1541C unusual "Shorter Board" version - J3?

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rmzalbar
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1541C unusual "Shorter Board" version - J3?

Post by rmzalbar » Wed Oct 16, 2019 5:11 am

Hi, I have acquired a 1541C that isn't working, due to an open bridge rectifier, which will be rectified shortly.

However, when I initially opened it up to see what was the matter, to my surprise, it has a 1984 made-in-Japan "shorter board" PCB 251830 rev. A assy 250442, rather than the "shortest board" typical to the 1541C.

At first I thought, oh darn, someone reused the shell and put a different drive inside. However, I can see the track 0 sensor and it is wired to pins 4,5,6 of P6. I looked at those pins and they don't connect anywhere useful on the PCB.

My questions are,
1. What the heck?
2. ...?



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eslapion
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Re: 1541C unusual "Shorter Board" version - J3?

Post by eslapion » Wed Oct 16, 2019 6:45 am

rmzalbar wrote:
Wed Oct 16, 2019 5:11 am
Hi, I have acquired a 1541C that isn't working, due to an open bridge rectifier, which will be rectified shortly.

However, when I initially opened it up to see what was the matter, to my surprise, it has a 1984 made-in-Japan "shorter board" PCB 251830 rev. A assy 250442, rather than the "shortest board" typical to the 1541C.

At first I thought, oh darn, someone reused the shell and put a different drive inside. However, I can see the track 0 sensor and it is wired to pins 4,5,6 of P6. I looked at those pins and they don't connect anywhere useful on the PCB.

My questions are,
1. What the heck?
2. ...?
Drives with the very short board of the 1541C had some issues...

1. They were a serious b..tch to repair. The big flat module combined virtually all discreet logic components and all you could do if it had problems was replace the whole thing and it was expensive and fully custom from Commodore
2. It had a bunch of compatibility issues.

Many people intentionally reverted to the older boards used on later standard beige 1541 drives assy 250442 and 250446. The only consequence, as you saw, was the track 0 sensor ended up connected to nothing.

Today, I don't know of anyone owning a 1541C with the very short board which is working okay. They all died and all of them because of the big flat custom SIP chip.
Wealth, like happiness, is never attained directly. It comes as a by-product of providing a useful service. -Harland D. Sanders

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Re: 1541C unusual "Shorter Board" version - J3?

Post by rmzalbar » Wed Oct 16, 2019 8:25 am

eslapion wrote:
Wed Oct 16, 2019 6:45 am
Drives with the very short board of the 1541C had some issues...

1. They were a serious b..tch to repair. The big flat module combined virtually all discreet logic components and all you could do if it had problems was replace the whole thing and it was expensive and fully custom from Commodore
2. It had a bunch of compatibility issues.

Many people intentionally reverted to the older boards used on later standard beige 1541 drives assy 250442 and 250446. The only consequence, as you saw, was the track 0 sensor ended up connected to nothing.

Today, I don't know of anyone owning a 1541C with the very short board which is working okay. They all died and all of them because of the big flat custom SIP chip.
Ugh, okay. I suppose I dodged a bullet, there. I bought it because I was curious about the track 0 sense behavior, but it was just $12, and if a $1 part gets me another solid unit then I can't be harmed. Thank you!

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Re: 1541C unusual "Shorter Board" version - J3?

Post by Zippy Zapp » Wed Oct 16, 2019 4:06 pm

eslapion wrote:
Wed Oct 16, 2019 6:45 am
Today, I don't know of anyone owning a 1541C with the very short board which is working okay. They all died and all of them because of the big flat custom SIP chip.
You do now. LOL. I still have my original 1541C with short board and it is my daily drive. I even kept the box, manual etc. as it was in my storage for a number of years. My original 1541 bought in 1983 was a VIC-1541, a white one and that one lasted through most of the 90's with daily use. It's funny because back then when the 1541C was bought with a 64C we did all kinds of stuff to it and didn't know that the drive was a piece of junk. I mean, the head chatter when you power it on or reset it is annoying.

I have another 1541C I picked up some years ago and enabled the track zero sensor so it no longer chatters. I should hook that one up and test it out for compatibility. Supposedly that is why it was shipped disabled.

At any rate my daily 1541C was one of the drives used to run our BBS from 1988-1991 and had a SuperCard+ installed in it for backing up our games at the time. Still have the SC+ but it is no longer installed in that drive.
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Re: 1541C unusual "Shorter Board" version - J3?

Post by rmzalbar » Wed Oct 16, 2019 5:42 pm

Eslapion, is the SIP you are referring to the "251853-02 IC Hybrid" at one end of the board? If so, I found one document that says it's a generic H36A2U57 of which there seem to be small surplus repositories here and there.

http://personalpages.tds.net/~rcarlsen/ ... /1541C.txt

Still, if it's a high failure part, still a pain. I am interested in the compatibility issues with the 1541C though - would it be different from the 1571?

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Re: 1541C unusual "Shorter Board" version - J3?

Post by eslapion » Wed Oct 16, 2019 8:07 pm

rmzalbar wrote:
Wed Oct 16, 2019 5:42 pm
If so, I found one document that says it's a generic H36A2U57 of which there seem to be small surplus repositories here and there.
There is absolutely nothing generic about that component. The number printed on it is sometimes different. See: https://www.nightfallcrew.com/wp-conten ... g_2165.jpg

Fundamentally, apart from the track 0 sensor, all VIC-1540, VIC-1541, 1541 and 1541C are almost exactly the same from an electronics point of view.

From the VIC1540/VIC1541 to the standard 1541, a group of 13 logic ICs were bunched together to create custom IC 325572-01. I studied the details of that IC and exactly which ICs were integrated and this allowed me to create GandALF to replace it.

From the 1541 to the 1541C, another bunch of ICs, a mixture of analog and digital were put together.

The large SIP has a lot more integrated into a single PCB which was dipped in plastic to seal it. If you were able to remove the plastic you'd find a cluster of surface mounted parts that are all essentially the same as those in the old VIC-1540. If I were to meticulously compare the schematics of a VIC-1541 with that of a 1541C, I could certainly determine exactly what's inside and make clones but it would be very long and tedious work.

Almost only the power components, the ROM, RAM, CPU and VIAs are left outside. Even the ROMs were combined in a single IC and glue logic changed accordingly.

If you want to modernize a 1541 board, change the 325572-01 to a GandALF (which uses modern CMOS and doesn't heat), change the 2 MOS 6522 for 2 new W65C22N (DON'T use Rockwell's R65C22 or W65C22S because the IRQ isn't made to be shared on these), change the MOS 6502 for a R65C02 and change the large capacitors for new ones. Your drive will no longer heat and will consume half the power.

Added edit:
Doing a top quality alignment on a 1541 drive with an oscilloscope usually involves tapping the output of the video amplifiers (NE592) and that's essentially impossible with the 1541C as they are sealed inside the large SIP. Doing the alignment on these drive usually involves taking the mecha and attaching it to an older drive or temporarily switching the board for an old one where you have access to the amplifiers. It sucks!
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Re: 1541C unusual "Shorter Board" version - J3?

Post by Zippy Zapp » Thu Oct 17, 2019 2:21 pm

eslapion wrote:
Wed Oct 16, 2019 8:07 pm

There is absolutely nothing generic about that component. The number printed on it is sometimes different. See: https://www.nightfallcrew.com/wp-conten ... g_2165.jpg
I think the confusion comes from this line in the above referenced text file on Ray's site:
UD1     251853-02 (GENERIC H36A2U57)  HEAD AMPLIFIER LSI
     Read and/or write failure
So out of all the 1541 drives which is the easiest to work on from your perspective? I have an original 1541 that is marked VIC-1540 on the rear and the board is 1540 full length. Tons of discrete logic chips on this board. From a trouble-shooting perspective I guess this could be good, since replacements are probably still made and bad since it is more to troubleshoot.

Have you come across anything in the past that mentions or notes the compatibility issues with the 1541C with or without the track zero sensor enabled? I think it would be an interesting read.
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Re: 1541C unusual "Shorter Board" version - J3?

Post by eslapion » Fri Oct 18, 2019 5:25 am

Zippy Zapp wrote:
Thu Oct 17, 2019 2:21 pm
So out of all the 1541 drives which is the easiest to work on from your perspective? I have an original 1541 that is marked VIC-1540 on the rear and the board is 1540 full length. Tons of discrete logic chips on this board. From a trouble-shooting perspective I guess this could be good, since replacements are probably still made and bad since it is more to troubleshoot.
It depends whether you want to fix a disk drive or experiment on it.

I have a VIC-1541 with the full length board too and it is with this disk drive that I was able to develop GandALF as you can attacha a scope or logic analyzer just about everywhere. If it had trouble, however, there is a lot of chips to check.

The standard 1541 is easier to diagnose and repair, assuming you can get all parts.
Have you come across anything in the past that mentions or notes the compatibility issues with the 1541C with or without the track zero sensor enabled? I think it would be an interesting read.
I'm a hardware guy. This is really a question for software guys. Pete Rittwage who has a lot of experience is probably the guy to ask.

Sidenote, I just bought a fully functional 1541C with very short board today.
Wealth, like happiness, is never attained directly. It comes as a by-product of providing a useful service. -Harland D. Sanders

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Re: 1541C unusual "Shorter Board" version - J3?

Post by eslapion » Fri Oct 18, 2019 3:13 pm

I just noted a problem with the 1541C. If you use the very short board, every time you turn on the drive, it will knock the stepper motor down the track 0 stop (like formatting does) even with the presence of a track 0 sensor. Guaranteed drive misalignment just by turning the drive off and on...

Another good reason to switch the mainboard to the older type.
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Re: 1541C unusual "Shorter Board" version - J3?

Post by rmzalbar » Sat Oct 19, 2019 1:54 am

I've read that the track 0 sensor is disabled from the factory - does this knocking happen even if you cut the disable jumper?

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