Tektronix TDS1002 - Page 1
Forum section: electronics / repair

author: Nixfried
author_rank: Contributor
timestamp: August 09, 2018, 08:03:48 pm
content: 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

author: james_s
author_rank: Super Contributor
timestamp: August 09, 2018, 08:09:23 pm
content: 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.

author: Nixfried
author_rank: Contributor
timestamp: August 09, 2018, 08:14:51 pm
content: Sorry i totally forgot to mention that i already checked the power supply. Nothing suspicious there, meaning every voltage is present and within spec...

author: james_s
author_rank: Super Contributor
timestamp: August 09, 2018, 08:17:49 pm
content: 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?

author: Nixfried
author_rank: Contributor
timestamp: August 09, 2018, 09:17:01 pm
content: 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?

author: james_s
author_rank: Super Contributor
timestamp: August 09, 2018, 09:45:05 pm
content: 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.

author: JFJ
author_rank: Regular Contributor
timestamp: August 11, 2018, 08:23:37 pm
content: Quote from: Nixfried on August 09, 2018, 09:17:01 pmIf 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.

author: Nixfried
author_rank: Contributor
timestamp: August 12, 2018, 02:58:49 pm
content: Thanks for the pinout!Here are the results:Pin 5 (VCC) = 3.43VPin 6 (CLK) = 7.58Mhz 3.76V peak to peakPin 9 (HALT) = 0.35VPin 10 (RESET) = 3.43VPin 41(VCC) = 3.44VAny idea what can cause the microprocessor to pull the HALT pin low? What should i do next?

author: james_s
author_rank: Super Contributor
timestamp: August 12, 2018, 04:49:52 pm
content: 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.

author: JFJ
author_rank: Regular Contributor
timestamp: August 12, 2018, 06:47:11 pm
content: Quote from: james_s on August 12, 2018, 04:49:52 pmThe HALT line is bidirectional.From page 3-7 of theM68000 8-/16-/32-BIT Microprocessor User's Manual:

author: JFJ
author_rank: Regular Contributor
timestamp: August 12, 2018, 06:50:05 pm
content: Quote from: Nixfried on August 12, 2018, 02:58:49 pmDuring 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.

author: james_s
author_rank: Super Contributor
timestamp: August 12, 2018, 08:43:11 pm
content: Quote from: JFJ on August 12, 2018, 06:47:11 pmInteresting, thanks. I've been working with the 6800 lately and assumed that the 68000 was similar in that regard.