Re: Dead 7603


Jim Ford
 

Thanks, Harvey.

So I checked out the 8 V supply feeding the 5 V supply - it looks really bad! Similar waveform to the 5 V rail but dips from a high of maybe 14 V down to about 2 V. That's at the junction of the cathodes of rectifiers CR820 and C821, C821, R821, and the collectors of Q829 and Q835 on the Rectifier Board.

Then I checked at the anodes of CR820 and CR821; more convenient to look across C820. I saw about a 30 Vp-p near-sinewave there, with the anode of CR820 180 degrees out of phase with the anode of CR821. That seemed OK, but the output is just wrong!

I tried to measure C821 (18,000 uF) in-circuit with my cheapo Chinese transistor/resistor/diode/capacitor checker, but it couldn't get a reading. Ditto for C820 (0.1 uF, 100 V). I assume the transformer effectively presents a short and throws off the checker.

Also, R821, from the cap/diode cathodes/transistor collectors point to GND, reads 5.5k ohms, when it's supposed to be 4.7k +/-10%. I was surprised to see it read high; most times the sneak paths make resistors read low in-circuit. Not that 5.5k probably makes a difference.

Anyway, should I suspect the cap C821 or one of the diodes CR820 or CR821? Or something else altogether?

Thanks, everybody.

Jim

------ Original Message ------
From: "Harvey White" <madyn@...>
To: TekScopes@groups.io
Sent: 7/15/2018 6:16:20 PM
Subject: Re: [TekScopes] Dead 7603

On Sun, 15 Jul 2018 23:32:06 +0000, you wrote:

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.
suspicious

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.
DING DING DING!

Problem here. Possible that one of two things is at fault (maybe
both, but not likely). FIRST: suspect the bulk capacitor. SECOND:
suspect the bridge rectifier. (On a 5440, had the same thing, as well
as a toasted resistor in the 15 volt supply. filter capacitor (10000
uf, 12 vols) read extremely high esr (couldn't measure), and did not
tell me what the capacitance was.)

Replacing main capacitor, will mention what the result was, bridge
tested OK.

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.
Could be an aluminum electrolytic, but high ripple on a supply points
you at either the main filter capacitor *or* the rectifier. I'm
betting on the capacitor.



Any ideas on where to look? I figure the crap on the 5 V rail may be
messing up the others - make sense?
The ripple may be causing all sorts of problems. Regulators do expect
voltages at their inputs that never drop below the output voltage.
(most of the ones in TEK equipment being series regulators,
effectively).


Harvey


Thanks.

Jim

------ Original Message ------
From: "Al Holt" <@wd4ah>
To: TekScopes@groups.io
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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