Tek. 7904A and 7854 trace too bright (Z board working right and dc restoration diodes replaced)

Mark Vincent

I have the same problem in both scopes. The trace was normal one time and
too bright the next time it was turned on. The bias on the crt is 0V. I
have changed the diodes in the dc restoration circuits and doubler in the
crt cathode supply with MUR1100G, the 22meg carbons replaced with 1/2W 1%
types as the carbons checked about 30meg. I have replaced C82, C84, C86,
C87, C89, C91, C66, 69 and C72. The voltages were increased from 2kv to 3kv
and 4kv to 6kv types. I changed the 13meg resistor, R112, because it
checked about 20meg. That is now a 10meg and 3meg 1% type in series. I have
tried a new 741 opamp. Q129 tests fine on both units. Pulling the PDA,
plugs from the Z axis and larger one to the crt base make no difference in
the bias voltage, 0V between the cathode and G1. Keeping only the PDA
disconnected with the other two connected the trace is reduced. The crt
bias is set at max., 130V (should be cutoff) and int. on A and B at
minimum. There is no change in intensity with either reduced to a normal
bias and intensity. Turning up the intensity only makes the trace out of
focus. The vertical and horizontal is working fine. A trace can be seen if
an input is to the vertical plugin. The crt cathode voltage is correct. The
100V at the collector of Q129 is there and will vary with the high voltage
adjust. The 130V supply is correct. The two electrolytics are new Nichicon
ULD 5.6mfd 200V. I do not suspect a bad transformer since voltages are

I am taking voltages from the 7904A here. The 7854 is likely the same in
voltages since the problem is exactly the same. The intensity is minimum
and bias at max. (130V). The 741 pin 2 -.02V, pin 3 .475V and pin 6 -1.63V.
The -2965 is there and the cathode and g! of the crt. The ac voltage at the
load side of R62 is 7.9v p-p. The ac voltage at the load side of R32 is
15.6V p-p. These two voltages seem too low. These voltages are in the
condition it is now with the PDA unplugged and other connectors connected.
If the PDA is connected, the trace is very bright and will fill the screen
as it should in normal operation. The power supply does not pulse when the
PDA is connected. It will if the brightness is too high in normal
operation. I have seen this when a restoration diode goes bad.The focus is
out when the PDA is connected. Likely the high brightness causing this. The
thick film resistor networks check fine. The proper operation voltages are
unknown (to me) and not on the schematic.The voltages from the Z board are
right. I adjusted them as said in the manual. Adjusting the focus does make
a difference in voltage and on the screen. The readout is working fine on
both units. That will adjust from off to full.

The 7854 was being used to test voltages between the two when it suddenly
developed the same problem(s) as the 7904A. It was on, working fine then
the problem happened. Voltages are now almost exactly the same as in the
7904A. Dust/dirt is not a problem as both are totally clean. Dust covers
are kept on them when not in use. The only difference between the boards is
the transistor is on the 7854 that goes back the power supply for beam
current detection. The 7904A board (A20) has a jumper between B and C where
the transistor could be and no other parts. This board was for both with
only some parts there or not depending upon which it was going in. Both
scopes and plugins have been overhauled, power supply voltages adjusted to
spec.. I know I have given a lot of details. I wanted to eliminate things I
know or suspected could be bad and to help anyone that knows what the
problem(s) is/are. Does anyone know what the problem(s) is/are? Thanks