51S-1 Service information

The following items are intended to be used as aids in trouble shooting

51S-1 Crystals in turret
You may find most of the crystals in the turret are off frequency, especially the higher frequency ones and they are all low (from 1.2 to 13KHz, mostly ~ -3KHz!). My guess is that they have simply aged, these are the smaller crystals and were not too common in the early sixties
These xtals all need to be pretty much bang on frequency as the fiducial only moves +/- 3KHz and the PTO tracking is ~ +/- 1KHz
First of all there are piston capacitors in the unit that will allow you to adjust the crystal freqency but only about 2KC as I remember so you might be able to correct some the bands this way. Also you might end up having to adjust all of them to get them all close together so the hairline does not have to be shifted on only but a few. It is very time consuming and you have to use some logic in your approach. If they are really out you will have to go for new ones. I have purchased them from International Crystal in the past and they have the 51S-1 as well as other in their data base. Also some of them are used on more than one band so you do not have one dedicated to each band. If only a few are out just buy them, but if it is a lot of them (ones that the adjustable caps will not bring in) you best replace all of them. I have heard of some people that have added capacitors to correct the problem when the adjustment caps will not do the job and you need more capacitance not less. This really would be very time consuming but it can be done.
Anyways the above are your options, the ideal 51S-1 is one that you do not have to adjust the hairline on any of the bands. By the way the first two bands are usually the exception in that you are using a conversion system using the two highest bands so there is some other stuff going on so do not be concerned with these two.
Dennis Brothers told me, much to my surprise, that this is far more common in radios that have not been used as opposed to those that have been. The one I had the most problem with in this area is one that was NOS, a very early winged one in the 200 serial number range, that sat on a shelf for 25 years. I ended up changing out all the crystals to do the job right in that the unit was as new condition.
Kurt Keller
David KM5TZ
The 6BF5 Almost everything has been said but for the curious there is some additional food for thoughts: not only is the 6BF5 operated close to or outside the published limits (higher AC voltage may be an additional cause) but the bias voltages on the circuit diagram do not compute when you take the resistor ratios... They will again be different when measured as an overheated 6BF5 might draw grid current.
My solution was to leave the unbypassed 39ohm cathode (AF and DC feedback) resistor in place and pull the screen grid voltage down by connecting a 27kohm (2W) resistor to gnd (there is a convenient tie point on the turret nearer the p.s. circuitry). This yields better performance and stability than increasing the screen grid resistor only.
I also replaced the original screen grid resistor with a higher wattage type and the control grid coupling capacitor (which might or might not be leaky).
With a good 6BF5 there is no grid current and the bias voltages measured should track what is supposed to be there according to the dividing ratios (starting with approx -37VDC).
Now the 6BF5 is operated well within its limits - still getting hot of course...
As I spent some time analyzing this matter armed by tube manuals, 6BF5s (which still is not an expensive tube) and various pieces of test equipment I thought this might be helpful. Should you want detailed test results please send mail.
Ulrich N Fierz
HB9AIK - VK6BQD
CH-8044 Zurich
Switzerland
hb9aik@yahoo.com
As you are aware, the 6BF5 is running too hot. For some reason the screen voltage is too high :over 150 volts and according to all the tube manuals it is rated at 110 typical and 117 volts Max. I did put a 12000 ohms in series at pin 6 on mine. Some from the military had the sockets replaced with ceramic ones.
Also I do cut pin 6 from the 6AK6 tube ( not used in my case ) in order to cut the current drain from the transformer (also running hot ).

Steve Wn4I
Looking through an old RCA receiving tube manual. Some interesting thoughts.
Technical series RC-18 1956 1956 1956 States "RCA Receiving tubes NOT recommended for new equipment design"
In it is a 6BF5, the very hot tube in the 75S Series receivers.
Why did Collins use this old outdated tube? (draws 1.2 amp heaters) I understand some people change out the socket to ceramic and other people install the fluted tube shield to get rid of heat.
Why did they not design around the 6AQ5, much cooler and would work just as good if designed correctly in the circuit ??
Charles K4QZO
Not only that,according to all the books the screen voltage is 110 and a maximum of 117. According to the latest 75S3B/C it is 140 and in reality today we are running over 150 volts on that poor screen. Same problems on the 51S1.
Yves Wn4i
Meter zero On the 51S-1 reversing the leads to the outer connections of the zero adjust pot moves the zero seting to another area of the pot. Thus old pots that are somewhat erratic with heating and adjustment are now being operated on a "virgin" region.
Ian, K6SDE

Service Bulletins 51S-1 Receiver

PUB NO.	NOTE	DATE	DESCRIPTION
SB 1		2/22/63	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
SB 2		6/22/64
                Revised
                6/15/73	Replace Transformers T14 and T15 With Mechanical Filter
SB 3		8/11/64
                Revised
                8/31/64	Improve Sensitivity of Lower Bands
SB 4		8/9/65	Addition of Dial Lock Assembly
SB 5		4/1/70	A: Converts 51S-1A/1AF to 51S-1/1F B: Converts 51S-1/1F to
                        51S-1A/1AF
SB 6		9/1/71	Provision for High-Speed Receiver Muting and Recovery
SB 7		1/1/72	Reduce 500-kHz Spurious Response
SIL 7-63	5/14/63	Reduced Hum in Audio Output With Use of 28-Volt DC
                        Power Supply

51S-1A
Receiver
PUB NO.	NOTE	DATE	DESCRIPTION
SB 1			Not Issued
SB 2		6/22/64
                Revised
                6/15/73	Replace Transformers T14 and T15 With Mechanical Filter
SB 3		8/11/64
                Revised
                8/31/64	Improve Sensitivity of Lower Bands
SB 4		8/9/65	Addition of Dial Lock Assembly
SB 5		4/1/70	A: Converts 51S-1A/1AF to 51S-1/1F B: Converts 51S-1/1F to
                        51S-1A/1AF
SB 6			Not Issued
SB 7		1/1/72	Reduce 500-kHz Spurious Response

51S-1AF
Receiver
PUB NO.	NOTE	DATE	DESCRIPTION
SB 1			Not Issued
SB 2		6/22/64
                Revised
                6/15/73	Replace Transformers T14 and T15 With Mechanical Filters
SB 3		8/11/64
                Revised
                8/31/64	Improve Sensitivity of Lower Bands
SB 4		8/9/65	Addition of Dial Lock Assembly
SB 5		4/1/70	A: Converts 51S-1A/1AF to 51S-1/1F B: Converts 51S-1/1F
                        to 51S-1A/1AF
SB 6			Not Issued
SB 7		1/1/72	Reduce 500-kHz Spurious Response

51S-1B
Receiver
PUB NO.	NOTE	DATE	DESCRIPTION
SB 1			Not Issued
SB 2		6/22/64
                Revised
                6/15/73	Replace Transformers T14 and T15 With Mechanical Filter
SB 3 - 6			Not Issued
SB 7		1/1/72	Reduce 500-kHz Spurious Response

51S-1F
Receiver
PUB NO.	NOTE	DATE	DESCRIPTION
SB 1		2/22/63	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
SB 2		6/22/64
                Revised
                6/15/73	Replace Transformers T14 and T15 With Mechanical Filter
SB 3		8/11/64
                Revised
                8/31/64	Improve Sensitivity of Lower Bands
SB 4		8/9/65	Addition of Dial Lock Assembly
SB 5		4/1/70	A: Converts 51S-1A/1AF to 51S-1/1F B: Converts 51S-1/1F
                        to 51S-1A/5AF
SB 6		9/1/71	Provision for High-Speed Receiver Muting and Recovery
SB 7		1/1/72	Reduce 500-kHz Spurious Response



55G-1
Low-Frequency Tuner
PUB NO.	NOTE	DATE	DESCRIPTION
SB 1		5/23/69
                Revised
                7/25/69	Increase Current Handling Capability of 
                        115-Volt Input Fuse

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