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7704A bottom readout out of position

Thierry Delaitre
 

Hello

The bottom readout line on my 7704A appears to be out of position and has gone up - it is located at one third of the screen from the bottom. The one at the top is ok and well positioned. I tried to use the vertical separation pot but if I use it, then the top line goes up outside the CRT. I've replaced the tantalum capacitors on the readout board and the voltages appear to be spot on. do you know what I could try to troubleshoot?

Thanks

Thierry

Mark Vincent
 

Thierry,

Pull the vertical plug-ins out, power the scope on with known good horizontal time base(s). This will see if the vertical is centered inside. If not, remove the left side cover then adjust R4493 on the vertical board. This the main centering pot. When that is centered, turn off the scope, install a vertical plug-in with its left side panel removed on the left for easier adjusting. See if the readout is centered, not with what you say is happening now. If centered, adjust R3490 (vertical separation) to see if that causes the readout to expand to the top and bottom. If it works, then adjust the Character Scan R3457 for the character height desired. If it is still off, see if contact problems are causing the problem. This is common. Clean the pins with 91% alcohol using an artist brush that is small. Pull the devices, ics and transistors out first. Put the alcohol on the pins of each device and sockets, insert the device, pull up some then down several times while keeping the pins in the socket holes. A few mm of up and down with the device usually cleans the contact problem. Doing this to the coax ends inside the unit should also be done. The center wire will oxidize enough to cause problems even though it cannot always be seen. A bad transistor, ic or both can cause readout problems. I replaced the 1500 and 510 ohm resistors to Q3493 and Q3499 with 1/2W and 1W respectively because the originals are too small in wattage, 1/4W. I use only 1% types for the low drift due to temperature, 50ppm/deg. C. I used 47mfd 25V Nichicon ULD types for the decoupling in place of the original 1mfd tantalums. These give higher decoupling while having a very low ESR and long lives. Some 1mfd tantalums were replaced with 1mfd 63V film types. The left panel removal on the left side of a vertical plug-in is for internally adjusting the vertical centering inside of that type once the trace is centered on the vertical board. I hope this helps.

Mark

Craig Sawyers
 

The bottom readout line on my 7704A appears to be out of position and has gone up - it is located at
one third of the screen from the bottom. The one at the top is ok and well positioned. I tried to
use
the vertical separation pot but if I use it, then the top line goes up outside the CRT. I've
replaced the
tantalum capacitors on the readout board and the voltages appear to be spot on. do you know what I
could try to troubleshoot?
Apart from the occasional dead readout chip (which has much more obvious problems) I once had a
resistor on the readout board go open. Very odd since it looked like metal film - but worth checking
on yours.

Craig

Thierry Delaitre
 

Thanks Mark/Craig



I've removed the vertical units in order to check the centering. I've
adjusted R93 on the vertical amplifier for the trace to be in the center.



It has slightly improved the readout but it seems the bottom character line
is still slightly too high. If it try to adjust the vertical separation,
then the linearity for the top and bottom appear to be different. The top
line disappears the screens a lot faster than the bottom one.



I've checked the resistors around the output of the X&Y of U3480 and all
appear to be ok. R84 to R99.


I attach a picture of the screen. Should I expect the bottom line to be
lower ?


Thanks


Thierry

On Fri, Aug 2, 2019 at 7:25 AM Craig Sawyers <c.sawyers@...>
wrote:

The bottom readout line on my 7704A appears to be out of position and
has gone up - it is located at
one third of the screen from the bottom. The one at the top is ok and
well positioned. I tried to
use
the vertical separation pot but if I use it, then the top line goes up
outside the CRT. I've
replaced the
tantalum capacitors on the readout board and the voltages appear to be
spot on. do you know what I
could try to troubleshoot?
Apart from the occasional dead readout chip (which has much more obvious
problems) I once had a
resistor on the readout board go open. Very odd since it looked like metal
film - but worth checking
on yours.

Craig





Craig Sawyers
 

I attach a picture of the screen. Should I expect the bottom line to be lower ?


Thanks


Thierry
Hi Thierry

You can't attach pictures to messages on this mail list. You need to host them somewhere, or upload
them to the pictures section of the Tekscopes io, and provide a link to where they are (there is no
search facility on io images).

Craig