Tektronix TDS210 input defect, devices burned

[Aheld]

Hi Forum,
I received a Tek TDS210 Oscilloscope with burned input devices (please see picture attached).
Can anybody tell me what devices are placed direct behind the BNC input?

Thanks
aheld

[Jeff eelcr]

Resistor? 50 ohm? 1M ohm?
Jeff

[alm]

The TDS210 doesn't have a 50 Ohm input impedance setting, so it won't be a 50 Ohm terminator. Probably a protective series resistor. My advice would be to look for pictures of the TDS 210 / 220 / 224 if the resistor has visible markings, and if that doesn't work look for the TDS-510B service manual, which isn't very similar but might just have a similar input circuit.

[james_s]

Wow, that poor scope, and both channels too. I wonder what somebody did to it? 

[bdunham7]

There should be a similar device placed in a similar position for the EXT TRIGGER input.  What do you see there?

[Aheld]

Thank you for the replies,
I don‘t know the history of this device and why both inputs burned. But I agree it is strange…
The ext trigger contains a 38 Ohm resistor. Maybe a good starting point. Thank you for this idea.

I made fiirst tests without resistors, an it seems that this oscilloscope is still working. So I‘ll give it a try. Maybe first measurement equipment for my son to start a great hobby?

[TERRA Operative]

Is it a newer revision with the flashRAM or the older type with socketed ROM chip?

I have some boards here I can check.

[David Hess]

Usually there is a low value resistor in series with the input BNC to both dampen reflections and act as a fusible link.

[Aheld]

@Terra Operative: If I need to guess, than I would expect the newer version?
Please double check picture in attachment.

It would be great If you cen make a picture or simple measure the value?

@David: looking to this deep burning marks. Do you have any idea how the current could flow through the circuit?
A didn't find additional defect components, which didn't resist the energy.

Thanks for helping :-)

[TERRA Operative]

I'm currently looking at an older version board here (yours is indeed a newer version), the trigger input has a 100ohm resistor (marked '1000'), and the two vertical inputs are 39.2ohms (marked '39R2')

I also just pulled my late version board out of my TDS220, and all three inputs are 39.2Ohms (I measured with my multimeter, I don't want to risk damaging my good scope by removing the shield plate).

For repair, I'd be inclined to remove the charred bits of the PCB as best as possible (with a Dremel or similar) and cut the two traces right back to the BNC center pin and the via. Maybe fill the holes with a bit of epoxy, or seal it with some clear enamel (like nail polish etc) if you feel like it.
Then scrape back the solder mask on the vias there to expose the copper and solder a replacement resistor directly from the BNC pin to the via.

Also, check the solder on the BNC ground posts isn't cracked, it's common for them to get loose and usually they just need a good bit of heat and some flux to reflow the solder to make them solid again.

Don't forget to give the board a good wash with isopropyl alcohol when you're done to remove all the flux and junk.

[Aheld]

Great, thank you!
Then I can start with this repair.
You‘re absolutely right that it make sense to clean and repair traces and pcb material. The idea with filler for nails is great.

I’ll keep you updated.

[David Hess]

@David: looking to this deep burning marks. Do you have any idea how the current could flow through the circuit?
A didn't find additional defect components, which didn't resist the energy.

The next parts are the switchable AC coupling capacitor and the high impedance attenuators.  After that is the 1 megohm termination, followed by a roughly 500 kilohm series resistor bypassed with about 1000 picofarads of capacitance, and then the protection diodes in front of the high impedance buffer JFET or MOSFET.

The 500 kilohm series resistance would limit the DC input current to negligible levels, but applying RF to the input could drive plenty of current through the fusible link resistor to burn it.  The input protection diodes and FET may or may not be damaged.  That can be tested by measuring the input leakage current after the fusible resistor is replaced.

In older oscilloscopes, the fusible link resistor was a 1/8 watt carbon composition part and typically 51 ohms.