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Hi all,

My trusty 7603 developed a fault yesterday while I was using it, and I
fear that a Tek special IC has expired.

Until yesterday morning it was working perfectly. I looked away to do
something else for a couple of minutes, and when I looked back the
display had shrunk horizontally. It now looks like this:



Oddly the readout seems to be compressed into the left side, but the
main trace is compressed into the middle. I've measured a few things
around the horizontal amplifier board. The power supplies are all OK and
ripple-free, and the resistor values seem plausible based on a few
in-circuit checks.

Switching off the readout makes no difference (other than the readout
disappearing). Pressing 'beam find' has the expected effect, and it's
not stuck on.

I had a look at the waveforms going in to and out of the horizontal
channel switch U510 (156-022) and, while the input (pins 2/15) looks right:



the output (pins 12/13) looks far from right:



Apologies for the lack of vertical scale on the photos: the output is at
400mV/div and the input was 100mV/div, I think.

The 'display limit command' line, which seems to bias the channel
switch, is firmly at -15V as the book says it should be. I tried
reseating U510 but that had no effect.

Has anyone seen a fault like this before? Should I bite the bullet and
find another 156-022?

Thanks
Chris

I believe that U510 is P/N 155-022 rather than the 156-022 that you have
quoted. At any rate did you notice that both the A3 Trigger Selector and
A4 Vertical Interface boards, U324 and U214, have the same device? It
would be simple to interchange with U510 and see what happens.
I did see this problem about 20 years ago but I didn't notice at the
time that there were other same devices in the mainframe. My problem was
intermittent and I also trouble shot down to U510. I ordered a new
device and installed it and nothing changed. I tried the usual
troubleshooting techniques but nothing worked, (freeze heat push and so
on). Since the boss was getting ticked off about the time I was spending
troubleshooting it, I ordered a new Horizontal Amp Board and that fixed
the problem. Of course that was in the days when Tek was interested in
supporting even their obsolete equipment. I still have the 7603 at home
now, along with some parts and that board and device, long sad story
there about the company folding.

Bill



---In TekScopes@..., <chris@...> wrote :

Hi all,

My trusty 7603 developed a fault yesterday while I was using it, and I
fear that a Tek special IC has expired.

Until yesterday morning it was working perfectly. I looked away to do
something else for a couple of minutes, and when I looked back the
display had shrunk horizontally. It now looks like this:



Oddly the readout seems to be compressed into the left side, but the
main trace is compressed into the middle. I've measured a few things
around the horizontal amplifier board. The power supplies are all OK and
ripple-free, and the resistor values seem plausible based on a few
in-circuit checks.

Switching off the readout makes no difference (other than the readout
disappearing). Pressing 'beam find' has the expected effect, and it's
not stuck on.

I had a look at the waveforms going in to and out of the horizontal
channel switch U510 (156-022) and, while the input (pins 2/15) looks right:




the output (pins 12/13) looks far from right:




Apologies for the lack of vertical scale on the photos: the output is at
400mV/div and the input was 100mV/div, I think.

The 'display limit command' line, which seems to bias the channel
switch, is firmly at -15V as the book says it should be. I tried
reseating U510 but that had no effect.

Has anyone seen a fault like this before? Should I bite the bullet and
find another 156-022?

Thanks
Chris

I believe that U510 is P/N 155-022 rather than the 156-022 that you have quoted. At any rate did you notice that both the A3 Trigger Selector and A4 Vertical Interface boards, U324 and U214, have the same device? It would be simple to interchange with U510 and see what happens.
I did see this problem about 20 years ago but I didn't notice at the time that there were other same devices in the mainframe. My problem was intermittent and I also trouble shot down to U510. I ordered a new device and installed it and nothing changed. I tried the usual troubleshooting techniques but nothing worked, (freeze heat push and so on). Since the boss was getting ticked off about the time I was spending troubleshooting it, I ordered a new Horizontal Amp Board and that fixed the problem. Of course that was in the days when Tek was interested in supporting even their obsolete equipment. I still have the 7603 at home now, along with some parts and that board and device, long sad story there about the company folding.

Bill

Bill,

Thank you so much - I hadn't realised that there were other parts the
same in the mainframe.

I swapped U324 with U510 and the fault is fixed! I tried swapping them
back and forth a bit to eliminate bad contacts, bent pins and so on, and
it's definitely one of the chips that's faulty.

Since U324 is the trigger selector, the faulty chip actually works
tolerably well in that position: its output may be a bit limited and
non-linear, but for triggering that's OK. I wouldn't notice the
difference as a user immediately but I suspect the range of the trigger
level control on the timebase will behave a bit oddly. I will still look
out for a replacement IC.

And you're right, the chip does say 155-0022 on it. The diagram in the
manual says 156-022 for U510 and nothing for U324, so I just got confused.

Thank you once again for your help. I didn't expect tracking the fault
down to be so easy!

Chris

Chris:

I still have the 155-0022-00 in the original package that I ordered from
TEK 20 years ago.
If you live in the US or Canada I can put it into a padded envelope and
send it to you. It sure isn't doing me any good right now and I have two
parts mules with at least 6 of these in those units.
Free, gratis and in the hope someone else can help me out someday. " Pay
it forward". It can't cost more than $1 for the postage. Free, how can
you beat a deal like that?
Bill