[AMRadio] SB620 repair

[CL in NC]

Well, seems that neon bulb is the key to the sweep centering problem since nothing else corrected the problem.  The one in there is a #83, and I tried several neon lamps and noticed their effect on the sweep.  The firing point and lamp current actually determines the range the horiz. position control can shift the trace, and if it is too high or low, you can't center it.  As the voltage rises to the fire point, it sets up the grid volts on the horiz. amp, and when it does fire to make the sawtooth, if the point it fires is correct the trace starts within the range of the horiz. pos. control.  Of course, when I tried to remove the one leg of the lamp for testing, the lead broke off right at the bulb.  I tried NE 2's, B1A, NE 51, a couple unknown, which made the sawtooth have a loop on one end, or fired at such a voltage the sweep was way off the side unseen and unadjustible.  One mystery neon worked almost correctly, but it generates a curved ramp on the sawtooth.  So, went looking for 83's and found a surplus company that had them, 5 for $15, another place had them for $3.75 each, or you could buy one on Epay for 24 bucks.  I have 4 Heath scopes that use the same neon lamp.   It seems like the charge circuit resistors could be changed to adjust it to operate with other neons, just in case you all have issues in the future and #83's are long gone.

Charlie, W4MEC in NC