Wierd Tek485 intensity problem


10papak01@...
 

Hi there,
I'm new here in this group, but I'm Tek user (and have to say hooked on their scopes) for quite a while now. Few days ago I acquired 485 in a pretty good mechanical shape, but only partially working. I've wanted 485 since it is quite fast (350MHz @ 50ohm Zin) and also was intrigued by this particular specimen since it came with P11 phosphorus CRT, and it is quite rare and pretty.
Oscilloscope had few problems. Broken cooling fan, missing CRT protective glass, missing a trace when time base is set to 50ms/div or slower, delayed sweep not working, badly uncalibrated vertical amplifier and one a little strange problem I found out about only after fixing it.
So, I glued fan and made a little metal clamp for it, made a new protective glass out of a blue plexy, replaced leaky CR1534 in beam current limiter amplifier, replaced bad Q1074 in B trigger circuit and calibrated whole vertical amplifier (it was a lot of work since someone played with every single trimmer, and to tell the truth there are a lot of 'em). After this,oscilloscope worked nicely, but I found yet another, and a little strange problem I could not fix, so I decided to write a little post here to see if any of You have some useful ideas...
When I turn on my 485 and set it to delayed sweep mode, intensified A or alternating, there are two problems with intensity. Firstly intensified part of A sweep is poorly intensified even with B intensity all the way up (if I do this i ALT mode, B trace is shining like crazy, so B intensity is working as expected), and also if A intensity is just a little turned up (so that A trace is not yet visible) B trace intensity can not be set to zero even with B intensity control set all the way down. So to make a long story short, if I set my 485 to delayed sweep ALT mode (for example) to make intensified portion of A sweep I nave to turn B intensity almost all the way up, and that makes B trace extremely bright.
I poke a bit around U1560, found its datasheet on the internet so I analyzed its inner workings and then measured resistor network connected to pins 16,1 and 2 (common emitters of 'A', 'B' and 'A-intensified' differential amplifiers) and all the values are good, so I have reached a dead end. I even tried replacing U1560 with the one from a well-working 485, and there was no change. Also tried putting U1560 form 'bad' 485 to working one, and it worked just fine, so U1560 is good, resistor network is good, and I have no other ideas how to fix the 'A intensified' problem I have!
Thank You for any ideas or help, and all the best from Ivan!

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