Re: Hello from newcomer Fabio Trevisan - My first Tek Scope 464 + DM44

Fabio Trevisan
 

Hi Dave, Thanks a lot for your comments (so promptly!!)

---In TekScopes@..., <davidwhess@...> you answered :
> Someone who has used a DM44 can say for sure but I need to take a closer look at the schematics to
> figure it out. The problem may be the ALT mode switch issue you found; see below. >Normally the 464 delay time control would move both intensified marks and the DM44 delta delay time >control would move the second one only changing the distance between them but there is an optional >change to the DM44 which makes the marks move independently and it seems that was done to yours. >I was kind of surprised that there are no videos on Youtube showing what the DM44 can do. All of them >look like people just randomly pressing buttons. Indeed you're right about the two different configurations for the "Delta Time" control.
I got to know about it from the manual before opening it up the first time, so I tried it both ways to see if one of the configurations would eliminate or change the problem somehow, but don't...
Both problems were present in either mode of operation.
As for your comment of YouTube, I haven't really searched YouTube for that as I`m not very much into Youtube for technical tutorials.
It's not that I`m that kind of "grumpy" either who wouldn't take anything knew... I like very much the videos from EEVBlog. That guy knows how to present things in video!


You're also right about the ALT mode switch. I wasn't sure of its upper side,which actually has its "pushed" contact connected to +5V and therefore could be very well being used for something else in the scope, but one way or the other, having +5 on the pushed position is the opposite than what the DM44 requires, so I knew that the DM44 no matter what would need to be connected to the lower side of the switch (which, apart from the wire going to the DM44, seemed to be completely unused).
Then, after checking it thoroughly, I connected the wiper of the lower side to the +5V and voilá, I can see two intensified "marks" in all vertical modes. That part of the problem was fixed already when I wrote yesterday but thanks for your comments as they helped me validate my conclusions..


>The ALT mode switch in the 464/466 is the default position for the


>switch bank so without the DM44, it does not connect to anything; when


>none of the other switches are pressed, then it defaults in ALT mode.


>When the DM44 is installed, it looks like the +5 volt connection needs


>to be added but the DM44 schematics do not show it. The late 466


>schematics do not show the ALT mode switch at all.






>Maybe someone retrofitted the DM44 to your 464 and they missed this


>needed change.


Once again, you are "on the spot" about the retrofitting... Today I found the ultimate proof of that!
They didn't just miss that jumper to +5V on the ALT switch!
While looking for why the "Time" function would only work on 2's timebases, I discovered that the 3 resistors that should make up for the 1-2-5 voltage divider ARE MISSING! from the 464's Hor.Timebase board (R1142, R1143 and R1147).
And not just missing... There are clear signs that they were removed.
Now it's getting funny and it reminds me of the story about Jim Williams at his first job at MIT, when he would ask colleagues from other departments to "inflict" defects to their instruments so that he could fix it.
Now anything is possible because I know I`m working on a set that has been obviously tampered with.

> That was probably a loose mechanical connection.
I Agree! The CH2 leak problem into CH1 is gone anyway
> I would reseat all of the ICs and anything else in a socket. I do not
know if it uses them but the edge wipe TI sockets are not very
reliable. I shall reseat the ICs when I have some time... I already reseat most transistors. Once I`m done with the DM44 and know it works in all functions and ranges (when I know I won't have to replace anything else), I`m even thinking of soldering them all to the board...
> Yep, that is a drop in the CRT acceleration voltage.


> I can understand that. There are some things which can be checked


> before resorting to a measurement while the high voltage supply has


> failed.


> Take a close look at C1487 which decouples the unregulated +15 volt
supply for the high voltage inverter. If it is bad, then the inverter will
have trouble supplying enough powerThanks for that last tip. I will check C1487, and the unreg +15V supply for voltage and ripple.
I`m thinking of monitoring the current drawn by the inverter, as it heats-up , to see if there's any noticeable bump or dip when it reaches the "dropout". I have a DMM with RS232 logging that will fit this bill.


Thanks again for your comments... I`ll keep all posted.
Rgrds,
Fabio

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