Service manual

Wells-Garnder Color Vector Monitor Guide
Page 17 of 73
Deflection Board
On the deflection board, the most common failures are Q100 and Q101 and when these fail they
take R100 and R101 (respectively) with them. These transistors are part of the + & - power
supply circuit and are often bad when the resistors are really burnt. To properly test these
transistors, they should be desoldered and removed from the PCB. Even in-circuit and not
isolated from other components you can still get a pretty good idea with a multi-meter (analog
meters work best since digital meters show infinite resistance most of the time) if the transistors
are bad as transistors tend to fail catastrophically. In other words, they usually completely short
(0 Ohms) or open. If you see 0 Ohms where there should be an open circuit or 2.5K Ohms, then
the transistor is probably shorted. If you see greater than 2.5K Ohms when the reading should be
in that range the transistor is probably open. When these transistors are bad, they usually show a
large crack in the case if you look closely at them.
Replace Q100 with the same type transistor, but if Q101 is bad (and even if it is not), you are
STRONGLY suggested to upgrade it to a larger transistor that will handle more current.
ALWAYS replace Q101 with a TO-202 package instead of the much smaller TO-92 package
that the board comes with. In fact, the P327, and P339 versions of the deflection board were
manufactured with this upgrade. Use an NTE50 or equivalent.
Even though these transistors (and resistors) are the most common failures on the deflection
board, you will most likely NEVER see them go bad after Q101 is upgraded to a NTE50. If you
cannot find the generic parts, a common modern day replacement for MPSA06 (Q100) is
NTE287 and a common modern day replacement for MPSA56 (Q101) is NTE159 (but I cannot
stress enough the utility of going with the larger replacement for Q101). Upgrading Q101 should
always be the first thing you do to any deflection board; replacing it before it fails will save you
from having to replace the other parts that go bad when it does fail.
It is unusual, but sometimes some of the four heat sinked transistors on the deflection board will
die (Q603; Q604; Q703; Q704). If any of these transistors are bad, you will usually get no
picture at all, but you will see "background brightness" that lets you know some electrons are
being thrown at the tube. Be sure to check the resistors and diodes around any bad transistors you
find.
Very infrequently, you may have problems with some other transistors in the X/Y amp section of
the board. Most of the other transistors that populate the deflection board (Q600-602, 700-702)
are type TPS98 and are not easily found anymore. The good news is that TPS98 is equivalent to
the PN3569 and the ECG or NTE 194. All Electronics (800-826-5432) usually has PN3569
transistors in stock.
Once in a while you will see D104 or D105 open or shorted as well.