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Bad Focus on CRT


Harvey Parent
 

First time poster here. I’ve been reading many posts and learned a lot from you guys.
I got a 465B from eBay and I’m having a problem with getting the trace to focus properly. Even after checking and adjusting all CRT circuits outlined in the service manual, the trace still looks ‘fuzzy’ on the horizontal top and bottom segments of the square wave calibration signal.


Tom Lee
 

Go to x-y mode. Can you make the dot nice and small by adjusting the focus and astig? Does the dot continue to look good as you move it up and down, and left and right?

Sent from my iThing, so please forgive typos and brevity.

On Feb 24, 2021, at 1:54 PM, harvey.parent@gmail.com wrote:

First time poster here. I’ve been reading many posts and learned a lot from you guys.
I got a 465B from eBay and I’m having a problem with getting the trace to focus properly. Even after checking and adjusting all CRT circuits outlined in the service manual, the trace still looks ‘fuzzy’ on the horizontal top and bottom segments of the square wave calibration signal.





robeughaas@...
 

Try pushing in the 20MHz bandwidth limit switch.

I prefer setting the focus and astigmatism controls with a sine wave displayed and the 20MHz bandwidth switch pushed in.

--
Bob Haas


Harvey Parent
 

Hi Tom,

I will test X-Y mode and get back shortly. Thanks for your reply.


alan.bain@...
 

I have a 2465 with a very similar sounding focus problem. In X-Y mode I cannot get completely down to a point only a short horizontal line (using astig & focus controls and focus if fully ccw so output of 0V from control pot) and I'm sure I shouldn't be working at the end of travel of the focus. I cannot also get a short vertical line!

The focus output is producing between -108 and -180V and supply rails look good (have replaced a few caps).

There is a not very obvious DC restorer circuit in the grid side which had a very slowly rising voltage out of an LF353 which does appear to work as per the diagram in the manual, but part of me wonders if this could be a bad CRT since


Harvey White
 

One thing you can do, in XY mode, is sweep the horizontal and then the vertical centering rapidly back and forth.  With input shorted to eliminate as much interference, you *might* get an idea of the waveshape.

Another option is to take a function generator, put it on triangle, and then use it as a fake horizontal sawtooth.  If your function generator does ramps, so much the better.  It will eliminate the internal sweep generators from contention.

Another option is to disconnect the vertical and horizontal plates if possible, (it may drift because of static buildup), but that eliminates pretty much the entire vertical and horizontal chain.

Reconnect and figure out what's coming from where.

In the vertical and horizontal amplifiers, generally shorting the two complementary signal rails is not only reasonable, but eliminates the contribution from previous stages.

Just thoughts, though.

Harvey

Failing that, perhaps you should blame the local FM stations, as has been indicated in other threads.

On 2/27/2021 7:18 PM, alan.bain@gmail.com wrote:
I have a 2465 with a very similar sounding focus problem. In X-Y mode I cannot get completely down to a point only a short horizontal line (using astig & focus controls and focus if fully ccw so output of 0V from control pot) and I'm sure I shouldn't be working at the end of travel of the focus. I cannot also get a short vertical line!

The focus output is producing between -108 and -180V and supply rails look good (have replaced a few caps).

There is a not very obvious DC restorer circuit in the grid side which had a very slowly rising voltage out of an LF353 which does appear to work as per the diagram in the manual, but part of me wonders if this could be a bad CRT since





Chuck Harris <cfharris@...>
 

I wouldn't condemn the CRT just yet.

Focus to a spot depends on a lot of things, and one is
that the spot isn't being deflected by other things.

The first place you should always start is the ripple on
the supplies, and the voltage on the supplies. Most all
2465's by now have several bad capacitors in the low voltage
power supply, particularly the -15V filter for the inverter.

Make sure they are in good shape first.

No controls in the 2465 should be all the way to one end.
That usually indicates a problem.

If you have another scope to help you, that will make
your job a lot easier.

When you are in X-Y mode all of the dynamic focus circuits
are not doing anything, so you should be able to make the
astigmatism and focus make a spot anywhere on the screen.

If not, look at the direction of the distortion in the spot.

If it is a vertical line, that says that the vertical deflection
plates are driving the spot that way... if it is horizontal,
that means it is a horizontal problem.

If it is diagonal, that is the universal indication of a common
problem, such as power supply ripple or noise.

-Chuck Harris

alan.bain@gmail.com wrote:

I have a 2465 with a very similar sounding focus problem. In X-Y mode I cannot get completely down to a point only a short horizontal line (using astig & focus controls and focus if fully ccw so output of 0V from control pot) and I'm sure I shouldn't be working at the end of travel of the focus. I cannot also get a short vertical line!

The focus output is producing between -108 and -180V and supply rails look good (have replaced a few caps).

There is a not very obvious DC restorer circuit in the grid side which had a very slowly rising voltage out of an LF353 which does appear to work as per the diagram in the manual, but part of me wonders if this could be a bad CRT since






alan.bain@...
 

Thanks. Supply rails are (now) good after replacing quite a few capacitors that weren't doing much (after tracing a few bad ones I ended up doing all electrolytics on the low & high voltage psu boards because dismantling took longer than resoldering). I also rewound the annoying noisy fan motor which is thankfully now silent but comes high on my list of not very fun jobs due to those very fiddly hall effect sensors. I do indeed have another scope to help - an HP183 (sorry Tek). But I am guilty as you say of a school boy error. Disconnect X deflection amp and presumably shorting the two sides on the CRT together is a sure way to eliminate X-deflection issues. I think though based on the shape this is a focus issue, but my experience tends to be with more basic crt types.

However I've also noticed that the edge focus is set hard against one endstop and there's something slightly strange with the drive to the third quadrupole lens (there is a LF353 and transistor driving the negative plate and I'm not convinced that is working as I'd expect symmetric voltages there) time for some component checks around that area.

Alan


Harvey Parent
 

The Focus problem of my 465B has significantly improved by powering it down and unplugging it from AC power.
Caution: -Do the following steps at your own risk. Be sure to wear safety glasses and leather gloves.
Next the CRT socket pins are discharged to ground and the socket is Carefully pulled back away from the CRT base. This is to avoid sending DC to the High Voltage transformer.

Leave the ‘scope switched off and disconnected from AC power during the whole process. Connect an external DC power supply voltage to a approximately 15 to 20 percent above normal rated 6.3 V filament. Note For my 465 the power supply was set to 6.3 V x 1.20 = 7.5 V to pins 1 and 14 directly to the CRT pins at the base of the CRT tube, not to the socket. Check the specs and schematics for your particular CRT. It could be different.
After an hour or so, everything was reassembled and the ‘scope powered up with a much sharper trace.

Thanks to the guidance of members on this forum, I was able to get the 465 fully functional.

Harvey Parent