[TMC] Low Serial GSB-1 Oddities

[Grant Youngman]

I acquired a GSB-1 a while back with a serial number of 131 (might be 181 -- hard to read).  While getting it cleaned up and working I noted that it has several differences from the common GSB-1 and the available manuals.  I haven't been able to locate a readable date code anywhere on the components.  It is marked GSB-1 and looks externally like every other one I've seen or owned.

1.  The location and part numbers of some components are different, including T1/C16/C24, tube socket positions on the chassis, and two dual-section electrolytic cans in the power supply instead of one quad-section.  Other minor differences such as several 0.1uF coupling caps where the common manual shows 120 pF.  I realize TMC wasn't very consistent in coupling cap values throughout the MSR-xx line, with 0.01 being the most popular … 

2.  A pi-section LC low pass filter a la the MSR-xx adapters rather than RC filter shown in the GSB-1 manuals.  It is discreet and not encapsulated, with a 720 mH toroidal inductor and two 2600 pF caps, terminated in 12K.

3.  An unusual method of switching Slow/Fast AGC -- at least different than I've seen in any other similar MSR-xxx product.  Instead of switching in low resistance to get to "fast", it switches in additional capacitance to get to "slow". 

These differences are clearly factory original, with no evidence of rework or molestation.

If you have a GSB-1 in the mid-100's serial range or lower I'd be interested in swapping notes -- especially if you have a manual/schematic that matches some or all of these differences.

Grant NQ5T