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From: Juhani Jaakola on 22 Mar 2010 09:46
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.

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

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.

Regards, JJ
From: Andreas Meerbann on 22 Mar 2010 11:24
Hi JJ,

why don't you just try to get another 1541 on ebay? You could swap
chips between both 1541s and check if that changes anything.
Few days ago I managed to get one (-II version) for 1€ (+5€ shipping)
- vendor said he could not test and it had no power supply. Once I had
it in my hands I found that it was actually working fine - so I
dropped my plan to use it as organ donor for another defective one and
now just use the new...

>
> 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.

That's true... but you've checked the voltages so that indicates that
capacitors are more or less OK. CBM stuff is not that old to have
dried capacitors but old enough to have the good caps that have been
made before the big capacitor plague (see http://en.wikipedia.org/wiki/Capacitor_plague)
Have you checked the board for bad soldering? have you removed and re-
inserted all socketed chips? CBM boards are quite rock solid by means
of soldering and contacts but you never know...

In case you have a scope you could check if there is some activity on
the CPU pins - starting from clock signals then continuing with the
data and address lines.

cheers,
Andreas


From: JPPBM on 26 Mar 2010 23:30

> 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.
>
> Regards, JJ

IC Chips are just like any other device. While you may feed it +5V;
Inside, it can become on OPEN circuit which can be detected by the
chip being unusually cool sometimes. Or the chip can develop a SHORT
circuit which would make the chip very hot --like how RAM usually goes
bad. You need to look at the outputs of the chip to determine if it is
working correctly rather then seeing if there is just a +5V feed to
it....
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