Tektronix TDS1002

[Nixfried]

Hi,

i recently bought a broken Tektronix TDS1002 from Ebay(80 Euros), hoping to get it fixed relatively easy...
I downloaded the service manual(https://de.tek.com/oscilloscope/tds1002-manual/tds1000-tds2000-series-service-manual) and followed the troubleshooting tree on page 110.

The scope is showing some signs of life, meaning the Backlight turns on but nothing else. Also, there is no 1 kHz square wave on the probe compensation port but a steady, unmodulated 2,5 VDC...
So, there seems to be a problem with the main board.

I desoldered all transistors and checked them with my dmm, none of them is faulty. I also checked the voltage regulator (LM2940CT) which is fine...
Maybe i should check the tantalum caps next?
Can someone help me identifying the value? It should be 6,8 uF 6V right?

And yes, i will touch up the solder joints from the parts i tested later...

What should i do next? Any recommendations?

Greetings from Germany

[james_s]

You should start by troubleshooting, don't just remove/replace parts blindly. The very first thing I would suggest doing is check all of the voltages from the power supply. If one or more voltages is missing or wrong then go from there. It's easy to create problems where none were before by blindly swapping parts.

[Nixfried]

Sorry i totally forgot to mention that i already checked the power supply. Nothing suspicious there, meaning every voltage is present and within spec...

[james_s]

If that's the case then capacitors probably aren't the issue, unless there are SMT electrolytic types which are notorious for leaking, I think Tek stopped using those though.

Can you find the reset line to the CPU? Perhaps it's stuck?

[Nixfried]

I have no idea how to locate the reset line, sorry.

What do you mean by electrolytic types? Tantalums are electrolytics, right? Or do you mean aluminium electrolytics?

[james_s]

I meant aluminum electrolytic. Tantalum capacitors are normally dry inside and won't leak. They do fail occasionally but normally the failure mode is a short which is easy to test.

[JFJ]

I have no idea how to locate the reset line...

If the CPU on your main board is a MC68SEC000 (in a 64-pin TQFP package) then the RESET line is connected to pin 10:



It's also worth checking the Vcc lines (pins 5 and 41), the CLK signal (pin 6) and the HALT line (pin 9). If the processor hits a problem, it sets the Halt line to low and all of its three-state lines to high impedance.

[Nixfried]

Thanks for the pinout!

Here are the results:

Pin 5 (VCC) = 3.43V
Pin 6 (CLK) = 7.58Mhz 3.76V peak to peak
Pin 9 (HALT) = 0.35V
Pin 10 (RESET) = 3.43V
Pin 41(VCC) = 3.44V

Any idea what can cause the microprocessor to pull the HALT pin low? What should i do next?

[james_s]

Halt is an input, the microprocessor doesn't pull it low, an external circuit does. You might try pulling it up with a resistor, something around 330R ought to be safe, connected to Vcc and see what happens.

[JFJ]

Halt is an input, the microprocessor doesn't pull it low...

The HALT line is bidirectional.

From page 3-7 of the M68000 8-/16-/32-BIT Microprocessor User's Manual:

[JFJ]

Any idea what can cause the microprocessor to pull the HALT pin low?

During the power-up sequence, the HALT line should go high at the same time as the RESET line. If it doesn't, then it is more likely that an external device is holding the HALT line low.

[james_s]

Halt is an input, the microprocessor doesn't pull it low...

The HALT line is bidirectional.

Interesting, thanks. I've been working with the 6800 lately and assumed that the 68000 was similar in that regard.