Hello
I have bought a 2445 for a reasonable price, checked visually, uncovered whole beast, cleaned it from dust and powered up.
The scope surprisingly started, WOW. I made some measurement and turned it off. Few days after i tried to power up again, but the scope didn´t started properly. Sometimes just diagonal traces and unreadable readout characters appeared and few leds were lighting. Sometimes only some LEDs were present and no display. Another time just the point appeared(timebase not running). So i digged and discovered some bad caps in PSU and mainboard too. The PSU now gives correct voltages. And then begun a series of troubleshooting. Finally i focused into A5 digital board, because all timing, control signals should going from there. But they don´t. I found(according to numbers on the image below) that:
1. ONE signal from address bus of a MCU is strange. On pin25 of U2092,(TP506) i measured the signal which can´t drop to logic L due to some reasons. I tried to test the U2480(74LS138), but this was good. The data line A15 is going only into this decoder, and to unused J500 connector.
This is the first reason for me to think, that the MCU has a faulty pin 25 The signal from the pin 37 of MCU is OK.
2. I also measured the signal on MCU IRQ(pin4 on U2092). Straight line, no pulses. They should come from 4020 counter and inverted by 74LS04. The clock signal on U2668(4020) was present, but no pulses on output. Reset input was HIGH. I tested the 4020 and it was also faulty. Replaced with new one, but no change. The reset signal is coming from U2034 which should be clocked by decoder U2480. This decoder doesn´t give any clock signal, probably because of bad signal from the MCU(TP506)
This is the second reason why i suggest the MCU is faulty3. The main clock for the MCU is OK.
4. I found that one data line(D0, pin 33 on MCU) has strange oveshoot, different from another 7lines of data bus.
This is the third reason why i suggest the MCU is faultyGuys, do you think my suggestion of faulty MCU(U2092) is right? Is there any chance to get somewhere the main MCU?
I hope somebody can help.