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| Beckman Industrial 9102 scope problem |
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| kf7pcl:
So I ended up picking up a Beckman Industrial 9102 20MHz scope for a very low price. It was sold with a known issue of "losing trace after warming up". I noticed the scope is almost completely unmentioned online. I couldn't find a manual or schematic. I turned it on and it seems to operate okay except the trace is somewhat dim. I have to put the brightness almost all the way up in order to get a moderate brightness trace. Also, if I increase the brightness pot past about 80%, it actually gets slightly dimmer. After just a 2-3 of running, a heatsink with three transistors and a 1.2K resistor becomes too hot to touch. It starts smelling like hot components not long after and I measured the resistor exceeds 100C. The transistors seem to be part of the voltage regulation circuit for the power supply. But I don't have a schematic and there aren't even any labeled voltage test points. Any ideas? |
| floobydust:
Welcome to the forum :) It looks like one of the South Korean Hung Chang/Protek scopes, just like model 5504 but a 20MHz version, not 40MHz. They all use a similar circuit, a bit difficult to repair. Start with finding the schematic/service manual. I could not find one; EEVblog member Armadillo showed pics of it (5504) here: https://www.eevblog.com/forum/repair/hung-chang-5502-oscilloscope-problem/msg1132684/#msg1132684 Hung Chang 5502 20MHz Dual trace, Single Timebase Hung Chang 5504 40MHz Dual trace, Dual Timebase (Beckman 9102?) Hung Chang 5510 100MHz Dual trace, Dual Timebase Hung Chang 5602 20MHz Dual trace, Dual Timebase, On-screen readout, aka Beckman 9202 Hung Chang 5604 40MHz Dual trace, Dual Timebase, On-screen readout https://www.eevblog.com/forum/repair/hung-chang-5510-oscilliscope-repair/ Mods maybe move this thread to Repair? |
| kf7pcl:
Thanks! I wondered if it was a rebrand. It is indeed a Hung Chang. I couldn't find the correct manual but I did find one for the 5602 which is certainly better than nothing |
| floobydust:
I would guess something is overloading the 140VDC power supply, which is then overheating. It's usually the deflection amplifiers (transistors) or some other component like a capacitor. Don't run it too much and keep a careful eye what happens, the beam may not be going out, but rather off screen. If it's over 140V then stop now. The Korean scopes, most parts are run at their max. rating, so any failures can cause a bunch of failures in other parts as a domino effect. The 140VDC regulator is extra confusing because the pass-transistor is on the -ve (GND) side and an assist resistor is used to get more current out. Both may be your hot parts. 5602 schematic |
| kf7pcl:
--- Quote from: floobydust on June 05, 2018, 10:43:10 pm ---I would guess something is overloading the 140VDC power supply, which is then overheating. It's usually the deflection amplifiers (transistors) or some other component like a capacitor. Don't run it too much and keep a careful eye what happens, the beam may not be going out, but rather off screen. If it's over 140V then stop now. The Korean scopes, most parts are run at their max. rating, so any failures can cause a bunch of failures in other parts as a domino effect. The 140VDC regulator is extra confusing because the pass-transistor is on the -ve (GND) side and an assist resistor is used to get more current out. Both may be your hot parts. 5602 schematic --- End quote --- Yes, those are indeed the hot parts. I tested the regulated rails: +12V: 11.99V -12V: -12.00V +5V: 5.15V +140V: 158.3V Obviously the 140V rail is way higher than it should be. What should the next action be? Check the pass transistor? |
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