开云体育

OK when I powered this 7854 on fixing the PSU, if failed POST with a ROM
failure (dratted Mostek mask ROMs).

So as to be able to continue testing, I "borrowed" the ROM board
(670-5047-02) from my personal 7854 and put it in the patient.

It passed POST, but this is what the screen looks like: <
/g/TekScopes/files/7854/7854%20Initial%20Display%20Problem.
MOV>

It sort of looks like it may be trying to display the normal Self Test
Complete screen, but it sure isn't right!

Any suggestions for likely culprits?

Thanks
David

link got split

</g/TekScopes/files/7854/7854%20Initial%20Display%20Problem
.MOV>

Sorry

Looks like it won't let me put the link on a single line - grrrr

D.

tinyurl.com

Sent from a small flat thingy

Or even easier, if you put down a "<" and ">" on a line,
and paste the link between them, you will be signaling
that it is a link, and it will not be broken when it wraps.

-Chuck Harris

Dave Daniel wrote:

This usually works,

/g/TekScopes/files/7854/7854%20Initial%20Display%20Problem,.MOV

Nice job on documenting your process and trials.? I'll definitely save your summary since I have a copy of the beast myself.

Bob.

I did that, but it still broke it across lines

Drat it - it still split the link!

<>

David

Could be a focus problem, high voltage, or still your power supply. I think the digital part should be fine, you can see a blob where the readout is supposed to be.

--
Best regards,

Fred S.

OK I just ran through the Z-Axis calibration and pretty much everything
looked just fine on the test scope.

The following test was done with readout on the 7854 turned off.

Looking at the waveform at TP183 on the Z-Axis board: With the 7854
intensity set to minimum, the base line is at about 9.5V, and while a sweep
is running the voltage is about 20V. If I increase the intensity to
maximum, the voltage is about 80V, which is correct. The signal has a
flat top (no variation), and the variation in the peak voltage is linear
with the intensity control.

With the intensity set to minimum there is no obvious trace, but there are
very short bright dots at fairly random locations along the line where the
trace would be. As I increase the intensity to the 270 degree position
(dot on knob pointing to left) the dots disappear but no trace appears.
Nothing is then visible until the intensity control reaches just before the
90 degree position (dot on knob to right). At this point a somewhat out of
focus trace with intensity modulation along its length appears, and finally
with intensity to maximum the trace is a bit more out of focus and somewhat
brighter but still modulated. The modulation appears to about 1-2 cycles
per sweep division when the timebase in the 7854 is set to 10mS/div.

At this stage I'm guessing it's probably some sort of problem in the HV
circuitry (possibly the cathode supply) or maybe the DC restorer, or even
the EHT supply to the anode? I suppose it might also be a dying CRT?

I have scoped the cathode supply using a P6015 and it's close to -3kV with a
square wave ripple of about 0.5Vpp at 25.5kHz.

Your thoughts most welcome ...

David

On Thu, 31 Jan 2019 18:35:46 -0000, you wrote:

I *seriously* have no idea what's going on, but I do have at least, a
question.

With the intensity at minimum, where are those intensity spikes coming
from?

1) could they be related to the readouts? (not sure, but changing
readout intensity ought to say something)

2) are they related to something breaking down in the intensity
circuits? (I'd expect anything on the CRT Z axis to spike and be
bright).

3) are they related to unblanking (in which case, I'd expect the
retrace to do something odd....)

Not sure what else is in there, since I don't have a 7854, although
I'd rather like one.

OK I just ran through the Z-Axis calibration and pretty much everything
looked just fine on the test scope.

which means that the relative voltage readings are fine, and the z
axis stuff is ok, but tells you little about the sources.

The following test was done with readout on the 7854 turned off.

Looking at the waveform at TP183 on the Z-Axis board: With the 7854
intensity set to minimum, the base line is at about 9.5V, and while a sweep
is running the voltage is about 20V. If I increase the intensity to
maximum, the voltage is about 80V, which is correct. The signal has a
flat top (no variation), and the variation in the peak voltage is linear
with the intensity control.

Then where are the dots (negative spikes in K-G voltage) coming from?

With the intensity set to minimum there is no obvious trace, but there are
very short bright dots at fairly random locations along the line where the
trace would be. As I increase the intensity to the 270 degree position
(dot on knob pointing to left) the dots disappear but no trace appears.

Hmmm, sounds odd, but what happens to the baseline K-G voltage or the
effective intensity modulation given to the CRT?

Nothing is then visible until the intensity control reaches just before the
90 degree position (dot on knob to right). At this point a somewhat out of
focus trace with intensity modulation along its length appears, and finally
with intensity to maximum the trace is a bit more out of focus and somewhat
brighter but still modulated. The modulation appears to about 1-2 cycles
per sweep division when the timebase in the 7854 is set to 10mS/div.

Then the modulation is still there regardless of intensity control,
and perhaps you're just seeing it?

At this stage I'm guessing it's probably some sort of problem in the HV
circuitry (possibly the cathode supply) or maybe the DC restorer, or even
the EHT supply to the anode? I suppose it might also be a dying CRT?

IF so, then I'd expect some variations in the voltages, HT, cathode
supply (where's it coming from) and grid supply (ditto).

If I didn't see any pulses at all (can you find a source?), then I'd
start to suspect the CRT.

Not readily, though.

I have scoped the cathode supply using a P6015 and it's close to -3kV with a
square wave ripple of about 0.5Vpp at 25.5kHz.

Then the HT is likely stable enough, and I'd start to probe the other
electrode voltages on the CRT.

Not sure you've gotten there quite yet.

From there, I'd look to see what causes it..

Elementary, of course, but completely looking from the outside in,
without any particular knowledge of the circuitry involved.

Just looking at this from an entirely logical viewpoint.....

Hope it helps... <grin>

Harvey

Harvey

Your thoughts most welcome ...

David
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of David C.
Partridge
Sent: 30 January 2019 18:30
To: [email protected]
Subject: Re: [TekScopes] 7854 display problem

Drat it - it still split the link!

<>

David
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of David C.
Partridge
Sent: 30 January 2019 15:43
To: TekScopes
Subject: [TekScopes] 7854 display problem

OK when I powered this 7854 on fixing the PSU, if failed POST with a ROM
failure (dratted Mostek mask ROMs).

So as to be able to continue testing, I "borrowed" the ROM board
(670-5047-02) from my personal 7854 and put it in the patient.

It passed POST, but this is what the screen looks like: <
/g/TekScopes/files/7854/7854%20Initial%20Display%20Problem.
MOV>

It sort of looks like it may be trying to display the normal Self Test
Complete screen, but it sure isn't right!

Any suggestions for likely culprits?

Thanks
David

1) Readout was turned off completely so no I don't *think* those spikes
relate to that.
If it had been turned on I believe I would've seen the RO Z signal mixed
with the main Z signal.

2) Maybe? I am befuddled.

3) Pretty sure not.

I don't think this is in any way related to the digital stuff.
If this were coming from there I'd see a crazy Z-Axis signal.

Cheers
David

On Fri, 1 Feb 2019 04:10:50 -0000, you wrote:

This would have been easier to follow had you interleaved this <grin>.

1) Readout was turned off completely so no I don't *think* those spikes
relate to that.

turned off or disabled so it didn't do anything? But I'd expect you'd
see something on the CRT unblanking.

If it had been turned on I believe I would've seen the RO Z signal mixed
with the main Z signal.

OK

2) Maybe? I am befuddled.

I'd think you should see this with an HV probe.

3) Pretty sure not.

OK, lets that one out.

I don't think this is in any way related to the digital stuff.
If this were coming from there I'd see a crazy Z-Axis signal.

Except.....

Have you looked at every digital input to the Z axis (and X and Y) to
make sure that they were correct (or at least, off) when you got that
dot?

You're seeing intensity variations in the trace.

Either....

1) it's on the Z axis (and if random, why not on the readouts?)

2) it's on the X axis (stopping the sweep)(yeah, how?)

3) on the x and Y axis (in a time where the beam is not multiplexed
for readouts... possible


4) I don't get this, but I *do* think it's something to do with the Y
axis (and perhaps X) that the Y axis ought to be doing something and
isn't. you can argue the x axis....

Harvey

Cheers
David

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Harvey
White
Sent: 01 February 2019 01:18
To: [email protected]
Subject: Re: [TekScopes] 7854 display problem

On Thu, 31 Jan 2019 18:35:46 -0000, you wrote:

I *seriously* have no idea what's going on, but I do have at least, a
question.

With the intensity at minimum, where are those intensity spikes coming
from?

1) could they be related to the readouts? (not sure, but changing
readout intensity ought to say something)

2) are they related to something breaking down in the intensity
circuits? (I'd expect anything on the CRT Z axis to spike and be
bright).

3) are they related to unblanking (in which case, I'd expect the
retrace to do something odd....)

Not sure what else is in there, since I don't have a 7854, although
I'd rather like one.

I received an offline email from Christian Servais who suggested that the
control grid connection inside the CRT was open resulting in the intensity
signal being capacitively coupled. It wasn't that but the problem was
definitely related to capacitive coupling ...

As I swung the Z-Axis board out to gain access to the HV board, I noticed
that pin 1 of P83 wasn't connected even though I had connected it when I
opened it up earlier. This pin is the one that carries the Z-Axis signal
from the Z-Axis board to the HV board.

The connector wasn't a good fit on the pin, and consistently came off when
the Z-Axis board was closed up to its normal position!

Once I removed the plastic cover, "tweaked" the locking tab, and
re-assembled it, all was well!

It's so nice when the problems are reasonably simple! Even if it drove me
slightly nuts trying to work out what could be causing it!

Cheers
David