Tektronix TDS 694C Trigger cooling & workaround

This 3Ghz scope suffers trigger chip burn out- this project suggests workarounds/repairs & mods to mitigate this problem.

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Tektronix technology delivers many years after equipment is outdated. But this gives the hobbyist a chance to access good quality gear at attainable prices. Sometimes gear comes with common problems. This is about mitigating one such common problem as applies to the TDS 694C

A custom laser cut template permits the direct mounting of TO-220 heat sinks (oversized for extra cooling) to the 4 x small TQFP trigger chips that run hot. Direct screw clamping pressure is achieved via plastic 6-32 hardware mounted on the bottom of the O'scope case aligned to the heat sinks for optimum results. Plastic screws limit the max. pressure that can be applied as a safety vs flexing the multilayer PCB. A 30°C improvement has been measured.

Just 10° temp rise.jpg

Results of cooling upgrade as seen thru case vent!

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  • 1 × 1mm acrylic cutout & matched spacer for To-220 Heatsinks laser cut req'd.
  • 4 × TO 220 heatsink with flat back , anodized black. jameco # 158051
  • 4 × Silicone electrical insulating pads 0.3mm thick for To-220 used between Trigger chip & h. sink
  • 4 × 1.5", 6-32 plastic machine screws for 'clamping' h-sinks.
  • 1 × 1/8" HSS drill bit drilling Oscope case.base

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  • 1
    Step 1

    Before you install the DIY cooling kit, (yellow sidebar links to this or Ebay search) ensure you have reliable triggering in the Scope! If the triggering fails after a few minutes you have borderline chip failure with likely solder pad degradation due to thermal cycling. In this case the first step should be to use a hot air rework @ 305°C to remove the TQFP Trigger chip, then clean the pads with solder wick, add fluxed solder paste to the center of the cooling land under the chip and seat the chip. Reflow the land paste/solder so the chip sits FLAT on the pin pads, THEN resolder the pins so the chip is properly installed and not tilted. Verify trigger function before proceeding. Access to the acq/trig PCB is easy via the underside of the scope. No PCB dismantling req'd, just pull off the case. You can run the scope for a short time ( 5 minutes) without the case before overheating sets in.

    You MAY have the common problem of CH3/4 appearing to cause a trace freeze when triggering occurs as I do. It's as though you are in Normal sweep mode (not AUTO) and the trigger goes out of range for a static waveform display. After much R&D I've developed (AFAIK, I am the first) an option to side step this problem via the front panel controls. Under the Horiz. sweep menu, activate the 'Intensity' function and this activates the CH3/4 acquisitions during triggered mode again!

    If you have NO triggering AT ALL: see the EEVBLOG link in my links section for a trigger chip swap solution. I have not tried this though as the Intensity function solves my issue.

    Ok, now that we're past the repairs or workaround for triggering issues, it's time to enhance the longevity of those overheated trigger chips to prevent further degradation.

    Note the step by step pics in the pictures section. I summarize:

    1) Remove the 4 case screws from the rear plastic panel and remove the case , it's a snug fit.

    2) Flip the scope so the bottom is up and you have good access to the Trig/Acq PCB, ensure you are antistatic grounded and the scope is unplugged.

    3) Loosen the two center torx screws (pictured) at the front of the PCB by about 1/8" to accept the acrylic template and spacer.

    4) Install the acrylic heatsink template & spacer (slots aligned under the torx screws) with inserted TO-220 x 4 h.sinks and sil-pads aligned on the trigger chips. You may need to gently bend the semi rigid CH1 input line to the left to make clearance. A bit of thermal paste can make the Sil-pads adhere to the H.sinks to simplify the process.

    5) Square up the fitment of the H.sinks over the trigger chips and secure the two torx PCB screws to lock it down.

    6) Mark and drill the bottom of the PCB case as per the images, use a 6-32 tap or screw to make threads for the plastic screws.

    6a) Verify fitment of the plastic screws, some 6-32 self tapping screws may cut a tight tapped hole making it difficult to hand tighten the plastic screws. Use a real 6-32 tap in this case or (if u must do a hack) wiggle the 6-32 tapping screw about to gain clearance.

    7) Clean up the drilled case CAREFULLY for any metal cuttings/filings. I used a paint brush to brush it out and an alcohol wipe down for the tiny stuff, both inside and outside. Cleans the dust as well.

    8) Reinstall the unit in the case, I suggest that you slide the scope slowly down into the edge standing case and ensure the front panel slot sits snug on the case.

    9) Run in the plastic screws, by hand, peering thru the nearby case vent holes so that the screws seat on the heat sinks without bending. Plastic screws lack the strength to distort the PCB or the ICs, they will strip or bend first, so your scope PCB is safe. Mark any excess screw length with a sharpie,. Then back out the screw, measure the true thread length required and snip (flush cutter) off the screw shaft excess allowing for 2 or 3 threads extra. Re install and hand tighten so the screw does not flex as seen thru the vent holes.

    10) After seating all 4 screws, reinstall the back plastic housing of the 'scope case and enjoy your cool running trigger chips and the extra life of your valuable Tektronix scope. Note the image (thru the case vents) showing the edge of the trigger heat sinks at ONLY 10°C over ambient. This is due to the faster airflow & cooling achieved by the system fan suction through the case vents.

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