Greetings,
I did fix my Test 04 Fail 44 problem with indispensable help from
George Kerber who had the same problem, he fixed his, then sent me
images and voltage references. I get an email about this problem at
least once a month, so I figured I'd post my results here so people
can find the information faster.
*** WARNING ***
Before I go any further, ALWAYS be sure to have a fan blowing on the
main hybrid board when the case is off and the scope is powered up!
You can easily destroy your scope if you don't! The case forces
airflow over the hybrids which does not happen with the case is off.
I know most of you know this, but I figure saying it one more time
can't hurt... :-)
THE SHORT (QUICK) ANSWER:
If you want the short answer, you need to look at the A5 Control Board
DAC and surrounding resistor network. My problem was leaky SMT
electrolytics and one open resistor in the DAC's reference voltage
divider. George Kerber had two open resistors and he replaced his
DAC, but found out later that his DAC was fine.
THE LONG ANSWER:
My first problem was getting the right schematics, which I actually
did not obtain until after I fixed the scope. George Kerber emailed
me about the problem and furnished me with partial images of the
correct schematics for the area of the A5 Control board I needed to
troubleshoot. The 2465 I have has a surface mount version of the A5
Control board, not the DIP version, but unfortunately the schematics
in the 2465 manual are for the DIP version only... Go figure. The
correct schematic for the SMT A5 Control board is not even in any
version/revision of the 2465 manuals! I can't even begin to tell you
how frustrating this was and how long it took me to find someone who
knew this little bit of information. You have to get the 2445B/2455B
service manual #070-6862-00. I found a source on ebay of a company in
Greece that makes very high quality physical manual reproductions, as
well as CD's with the manuals in PDF format. It was well worth it,
the quality is excellent, and I highly recommend you get a copy if you
are fixing you own scope. Here is a link to their web site:
Initial troubleshooting, via the flowcharts in the manual, indicated
my problem was either the DAC on the A5 Control board, or the
downstream hybrid (can't remember which one exactly, Vert Amp I
think.) I had never done SMT soldering before so I was hoping it was
not the DAC, but then again I didn't want to pay for a replacement
hybrid either. My solution came in the form of an email from George
Kerber who had a similar problem. He had replaced his DAC and two
resistors in the DAC bias voltage divider, and he sent me correct
voltage readings for the whole bias circuit (as well as a partial
schematic for that part of the circuit.) What I found was one open
resistor, and when I jumped it temporarily with a 1/4 watt resistor of
the correct value, the whole scope started working! So that was it,
one open resistor in the DAC bias circuit and 4 leaky caps!
Order of Events:
1. Initial symptoms: Display jittering, squashing down to about half
size, and one day the display quit all together.
2. Upon initial inspection I found leaky surface mount electrolytics
(4 of them) on the SMT A5 Control board. People on the forum said
there was a span of time where "crap caps" hand been made. Everyone
seemed to say "yeah, those bad caps again..."
3. I finally figured out that the front panel was telling me something
with the lights during start up. Once I had the service manual (2465
with wrong A5 schematic), I could follow the troubleshooting charts to
reveal several possible problems.
4. While following the troubleshooting charts, I adjusted the Grid
Bias (R1878 I think) *just slightly* and my display came back! It was
over-driven of course, but now at least I could see the error message
someplace other than the front panel indicators.
5. This is where the repair sat idle for several months. I could not
find the proper schematics for the SMT board, so I could not determine
part placement or make any kind of measurements since my board did not
match the schematics. I had ordered a new DAC and caps from DigiKey,
but they were just sitting there as I was too nervous about doing the
SMT work. Also, I did not know for sure that the DAC was actually
bad, and I did not want to replace it if it was not faulty.
6. I received the email from George Kerber. This is what literally
saved my scope! He sent me voltage readings for the entire DAC
reference voltage divider network, as well as an image of the
schematic for the DAC circuit on the SMT A5 Control board. Now I
could troubleshoot!
7. I started taking voltage measurements and found an open resistor!
I jumped the open SMT resistor with a normal size standard 1/4 watt
resistor of the correct value and the scope started working! Moment
of revelation! The resistor for me was R2013, 10.0k. It was open.
There is no way it could be that simple! I don't have that kind of luck.
8. I ordered every value of resistor in the DAC bias network since the
acid from the SMT caps had leaked on to other components and could
have easily caused the failure I had. Two of the caps sit right above
the DAC in the normal orientation of the scope. Cleanup was a pain in
the !@#.
9. During the downtime on the repair, before I received George
Kerber's email, I did manage to read up on SMT soldering and common
rework procedures. Everything was making me nervous until I found a
web site that changed everything for me! See my notes below about SMT
rework.
10. All parts received, it was time to fix this puppy. First I
removed the leaky caps and open resistor. Then I cleaned the heck out
of the board as best I could, being careful not to lift any pads or
traces.
11. After replacing the caps and resistor, I did the DAC cal procedure
according to the manual.
12. Because I had tweaked the Grid Bias, I went ahead and did most of
the display calibration, up till the point where a sig-gen is needed
(I don't have one.)
13. Buttoned it up and tested it.
Here are the notes and voltage references sent to me by George Kerber:
***
Here are the voltage measurements (reference to ground) at the DAC pins:
DAC Pin 14 = 0.151V
DAC Pin 15 = 0.147V
More voltages:
Voltage drop across R2013 (10K) = 8.82V
Voltage drop across R2016 (10K) = 1.02V
Voltage drop across R2011 (4.75K) = 0.639V
Voltage drop across R2014 (221K) = 7.97V
Voltage drop across R2015 (82.5K) = 8.18V
Also, the sum of the complementary DAC currents (+I + -I) through
R2520 (681 ohm) & R2521 (681 ohm) produces a fixed voltage of +1.36V
at the positive terminal of C2420. This voltage is constant and
independent of DAC digital code. If the DAC reference is not correct,
this voltage will not be +1.36V.
***
SMT Rework Notes:
If you work with SMT, I strongly recommend ChipQik or something
similar. It's a way to removed solder at low temperature by mixing
their product with the solder to be removed. Basically it lowers the
melting point of the solder to be way below normal, so you can work at
a lower temperature to removed just the parts you want, without
affecting the surrounding components. It made it possible for me to
fix the SMT board. Check out this site:
I suggest you read all the info, the technical papers, and the
removal/solder processes. Here is the removal process link since it's
kind of hard to find on their site:
The air bath was the major part of the process that really made the
difference. You remove your parts without a soldering iron! When you
are done, the removal is so clean you can't believe it! It also makes
soldering in the new parts very easy, and helps prevent stress on the
parts and board. Their are other benefits as well, so check out the
site and read their white papers.
I can't afford any of their products right now (I wish I could.) I
can, however, follow their procedures. I made my own circuit board
stand from 13" joist hangers and a few other parts I found at Lowes.
I used my Makita heat gun for the air bath. To set the temperature I
turned on my heat gun and noted the setting (it has a dial with
numbers 1 to 5... not very handy when you need a temperature.) Then I
used an oven thermometer to find the height above the heat gun where
the temp was 150C (302F.) This worked very well for me. I had bought
ChipQik from DigiKey before I knew about Zeph's products, but either
way the stuff is kind of expensive, but well worth it!!
Anyway, that's pretty much it, I hope it helps. Let me know if you
have any other questions.
Matthew
