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1541 red light constantly on, spindle behaving as expected, drive not responding.

BennehBoy

Experienced Member
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Nov 2, 2021
Messages
61
Hi,

I have a 1540050 Rev C 1541 that is exhibiting a behaviour that I can't find any information on.

When powered on (connected via serial to a C64) the red light comes on, spindle turns for usual second or so then stops, red light stays on.

I swapped in all the main IC's ( UB4, UB3, UB2, UC4, UC3, UC2, UC1) from another working drive (and tried the ones from non working in that) and there are no issues with them - I even socketed the SRAM to test that.

Board inspection shows no obvious blown caps.

Initially any command issued to the drive would hang, I replaced UC5 and now the hangs have stopped but I get DEVICE NOT PRESENT ERRORS. Neither of the device pads have been cut so it's not a device ID issue.

So far I have replaced:
UA1, with a quality ST part (tested working in an IC tester - the removed component tested OK)
UB1, which was a MOS 7707 (which tested OK when removed)
UC5, this part was bad

There is a bodge wire running from 6502 pin 39 to UC6 pin 2 - the track from UC5 pin 12 to here is cut (not sure why - any ideas?). Reverting this bodge results in no appreciable change.

I've replaced the ROM with a freshly burned EEPROM which works in the other drive.

Next I'll be pulling UC7 & UD2 to test these.

Voltages are good.

Clock looks good just a hair below 1mhz.

Address bus on the 6502 shows activity.

Reset on 6502 is high.

Thoughts?

PS Hello, this is my first post.
 

ajacocks

VCF MA Committee Member
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Welcome, BennehBoy!

You might want to watch one of the 1541 troubleshooting videos on the "Adrian's Digital Basement" or "Jan Beta" YouTube channels. IIRC, the light never going out means that the 6502 is never leaving reset but I'm far from the best C= troubleshooter.

- Alex
 

BennehBoy

Experienced Member
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Nov 2, 2021
Messages
61
Thanks - I'll have a look - although the reset pin is high, so it doesn't appear to be stuck in reset....

UC7 tests OK btw.
 

BennehBoy

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Nov 2, 2021
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I've worked through one of Adrian's Digital Basement videos with my scope, and CPU appears to be running code. Reset is High, clock is good, IRQ is high with a regular v short pulse low. R/W is pulsing. A15 starts as a regular repetition (FFFx vector?) then pulses randomly. I'm thinking the CPU is running code but that maybe something on the serial side is not working correctly/preventing a reset coming through from the C64?

I just tested all the new chips I inserted and the 74LS04's are tesing as 74HC14's. although they work fine in the drive which works...
 

Ruud

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I would check the 7406 and 74LS14 near the end of the left side of the drive.
 

BennehBoy

Experienced Member
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Nov 2, 2021
Messages
61
I've replaced both of those parts - the removed parts test fine though. UB1 has the factory reset fix ups so I haven't socketed the replacement as it was too fiddly to get the wires on the upper side under the socket - any tips there?

BTW when hooked up to the C64, and the computer is power cycled, the drive spindle will fire up briefly, so the drive is responding to reset from via Serial.
 

BennehBoy

Experienced Member
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Nov 2, 2021
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Well, I guess I'll keep pulling the other cmos logic and testing. As far as I can tell with the scope, the CPU is executing code. But the red light stays and the 64 reports device not present.
 

jafir

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Do other drives work on the C64? I had a C64 with a bad CIA that couldn’t communicate correctly with any 1541, but it seemed to work mostly normally otherwise.
 

ajacocks

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You might want to check Ray Carlsen’s 1541 diagnosis procedure:

https://portcommodore.com/rcarlsen/cbm/1541/fix1541.txt

He sways, for the red-light-on symptom:

At power up (without the computer connected) if the red activity LED stays on and the motor runs continuously, it means that the drive failed to complete its startup sequence. The most common causes are a bad DOS ROM UB4 (901229-xx) or failing 5V bridge rectifier, but other bad chips can produce those symptoms. The easiest place to check for correct voltages in and out of the power supply regulators is at the diodes CR2 and CR4 located near the two rectifiers. The anodes of those diodes are connected to the outputs of the +5V and +12V regulators. The cathodes (designated by a line or stripe at one end) show the unregulated source from the rectifiers that feed the regulators... about 10V for the 5V source and 20V for the 12V source. With drive startup problems, some chips to check are: UC4 (6502 MPU) and UC2 (6522 VIA). The smaller "glue logic" chips are pretty rugged, but do sometimes fail. Check UA1 (74LS14) and UD2 (7407)... they have also been known to cause those symptoms.

- Alex
 

BennehBoy

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Hi, thanks for that - I already went over Ray's diagnosis procedure and my drive's symptoms appear to fall outside of the scenario's he describes. The motor behaves exactly as it would for a drive that is fully working, it spins for a second or so after power on then goes off. The red light stays lit and the computer cannot find a device on the bus. When the computer is power cycled the drive responds by spinning the motor briefly as expected. A scope on the cpu seems to indicate that everything is as expected.

So far UC5 is the only part I've found that is bad - I've yet to pull some of the CMOS logic. I may go full on and start replacing discrete components - eyeballing the caps (ceramic & tantalum) shows no obvious offenders.
 

BennehBoy

Experienced Member
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Nov 2, 2021
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61
OK, some progress, replaced UD1 (MOS 7707) which appears to have failed. Red light goes out, 64 can communicate with the drive. Head stepping control seems to be faulty now - this board exhibits the same behaviour with the known good mechanism, the good board will drive this mechanism just fine. So, back to Ray's guide...
 

BennehBoy

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Nov 2, 2021
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I tested UE6 as per Ray's guide and it checks out OK. That means that UD4, a 9602, is probably the fault - I've ordered a few of these but they are only available overseas (I'm in the UK), so now it's a waiting game as I don't want to remove the one from the working drive to see if that resolves the issue.
 

VintageVic

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Statistically (with my say, a few dozen fixed 1541's) I'd say UD4 is unlikely suspect. Never seen one that failed, yet
it is still a possibility.

So, you're 1541 boots up now, great! it is most likely salvageable unit.

When you give a load command to the drive, does it spin the spindle and red light goes back on but the head
is not moving (or is very slightly trying to move but fails to seek track)?

If so, first focus on UC2. It may be on old ic socket that has corrosion. Check that every leg has a good contact and that
there are no broken traces. Also check traces and connections on every chip you have pulled (and hopefully installed a new ic socket).

Also check motor controller UC1. If you are skilled enough to pull these chips from the failed board, test them in a working board to make
sure the chips themselves are ok.

I have chased my own tail often because I have broken a trace or had a bad solder joint on IC socket that myself have put there.
Your board is fairly early model that had some factory defect in it. Probably the botch wire is there because someone has repaired
that factory defect.

Good luck!
 

BennehBoy

Experienced Member
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Nov 2, 2021
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When you give a load command to the drive, does it spin the spindle and red light goes back on but the head
is not moving (or is very slightly trying to move but fails to seek track)?

This is the exact symptom now. UD2 was failed, but only one gate - replacing it didn't really make a difference.

Unhappily, I now have the same issue on my previously working drive - pretty sure I misaligned P5 or P5 whilst rushing

I'll check continuity and traces - but given that I now don't have a full working drive, substitution won't help.​

Following this guide -> https://www.youtube.com/watch?v=3dc3xH_W8QA , I see pulse on pins 10 & 11 of any of the VIA's that I swap in there, but the pulses on pin 11 are only 1v peak to peak...
 

BennehBoy

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Nov 2, 2021
Messages
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I've managed to find a couple of NOS chips from Germany for not much money - fingers crossed.
 

VintageVic

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Going back over that video guide, I guess I've got 2 fried UC1's :(

it is a possibility, but not the only one.

The exact symptom I described earlier (and you confirmed to have) has occured three times on my drives.
In every case it was either bad 6522 IC at UC2 - or bad contact of that chip to its IC socket. Make sure you do not have
corrosion on that ic socket and that every leg gets a good contact.

Also, checking from the schematics, UD1 (7406) appears to be between UC1 (drive motor) and P7 (track select) connector.
Thus, it might be good idea to check UD1 as well.
 

BennehBoy

Experienced Member
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Nov 2, 2021
Messages
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UD1 is good.

I continuity tested the underside of the socket to the ic pins when inserted for UC2 and all is good. To be doubly sure I replaced the socket with a brand new double wipe one, no change. Swapped 3 other 6522's in and no change.

It's possible all the 6522's could be bad, but the one I'm testing with I bought as 'tested working', nothing to prove that it is of course.

And my thinking now is that perhaps the failure I've caused on my working drive, is what the other drive had actually had done to it at some point. Anyway, the UC1's should arrive in a few days, let's see how that goes.

Both drives may be in the bin if there's no change :D
 

VintageVic

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Location
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Absolutely not to the bin! Even if you fail to fix them, someone else might succeed and will give you
money for the drives even when they are defective.

The drive will not boot, if all of your 6522's are defective. There must be some life in some of those chips,
because your drive does boot up.
 
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