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SC502 With Display Issues


Stephen
 

Next project up is this little SC502 that came with the TM506 I bought.
At first the display was only showing 2 shorts lines that wouldn’t move very much from left to right.
Other that that, the scope would stay on forever.
I’ll spare you the details, but it turns out that one of both the horizontal and vertical plates were disconnected.
After reconnecting the plates, all was back to normal... But...
Now, after a few seconds, the beam is bouncing off the left wall of the CRT. Hence no trace is visible. The screen is just glowing with an emphasis on the left side. At the exact same time, the power indicator LED turns off.
It can happen right away after being powered on, or I can just see a trace for just a few seconds.
Also, when I turn the horizontal position, I can sometimes see the flash of what seems to be a single dot.
It’s very fast and lasts only a few ms. Barely visible if you’re not paying attention to the screen.

I suspect the horizontal amp. However, there’s also the VR that controls the trace rotation that is missing it’s wiper. I plan to replace it.

I don’t think that could cause this, but I’m no expert.

Any clues to set me on the right track would be appreciated.

Thank you.


teamlarryohio
 

Trace rotation adjusts the planes of V & H deflection to be parallel to the center lines of the graticule. It affects both axes. The Y-Axis coil does the same thing, but only affects the vertical plane. They twist the trace but do not slide it. I hope that's not as confusing aa it looks to me.
-ls-


Roger Evans
 

You need to check the state of all low voltage supplies before and after the fault appears. The power light is an LED fed from the +5V rail, so try watching the 5V rail during the transition from working to not working. You can measure the current supplied by +5V by measuring the voltage across R982, at about 250mA it is going to shut down the regulator via Q965. +5V will also shutdown if +20V disappears.

Regards,

Roger


Stephen
 

Thank you Roger.
I’ll try to check that. But it’s extremely fast, and most of the time instantaneous upon power on.


Roger Evans
 

Your first email seemed to imply that sometimes it was working for a few seconds, hence the before and after suggestion. Anyway, measure all the low voltage rails and if any are significantly low measure the voltage across the current monitoring resistors R982 for +5V and R940 and R943 for +/-20V. If you can get at the deflection plates try to measure the voltages there as well. The high voltage supply to the CRT is from the unregulated 33V so that can continue to run despite a fault on the regulated supplies.

Regards,

Roger


Stephen
 

Yes Roger, you’re right.
I forgot to mention that when it does work for a few seconds, it’s usually 1 or 2, and it’s extremely rare. 99% of the time it’s instantaneous.


Stephen
 

Accès to the plates is quite easy.
Will do what you suggested and report back.
Thank you Roger


tenareze32@...
 

Be careful getting probes on and off scope deflection plates. I got a probe snagged and broke off the pin on the scope! wailing and gnashing of teeth.
Simon


Stephen
 

On Sat, Aug 15, 2020 at 02:08 AM, <tenareze32@...> wrote:


Be careful getting probes on and off scope deflection plates. I got a probe
snagged and broke off the pin on the scope! wailing and gnashing of teeth.
Simon
Will definitely be extra careful.
Thank you.


teamlarryohio
 

My point was that this is not going to be the cause of the trace moving offscreen....


Stephen
 

On Sat, Aug 15, 2020 at 02:38 AM, teamlarryohio wrote:


My point was that this is not going to be the cause of the trace moving
offscreen....
Ohh, ok.
Thank you for making that clear.
So why do I need to check the voltage at the plates? I’m a bit confused now...
Trying to learn something in the process too.


Roger Evans
 

I could be wrong, but I thought Larry's point was that the rotation and Y adjust coils are in the nature of fine geometry adjustments and are not going to send the beam off screen unless you put a crazy current through them.

My point is that the beam is off screen because there is an electric field between the two X plates and it would be useful to decide if the electric field is caused by a wrong voltage on one plate or (different) wrong voltages on both plates.

Roger


Michael W. Lynch
 

On Sat, Aug 15, 2020 at 09:22 AM, Stephen wrote:


Ohh, ok.
Thank you for making that clear.
So why do I need to check the voltage at the plates? I’m a bit confused
now...
Trying to learn something in the process too.
The beam is deflected by differing potentials between the two plates. if the dot is off the screen, then one plate is much higher or lower than the other. Depends on which way the beam is being deflected. When centered, the plates should be equal in potential. The + and - horizontal plates will have a complementary rising or falling triangle waveform present at the plates, depends on the time/div setting. The + and - vertical plates should have a complementary signal present, with the shape of the trace dependent on the form and amplitude of input signal.

--
Michael Lynch
Dardanelle, AR


Michael W. Lynch
 

The + and - horizontal plates will have a complementary rising or falling triangle waveform<
Actually a ramp, more than a triangle.

--
Michael Lynch
Dardanelle, AR


Stephen
 

Thank you Larry, Michael and Roger for your explanations and help.
I will report back when I get to it shortly.


teamlarryohio
 

Perzackly! That electric field is what does the deflection when it is the right polarity and magnitude at any given instant. When the voltages at the plates are not correct, neither is the position of the spot that draws the trace. In the extreme, that is often offscreen.


DaveH52
 

If you have a service manual, spend the time to read the functional description while following along in the schematics.
My starting point was smoke and lots of it. I'm going to be busy for a while replacing every tantalum capacitor and all of the A1-A5 HV caps because one of them had arced to a wet tantalum that spewed sulfuric acid. Once that was repaired, each power-up after that, would fail because another tantalum cap had failed and burned up a resistor. Therefore, not they're all getting replaced.


Stephen
 

I’ve made quite a mess...
I wanted to check the voltages. I plugged in the extender and forgot to power off the TM, and didn’t insert it correctly. I blew the 2 +/- 35v fuses. Replaced them and now all the plugins work fine except this one. Nothing... No power at all. All the other plugins that stopped working when that happened, are now ok in all bays.
I checked the 2 fuses inside the unit, and they’re good...
BTW, the SC was in the OFF position when that happened.
Grrrr. I shouldn’t be doing these things at 1:00am.

PS: I was planning on replacing all the tantalums anyway, just for good measure.


Harvey White
 

responses interleaved

On 8/15/2020 7:17 PM, Stephen wrote:
I’ve made quite a mess...
not sounding so good
I wanted to check the voltages. I plugged in the extender and forgot to power off the TM, and didn’t insert it correctly.
To quote RJS:  Gee Bullwinkle, that never works.
I blew the 2 +/- 35v fuses. Replaced them and now all the plugins work fine except this one.
Ok, now disaster control.
Nothing... No power at all. All the other plugins that stopped working when that happened, are now ok in all bays.
Ok, that's actually part good news.  Not likely that you've damaged the frame *if* the other modules use all of the frame slot's capabilities.
I checked the 2 fuses inside the unit, and they’re good...
BTW, the SC was in the OFF position when that happened.
Then the thing to check is what's in the unit that happens before that on/off switch.  Another question, however embarrassing, is if you can figure out what went wrong (in terms of how the extender is put in), and what the possible results might be.  That could tell you what might be damaged.

Another possibility when you do plug things in with an extender is to put the frame end first.  That protects the plugin.  That way, if you have BIG RED LEDS on the extender (hint... hint...) you can tell that power is on.


Grrrr. I shouldn’t be doing these things at 1:00am.
Been there, done that.  That includes soldering a 100 pin chip rotated 90 degrees from what it was supposed to be (IIRC).  We won't mention what happens on some designs...  Everything gets reviewed in the morning. (and I'll still need revisions).

You might look into some tektronix breadboard units, those might be an answer to homemade extenders.



PS: I was planning on replacing all the tantalums anyway, just for good measure.
not a bad idea, but consider the type (gumdrops likely will fail) and 7 volt parts used on 5 volt supply rails will likely fail as well.  On the other hand, a 35 volt part used on 20 volts is less likely to give you problems.

Harvey





Stephen
 

On Sat, Aug 15, 2020 at 01:59 PM, Harvey White wrote:

Interleaved

Then the thing to check is what's in the unit that happens before that
on/off switch.  Another question, however embarrassing, is if you can
figure out what went wrong (in terms of how the extender is put in), and
what the possible results might be.  That could tell you what might be
damaged.
I’m a bit out of my depth here, if not completely...
I’ve check the Vcc of U950 and I have -30.62 VDC at pin 4.
Output at pin 6 is -1.2VDC
U960 has nothing anywhere.

I suspect Q960 pass transistor (2N5401) to have blown along the the Mainframe fuses...

I hope no other transistors down the line blew as well, as I have no source for Q950 and Q955 (2N5086) and I’m not sure I can find equivalents (if there are any suitable for this application)...