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 Post subject: Fixing a Tektronix TDS-420 Attenuator
PostPosted: Feb Fri 22, 2013 9:49 pm 
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Posts: 2348
Location: Seattle, WA, USA
All -
A couple months ago I rebuilt a couple of TDS-420 oscilloscopes for the lab
at work. Mostly the task just consisted of replacing all the SMD electrolytic
capacitors and repairing all the traces that were etched through by the leaked
electrolyte. However, one of them was rather more 'interesting' in that it had a
blown attenuator. At first I assumed that this could not be fixed, after all, on
such a 'modern' scope the attenuators are all made using thick-film hybrid
technology. However, I decided to try anyway and was successful so I thought I
would share my methods here so that it might help someone else.

The first step was to remove the ceramic attenuator hybrid from the
attenuator PC board. It has 35 through-hole connections so this in itself was a
bit of a chore. However, using my Hakko 472 vacuum desoldering station I was
able to unsolder them all without damage to the PC board or attenuator
substrate. Once I got it out, the damage was obvious - a big burn mark on top of
one of the thick-film resistors.

At this point, I used acetone and a swab to remove the protective insulating
coating from the damaged resistor. This nicely exposed the damaged resistor and
the traces that connect to it. See the picture at the link below.

Image

The next step was to determine what the resistor value was originally. This
was a bit of a challenge because no schematics were published for the TDS420 and
I didn't want to have to unsolder one of the other attenuators just to measure
it. The solution was to figure out what points on the top of the attenuator
substrate corresponded to the resistor on the bottom which I was able to do by
looking carefully at the damaged attenuator. Once I knew where these points
were, I just measured them on one of the working attenuators and determined that
the resistor was originally 50-ohms.

Knowing the resistor value, it was then an easy task to solder in an 0603
SMD part to replace it. The resulting repair is shown in the picture at the link
below.

Image

After that I just soldered the attenuator back onto the board and put it
back into the scope. Sure enough the problem was gone and as far as I could
measure the repair did not adversely affect the calibration or bandwidth of the
repaired channel at all.

Hope this helps someone!

-Matthew

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 Post subject: Re: Fixing a Tektronix TDS-420 Attenuator
PostPosted: Feb Fri 22, 2013 11:03 pm 
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Awe-inspiring work, Matthew!

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 Post subject: Re: Fixing a Tektronix TDS-420 Attenuator
PostPosted: Sep Mon 29, 2014 2:03 pm 
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Joined: Sep Mon 29, 2014 1:42 pm
Posts: 2
I know this is an old post but I still want to say thank you. I first tackled the electrolytic capacitor issue thanks to your post. When I first fired the scope up after repair, I had no display and a .b code on the 7 segment on the cpu board. Following the troubleshooting tree in the manual, I found that I had inadvertently flipped the two switches at the rear of the cpu board. After returning those switches to the correct position, I had a clean self test w/o errors! Even though these scopes were early in the digital age of test equipment they were well engineered.

Most electronics from that era had the capacitor problem and most cpu boards from pc's failed in a year or less. Intel had a massive replacement problem and in our business we were shipping back 1-10 boards every day. Some boards had caps that were "pooched" (swollen) before we installed them in a system.

Now that I have the scope passing the self test I am inspired to tackle the attenuators with bad 50 ohm resistors. True to the nature of Ebay, I bought an "untested" attenuator board for parts and found it had three channels with bad 50 ohm input terminating resistors. Untested often means "I know it's broken I just didn't use any test equipment to prove it".
Thanks again for the inspiration!


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 Post subject: Re: Fixing a Tektronix TDS-420 Attenuator
PostPosted: Sep Mon 29, 2014 4:00 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2348
Location: Seattle, WA, USA
Wow! Thanks for the feedback. I always really appreciate hearing that my posts actually helped someone. Congrats on fixing the capacitors and good luck with the attenuators. A couple pieces of advice:

1) If you don't have a vacuum desoldering station, get one! I don't know of any other way to remove the ceramic hybrid without damage.
2) Work very slowly and carefully desoldering the ceramic hybrid. It is super easy to crack, so DON'T FORCE IT. If it won't come loose on its own, use a probe to wiggle each pin in its hole to figure out which isn't free yet, then add more solder to that one and use the desoldering tool again on it. Repeat until the hybrid will lift out.

-Matthew

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 Post subject: Re: Fixing a Tektronix TDS-420 Attenuator
PostPosted: Sep Mon 29, 2014 5:56 pm 
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Location: Maryland 20709, USA
Nice job, Matthew. You have more patience than I. :mrgreen:

- Leigh

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 Post subject: Re: Fixing a Tektronix TDS-420 Attenuator
PostPosted: Sep Mon 29, 2014 7:27 pm 
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Joined: Sep Mon 29, 2014 1:42 pm
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Thanks again. Looks to be pretty difficult. My Hakko tip is too big and only have one. I will try my ungar or get my old Pace working and then practice on a known bad channel on the parts board. I may just live with one channel not switchable to 50 ohm. Looking at it, you could not apply much voltage without taking that 1/8 watt (?) resistor out. That is why in the "old days" you bought a 1-5watt 50 ohm Tek feed through.
John


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 Post subject: Re: Fixing a Tektronix TDS-420 Attenuator
PostPosted: Feb Sun 07, 2016 9:16 pm 
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Joined: Nov Mon 11, 2013 5:47 pm
Posts: 4
7jp4-guy wrote:
All -
However, one of them was rather more 'interesting' in that it had a
blown attenuator.


Hey Matthew,

sorry about the thread necro, but do you happen to remember the symptoms this caused? On the TekScopes forum we have a member with a TDS380, where both attenuators let no DC through in 500/200/100mV/div, but otherwise seem to work fine.

Thanks,
Siggi


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