4831 – 5842 CR/Anamosa
1969 Winged; RE-transition 5843 – 6721 CR/Anamosa
1970 RE Large & Small 6722 – 7433 CR/Anamosa
1971 RE Large & Small 7434 – 8197 CR/Anamosa
1972 RE Large & Small 8198 – 8925 CR/Anamosa
1973 RE Large & Small 8926 – 9363 CR/Anamosa
1974 RE Large & Small 9364 – 10229 CR/Anamosa
1975 RE Large & Small MCN12000 – MCN12224 (note 1) CR/Anamosa
1976 RC & variations 13000D – 13985D (note 2) Richardson, TX
1977 RC & variations C30,000 – C30,099 (note 3) Toronto, ON
1978 RC & variations C30,100 – C30,269 (note 3) Toronto, ON
1979 RC & variations T5001 – T5250 El Paso, TX
1980 RC & variations ? Salt Lake City, UT
1981 RC & variations ? Salt Lake City, UT
1982 RC & variations ? Toronto, ON

Note 1 – There is evidence that some 124 random unused serial numbers were used in FY1975 in addition to the series shown.

Note 2 – It is unlikely Richardson produced nearly 1,000 radios, therefore assume some numbers were skipped.

Note 3 – There is evidence of the same serial number in this range being assigned to different receiver type numbers e.g. 51S-1 and 51S-1F.


The 51S-1 series receivers consist of several different types and within each type there are minor variations denoted by a difference in the last three digits of the Top Level Collins Part Number. Some also have military nomenclature as shown in Table 3.

Table 3 – 51S-1 Receiver Models and Variations

Model Top Level CPN No. of CPN Package PS Notes
51S-1 522-2245-xxx 18 cabinet AC R1122/GR
51S-1A 522-2546-xxx 17 cabinet DC R1433/UR
51S-1B 522-3857-xxx 2 cabinet DC Mil Connectors; USAF – no mil nomenclature
51S-1F 522-2498-xxx 18 Rack mount AC R1156/GR (522-2498-00)

R1156A/GR (522-2498-030)

51S-1AF 522-3156-xxx 17 Rack mount DC
51S-1BF 522-3850-xxx 1 Rack mount DC Mil Connectors

Very early units (about the first 130 units) sported a 5-fluted Megacycle knob and wide spaced lines around the Rejection Tuning knob.  These early radios also used the 70K-4 two-tube PTO.  After that, all receivers used the 70K-7 single-tube PTO.

There were a variety of meter faces scattered throughout the production history.  In general they follow this pattern:

Early WE – White Meter Face

Late WE – Yellow/Amber Meter Face

Late RE and RC – Red or Orange Meter Face

The Collins Logo emblems run the gamut – Winged Emblem from 1961 to the fall of 1969; Round Emblem Transition fall of 1969, Small RE (about size of a dime) from fall 1969 to about 1975, Large RE (about size of a nickel) through FY 1976, Rockwell Collins with terra cotta “Collins” & white “Rockwell” to the end except for a few with blue “Rockwell” from about 1980.    A third Rockwell-Collins emblem has recently surfaced.  It is square with all terra cotta color and the only word is “Rockwell”.  It carries a serial number indicating it was manufactured in Richardson, TX probably in 1976.  So there you have it – a grand total of seven different emblems to collect!

Front panel variations abound:  Green front panels; Panels labeled in Spanish, and (hold onto your hats) the G-133F receiver from Ling-Temco-Vaught (LTV) began life in the Collins factory as a 51S-1.  It is unknown if these units carried serial numbers in the 51S-1 sequence or even how many were produced.

How many front panel colors can a 51S-1 come dressed in?  Believe it or not – there were a total of six colors per the list below.

Dark Gray:  Per Collins PN 580-0182-00 (The most common color)

ICF Gray:  Color #26440

FAA Gray-Green:  Per Collins PN 580-0342-00

Gray:  Per Collins PN 580-0255-00

Green:  Color #24325

Gray Enamel:  Per Collins PN 580-0236-00

Only a few units probably used these variations.  But if it looks strange – don’t repaint it – it just might be original!


Figure 3 – Winged Emblem 51S-1 Receiver 


Figure 4 – Large Round Emblem 51S-1 Receiver


Figure 5 – Rockwell Collins Emblem 51S-1 Receiver


Figure 6 – WE, RE, and RC 51S-1 Receivers Lined up for Inspection

Tables 4 and 5 provide a list of publications on the 51S-1 receivers. Now that you can place your receiver in the production time frame, you can find the appropriate publications for it.


Table 4 – 51S-1 Instruction Manuals

Models in Title Edition Date Comments
51S-1/1A/1F 1st Edition 1 Apr 1961 70K-4 PTO
Addendum to 1st Ed. 28 Mar 1961 AF Gain Control Change
2nd Edition 1 Aug 1961
2nd Edition, Rev 1 15 Oct 1961
3rd Edition 15 Dec 1961 70K-7 PTO
Addendum to 3rd Ed. 15 Feb 1962
51S-1/1A/1F/1AF 4th Edition 15 July 1962
Addendum to 4th Ed. 1 Jan 1963 Alternate L108 assemblies
5th Edition 15 Aug 1963
Supplement to 5th Ed. 15 Feb 1965
6th Edition 1 Mar 1965
Supplement to 6th Ed. 15 Feb 1965 Identifies differences in 51S-1B & 51S-1
7th Edition 1 Nov 1965
8th Edition 1 June 1967
51S-1/1A/1F/1AF/1B 9th Edition 1 July 1969
Supplement 1 15 Mar 1970
19th Edition 15 Jan 1974
11th Edition 15 Sep 1975

Table 5 – 51S-1 Service Bulletins and Service Information Letters

Pub Date Applies To: Subject
SB 1 02/22/63 51S-1 Suppression of Unwanted Oscillations in Low-Level Audio Output Lines and Allow Use of Wiring Configuration Shown in Figure 1.2.a of Instruction Book
SIL 7-63 05/14/63 51S-1 Reduced Hum in Audio Output With Use of 28-Volt DC Power Supply
SB 2 06/22/64



51S-1/1A/1AF/1B/1F Replace Transformers T14 and T15 With Mechanical Filter
SB 3 08/11/64



51S-1/1A/1AF/1F Improve Sensitivity of Lower Bands
SB 4 08/09/65 51S-1/1A/1AF/1F Addition of Dial Lock Assembly
SB 5 04/01/70 51S-1/1A/1AF/1F A:  Converts 51S-1A/1AF to 51S-1/1F

B:  Converts 51S-1/1F to 51S-1A/1AF

SB 6 09/01/71 51S-1//1F Provision for High-Speed Receiver Muting and Recovery
SB 7 01/01/72 51S-1/1A/1AF/1B/1F Reduce 500-kHz Spurious Response