Tek 7603 missing readout but present when beam finder pressed


Roy Kalawsky
 

I’ve just acquired a 7603 after many years of searching. Everything spot on except the readout display is missing. However, when the beam finder pressed it appears compressed with the other traces and its brightness can be controlled with the readout control. Anyone any clues?

Cheers Roy


satbeginner
 

Just to be sure: you did adjust the readout intensity knob beneath the crt?
Only on half of the dial of that potmeter the readout is actually "on".

Sorry if you already adjusted that one .

Leo


 

Check the CRT Grid Bias - step 4A of the calibration procedure. This adjustment changes as CRT's age and affects the normal vs beam finder behavior.

--
Bob Haas


Karin Johnson
 

Is it possible that the positioning of the readout is off screen in normal mode? Then when pushing the beamfinder the readout is brought on screen?
I believe there is an adjustment on the readout board to position the readout on screen at the proper locations.
Karin


Roger Evans
 

There are two adjustments that affect the vertical position of the readout. R2291 on the readout board sets the vertical separation of the top and bottom readouts and R403 on the vertical amplifier board is the vertical centering control which is primarily for centering the Y trace with no input signal.

Regards,

Roger


 

Roger,

Does that mean that the readout lines are actually positioned relative to the vertical center position? I'm a little surprised that top and bottom readout separation was not made a user control on the front panel.

-- Jeff Dutky


Roy Kalawsky
 

Jeff

With readout control fully clockwise I suspect the readout is actually offset vertically because there is a faint brightness at very top of the crt (just out of the visible area). If you reduce readout brightness it disappears . When the beam finder button is pressed you get a compressed display of the traces and the readout as expected. The readout line text appears to change when tge input ranges are changed via the front panels. So Im guessing the vertical output from the readout board may be the issue here.

Cheers Roy


Albert Otten
 

With readout control fully clockwise I suspect the readout is actually offset
vertically because there is a faint brightness at very top of the crt (just
out of the visible area). If you reduce readout brightness it disappears .
Why this conclusion Roy? I would think of to large vertical readout gain, and try to correct that with R2291 (mentioned by Roger).
In case of (just) wrong offset there would at the same time be a terribly bright readout between bottom and center of the CRT unless the bottom readout line is not needed. So did you set an amplifier for CH2 or ALT or CHOP to test this?
Albert


Roger Evans
 

Roy,

If adjusting R2291 does not solve the problem then there are a couple of other tests that would help to tie down the fault, don't yet adjust R403 on the vertical amplifier board.

Remove both vertical amplifier plugins and leave the time base plugin in place, set to auto-trigger. Turn the scope on and you should see a horizontal trace very close to the centre of the CRT. If it is more than a couple of minor divisions out it will need resetting later, if it is seriously out then it may indicate a fault in the vertical amplifier.

Replace the vertical plugins with no input signals and arrange the display to have two horizontal lines, if you have a dual trace plugin then use both sections of the one plugin, this will activate an upper and a lower readout slot. Set the upper line to one division below the top of the screen and the lower line to one division above the bottom of the screen. Now press the beam finder and turn the readout intensity up so that you can see the compressed readout. Where are the compressed readouts relative to the two traces?

When you said that the scope was working fine apart from the lack of readout, had you checked with a ramp or sawtooth waveform that there is no distortion in the vertical signal? If you don't have a suitable waveform generator then check that a two division square wave does not change amplitude as you move it up and down the display. It is always possible that a previous owner has tried to adjust out a vertical deflection fault using R403 and moved the readout off screen in the process.

Jeff, the readout signal is single ended, not balanced like the main Y signal, but it should move symmetrically about ground provided the components and ICs on the readout board have not drifted over time. Other 7000 scopes like the 7904 do have a preset control on the vertical amplifier board to centre the readout separately from the main Y signal.

Regards,

Roger


Roy Kalawsky
 

Hi Roger

Thank you coming back to me on this.
1. Removing both plugins produces a perfectly centred horizontal line.

2. Replacing both plugins and positioning channel 1 snd 2 as you suggest and then pressing beam finder produces a compressed display with the upper read out line above the upper line snd the lower readout line below the lower line.

3. When inputting a square wave and moving this up snd down the display there is no change in amplitude.

Cheers Roy


Roger Evans
 

Roy,

That looks all OK apart from the readout positioning. You should be able to adjust R2291 and bring the readouts back to their proper position.

Roger


Roy Kalawsky
 

Roger, adjusting R2291 now brings the readout onto the crt screen. However, I had to set R2291 to an end stop. I guess I should check out possible out of spec resistors etc. It is possible that the previous owner may have tweaked gain controls etc so I reckon I should go through the calibration sequence. The plug-ins still have unbroken calibration seals.

Thank you again for your help - very much appreciated. Roy (proudness owner of a RollsRoyce oscilloscope.


Mark Vincent
 

Roy,

I have increased the wattage of some resistors in the readout boards and the replacements are 1%. R2107, 2139 and 2297 to 1/2W, R2112, 2298 and 2299 to 1W. The 1W resistors are mounted above the board and run warm in operation. You will feel the heat touching them after being on a few minutes. I did replace R2217 with a 1/2W 1% type for low noise and drift.I replaced these resistors on the readout boards in my scopes. The electrolytics I replaced with higher capacitance Nichicon ULD or tantalum types, 10-22mfd. My R2291 is now not at one end on any readout board. Cleaning the sockets and pins is a good idea.

Mark