Excelerator plus schematic?

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Gyro Gearloose
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Excelerator plus schematic?

Post by Gyro Gearloose »

Can't find a schematic for this drive. Visual inspection showed something weird. Pin 9 of this 74LS133 has been cut. Since this is the output of the logic gate, I can't see the value of cutting this pin off. It looks really neatly cut too, like someone put some effort into it.

...but why?
Image

Of course, I can't find a 74LS133 in my parts... Maybe I should call it a night.


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Re: Excelerator plus schematic?

Post by Gyro Gearloose »

Really? No schematic? Plus there seems to be at least two versions of this drive. I saw a video

where the board is clearly different; the DIP switch has 3 positions instead of 2; there is only one ROM installed (there are jumpers under the board for different address lines I guess), and some chips are not installed.
The daughterboard also comes in different flavors.

But what I can't figure out is the reset circuit of the drive. All the reset lines are connected together but nothing is driving them. The reset signal also goes to a 74LS86 XOR input... and that output goes to a 100uF capacitor?!? It's a mess.
My drive powered up once "normally" but since then has been unpredictable. If the reset line is floating, well that explains that.

What a whacky drive.
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Re: Excelerator plus schematic?

Post by Gyro Gearloose »

OK my mistake, I swapped U17 for U15 on my preliminary hand-drawn reverse engineered schematic. So there is something driving the board's RESET line, it's just a piss-poor circuit. Once I drew it out I tried to see if I was drunk or what but no, that's the circuit...

Image

You'd expect to see all kinds of weird oscillations and unpredictable waveforms at the output of U17 pin 6 as that cap sloooowly charges at a non-Scmitt input; indeed there's a lot of crap and runt pulses and garbage on pin 6 for a brief moment but eventually it snaps out of it and goes high. This is crap, I'll try to shove in a DS1233 chip instead of the cap. It was a weird design to use that XOR as a buffer, it would have been better to use a transistor circuit or try to swap a LS14 gate for a LS86.

But the drive was still wonky as it would sometimes keep the motor spinning, sometimes stop after 5-10 seconds, sometimes try to seek the head a bit, never the same. Usually I troubleshoot power supply, reset line, clock lines, so now I looked at the clock lines. The 6502 has a nice 1.000MHz clock, but the two 6522s had a weird undefined 0.3 voltish noisy line at their clock inputs.

First I thought those clocks would come from the daughterboard but instead the 6522 clocks are driven from the 6502 clock output. I don't know much about 1541s so maybe that's the way they all work. In any case I traced out the line and the line does connect the 6502 clock out to both 6522 clock in.

I noticed there was some awful bodge wire on the 6502 with a really bad soldering job so I decided to take a real close look at it, and yup, one of the strands of the wire managed to sneak behind the 6502 on pin 39; the clock out.

Image

I'm done for tonight but I hope I don't need to remove that 6502. If I can remove that short/intermittent contact caused by that strand, I hope the 6502 clock output wasn't damaged by that.
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Re: Excelerator plus schematic?

Post by Gyro Gearloose »

I removed that crappy wire, it wasn't even properly soldered. Once I unsoldered the wire from the 6502 the wire just popped off from the other end.
That bit of stray wire under the 6502 was removed and now I have a clock signal on both 6522s.
Great, now it powers up the same way every time.
Without a computer attached, the drive powers up and blinks red twice and keeps spinning. It continually blinks red twice.
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Re: Excelerator plus schematic?

Post by Gyro Gearloose »

Hey what's up everyone, hope I'm not too tedious with this drive nonsense I've been exploring.
I found the 1541 service manual here
http://personalpages.tds.net/~rcarlsen/ ... (1985).pdf
and it's quite interesting.
First it mentions information about the head:
The Read/Write head uses a glass-bonded, ferrite/ceramic head. Track-to-track erasing is
accomplished by the straddle erase method. The surface of the Read/Write head is mirror-
ground to minimize wear of the head and media. Also, the head is designed in such a way that
the maximum signal can be obtained from the media surface.


Then it says the 1541 has flash codes. I never knew that. :lol: Or I forgot! :o

The 1541, upon power-up, goes through its own internal diagnostic. If an electronic problem is detected,
it's indicated by flash code. The led's will blink a number of times, pause, and then flash again until the
problem is corrected.

Number of flashes 2 Possible failure Zero page


Zero page is in RAM, so this sucks for me, either the 6502 was damaged from that little short circuit or the SRAM chip is bad. I do not have a 2Kx8 SRAM in my parts bin. Worse, I would have to desolder it and test it. I guess before I go that route I should check all address and data lines and chip select logic.

It also shows the 1541 reset circuit, it is better than what's in the Excelerator.
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Re: Excelerator plus schematic?

Post by Gyro Gearloose »

Next chapter in my boring saga. I ordered some DS1233s from eBay, instead I received the DS1233M; it generates a reset pulse but does not include the pushbutton debounce feature. Good enough, I removed the dodgy capacitor reset and slapped in a DS1233.

I get a nice crisp reset pulse. Blue trace is the 5V power supply, trigger is set to its rising edge, the yellow is the reset line.

Image

Image
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Re: Excelerator plus schematic?

Post by wes256 »

Sorry for dredging up an old post but has anyone verified/validated that the 3rd switch is for selecting the empty ROM position ?
I tried installing a 27128 16K rom in the socket as the operating manual suggested but the drive won't boot successfully.
Tried both a Jiffydos rom and another CBM dos rom from a 1541-II (as was the original dos in this drive) but it hangs on power up.
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Re: Excelerator plus schematic?

Post by Gyro Gearloose »

Don't forget the data pin swapping. These drives are REMOVED, poorly designed, badly built, unreliable.
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Re: Excelerator plus schematic?

Post by Gyro Gearloose »

How the hell is "little" "monsters" become "REMOVED"? Don't like Lady Gaga?
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Re: Excelerator plus schematic?

Post by rmzalbar »

They do have swapped pins; D15 and D14.

https://portcommodore.com/rcarlsen/cbm/fsd-2/FSD-2.txt

Ray Carlsen suggests a straightforward way of getting normal CBM rom to work by swapping the two data pins, but since you have an eprom you might prefer to be clean about it and burn the correctly scrambled 16K ROM, which Gyro posted earlier:

https://www.commodoreserver.com/PublicD ... 950E5F02EB

If that's no good, I'll read one of mine out and post it here. Or if you prefer I can mail you one of my original ROM.

I have two of these REMOVED. They both run perfectly, upgraded to Jiffydos kits directly from Jim Brain. They are direct drive and relatively small and quiet, making them nice to use. Forget anything you read in the manual about being faster or mostly compatible: they perform identical to the 1541 and are fully compatible. This works in our favor because of the many alternative DOS for the 1541, although mod space is rather tight in the case. I ended up soldering in my Jiffy because I felt space was too tight for a socket.
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