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From: tekman on 4 Mar 2005 02:54
I have to agree with mike: The service manuals which can be downloaded
from the usual places (www.tektronix.com, www.teknetelectronics.com)
have NO detailed schematics. The go down to moduel level only. So, I'd
recommend you try to inspect the board and when you are nesr a D/A
converter (AFAIK they used devices from AnaloDevices, marked AD) then
try to find the cps nearby.

To Juerg: I know this is not a perfect way to diagnose. But what
options do you have? The instrument is out of long term support from
Tektronix. No schematic avail for the public.
I guess then you have to do it the hard way. When you are located in
Europe, I might be able to help via phone.


hth,
Andreas

From: Juerg on 5 Mar 2005 00:45
That's what I figured from searching the web...

Anyways, I took a closer look today and found the D/A converter (AD667)
for the bias voltages and a bunch (5) of analog 8:1 multiplexers (don't
remember the part number)... And yes, as Mike suspected, the frequency
of the sawtooth is the same as the refresh rate of the D/A. The
sawtooth is also visible on the output of a couple of the multiplexers
as well as on the input and output of a couple of the OpAmps (TL074C).
But I haven't found any holding caps yet, that also show the sawtooth
across them.

I'll keep hunting around. Maybe I just replace all the electrolytic
caps in that area...

Thanks for your help, guys
....juerg





mike wrote:
> Juerg wrote:
> > Hi Mike,
> >
> > Thanks for the tips, I'll check them out. You don't happen to have
> > schematics for the TD540, do you? That might help me figuring out
the
> > 640.
>
> TDS schematics are unpublished and likely not found anywhere.
> Anybody who had them would be sitting on a repair bonanza and
unlikely
> to give them up.
> mike
>
> >
> > ....juerg
> >
> > mike wrote:
> >
> >>Juerg wrote:
> >>
> >>>Hi,
> >>>
> >>>I could use some help fixing a TDS640 digital Tek scope.
> >>>
> >>>The baselines of all four channels show an offset'ed sawtooth
> >>>(different amplitudes and offsets for each channel) with a period
> >>
> > of
> >
> >>>4.5ms rather than a flat 0. When applying an input signal, it gets
> >>>overlaid over the sawtooth but otherwise looks fine.
> >>>
> >>>I was following Teks troubleshooting guide and located the problem
> >>
> > to
> >
> >>>the acquisition board. The input attenuator is believed to be OK
> >>
> > (input
> >
> >>>to the AD converters look fine). All the low voltages (+/-15V,
> >>
> > +/-5V)
> >
> >>>are OK and within spec (checked with another scope, no ripples).
> >>>
> >>>What I noticed is that over time when the scope warms up the
> >>
> > amplitude
> >
> >>>of the sawtooth decreases until it becomes zero, but there's still
> >>
> > an
> >
> >>>offset that won't go away. I was using some freeze spray to try to
> >>>isolate the problem further and ended up pin-pointing it to the
> >>
> > section
> >
> >>>around the AD converters. When I cold spray that area, the
sawtooth
> >>>comes back and disappears again after a while. There's a bunch of
> >>>OpAmps, resisors, caps and diodes in that area.
> >>>
> >>>Any ideas what to look at first?
> >>>
> >>>Thanks in advance for any suggestions
> >>>....juerg
> >>>
> >>
> >>All my experience is with the TDS540, so my comments may or may not
> >
> > be
> >
> >>relevant.
> >>All the bias voltages are set up by a single D/A converter that
gets
> >>multiplexed onto hold caps followed by op-amps.
> >>Leakage to any other part of the circuit or a defective op-amp will
> >>cause the voltage to decay between refreshes. The resultant
triangle
> >
> >
> >>will appear on the acquired signal.
> >>
> >>I'd compare the triangle on the waveform to the refresh rate of the
> >
> > D/A
> >
> >>hold circuits. Another simple thing to do is dig out your data
> >
> > sheets
> >
> >>and check the power supply voltages on all the op-amps and
> >
> > multiplexer
> >
> >>chips. I had several +15V supply pins to op-amps go open.
> >>
> >>ON the 540 series, a major cause of this leakage was failed
> >
> > electrolytic
> >
> >>caps that leaked caustic goop onto the board. It can be too little
> >
> > to
> >
> >>see and still cause problems. Another problem I've seen is
corrosion
> >
> >
> >>between op-amp and multiplexer pins facilitated by this
electrolyte.
> >>Sometimes it gets down tiny blind via holes and eats out the via.
> >
> > You
> >
> >>can't get to the other side to test it.
> >>
> >>Don't know if the 640 series is afflicted with leaky caps.
> >>
> >>mike
> >>
> >>--
> >>Return address is VALID but some sites block emails
> >>with links. Delete this sig when replying.
> >>.
> >>Wanted, PCMCIA SCSI Card for HP m820 CDRW.
> >>FS 500MHz Tek DSOscilloscope TDS540 Make Offer
> >>Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
> >>Bunch of stuff For Sale and Wanted at the link below.
> >>MAKE THE OBVIOUS CHANGES TO THE LINK
> >>ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/
> >
> >
>
>
>
> --
> Return address is VALID but some sites block emails
> with links. Delete this sig when replying.
> .
> Wanted, PCMCIA SCSI Card for HP m820 CDRW.
> FS 500MHz Tek DSOscilloscope TDS540 Make Offer
> Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
> Bunch of stuff For Sale and Wanted at the link below.
> MAKE THE OBVIOUS CHANGES TO THE LINK
> ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/

From: James Sweet on 6 Mar 2005 13:39


>
> If you really want to clean it,run it through your dishwasher(alone,no
> dishes!) using Calgonite,then dry thoroughly,repeat;thoroughly.
> We used to run 2236 DMM boards thru a DW to cure a Hi-Z leakage problem.
> Then 3 days in the drying oven.
>


I pretty regularly run circuit boards and even entire open-frame video
monitors through the dishwasher, people always think I'm crazy but it works
so well, the stuff comes out looking like new and is far easier to work on
not to mention it runs cooler when I'm done.


From: Michael A. Terrell on 9 Mar 2005 02:47
Jim Yanik wrote:
>
> If you really want to clean it,run it through your dishwasher(alone,no
> dishes!) using Calgonite,then dry thoroughly,repeat;thoroughly.
> We used to run 2236 DMM boards thru a DW to cure a Hi-Z leakage problem.
> Then 3 days in the drying oven.
>
> --
> Jim Yanik

Jim, have you seen the small powder coat oven at Harbor Freight? The
temperature range is too high, but it looks like a small environmental
chamber. For the $399 price it wouldn't be hard to change or modify the
controller to use for circuit boards, if needed.

< http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=46300
>


--
?

Michael A. Terrell
Central Florida
From: Jim Yanik on 9 Mar 2005 12:20
"Michael A. Terrell" <mike.terrell(a)earthlink.net> wrote in
news:422EAA10.B093F0F3(a)earthlink.net:

> Jim Yanik wrote:
>>
>> If you really want to clean it,run it through your dishwasher(alone,no
>> dishes!) using Calgonite,then dry thoroughly,repeat;thoroughly.
>> We used to run 2236 DMM boards thru a DW to cure a Hi-Z leakage problem.
>> Then 3 days in the drying oven.
>>
>> --
>> Jim Yanik
>
> Jim, have you seen the small powder coat oven at Harbor Freight? The
> temperature range is too high, but it looks like a small environmental
> chamber. For the $399 price it wouldn't be hard to change or modify the
> controller to use for circuit boards, if needed.

It is only large enough for boards or small instruments.But it would be a
nice oven for a small business.
>
>< http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=46300
>>
>
>

You could make a large plywood box,put a bank or two of 100W light bulbs in
it,and a couple of fans drawing air out of the box,for a lot less,IMO.
And size it large enough to fit a 7000 series scope mainframe.
Gets too hot inside,unscrew a light bulb or two.

Not fancy,but workable.

--
Jim Yanik
jyanik
at
kua.net
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