Re: Tektronix 7603 won't trigger

On Mon, 22 Jun 2009 05:08:14 -0700 (PDT), GanjaTron
<ganjatron@xxxxxxx> wrote:

Thanks for all the replies! I did check for ripple with a working
scope, and found a few lines with obvious sine waves (like 4Vpp), but
I understand the PS also has unregulated outputs, so I didn't
investigate any further. The cap voltages themselves checked OK with a
voltmeter.

If the caps are bad, the voltage will be temporarily within limits,
but the ripple voltage will be rather high. That causes blurred
traces that can't be adjusted out with the focus and astigmatism
controls. Use another scope to verify that you're getting DC on the
PS voltages.

Jeff, when you say the caps "blow", you mean they literally
explode? :^O

No. They got hot and eventually blew a series resistor or fuse. I
forgot which. I got lucky and caught it before it destroyed any
active components. The bakelite bottom of the can capacitors were
obviously bulging and had turned from brown to black. My capacitance
and ESR capacitor tester both showed that they were under value and
with a very high internal series resistance.

I won't mention what I used to replace the caps, or how the
replacements were installed. I'm not very proud of the work.

The caps on the interface board would be the next suspects, if they
weren't so tough to get at (on top of which I'd have to lift them out
at one end to check).

If it were easy, it would be no fun. If you don't want to do it
twice, just find the replacement caps and install them instead of
testing the originals. This way you're sure that you won't need to
dive in again later.

This particular 7603 has the fan mod, btw. The TO-3s show no signs of
cracks, and I didn't find the gold dot connectors you mentioned. I
guess this is a revision. Mainframe serial# is B377720 with option 03,
which according to the service manual means EMI shielding, though I
can't find the bits mentioned there. Oh, and it has the readout. Trace
is mushy and the scope stinks. ;^)

The stink is a clue. It's probably boiled electrolyte.

Regards,

--Roland
--
Jeff Liebermann jeffl@xxxxxxxxxx
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
.

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