Hi Jim,
I would be 99% certain that changing the cap will fix it. I've seen identical symptoms and faults in a 7912AD. Browned off 10 ohm resistors and bead tants all over the thing.
?I actually watched one fail, I found two shorted ones, fired it up with the side covers off and it ran fine for about 15 minutes then I thought I noticed a wisp of smoke out of the corner of my eye by the time I saw which resistor was hot the screen blanked!
I've actually replaced them with 35V tants on the grounds that, as someone else who's running in 'bonus time', in another 50 years if they fail a second time my 121st birthday present to myself will be to pay someone else to change them!
Adrian
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On 8/29/2019 5:44 PM, James R. Bartlett wrote:
Not to worry Michael,
I know the feeling only too well, having used up my 3 score and ten years,
everything else is a bonus? Interesting it is shown in the 475A but not
the 475. Who knows what other errors
might be there, but the resistor IS there !!!! and SMOKED but obviously
not destroyed., glad to have it confirmed as 10 ohms. Might try
disconnecting C 1448 the 10uf Tant and see what happens
Don't like those things and modern electrolyic types are very smaall now
All good fun and a nice way to meet new friends
Thanks for your help.
Regards
Jim
On Thu, 29 Aug 2019 at 16:48, Michael W. Lynch via Groups.Io <mlynch003=
[email protected]> wrote:
Jim,
I guess I need to go back to reading comprehension school! I see that
now. FYI, I have a 475A manual and in that model schematic, that resistor
as R963, Indeed it is a 10Ohm 1/4W 1/2% Carbon Comp resistor. As Chuck
stated EXACTLY, this appears to be a Filter in conjunction with C963 on
+15V (DCPL2). At least this is what the 475A schematic indicates. I did
not find this in the 475 schematic. I will sit down quietly at the back of
the class, speak when spoken to and try to learn from the teachers!
Sincerely,
Michael Lynch 479-226-0126 Home Phone479-477-1115 Cell
Phonemlynch001@[email protected]@...
On Thursday, August 29, 2019, 10:22:59 AM CDT, James R. Bartlett <
james.r.bart@...> wrote:
Hi Michael,
Thanks for the reply.
My query was not about R956 but the one to the left of it and is situated
in the middle
between VR956 and R956, on my PCB component layout most components are
identified
but not this resistorThat is the one that measures 10 ohms.Seems to go back
to C1448 a 10uf Tant
Measuring across C1448 I also get 10 ohms.So need to find out where it
goes and its component number.
I am a patient man and wil keep trying.
Thanks again
Regards
Jim
On Thu, 29 Aug 2019 at 15:40, Michael W. Lynch via Groups.Io <mlynch003=
[email protected]> wrote:
Jim,
Gotta Love that magic smoke! I don't think that is a filter resistor as
described in Chuck's last reply, as I do not see a Tantalum cap adjacent
to
the resistor in that part of the circuit. R956 is the current limiting
resistor for the base of Q956 which appears to be part of the sweep
generator circuitry. In the schematic, R956 is specified as a 10K, 1/4W
5% resistor, NOT 10 ohms, so if you are seeing only 10 ohms, that is a
problem. What I would also suggest is to check Q956, plus CR952,CR955,
CR956 and VR956 and verify that these components are all good. Chuck's
advice about Tantalum Caps is spot on and you should check the entire
machine for blown or otherwise defective caps. My brother calls them
"devil caps", they are prone to failure at any moment.
Michael Lynch 479-226-0126 Home Phone479-477-1115 Cell
Phonemlynch001@[email protected]@...
On Thursday, August 29, 2019, 6:55:39 AM CDT, James R. Bartlett <
james.r.bart@...> wrote:
Hello to the group.
I find that I am in need of your assistance. Firsty repairing scopes is
not
my forte but I
want to do and learn.
I have a Tek 475, which was working.
However I went to use it the other day and
got the magic smoke. Problem seems to be related to the +15 supply/
Scope switches on,
The smoke came fro a resistor on the A9 board.
However it does not appear to be numbered on the PCB layout as shown on
the
downloaded
service manual.
It is located to the left and between VR956 and R956 it meters out at 10
ohms
Is this the correct value ? and what is its circuit number ?
I checked C1448 and that meters out also at 10 ohms.
+ 15 volt supply is 0 Volt
Any thoughts suggestions appreciated .
Many thanks
Jim
