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I recently acquired a Tektronix 2246 oscilloscope. It seemed to be working fine until I opened it and tried to check the test and self-calibration voltages (some of which out of the limits, but it is the same before and after the issue happened). Now no text is displayed, and the Readout intensity knob makes a dimmer version of the trace show up above it. If I try and open the Service Menu, etc. the screen just turns blank but I am able to quit out of it by pressing the right button.

Photo in /g/TekScopes/album?id=300837

Any suggestions for how to fix this? This is my first time repairing an oscilloscope, so any pointers would be appreciated.

Thanks for your time.

Aneesh,

I suspect you pulled a connector or more loose by removing the case. Check and reseat connectors. This can easily happen removing the case and/or doing any checking without you noticing. A couple of mm movement will disconnect. Dirty switch contacts will cause problems like this.

Having a 2252, the condensers in yours are likely the same in the scope. I did recap mine with NIchicon, ULD, UHE, UCY and LGR. The power supply board is tedious to remove. I did oil the fan with the supply removed. Bad caps will cause problems. If you have any RIFAs in there, replace them on-sight.

Mark

Hi Mark,

Thanks for your advice. I tried reseating the ribbon cables and the text came back, but was now wobbling. The only adjustments I remember significantly changing were C114/C124 (CH1/2 input capacitance), because I mistook it for the compensation.

However, I tried turning on the x10 magnifier and it started shaking much more violently, but then resistor A10R801 (part of the Horizontal Output Amplifier) blew up/melted. The unit can power on briefly, and I can see the resistor glow for a moment before it shuts itself off. If it's worth anything, Q801 (the transistor connected to it) got so hot that it burned my thumb, but that was before I turned on the x10 magnification.

Is it likely that this resistor was just on its way out, or will it fail again when I replace it? And if it's the latter, how do I go about figuring out the problem, especially since I can't get the unit to stay on to test voltages, etc.?

The 2246 service manual can be found at . The schematic with R801 is on page 253.

Thanks again for the help.
Aneesh

In order to replace the resistor, I think I'll need to remove everything so I can access the back:

On instruments with serial numbers below B040909, examine the circuitry near A10U802 (the Horizontal Preamplifier IC, shown on diagram 6) to determine if a diode (CR801) has been connected between C816 and W804. The anode of the diode connects to the end of C816 nearest pin 9 (-5 Vcc) of U802, and the cathode connects to to the end of W804 toward the rear of the instrument (the horizontal RO input—pin 1 of U802). For static discharge protection, the diode must be in place or added (and left) to the circuit before disconnecting the ribbon cable to J2302 on the Processor board.

I think I already removed that ribbon cable while reseating it. Could that be the cause of this issue? And now that it happened does it really matter where I put that diode in or not? I'm not sure how I would install the diode without access to the back of the circuit board

Aneesh,

Removing the ribbon connector while on could cause the IC to be defective causing a spike to damage the mosfet(s) in the upper part of the schematic. C807 could be shorted. Q802 could be bad making Q801 overheat. The 430 ohm resistor will need replacing. Both mosfets should be replaced after you find the problem. See if W17 is shorting to the shield.

Mark