On Feb 15, 1:08 am, mike wrote:
Smeghead wrote:
This is my first newsgroup post so maybe I could ask for some help
from the newsgroup. Please feel free to suggest if there is another
newsgroup full of Tektronix gurus.

Is there anyone who has experience of repairing a Tektronix TDS 460 or
same series.?
( 4 channel 350MHz DSO )
Mine shows an acquisition fail in the self test. It behaves OKish in
free run outside the waveform trigger region and one gets an
untriggered free running trace activity on screen.
However when the trigger level is in the correct active part of the
waveform no triggered capture takes place..ie the trig light goes
on..good and the waveform disappears..not so good.
Having tried to read up any leads from the web I suspect a capacitor
may have gone down in either the trigger circuit or on the a/d
acquisition board.
The snag is that these scopes were module repair only as far as I can
tell,
so the usual wonderful Tek service manuals which I now have, have no
PCB level schematics. I have yet to disassemble the beast, but some
pre-advice would be great if there are any stock faults that develop
with these scopes..like me I guess they are getting on a bit! My 475A
still keeps going well.

Any help here would be very gratefully received.
Thanks for looking Mike K

Many of the scopes of that vintage have one D/A converter.
It's multiplexed among about a dozen caps followed by high
impedance op-amps to generate all the analog settings from the front panel.

The electrolytic caps leak conductive electrolyte onto the board.
In early stages, the control voltages are just wrong. In advanced
cases, there's crusty corrosion all over the op-amps. But if left too
long, the stuff finds its way into a via hole and eats out the
plating. At that point you're screwed. Almost no chance of figuring
out where to put the jumper wire.

The fix for this problem is simple. Locate all the silver, cylindrical
surface mount electrolytics. Remove them. Clean the boards several times
with Simple Green, then Isopropyl alcohol. Put back new caps.
I didn't get the right caps. Just used a 0.1uF 1206 SMT cap for mechanical
stability and bridged a leaded electrolytic cap across that.
IF any of them are leaky, the rest aren't far behind. Change 'em ALL.
I think I counted almost 100 on the last one I fixed. Don't forget all
the boards and the front panel board.

It's also possible that you have only one electrically leaky cap.
Change 'em ALL anyway.

And you MUST clean the board, even if you can't see any electrolyte.
It's under the chips too.

YMMV
mike
--
Return address is VALID!

Brilliant, thanks for that. The prospect of having to deal with so
many small caps is a bit of a pain but I guess it needs doing.
The previous owner did say that the fault started as an intermittent
self test fail and I think it is very likely to be that the caps have
got to go. Not having worked on one of these boards I have a couple
of questions. Is the removal technique to use 2 soldering irons with
fine point tips to do the removal or what is a safe way?

Also I don't recognise what you are calling Simple Green? I wonder
what the equivalent is in the UK and I have been reading the very good
posts on PCB cleaning / different sorts of flux issues etc, I need to
do it right and I guess the cleaning stage is vital not to use the
wrong stuff and make matters worse. I wonder if the cap values are
critical...with 100 or so to replace it sounds a bit of a nightmare to
keep track of which ones are which value...made worse by not having
pcb level part lists or schematics.
I guess you do the replacement one at a time..or are the values and
voltage ratings likely to be all similar?

I am most impressed that you have seen and replied to my cry for help
so quickly as I guess this type of scope problem is hopefully not that
common...but I would not know that for sure. I have just been away
over most of the weekend and to see a reply in so quickly when I
thought the likelyhood of any reply being a) unlikely at all and b)
might happen at some unspecified time many weeks later.
Many thanks again...it means I really must find the time to get the
beast to bits and get the boards sorted. I also have a Tek 2235 and a
2215 both needing psu work ...maybe I should cut my teeth on those
first before getting stuck into the TDS.
Is there a good make / supplier that I should be using for the
replacement caps? Once the far eastern folks had captured the
capacitor market all our old makers such as ITT and Plessey could not
compete, so it is a bit sad that test gear can be put off the air
because the caps have a poor quality and MBTF. Usually I am quite
happy that old is still often very good. eg Marconi Instruments,
Racal, Wavetek etc kit seems to last well.
Many thanks again and I will hope to report good progress in a week or
3 .

Your words about the electrolyte getting under chips and onto vias
will stay with me for a scarey while :-(
If I get stuck can I email you direct to save a "war & peace" on the
net?
Best regards Mike