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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!


Mark Vincent
 

Ivan,

Welcome to the group. I too am a Tek. fan. I pulled my 485 out and set up to what you said you did. I had to adjust the intensity of A and B to get it to look right. The left portion of the trace is supposed to be bright. As the 10 turn pot Delay Time Position is turned from 0, the bright part of the trace moves. Increase the grid bias before the other controls in the previous sentence so B can be cutoff. It sounds like the bias is too low in your scope. You may have to adjust the max. Z (R1560), max. focus, focus gain or any combination of these. Adjust the other(s) as necessary after the grid bias is set to what you want. R1651 and R1669 22meg carbons are always way out of tolerance. The ones I have checked in various scopes are 30meg or higher. When a 1/2w 1% is put in, that can fix a problem that can appear to be coming from another part of the circuit. I used the Vishay VR37 series for this resistor. Check the voltage at pin 1 of U1710. It is supposed to be -,18V. I changed C1537 to a film type. Those small axial tantalums at 1mfd or lower have a higher ESR now. Films work well as replacements.

Q1544 will be leaky from C to E. They will check fine in the standard B to E and B to C. A beta tester will give a reading as though the transistor is bad, which it is. ALL 151-0367-00 that have round pins (the part that is above the socket will have a black oxide coating on the leads) are leaky while likely the square pin types can be leaky. The ones with a blue tint "face" where the part number is are rarely leaky. The leaky ones will check as a diode. Check ALL 151-0367-00 transistors and replace leaky ones. I used KSP10BU as replacements. The pinout is BEC instead of the common EBC. This 151-0367-00 number is in 7B92/A, 475/A, etc.. After replacing the leaky transistors, the pieces worked right. The leakage did cause problems at times (when a function was on that this transistor was in the circuit for that function) to all the time. I hope my suggestions help.

I have P11 (Opt. 78) in most of my scopes. I have no problem with my eyes focusing on blue to violet. I also do not get eye strain. I am the only one I have encountered yet that does not have these two problems with high frequency light. Some like the blue (P11) trace for a brief time and prefer P1 , P31, etc. for longer viewing due to their eyes not being able to focus and the eye strain. Nothing negative to those that prefer other colours. I also prefer the finer trace and fast decay of blue.

Mark