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

 

On 13 Sep 2016 08:46:02 -0700, you wrote:

Sorry Dave, My reply to you ended up with the formatting completely screwed up.
I`ll try to make it better the next time.
Rgrds, Fabio
It was no problem; many replies on the list get mangled for whatever
reason.

On 13 Sep 2016 08:42:37 -0700, you wrote:

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

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!
I had a general idea about how the DM44 delta delayed sweep function
should work but I found the description in the manual was ambiguous so
I wanted to find an example on video. I suspect most users who pick
up a DM44 have no idea what delta delayed sweep is.

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).
I was surprised that the ALT switch is not used except as a
placeholder in these oscilloscopes. The late 466 manual does not show
it at all and the 464 and early 466 manuals show it but with no
connections.

The DM44 is suppose to be a factory installed option so it does not
quite surprise me that the DM44 service manual does not cover the
needed modification to the ALT switch.

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 DM44 has a toggle set/reset flip-flop which determines if the
delayed or delta delayed sweep control is used but it was not
immediately clear to me from only the DM44 schematic under which
conditions it toggles. I would have to study the oscilloscope
schematics as well to figure it out.

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.
I remember that story about Jim Williams. They also had differential
diagnostic competitions where the objective was to figure out the
problem with a minimum number of tests.

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...
I do not think soldering the ICs or other socketted parts in is
warranted. If you remove the sockets, they can be replaced with
better sockets. Machined collet pin sockets are very reliable and if
you want to spend a little extra time on it, the collect pins can be
cut out of the replacement sockets and used individually:

<https://s14.postimg.io/3og36ice9/DC505_Lowest_Profile_Collet_Socket_2a.jpg>
<https://s14.postimg.io/l0gfry5vl/DC505_723_Socket.jpg>

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.
Some people put a 1 ohm resistor in series with the fuse F1487 to
monitor the collector current of the output transistor.

If you want to monitor the error amplifier, I think the voltage at the
collector of Q1476 is the best spot.

Another failure mode has occurred to me; Q1486 could be failing as it
heats up which is something I have seen before in transistors.

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