Tektronix 576 (Tek 576) Curve Tracer – Repair/Restore

I never wanted to own a curve tracer per se, but I came across one in a dusty shed, which was being demolished. Abandoned by the previous owner, along with hundreds of pieces of other test equipment (HP, Tektronix and few others), this one caught my eye as I had never possessed one or seen in real ;). The new owner of the property was into a car business, and he was getting rid of all this, and I ended up buying it from him.

More details, Ref Pics @ Tek Wiki – http://w140.com/tekwiki/wiki/576

As always, here is a picture of the horrible state in which I acquired it –

Rust. Mud, mold, mushrooms, and small zoo of insects.

First task is to clean this thing inside out, without breaking anything. Gallons of isopropyl alcohol and days later, the unit as well as the plugin is in a condition to work on and can be taken it inside the house.

Next task is to fix the heat-sinks for the power transistors. All of them are rusted and I didn’t see any heat-sink compound.

 

So I devised a plan to remove each transistor, clean, and replace the mica insulator,  apply new heat sink compound, use new screws, a new plastic Cap.

I use the heat shrink tube to insulate the screws/terminals and ensure they don’t touch the chassis on the mounting hole. Out of the two screws, one is used to connect the collector of the transistor back to the transistor socket.

One hole for the plastic cover needs to be expanded to accommodate the bottom screw to directly seat it on the transistor case for collector connection.

And the screws also needed a band to ensure they don’t touch the body.

Here is how it progresses –

and the final result, as compared to the initial state.

Rest of the instrument is thoroughly cleaned again, and now it’s time to do the electrical troubleshooting.

Used an oscilloscope trolley to work on it as its too heavy to move around or keep in a table.

CAUTION:

This instrument contains potentially deadly voltages, up to 1500V for the collector supply. It can deliver lethal DC and AC Voltages at substantial current levels to make you RIP in one shot. There are better ways to die if you are up to that.

DO NOT ATTEMPT TO REPAIR/RESTORE ONE OF THESE UNLESS YOU ARE QUALIFIED AND KNOW WHAT YOU ARE DOING AND HAVE THE RIGHT TOOLS, PROBES AND FOLLOW ALL SAFETY PRECAUTIONS.

EVEN IF YOU ARE QUALIFIED, PLEASE DO TAKE GOOD CARE AND ATTENTION WHILE WORKING ON THIS.

The unit is really heavy, 70lb or 32Kg. Be careful while moving this around. Suggest two pair of hands to move this unit. If you drop it you may/can get another leg/foot but some of the parts inside it will be impossible to source.

Next step is to visually inspect the whole unit for faulty/broken parts/components. To my surprise, this unit seems to be in good shape, I didn’t notice anything really bad or broken. Next is to check for leaky caps. First stop is the LV regulator Board, where I find one bad boy. Replaced right away.

The main filter caps are behind the rectifier board which is stacked behind the LV regulator board.

The Paper (or Mica, I am not sure) Caps in the rectifier tested good.

To access the main filter caps for power supply, you need to remove the  LV Rectifier board. Unfortunately it has cables connected on both sides that you can’t hinge them on onside using the cables.

Only option is to tag and remove cables from one side – with least amount of connectors – and open the board up.

Now you have access to the main filter caps on the back side. The capacitor number is marked on the body for easy identification.

Now each cap needs to be disconnected from the circuit so that they can be tested.

I tested each cap for Leakage, ESR and Value. Combination of Eico 950B, Transistor Tester and Sencore Cap tester was used to confirm the condition of the caps.

Surprisingly, all caps tested good and no leak.
I scanned through the service manual and parts list for all electrolytic and tested all of them and all were good.

There is one cap inside HV module, not easily visible, unless you trace it through the schematic.

 

Time to power up, all rails were connected to meters to monitor all of them in one go and input supply via Variac.

All the output rails are available on the LV regulator board. Check for the connector identification on the schematic. Wired up meters to track all outputs.

 

I powered it on (the unit gets power once the input crosses a limit), and heard a relay clicking and observed all rails behaving well. Raised input to 115V and here is the result from all power rails, +12.5, -12.5, -75, +100 and +5, all in control and stable.

and the front panel (sorry for the dark pic)

CAUTION: ALWAYS KEEP THE COLLECTOR SUPPLY SWITCH TO 15V WHILE TESTING/TROUBLESHOOTING. DO NOT BYPASS THE SAFETY SWITCH (LIKE IN THE PIC BELOW) UNLESS YOU ARE SURE ALL SAFETY PRECAUTIONS ARE IN PLACE.

And guess what, its all up and running. Tested few diodes and transistor to check its function and all tested good. Will try to calibrate it later.

This was an easy one compared to the oscilloscope projects,

Here is the Before and After

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