87 t-66
Well-known member
I am refreshing this 6100 deflection PCB and am curious as to why these diodes were placed on the bottom. It appears someone has previously worked on this board which often worries me. Any ideas? Should I leave them?
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BTW, your deflection board also has the two 1N754A zener mods added. So it is consistent with the upgrade instructions. See Figure 7 in the TM-268.
Agree with your thoughts, VectorCollector.It has been a long time, you cannot really assume the history of the cabinet, nor the monitor, nor the individual boards in either. Also, the owner(s) might have decided to do the protection upgrades even if not being used with a MH boardset.
The back-to-back zeners are to clamp the X and Y inputs to the deflection board. This protects both positive and negative transitions. Also protects the input board (i.e. logic board) from any reverse transients.
The 1N4002 are attached at P600/700 and are thus back-biased across the main deflection X/Y transistors. Due to the coil inductances, these "body diodes" for the BJT switches likely help in reducing inductive kick that might, over time, fail the transistors.
Agree with your thoughts, VectorCollector.
One thing I've not heard over the years is whether the later P327 and P339 deflection boards would benefit from these modifications. My assumption is yes as there is little difference in this part of the deflection board between the various revisions. Obviously, I appreciate hearing a more detailed and technical response from those more versed in these boards than me.
Scott C.