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Sorry to respond to an old thread, but I do have a pdf of "Typical Oscilloscope Circuitry" you can download here:



I've tried writing to a couple people on the wiki to get it uploaded but haven't received a response from anyone.

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

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

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

Hi Chuck , I guess you came in on the middle of this , I suggest you read my old posts before you comment, one of my early post is the very first thing I did was recap the two variable supplies they are nearly identical, the slave side has an extra trimmer, no the relays are not motorboating it is dependent on where the voltage pot is , please do not jump to conclusions, I know you better than this!
The fixed 5 volt was ok so did not recap it. The 5 volt regulator uses A LM 723as a reference, a little different than the 2 variable sides , Yes it would be nice if one side was working , but no they were both out from the beginning.
Another thing to consider is if this supply is so simple as you say why are there tons of bad ones on ebay, a guy like you could clean up and make a fortune fixing them and putting them back in service?
That is why I asked the group , how Reliable they were. Larry has 2 working same revision as mine and said they were pretty reliable, I'll accept this answer , until I know better. Actually the 470uf electrolytics ESR wasn't that bad , but since I'm working one handed and themain board was so difficult to get out, I changed them all, using New Nichicon , that was nearly a month ago, and the first thing I did!
I found the esr on the smaller electrolytics to be worse than the larger value ones, the 4- 4.7 uf 50 volt were the worst @ 1ohm and the 47uf were bad too. The rest seemed to be acceptable but were changed for reliability sake.
At this point I'm up to the references on both sides including the 7815's everything is is good on the +and - 15volts aU 101,Q101,and Q102,from here on is where Ineed help understanding whats going on.
John was kind enough to help, when You, Chuck chimed in again, So I am waiting on John to reply back and see if he is willing to walk me through the rest of the way, I am also studying the Theroy of Operation in the manual on Tekwiki even though it's a different revision, I believe all revision's share the same circuitry ,the only difference is the number of circuit boards, earlier ones had a seperate fuse board and all the jumpers and connectors are labeled differently and the looks and heatsink.
If John is willing to accept my terms of working at my own pace and time, We can proceed when I am able to in about a week or so, Chuck please Let John lead the Thread and if you have something to add, than go from there.
I have to pick up my stepson tonight , and we have plans until after the labor day weekend my wife Esther is off all next week so I probably won't be able to do much until after that John if your willing you can talk me through the rest of the way now, if not maybe Chuck can take over if he is willing and I will try to study up on the Theroy and will report back , when able.
Many Thanks guys!
Dave

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

Hi, yes have also used the hot water method.
Incidentally its best to test this on a failed transformer of the same time first to get the technique right.I also have used a very small amount IPA to release stubborn windings, this shouldn't damage them butif you use anything else be aware that winding damage is likely.If you're worried about the risks simply do what I did and use the water to make Plaster of Paris.

-Andre
#include "802701.h"

Thanks Chuck!

I put transformers in boiling water some minutes, with the same results

Regards

Miguel

Hi Bruce, thanks for the input!
If I sent you a photo of the schematic off list, could you make an educated guess (or perhaps see something I've missed?). I've read schematics since my Radio Shack 101 kit, but power control, especially 3PH, is all new to me. Vintage makes it even more interesting.
Regards,
-Dave
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