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Old 22 November 2020, 01:59   #1
B93
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Amiga 500 disk light always active

I am encountering an odd issue where the floppy light is always active on one of my amiga 500s. I have tried 2 different disk drives and different data and power cables. In this machine the light is always active however the drive functions as normal.

Both drives work normally in my other amiga 500 including the activity light.

Also tried two different keyboards just in case

Not sure what to try next?
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Old 22 November 2020, 07:20   #2
lesta_smsc
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Sometimes this is not a problem with hardware per say but the cable is connected the wrong way round. Match up Pin1 on motherboard to Pin1 on floppy disk drive. Usually indicated by printed white square block.
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Old 22 November 2020, 08:12   #3
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Quote:
Originally Posted by lesta_smsc View Post
Sometimes this is not a problem with hardware per say but the cable is connected the wrong way round. Match up Pin1 on motherboard to Pin1 on floppy disk drive. Usually indicated by printed white square block.
The OP wrote that the drive functions normal despite the permanently lit LED. Thus, the cable must be connected correctly.
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Old 22 November 2020, 08:32   #4
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I've heard it can work normally when connected wrong way around except it can damage the port... never personally tried this.

Swapping CIAs may be worthwhile.
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Old 22 November 2020, 09:42   #5
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The floppy light is the /INUSE signal, which is connected to the /MTR0 signal - so it could be that the /MTR0 signal is always active (low).

The /MTR0 comes via the Even CIA (U8), Gary (U5) and the 74LS38 (U36) and various other smaller components. Use the schematics and amigapcb.org to find the traces.

Inspect and reseat the socketed chips. Swap CIA chips to see if it changes behaviour.

I have a summarised diagram of the floppy drive connections, which you might also find useful:

https://sites.google.com/one-n.co.uk...ve-connections
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Old 22 November 2020, 10:50   #6
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Quote:
Originally Posted by lesta_smsc View Post
I've heard it can work normally when connected wrong way around except it can damage the port... never personally tried this.

Swapping CIAs may be worthwhile.
This can't work. On the floppy port, all odd lines are GND (thus shorted together) and (almost) all even lines carry signals. By reversing the cable, you effectively short all floppy signal lines to GND (via the power connector).
And because the signals are active low, this will pull down the MTR0 line (as mentioned in the previous post), making the floppy LED light up.

Last edited by torsti76; 22 November 2020 at 10:55.
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Old 22 November 2020, 11:37   #7
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Normally, when the floppy LED is on all the time it means either the floppy data cable is reversed or there is no power getting to the floppy drive.

But in both cases the floppy drive wouldn't be working.

The OP suggests the drive is working, but the floppy LED is on all the time.
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Old 23 November 2020, 21:34   #8
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Have double checked and cable is connected correctly and have swapped cias and reseated Garry and no change still same issue
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Old 23 November 2020, 22:06   #9
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Have beenfollowing the traces from u36 and noticed that there is an issue with c711. Would this be the issue?

Thanks for the replies so far
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Old 23 November 2020, 22:54   #10
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C711 is part of the reset circuit, nothing to do with the disk drive.

Confirm that you mean C711 and what the 'issue' with it is.
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Old 24 November 2020, 00:22   #11
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I was checking for an broken circuits following every race that I could see off of u36 but never mind I’m not that good with a multimeter dunno what I was doing but it is fine now.
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Old 24 November 2020, 03:17   #12
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Also tried taking Gary from another a500 and still same issue so Gary and cia appear to be ok
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Old 24 November 2020, 11:06   #13
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If you look at the (A500 rev 6A) schematics:

https://www.amigawiki.org/dnl/schematics/A500_R6.pdf

You will find the circuit for the keyboard:



You can see how the _MTR0 is related to the INUSE signal, which is used for the Floppy LED.

I would inspect Q503 transistor (and R504 and R506), which is responsible for turning the Floppy LED on and off, based on the _MTR0 signal.

Use amigapcb.org to help location the components and traces (although this only has board view for an A500plus rev8A).

Last edited by solarmon; 24 November 2020 at 11:15.
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Old 24 November 2020, 19:36   #14
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Have tried swapping the keyboards over and no change. On this Amiga still have the light stuck on. On my other good Amiga both keyboards work normally.
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Old 24 November 2020, 19:45   #15
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Either transistor is dead, or a short to ground on the base or R504. Do a continuity test to ground on both sides of R504, it should not be closed circuit.
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Old 24 November 2020, 21:42   #16
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Quote:
Originally Posted by B93 View Post
Have tried swapping the keyboards over and no change. On this Amiga still have the light stuck on. On my other good Amiga both keyboards work normally.
Sorry, I did not mean it was a circuit for the keyboard itself. It is for the keyboard connector on the A500 motherboard. You need to check those components on the A500 motherboard.
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Old 24 November 2020, 21:45   #17
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All good I just opened up again and realised what you meant. Am just taking the board fully out of the case as I can’t get a good angle at one of the legs of Q503.
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Old 24 November 2020, 22:47   #18
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So started buzz testing Q503 to keyboard ground pin and looking from top down the left leg does not buzz right and center do. Nothing from r504 either side. R506 top side does buzz to ground

Last edited by B93; 24 November 2020 at 23:18. Reason: Correction
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Old 25 November 2020, 01:00   #19
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So have given the components a good clean with ipl alcohol and all resistors buzz to ground now from both sides left leg of q503 buzzes to pin 8 but not ground
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Old 25 November 2020, 03:40   #20
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So I desoldered cleaned and re soldered the left leg and now get a buzz to ground but still light is stuck on
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