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

Fabio Trevisan
 

Hello folks at Tekscopes,
First of all, apologies for my first message being so long, but I`m excited!
I finally own my first Tektronix!
I just got a Tek 464 with DM44 option!!! S.N. B132839
It's looking really great for a 40 years fellow and, what's better, it was
cheap if you consider that I bought it in Brazil, from a Brazilian and
surprisingly.... It's EVEN WORKING!!!.
Given its price of BRL 465,00 (roughly USD 140,00, to be paid in 12 months,
shipping included).
You might be surprised by my remark, but it comes from the fact that in
Brazil these things are very expensive (even when used, even when in crappy
condition) so, it's very rare to find anything from Tek for this price and
still working.

I plan to restore it to mint condition as much as I possibly can and use it
for my hobbyist projects.

For now, I`ll just share the photo (at the end) that I took after I just
unpack it (last week) .

Since then, I already fixed 3 problems in it and there are basically 2
major problems left:

1. A problem in DM44. It would only show the two Intensified marks on
vertical mode "ALT"... in CH1 mode it would only show the "Delay Time
Position" mark and in CH2 mode it would only show the "Delta time" mark...
but never both of them on the same trace.
I found it to be a missing connection to +5V to the ALT mode switch (it had
the wire going to the DM44 on pin 1, but no +5V in the wiper). I`m still in
doubt if it's missing since it came from factory and nobody noticed it or
missed the functionality of measuring time with only one trace (whether
CH1, CH2, or ADD).

2. A problem is the vertical preamp. CH1 would show superimposed to its own
signal, about 25% of whatever signal that would be displaying at channel 2,
including CH2 position,... Besides... there was a sort of flicker
Problem got fixed by itself after I had to move the vert. preamp board from
my way, to be able to access the Vert. Mode switch board. When I put the
Vert Preamp board back in place, the problem was gone...

3. Calibrator was showing a trapezoidal waveform... More or less like if
the probe would be under compensated, but not caused by the probe...
Problem was a loose ground connection from the 10R resistor that sets the
30mA current. The resistor goes to ground not on the interface board, but
at the back of the chassis, by means of a bolt and interlocked nuts. The
nuts where tightly interlocked, but the bolt/nuts set were not pressing the
chassis and ground wire good enough.

*Problems remaining (that I appreciate any suggestion from you guys)*:

4. DM44 is acting unpredictably... sometimes it freezes up or just show
random numbers... sometimes it works fine for Volts and Ohms... but even
then, when switched to mode "Time", it would only show valid readings in
ranges multiple of 2 (0.2s, 20ms, 2ms, 0.2ms, 20us, 2us and 0.2us). Even
though, the value displayed is 5 times bigger than it should be for that
scale (a 1 division measurement in the 0.2ms range reads, that shoould read
0.2ms, reads actually 1ms on the display).
I`m suspicious of false contacts from all too many ICs that are socketed,
and possibly some flat cable incorrectly connected between the horizontal
timing board and the DM44. I`m still going to look at it further this week
and will let you know.

5. Last but not least, and most worrisome to me... The CRT beam works just
fine for about 30~40 minutes and after that it fades abruptly. It's still
visible for a couple of minutes more with the intensity at MAX and there's
a great increase of the deflection factor in both directions (Horizontal
and Vertical deflections increase by about 30%).
I know already, from these 2 symptoms that the acceleration voltage must be
going down for some reason, after H.V. step up power supply heats up, but
I'm still not sure the actual culprit.
I already did some cleaning work in the whole H.V. area, to eliminate
eventual leakages caused by the black dust deposits and moisture that could
be causing the HV to overheat and give up... but this "care" didn't help
whatsoever.

At this point, of course that I`m suspicious that the H.V transformer is
probably guilty... (I hope not but I know the odds are against me! Sigh!).
I haven't done much troubleshooting so far.... Only thing I was able to do
is to check the Cathode voltage, and it's rock steady at -1476V for several
minutes, from the moment I turn it on up to about 20 minutes of power.
I didn't measure it yet after it's faded because I don't want to leave it
on for too long after that, to avoid frying anything (I`m not sure it would
fry anyway, but better be sure than sorry).

I`m looking forward to sharing with you the results of this restoration,
the problems found and the fixes that will be applied in the process.




Best regards,

Fabio Trevisan

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