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Re: Dead 7603

 

Yep, a 3300 uF cap jumpered across C821 smoothed out the voltage fairly
nicely. The 18,000 uF C821 is toast! I have a feeling removing it is
going to be a two-person job.

Next order of business: there are dozens of 18,000 uF caps with voltage
ratings between 16 V and 100 V listed at Digi-Key and Mouser. Should I
just go with Nichicon or United Chemicon and look for the longest life
one at 105 degrees C without breaking the bank? After all, I did see
one for >$100! Most seem to be $4-6 apiece. Size does not seem to be a
problem, as any modern cap is much smaller than the monster that is in
there; it's about the size of 2 D-cell batteries end to end.

BTW, I did try a 7A22, a 7A26, and a 7B85 in the plug-in slots last
night. Readout was a bit grainy, and it was impossible to get a stable
trigger on the calibration squarewave except at the highest amplitude, a
few volts. Just to be sure, I moved those plug-ins to my trusty 7904,
and triggering was possible down to very low amplitudes. Also, there
was a vertical line at the center bottom of the screen that didn't go
away, AFAIK under any circumstances; I guess we'll cross that bridge
when we come to it. It doesn't seem like replacing C821 will fix that.

Thanks for your help.

Jim

------ Original Message ------
From: "Dave Hills" <dadhills@...>
To: [email protected]
Sent: 7/21/2018 9:44:06 AM
Subject: Re: [TekScopes] Dead 7603


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.
If either of the rectifiers were shorted, you would see the waveform go
negative across C821. The minimum value of 2v indicates they are OK.


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.
This is OK.


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.
This is what I would expect. Don't expect in-circuit measurements to
be meaningful with respect to a single component in a network. The
measuring instrument can't sort out a single component, as it see's
everything connected to it and makes a futile attempt to make sense of
the measurement.


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.
Either due to residual charge on C821, or R821 has drifted high, which
is not unusual if it is an aged carbon composition resistor.


Anyway, should I suspect the cap C821 or one of the diodes CR820 or
CR821? Or something else altogether?
All the info you have provided points to an open C821. Easy to confirm
by tacking an additional electrolytic cap across it and looking at the
waveform again. Even a small value, say, 100uF should make a
measurable difference. If the lowest level, 2v, increases, then you
have confirmed it is likely bad.

Dave



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