February 21, 1975
SB-644							Bulletin
No:
Remote VFO						SB-644-1

		 	    Switches JAM

The first run of SB-644's include hardware that may limit the travel of
the cancel bar in the pushbutton switch assembly.  You may receive
requests to replace a switch which appears faulty.  Advise checking
mounting hardware and replace with four screws to [PN 250-172].
-----------------------------------------------------------------------------------------------------------------
			May 20, 1975
SB-644							Bulletin
No:
Remote VFO						SB-644-2

			  VFO FM-ING

The following changes are being made to revive uniformity between the
SB-644 and SB-104 VFO's:

On the VFO oscillator board, Change:

	R1201 	from	4700	to 	1200	[PN
1-10]
	R1202	from	10K	to	R.F.C.	[PN
45-82]
	R1203	from	1000	to 	470	[PN
1-6]
	R1206	from	47	to	150	[PN 1-66]
	D1202	from	1N191	to	1N485	[PN
56-24]
	ZD1201	from	10V	to	7.5V	[PN
56-97]

Although the FM-ing problem may not exist when operating from a 13.8V DC
power supply, the FM-ing problem might occur if it is being operated from
a "field day" supply.  Make these changes only when a complaint is
received.
-------------------------------------------------------------------------------------------------------------------
			May 11, 1976
SB-644							Bulletin
No:
Remote VFO						SB-644-3

		           Schematic Correction

The following correction is to be made to the schematic in the [PN
595-1656-03] Manual:

On the SWITCH circuit board the line from junction of C1251 and C1252
whuld connect to the cathode of D1262 instead of to coil L12.
------------------------------------------------------------------------------------------------------------------
			February 2, 1978
SB-644, A							Bulletin
No:
Remote VFO						SB-644-4

			      VFO Dead

Check the solder pads at each corner of the [PN 85-1205-3] circuit board
to confirm that all solder resist has been scraped away.  If the solder
resist is not removed at these points, the oscillator will not run since
the board is not grounded.
-------------------------------------------------------------------------------------------------------------------
			May 30, 1978
SB-644, A							Bulletin
No:
Remote VFO						SB-644-5

	Oscillations Caused by Overdriven Switching Transistors

Switching transistors Q1255, Q1256 and Q1257 may sometimes oscillate due
to their emitters being at zero potential, grounded by L1255.

To prevent this, replace L1255 with a 470 ohm resistor [PN 6-471].  The
resistor will have to be installed on end (vertically) due to narrow hole
spacing of L1255.

Typical emitter voltage will now be about 1 volt.

This will be covered by insert [PN 591-2657] in the latest production
run.

Circuit board [PN 85-2037-3] will incorporate this change.
--------------------------------------------------------------------------------------------------------------------
			August 15, 1979
SB-644A							Bulletin
No:
Remote VFO						SB-644-6

		    Schematic Correction:  ADD R1227

The schematic in the SB-644A Manual [PN 595-2055-02] doesn't show R1227
[3900 Ohm] from source of Q1203 to ground on the Buffer board.  Add this
to your schematic.
------------------------------------------------------------------------------------------------------------------
			August 15, 1979
SB-644A							Bulletin
No:
Remote VFO						SB-644-7

	        Excessive Second Harmonics on VFO Frequency

The following changes will lower the second harmonics of the VFO
frequency and reduce cross-coupling between Q1256 and Q1257.  This will,
in turn, reduce the RF leakage on the VFO output jack when that line is
switched off.

Change:

R1268	from 	470 Ohm		to	Toroid Coil
[PN 40-1882]

Q1256	from	[PN 417-205]	to	[PN 417-134]

Q1257	from	[PN 417-205]	to	[PN 417-134]

Install:

Shield [PN 206-1257] as follows:

- Drill two .070 holes in ground foils at indicated spots.  ((one hole
shown at what looks like between L1253 and C1287[??]-the component that
is between L1253 & L1254.  The other hole is at the end of R12, right
down from 'K' & 'J'))

- Mount shield so that it does not touch transistors, coils or other
leads.
-------------------------------------------------------------------------------------------------------------------
That's all I have on the SB-644, A [1966-89].  Hope it helps.


73 de Joe  W7LPF/4  [NNN0KUU]
QWCA - SOWP - NCVA - FISTS - RCC
Gordonsville, Va 22942 [Orange Co]
Wanted:   SB-644[A]