OK, here's where I am now: Power light comes on, and all LV DC voltages
are within tolerance except +5 V, which reads ~+4.4 V. Ripple is out of
spec on all except for +130 V. +5 V rail looks like a high duty ratio
dipping from 5 V down to about 2 V periodically, synchronous with the AC
mains. I figured a tantalum capacitor on the 5 V rail was a nominal
short, but I don't see any tantalums anywhere in this scope, a late
model 7603, Serial Number B345179. Just ceramic discs and aluminum
electrolytics.
Any ideas on where to look? I figure the crap on the 5 V rail may be
messing up the others - make sense?
Thanks.
Jim
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------ Original Message ------
From: "Al Holt" <grovekid2@...>
To:
[email protected]Sent: 6/25/2018 4:21:40 AM
Subject: Re: [TekScopes] Dead 7603
You've got a dead 7603 with no lights. Definitely look at the low
voltages first. Check the 0.15A (150mA) fuse, F855, just because its
'filament' is so delicate it can easily become disconnected from its
end caps from heat and age. In my case this wasn't the 'magic bullet'
to fixing my 7603, but losing the 130V line kills the HV system.
Getting the front panel power indicator to light would be the next
challenge. The LV regulators are interconnected in a way that will
prevent this lamp from lighting if one of them is bad. I can't remember
how it all hangs together now, but there are a number of posts here
that spell it out.
Sorry, about being too vague, but I'm writing this from memory. I think
the LV rectifier board is going to be the key to this, and it's worth
the time to pull it, give it a visual and check the condition of the
electrolytic caps. And be sure to mark the ribbon cables as you
disassemble things!
Good luck!
--Al
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