Tektronix 2465 toasted parts on regulator board - photo

[TTWhy]

Thank you for welcoming me to a forum I've used for years. My turn to contribute a case for you geniuses. 300 MHz scope from later prod run before 2465A or concurrent prod. Conditions: 120V socket shared with very old HP 10 MHz Pulse generator running with knobs on 11  . Oscilloscope was set on top of this solid state toaster/pulse genny. Coax strung btw 50  genny out and megohm-in on the scope. After about 20 minutes, 'scope lets out a smoke smell and lost her trace for just a moment. I pulled the power cords quickly.

An older, but compatible rebuilt Regulator Board is in my possession. It is buckled (but a single layer board) and needs the power switch swapped in. After opening the Power Supply assembly, I see an Inverter Board with lots of old capacitors, even the flammable ones with crazed plastic (photo shown).

Since I really like this scope, I'd like to ask:

• Does the problem go deeper beyond an old flamed out cap?
• Should I replace electrolytics on the Inverter board, even the large 200V axial caps next to the fan motor? 54x25.5 mm
• Will these new components result in a need for adjustments?
• Do I have to be concerned about impedance mismatches when juicing the scope input with with 10 MHz 5V P-P signals?     

Thanks a lot, Earle.

[tggzzz]

Does the problem go deeper beyond an old flamed out cap?

In mine, the RIFA cap caused the series resistor to remove a 1cm diameter area of the PCB's prepreg. Replace all those b_____s. See
http://www.condoraudio.com/wp-content/uploads/Projects/Tektronix-2465B-Oscilloscope-Restoration-Repair.pdf

Also check the infamous electrolytics on the A5 controller board, and probably replace those.

Should I replace electrolytics on the Inverter board, even the large 200V axial caps next to the fan motor? 54x25.5 mm

Only if necessary.

Will these new components result in a need for adjustments?

No.

Do I have to be concerned about impedance mismatches when juicing the scope input with with 10 MHz 5V P-P signals?

The problem is power dissipation in the 50ohm termination resistor; if the voltage across that is too high, the scope should protect itself by breaking the circuit to the resistor and returning it to a "high" impedance input. Reflections from an open circuit might or might not damage whatever is generating the signal.

[macboy]

Do I have to be concerned about impedance mismatches when juicing the scope input with with 10 MHz 5V P-P signals?

The problem is power dissipation in the 50ohm termination resistor; if the voltage across that is too high, the scope should protect itself by breaking the circuit to the resistor and returning it to a "high" impedance input. Reflections from an open circuit might or might not damage whatever is generating the signal.

A 5 Vpp sine is just 1.8 V_RMS, and a square is just 2.5 V_RMS.  In the worst case, the reflection is anti-phase with the output, so the voltage across the generator's terminating resistor is double the nominal output voltage, so 5 V_RMS.  This will correspond to 0.5 W dissipation in that resistor, probably not an issue. The generator's output amp also sees an effective 25 load (due to reflected power) rather than the 100 ohm nominal that it is designed for, but it's an HP, so meh, no worries. Best case, the reflection is in phase with the output, and there is nearly no voltage across the resistor and very high effective impedance (similar to an unconnected output).


Good luck fixing the 2465, it is definitely worth the effort.

[TTWhy]

tjjww, I love the write up regarding improvements with mainboard capaccitor replaccement. Maybe next time.

macoy, my head has been concentrating on radio circuits and I hadn't thought how 1M  on an HP is different than a transceiver 50 I saw in the manual an overload lamp will indicate if 50 input is too great.

Thanks for the encouragement. Oscilloscope is buttoned back up and working like before. Had to use some 3m 77 adhesive spray to make the fan collet  stick again, but by the third reassembly, I finally had no extra parts.