naujoks

I had a look at it when I had the caps in the area out and I also confirmed the correct connections by beeping it out on my working board.

The bodge wire is the grey wire, the black trace you see is where the previous owner marked the original trace with a sharpie.

The bodge wire goes to the + of C43. Oooh I've never actually used the ammeter part of my MM! So how do I do this?

I thought U2 / U3 are only relevant to the stepper motor? Q7 and Q8 are not insulated. U6 is a new (confirmed good) part, The writing is just not visible. The original LS00 was not faulty either, incidentally. Q4 is tested and working, the blob is just my suboptimal solder work.

Incidentally, no parts get unduly hot (not L6 or Q6, nor the voltage regulators). There's no sign of burns anywhere.

As mentioned, I've tried three different TIP110 by now, and I had bent it away from the heatsink Here's a picture anyway. Note that the collector's trace was ripped up and there's a bodge wire running from it to one of the caps.

On a previous occasion I bent the transitor away from the heatsink altogether, without any screw. So there was definitely not connection.

What do you mean?

PS: Shorting collector and emitter of Q6 DOES activate the motor.

I replaced CR17-20 - no change. I'm waiting for replacements for CR15 & 16, but I don't expect any problem with those, they tested fine in my tester. Got onto replacing a few of the ceramic caps (tested the ones I took out, they were all good) and gave it a go, and something very weird happened: I had my multimeter with the negative against the heatsink (ground) and the positive on C70 (12V) and when I switched it on, it was spinning the motor for a second, then off, then on, so like going through the power on sequence when the drive is encountering an error. The 12V did NOT collapse during this time! Then I switched it off and on again, and boom, back to the original behaviour, 12V collapses down to 4V. Switching it on and off again didn't get it to go in the test cycle again. Remember that all the socketed ICs are known to be good. It's getting curiouser and curiouser.

The diodes appear to be fine. CR17-CR20 seem to be for the 5V line anyway, the other two are for the 12V. What's the modern day equivalent to replace these with? I have some BY 251 lying around, they're rated for 200V/3A. Is that overkill? I'm now thinking one of the smaller caps (they're tantalum, it appears) might be shorted, having read a post on that on here. Can I detect a faulty cap without taking it out? There's so many of them!

I have done more work on this today. So to recap, I replaced Q4, Q6, now also U5, U6, L6, all electrolytic caps in the area, checked diodes, replaced the 12V voltage regulator now with a third one. So far I haven't found a single faulty component. I checked the traces for shorts and have reflown the connector pins, all to no avail. Anyone has some novel ideas? Have I missed something really obvious? If something is shorting the 12v line to ground, should there be something that's getting warm and that I could identify like that somehow?

I replaced C44 and C45, CR14 tested ok, I replaced it with a new one anway. I socketed and replaced LM2917 just for fun as well. All to no avail. There really isn't anything else in the area that I haven't tested or replaced. This has turned into a real head scratcher.

The power drops to under 4V on either side of L6 when the motor is supposed to spin. On the working drive it stays constant at 12V. Could there be something wrong further up on the way to the voltage regulator? What are the SN75478 at U2 and U3 doing? Or could it be the LM2917 after all?

Yes, it is insulated.

I swapped L6 for a new one, no change. The question wether 12v is ok, I've measured it at various points, Q4, Q6, the L7812. Not sure I can test it further?

Here are the measurements (motor ON) Q4 Base 0.1V Q4 Collector 3.1 Q6 Base 3.5V Q6 Collector 3.8V Q6 Emitter starts out at ca. 2.2V, going down to 1.5V quickly Pin 11 U7 0V So it's Q6 emitter being suspect, which should be 3V, but maybe it starts out higher and goes down to 1.5V quickly, quicker than my MM can capture.

The chips are all known to be working well. I've checked caps C44 and C45, R80, diode CR14 - all ok. On pin 1 (red wire) of J1 (the motor connector) I'm only getting 2.3V. On my working drive it's 8V, so somewhere along the line I'm losing 6V, I have no idea where though.

I think pin 11 does change state. I'll check again later. But since the motor DOES spin up, just seemingly not with enough power, doesn't that point to a different issue?

Did you mean that? On the diagram, it says there should be 3V on the emitter of Q6.

I measured the points in the diagram, and all the readings are spot on. Only the collector of Q4 is a bit high, 4.1V instead of the expected 3V, but I don't imagine that's the issue. Where can we go from here?

TIP110 Q6 is new, Q4 was tested ok and the 6532 is a known good component. I just noticed that the spindle was moving a teeny tiny bit when switching it on, so I gave it a little push and it actually did move for a second! When a disk is in, it's not moving though. So is the motor not getting enough juice? Where is actually the best point to measure the voltages? So far I got them directly off the power transistors, but maybe that's not the best place. I also noticed that the activity LED flashes for a second, but then goes a bit dim, but not entirely off.

Measured against ground I'm getting around 140mV on both base and emitter and 11.9V on collector.

PS: Is it safe to measure voltages on the board without the mechanical drive bit connected?

I measured the TIP110 and I'm getting 11.9V between collector and emitter, but 0V between base and collector. On my working board I'm getting the same voltages on both pins (interestingly on the working board the voltage between base and collector go to 5V or so when the spindle motor is spinning). The TIP110 is a new part, I doubt it's faulty, but I will check. In the meantime, does this fault description point to something different?