@patpin --- While preforming the test suggested in post #120 please make sure you can move the 'dot' around the screen via adjustment of R69...

 

I concur with your assesment that the Scope's return to 'partial life' pretty much rules out such damage!



This is truly bizarre! -- Ok... Now that the scope is "working" again -- please check check X1 and X2 (pins #10 and #12) to each other and to chassis ground again --- Please remember to set the 'Time/Div' control to 'Ext X' prior to the test

between F-G: in ext-X: 100V; other than EXT X: ranges from 100 to 70 depending on knob HOR POS(R69);
to framE: pin 12: (in EXT X): remains at 110; other than EXT X: range 100 to 110 depend on knob
pin 10; ; (in EXT X): remains at 0; other than EXT X: range 0 to 20 depend on knob

 

Does adjustment of R69 move the 'dot' in 'EXT X' mode?

 

@patpin --- While preforming the test suggested in post #120 please make sure you can move the 'dot' around the screen via adjustment of R69...

I cannot move the dot in EXT X; there is no dot. I guess it is off screen even in most CW position of R69.

 

I cannot move the dot in EXT X; there is no dot. I guess it is off screen even in most CW position of R69.

The baselines also disappear (travelling to the left of the screen) (in other than EXT X) when the R69 is half way or more to CCW

 

I cannot move the dot in EXT X; there is no dot. I guess it is off screen even in most CW position of R69.

Based upon your readings, I think it's normal that you have no X-Pos adjustment in 'EXT X' mode -- The fact that the dot is off screen is probably related to original problem...

So, back to troubleshooting the original problem

Your description of the scope's behaviour during warm-up is strongly suggestive of power supply issues! -- Please give me a few minutes to look at the schematic -- Be back to you shortly!

 

Here's a schematic of the PSU, Etc...

 

Note that:
C402
C403
C406
C408

Are the 'prime suspects'

Can you locate the above components on the scope?

EDIT Added C408 to the list...

 

@patpin --- Please note the edit to post #128 (i.e. removal of C409 ,C411 and C412 from 'suspicion')

 

Note that:
C402
C403
C406
C408
Are the 'prime suspects'
Can you locate the above components on the scope?

If so please look for signs of bulging, leaking or overheating...

 

If so please look for signs of bulging, leaking or overheating...

No exterior signs of aging as U described. Whats the reasoning behind yr conclusion. Why is the baseline moved to the left even in the most CW position of R69.

 

Whats the reasoning behind yr conclusion

In an earlier post, you described intermittent phenomena during warm-up --- such symptoms are generally power supply, and, specifically, filter capacitor related...

Why is the baseline moved to the left even in the most CW position of R69.

That would appear to be issue of DC imbalance...

Important -- In an earlier post I understood you to say that the scope occasionally functions properly during warm-up -- Is that correct???

 

In an earlier post, you described intermittent phenomena during warm-up --- such symptoms are generally power supply, and, specifically, filter capacitor related...


That would appear to be issue of DC imbalance...

Important -- In an earlier post I understood you to say that the scope occasionally functions properly during warm-up -- Is that correct???

 

Yes, now at the moment it is full screen!!; I measured PIN10-PIN12(direct DMM): 14V...... and now it is as the decibed error (2/5th's of the screen) and PIN 10-12=0V (both 500 time/div); every thing was OK for 2-3 minutes.

 

I juist discovered som black dust on both external connections of R104 (not present on other potmetrs); looks like coal deposition. Could this be the one?

 

I juist discovered som black dust on both external connections of R104 (not present on other potmetrs); looks like coal deposition. Could this be the one?

If R104 is faulty that could certainly cause problems with trace position! -- Does R98 look as if it's been overheating?

 

I juist discovered som black dust on both external connections of R104 (not present on other potmetrs); looks like coal deposition. Could this be the one?

turned the whiper: doesn'help either

 

If R104 is faulty that could certainly cause problems with trace position! -- Does R98 look as if it's been overheating?

The coal dust could suggest it; otherwise no signs of melting.

 

turned the whiper: doesn'help either

If there's soot on the pot you are advised to check it's resistance and wiper connection integrity...

 

The coal dust could suggest it; otherwise no signs of melting.

Please check R104 and R98 out of circuit -- make sure R104's wiper maintains proper 'connection' to each 'end' throughout its travel...