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1541C drive stuck at loading

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powerlot

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Hi all

I've got a 1541C here. While I am familiar with the older models I have trouble finding the schematics for this one. My board revision is 251854 REV A, it's the one that is half-size the drive and has a hybrid IC. The heads measure ok therefore I want to try to repair the board.


The symptoms are exactly as described here:


When I switch it on it sounds normal, but when its reset by switching on the C64 it spins longer than usual. Trying to load from it results in everything not responding anymore.

The 7406 next to the serial connectors appears to be ok, I'm thankful for any advice in narrowing it down where else to look.
 
Yep, it turns off. It behaves like a working 1541 until you attempt a LOAD, or until you turn on the computer if the longer spin time is considered abnormal when it gets reset.
 
No I haven't, a dirty head should result in a "File not found" error, meanwhile I don't get any response from it when I issue a command, it's just hanging up. Even without a floppy inside it should result in an error, which it doesn't
 
So your drive boots up fine, but gets stuck at 'searching for...' and you're stuck with that situation?

A good candidate is to replace UB1 (74LS14) to a new one. This often fails causing this issue.
However, it also possible and likely that your UC1 (6522) has failed, it can cause the same issue.
Failing 6522s are also common.

If your UC1 and UC3 are on sockets, you can easily swap them and see if the drive boots or if the
behaviour changes in any way. If no change you might want to replace that UB1 next.
 
VintageVic said:
So your drive boots up fine, but gets stuck at 'searching for...' and you're stuck with that situation?
Click to expand...
Correct, but it would be nice to measure and understand what exactly causes this issue instead of replacing chips and hope for the best. I'll try to find a working board / drive and see if I can find some differences in the serial communication or drive control (where I guess is the fault when looking at the parts you mentioned)
 
In my experience (in repairing several dozen 1541s) it is often those ICs that I mention that
cause 'searching for...' error. Granted, these are probably not the only possible causes for it.
Are you familiar with Ray Carlsens work and documentation? Here:


Yes, a working board is very useful to check if any certain IC will work or not. I always use that
when repairing a drive.

Sometimes shorted larger ICs like the 6522 can get hot, but it does not always happen.
 
Thanks for the link, it appears to have documentation for the board I have :)
I'll study it and get back with further questions or results
 
I looked at the schematic today and started scoping the signals coming in, starting with RST, since it sometimes spins very long during power up. The capacitor in the reset circuit was out of spec (C13, 47uf/16V), but changing it didn't make a difference.

Next was the ATN line. Here I found that Pin 6 of the 74LS14 didn't invert the signal (as @VintageVic mentioned it's likely to fail):

1704130874195.png

I don't have a spare for this IC, but my TL899 seems to agree:
1704130952839.png

I'll update the thread when I managed to find a replacement and change it.
 
Great, you'll get it to work after replacing that IC. That would have been my first guess as I mentioned
above. It fails easily, if the C64 and/or 1541 are powered on and the serial cable is connected. Static
electricity is the killer, so always connect cables when the devices are powered off.

While at it, clean the head and lube the rails and you'll probably have perfectly working drive for a long time.
 
Yup, as you said: everything working well now. I noticed the reset behaviour on these type of boards is different. The reset sequence takes longer, but returns the heads to the track 0 position (and stops the head when reaching track 0 sensor). This didn't happen with the fautly 74LS14.

Thanks for your help! Cleaned, lubricated and closed up again.
 
Indeed, the head 'rattles' or bangs the head a long time during boot.
It almost feels like there is something wrong with it after you have used the older version
of the drive. And it annoys me at least. But this is intentional, coded to the newer ROM.

Why you might ask? Well the older drives may end up in situation where the head 'gets lost', it goes too far
into the other end of the rails. Then, those drives may result load errors or read errors. Nothing
actually wrong with the drive but it refuses to load programs. Operation can be returned to normal by physically moving the head
(while drive is off) or by initializing the drive by program. Cheap fix - reprogram the ROM to return the head
with each reboot! Problem solved, commodore style.
 
It is interesting the way TTL IC's connected to connectors which interface with other equipment can fail. It probably is related to charge storage in the cables, or hot plugging effects where the two system grounds are not initially at the same potential initially and a discharge current flows with hot plugging. One thing if you are forced into hot plugging it pays to touch the connector shells together first for a moment, before plugging it in properly.

It would probably help if a pair of protection diodes were added to the input pins of the TTL gates to prevent the pin being forced more than +0.6V above the +5V rail or pulled more than -0.6V below ground. And a low value series resistor such as 47 Ohms would help limit the peak current too. For IC's driving output lines on a cable, a low value series resistor can help a little too, 15 to 33 Ohms and it can suppress ringing effects. It is not uncommon that receiver/driver IC's on serial cards fail too.
 
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