Repairing a 466 oscilloscope

Nikolay Belikov
 

Hello there!

I'm a new member here and first of all I would like to say big thank you to whoever maintains this group and to everyone who fills it with invaluable information.

Recently I managed do get myself a 466 oscilloscope with DM43 in non-working condition. I'm a total noob in electronics engineering and this is my first scope so please be ready for a bunch of stupid questions.
I managed to fix the low voltage power supply by replacing faulty C1761, C1771 (as well as other big cans, as preemptive measures), C1712 and CR1712. Now the device shows something meaningful on the display, although I don't dare to run it for long periods to test it extensively because the fan does not work and I'm afraid of thermal damage, even with open cabinet. So fixing the fan is where I am stuck now.

First, I measured the transistors in U1690 assembly (the board is early revision, with 1973 printed on it), and found that transistors A (pins 1-3) and D (12-14) were dead. I replaced the IC with four 2N2222A transistors and tried to power it on. The fan worked for few minutes and stopped. The same transistors were very hot, although haven't failed. I removed the fan from the frame and tested the current draw with the impeller mounted. When cold, it consumes about 80 mA (which, from what I found in previous discussions, is normal), but the current immediately starts to ramp up to about 180 mA over several minutes, the aforementioned transistors overheat, the motor begins to vibrate and can be easily stopped by hand.

So what I fruitlessly did in order to fix the problem:
- Shuffled the transistors that replace U1690 - no effect. A (pins 1-3) heats up the most, D (12-14) is the second.
- Checked all the resistors and diodes - they seem to be in spec.
- Replaced the Q1698 with a 2N2907A - the new one does not get warm, although the old one did, but this didn't affect the problem.
- Replaced the C1698 with a new tantalum - no effect.
- Desoldered the motor from the board, lubed it at both ends - no noticeable increase in how easily the shaft rotates.
- Soldered the motor back through a set of wires. This allowed me to do the following:
- I left the windings (pins 10-13 on the motor) open and could measure the output of the Hall sensors, rotating the shaft by hand. The peak voltages were nearly the same across all four outputs.
- I swapped the connections of both windings and Hall sensors, so that windings 10 and 12 were driven by transistors A and D, and windings 11 and 13 by B and C, in case that I've missed something on the board - the transistors B and C predictably got hot.

So at this point I have no ideas left (except purchasing a spare motor, which I would like to avoid given how rare they are) and I'd appreciate any feedback or thoughts.

Thanks a lot,
Nik

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