Hamfest Find Tek TDS-220 White Screen

[The_PCB_Guy]

Greetings folks,

I have found a number of topics here already where other members discuss their TDS-220 scope with the same issue as mine, but so far none of the solutions have panned out.

Note: For the below troubleshooting process I refer to the TDS-200 series service manual:
https://download.tek.com/manual/071049203.pdf
Page 6-42

I purchased a TDS-220 from a hamfest for $20 with the knowledge that it most likely was not functional. When I powered it on all I saw was a blank white screen. I brought it home and did some troubleshooting, during which I probed something I shouldn't have and blew the power supply board fuse and a zener diode. After this the screen turned black, but replacing the fuse and diode brought it back to life (solid white again). However, there is still no display - just a white screen. I confirmed that the main system is functioning by checking the waveform on the probe compensation hooks - all looks good. I made sure the LCD connector pins were not damaged and were correctly positioned. I checked to make sure the LCD flat flex cable contacts looked good as well. I then began probing the signals on the LCD connector and verified the voltages and clocks were present. However, the 1.89MHz clock was measuring closer to 1.37MHz, the 19.3kHz clock was measuring closer to 17.4kHz, and the 75.5Hz clock was measuring closer to 70Hz. I am not sure what the tolerance is on these, so this may or may not be acceptable. However, there was nothing on the four data lines - they were all permanently high (5V).

Has anyone else seen this issue and/or have any other suggestions for troubleshooting? I am not sure where to go from here.

Thank you very much,
Matt

[The_PCB_Guy]

As a follow-up, I reread the below thread and realized I never actually checked the frequency of the probe compensation output, just whether or not there was a signal on it.

https://www.eevblog.com/forum/repair/tektronix-tds220/25/

There was a signal, but I see now it is 5kHz and remains there. According to the above thread, this suggests a ROM or RAM issue? Is there any additional testing I can do to narrow this down?

Much appreciated,
Matt

[The_PCB_Guy]

Anybody? Perhaps there is somewhere else I can look?

Thanks folks,
Matt

[bd139]

There are some voltage regulators on the main board. Worth checking that they are actually outputting the desired voltages first which are outside the scope of the power supply itself. Nothing will work if any of them are borked.

Some reverse engineering here: https://www.eevblog.com/forum/testgear/tektronix-tds220-repair/

TDS210 and TDS220 are almost identical.

Worst case, keep an eye out for main boards for it. A lot of the time there are beat up ones with good boards still.

[The_PCB_Guy]

Greetings, thanks very much for the reply! I do remember reading through that thread before creating my own topic. I checked the regulators (well, transistors) and found the following. Please note that these transistors are different part numbers, so below are the voltages on each of the pins with respect to ground:

Q140: 1 - 3.32V, 2/tab - 4.98V, 3 - 2.62V
Q131: 1 - -3.3V, 2/tab - -4.59V, 3 - -2.62V
Q141: 1 - -3.28V, 2/tab - -4.59V, 3 - -2.62V
Q143: 1 - 3.35V, 2/tab - 4.98V, 3 - 2.62V

Unfortunately I have not been able to find the datasheet for these transistors based on their markings:

Q140: "R70 P1F"
Q131: "R7N CE"
Q141: "R7N CE"
Q143: "R70 P1F"

All parts appear to have the Motorola logo.

Best,
Matt

UPDATE: Apologies, I was searching on the wrong term (R70 and R7N instead of P1F and CE).

P1F is a PZT2222A transistor:
https://www.onsemi.com/pdf/datasheet/pzt2222at1-d.pdf

CE is a BCP69T1 transistor:
https://www.onsemi.com/pdf/datasheet/bcp69t1-d.pdf

This information is also captured in the thread you linked.

[bd139]

That looks about right. Might be a duff ASIC. I’d keep an eye out for a spare board or another dead or dying or abused one and make a frankenscope. It was worth $20 either way!

[The_PCB_Guy]

Thanks for the input. Unfortunately even these scopes that have been destroyed tend to be expensive. I would much rather try and track down the specific issue rather than replacing the whole board. I've always been more interested in component-level repair than board replacement where practical.

Best,
Matt

[bd139]

Yeah I’m the same with me. Would prefer to fix. It’s difficult without a proper SM so may require reverse engineering somewhat.

I did some repairs on another one here a long time ago https://www.eevblog.com/forum/testgear/tektronix-tds210-teardown-and-bnc-replacement/