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7603 with no trace

Dave Seiter
 

A friend of mine would like a scope to start learning with, so I went through my stash of non-working 7xx3s, and decided to set him up with 7603 (I was thinking about a 5110, but while I have carcasses, I don't know if I have enough for a fully functioning system).  It's a later one with a fan, fairly clean, and my previous notes indicted that it wasn't totally dead.
The readout works fine, but I can't for the life of me get it to produce a trace of any kind.  The beam finder only shows the RO.  The plugins were verified in in a 7844, and the 7B53A is triggering properly.  The signals from the 7B53B are getting to the H amp, the main intensity control seems to work correctly out of circuit, and since the RO works fine, I have to assume that the V amp, H amp and all the CRT-related items are working fine.  My guess was that it was probably U99, or maybe Q90 or Q108 on the logic board, but even though U99 felt loose, swapping them out one at a time made no difference.  PS voltages are all spot on.
Ideas?  I haven't done much debugging in the past few years, so I'll admit I'm a little rusty.  
-Dave

John Griessen
 

On 6/17/19 7:18 PM, Dave Seiter wrote:
The readout works fine, but I can't for the life of me get it to produce a trace of any kind.
My shoot from hip idea is to clean card edge pads with alcohol and cotton rag, and try again.

No schematic reading done for this quickie opinion.

Others may trounce it.

Roger Evans
 

Try turning the readout off and probing R1101 on the Z axis board with one of your working scopes looking for the unblanking signal. If you don't see the unblanking, try adjusting the grid bias R1261 on the Z axis board and you should be able to see the trace. Return the grid bias to its original setting. Since the unblanking for the trace and for the readout have a common path after R1101, R1103 the rest of the Zaxis amplifier should be OK.

You can also look at the A+ gate out, sweep out and Y axis out on the back panel to confirm that signals are getting from the plugins into the mainframe.

Try exercising the mainframe push buttons for trigger select and left right Y axis select.

Regards,

Roger

Roger Evans
 

Correction - R1101 and the emitter of Q1107 may be sufficiently low impedance that the unblanking signal does not show up well as a voltage signal. Check the collector of Q1107 or farther down the Z axis amplifier for the trace unblanking signal. We know the Zaxis amplifier as a whole is OK since the readout works.

Roger

Dave Seiter
 

Hi Roger, All-
I was able to see the unblanking  signal, and was happy to see the actual trace when tweaking the grid bias.  Wanting to eliminate the intensity pot as a source of the problem, I detached it for closer inspection (it seems totally fine), then found that the grid bias needed to be adjusted again when I reattached it.  Since something had changed, I decided to run through the cal procedure to make sure I had a base line to start from.  Everything was great until I got to step 4Af; adjusting the intensity had no effect on the display- the dot doesn't move vertically at all.  
Tracing the the path of the intensity control, I can see an effect up until the cathode of CR93; on the collector of U99C, the signal changes wrt RO, but Intensity doesn't appear to have any effect on the signal.  (To be fair, the T912 is really struggling to trigger at this point, so something may be happening, I just can't see it).  I tried swapping out the logic board, and even tried a third U99 just for kicks, but there was no change.  
Time to call it a night! 
-Dave

On Tuesday, June 18, 2019, 4:13:39 AM PDT, Roger Evans via Groups.Io <very_fuzzy_logic=yahoo.com@groups.io> wrote:

Correction - R1101 and the emitter of Q1107 may be sufficiently low impedance that the unblanking signal does not show up well as a voltage signal.  Check the collector of Q1107 or farther down the Z axis amplifier for the trace unblanking signal.  We know the Zaxis amplifier as a whole is OK since the readout works.

Roger

Albert Otten
 

Dave,

The RO signal that you see at U99C collector originates from R1101, so that tells nothing about the Z-axis logic at the lower half of diagram <2>.
Did you test for continuity between Intensity wiper and R101? Or that R101 bottom can be pulled down to -15 V by Intensity?

In stead of the test scope you might use a DMM to DC measure Vbe of transistors. If you set the sweep rate for 1 s/div or so then during the sweep you can even change Intensity and see the effect on the reading. [At 1 s/div Limiting is active, but that's not problematic.] Is the difference between base voltages of U99A and B sufficient to guarantee complete switching?
The voltage across R112 determines the current drawn from the node CR93 - U99C emitter. If my crude calculations are correct then about 20 mA flows in R112 when Limiting is not active (sweep 50 ms/div or faster). A too low current there would result in a blanked trace.

In stead of playing with Intensity you could also try an external Z axis AC signal. The ext Z signal follows the path via Q90.

Albert

On Wed, Jun 19, 2019 at 05:41 AM, Dave Seiter wrote:


Hi Roger, All-
I was able to see the unblanking  signal, and was happy to see the actual
trace when tweaking the grid bias.  Wanting to eliminate the intensity pot as
a source of the problem, I detached it for closer inspection (it seems totally
fine), then found that the grid bias needed to be adjusted again when I
reattached it.  Since something had changed, I decided to run through the cal
procedure to make sure I had a base line to start from.  Everything was great
until I got to step 4Af; adjusting the intensity had no effect on the display-
the dot doesn't move vertically at all.  
Tracing the the path of the intensity control, I can see an effect up until
the cathode of CR93; on the collector of U99C, the signal changes wrt RO, but
Intensity doesn't appear to have any effect on the signal.  (To be fair, the
T912 is really struggling to trigger at this point, so something may be
happening, I just can't see it).  I tried swapping out the logic board, and
even tried a third U99 just for kicks, but there was no change.  
Time to call it a night! 
-Dave
On Tuesday, June 18, 2019, 4:13:39 AM PDT, Roger Evans via Groups.Io
<very_fuzzy_logic=yahoo.com@groups.io> wrote:

Correction - R1101 and the emitter of Q1107 may be sufficiently low impedance
that the unblanking signal does not show up well as a voltage signal.  Check
the collector of Q1107 or farther down the Z axis amplifier for the trace
unblanking signal.  We know the Zaxis amplifier as a whole is OK since the
readout works.

Roger


Albert Otten
 

(continued) Sorry, mistake, current via R112 is about 10 mA, not 20 mA . Still with disclaimer...
Albert

Albert Otten
 

Dave,

Please forget my "calculated" value for the current in R112. I have no 7603, but my 7633 has the same schematic for this purpose. Also some voltages are shown in my 7633 manual.
I did DMM measurements at locations that were easy to reach in my 7633/7B53A. RO turned OFF. AUTO triggering. No signal.
Across R112: 0.38 V (unlimited) and 0.21 V (limited, 0.1 s/div or slower). R112 is exactly 100 Ohm.
At very slow speed and during the sweep: Vbe of U99C 0.66 V (Int CCW) to 0.80 V (Int CW, trace off-screen). But most of the change appears to be in the below-visible part, as shown next.
Now 1 ms/div, so unlimited, RO still off, trace off-screen only at high intensity.
Vbe U99C pin EK
CCW 0.65 V 7.70 V
just visible 0.75 V 7.52 V
CW 0.78 V 7.31 V

Of course these DMM voltages are weighted averages over sweep and hold-off time, but sweep time has (by far) the highest weight. Neglecting the change in Vbe of Q1107, the difference of about 0.19 V at EK would correspond to about 5 mA current change in R1101. So also 5 mA change in the current drawn by the wiper of the intensity pot. That looks realistic.

I hope this helps you somewhat further.

Albert