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From: John Smith on 27 Jul 2010 12:34

"Juhani Jaakola" <juhani.jaakola(a)kolumbus.fi> wrote in message
news:6a343a56-ac5a-4072-92cc-42dcf24acb2a(a)q21g2000yqm.googlegroups.com...
> After been several years in my attic my 1541 drive is failing: when I
> power up the drive (without computer connection) the green and red
> LEDs stay on and the motor keeps on spinning.
>
> I have read http://personalpages.tds.net/~rcarlsen/cbm/fix1541.txt by
> Ray Carlsen.
>
> I have checked that my 1541 has correct 5V and 12V voltages on diodes
> CR2 and CR4.
>
> And I have checked that when I remove the ROM (901229-05) I can read
> its contents with GQ-4X True USB Universal Programmer, and its
> contents match exactly the image file that I have downloaded from
> http://www.zimmers.net/anonftp/pub/cbm/firmware/drives/new/1541/
>
> Anyway, I have ordered a new 901229-05 ROM and I'll try that when I
> receive it.
>
> I have removed every IC that is mounted on a socket and re-mounted it
> again in order to correct any bad contact. But the symptoms remain the
> same!
>
> Please suggest me what I should try next!
>

The 6502, swap with a known good one.


From: Clocky on 27 Jul 2010 12:52

"Juhani Jaakola" <juhani.jaakola(a)kolumbus.fi> wrote in message
news:ca611a5d-c96a-4320-8137-5f7afd93f9e2(a)g19g2000yqe.googlegroups.com...
On Mar 18, 10:54 pm, Sam <siemappel...(a)quicknet.nl> wrote:
> I had the same symtoms with my 1541. But it is normal that the green
> led stays on.. :-)
>
> And I'm sure your 901229-05 ROM is allright.
>
> I had the same symptoms with a 1541:
>
> The red led stays on and the motor runs continously. After swapping
> the DOS rom UB4, the 6502 MPU and the 6522 VIA at UC2 with known good
> ones I got no result. After that I unsoldered UA1 (74LS14) and UD2
> (7407), who also been known to cause this symptoms, placed sockets and
> new chips and found out that UA1 was bad. I repaired a lot of 1541's
> and a bad logic chip is rare in my experience.
>
> Mostly the VIA (6522) at UC2 is bad. If you don't have a spare 6522
> you can try to swap the both VIA's at UC2 and UC3. When UC3 is bad the
> drive powers up and resets normally. The screen indicates "Searching
> for......" when a load is attempted but no motor runs and the red led
> doesn't light.
>
> Greetings, SAM

Thanks for your help to everybody! However, I still have the problem.

I do not have a spare 6502, so I haven't been able to swap it yet. But
I measured that its VCC has +5V.

I swapped the two 6522's in my 1541, but it didn't change anything.
Both 6522's have +5V on their VCC pins.

+5V on VCC is expected whether the chip is bad or not, that in itself proves
nothing.


Is there anything other I can do except soldering (and swapping the
6502)? Is there any measurements that I could make?

Yes, the last two 6502's I diagnosed where by simply taking voltage readings
at the address lines. On both occasions one of the address lines stayed low
at all times indicating that there was a problem with either that address
line being pulled low by something external or a faulty 6502. I then
isolated that CPU pin and remeasured the voltage which confirmed that the
CPU was open on the address line.


By the way, is there any way to reset the drive when it has been
powered up? Perhaps a warm boot could succeed??? Should I try
grounding the NMI pin of the 6502 or is there a better way?

It's strange that a broken chip is the most common cause for a
failure... I thought that the most common reasons for failures would
be bad contacts (for example in IC sockets), bad soldering or dried
electrolytic capacitors.

Not really, IC's, particularly those that get warm suffer from thermal
cycling which over time will cause an IC to fail, and power supplies with
marginal capacitors (due to age) increase the voltage spiking effect on
powerup.
Dry (or cold as they're also known as) solder joints can be the problem but
they can usually be found by close visual inspection.


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