C64 Diagnose: Problems I’ve repaired

  • BAS ROM or RAM U3. Pwr up: Title page — Char (garbage) „flash“ but

disappear (blank „page“)

  • PLA U17, KER ROM U4. Pwr up: No title page (blank raster)
  • VIC U19. Garbage screen but loads/runs pgm
  • RAM chip. All garbage screen — no load or run
  • OSC U30. After warmup TV loses horiz sync
  • Pwr supply. No +5V, raster — no title page — no pwr LED
  • C85, 47 [ ?? ]. Pix goes out of horiz hold after 4 hrs+
  • RAM chip. Garbage/flashing chars. Within 1/2 sec of pwr up — Bad

RAM IC (hot)

  • SID chip. Music player: note „hang up“ — „muddy“ sound
  • SID chip. Blown by shorting Audio-In after pwr up — Title page:

goes to garbage

  • U8 buffer. C-64 initializes drive at pwr up but „device not

present“ error

  • U17 PLA. Blank raster
  • U17 PLA. Intermittent cursor on opening screen … it will not move
  • U2 CIA. Semi-random „squares“ on screen at pwr up (no title page)

U2 in 64C. Was 8521RO, I subbed 6526A, works OK (Old IC got hot, too)

  • U17 PLA. „Rainbow“ effect on opening screen characters
  • 1 RAM (not hot!). „Out of memory error“ at pwr up instead of

opening screen

  • SID chip. Erratic mouse pointer, music: notes „hang up“, „muddy“

sound

  • U31 Clock Gen. Horiz way off freq. on monitor
  • U1 CIA. Partial pgm load or screen freeze after run a while, „5“

key repeats if held down, „flashing“ cursor

  • U17 PLA. Pgm crash — back to Basic screen, new chip bad!

End of the Project 64 etext of the Ray’s C-64 Problems Solved.


Various usenet articles about 1541 repair.


Newsgroups: comp.sys.cbm
Subject: Re: 1541 Repair
From: Raymond Carlsen <rrcc@u.washington.edu>
Date: Tue, 4 Jun 1996 17:35:56 -0700

> I have a 1541 that just spins forever when turned on. Both LED’s
> are on. Anyone have any pointers to repair info on this model?

Steve,

    That usually points to a bad DOS ROM chip. It's CBM # 901229-03

(old drives) or -05. A bad 6522 is also a possibility.

Ray Carlsen
CARLSEN ELECTRONICS… A leader in trailing-edge technology.


Newsgroups: comp.sys.cbm
Subject: Re: 1541 Repair
From: Brian Heyboer <bjheyboer@space.honeywell.com>
Date: Wed, 05 Jun 1996 12:08:15 -0400

Steven J Tucker (dh395@cleveland.Freenet.Edu) wrote:

> I have a 1541 that just spins forever when turned on. Both LED’s
> are on. Anyone have any pointers to repair info on this model?

There are several things that can cause this, but the most common is a
bad 901229-05 ROM. The 6502, 6522, RAM, and a couple of the TTL chips
in the reset circuit can also cause it, pretty much in that order of
likelyhood, assuming it just „went bad“ and didn’t fail because of
something you did to it.


Newsgroups: comp.sys.cbm
Date: Wed, 5 Jun 1996 06:54:24 -0700 (PDT)
From: Raymond Carlsen <rrcc@u.washington.edu>
To: Cris Berneburg <74171.2136@compuserve.com>
Subject: Re: 1541 help!

> Well, I’ve done it again. I bought another 1541 drive that doesn’t
> work. The guy told me it was „out of alignment“. Needless to say, that
> was an understatement. So, I need some help trying to fix it.
>
> Here are the symptoms. When I power it on, the red and green LED’s
> both flicker on for about 1/4 of a second, then stay off. The drive
> motor spins constantly when powered on, and will not respond to
> commands. Do you why that is?

The power supply is probably failing. Note that the power indicator is
going out under load. Since the +12 volts runs the motors, try the +5v
line… bet you’ll find a bad bridge rectifier or flaky regulator.

Ray Carlsen
CARLSEN ELECTRONICS… A leader in trailing-edge technology.


From: <judd@merle.acns.nwu.edu>
Date: Wed, 5 Jun 1996 09:27:35 -0500
To: 74171.2136@compuserve.com
Subject: Re: 1541 help!
Newsgroups: comp.sys.cbm
Organization: Northwestern University, Evanston IL

In article <4p3vbp$s7i@dub-news-svc-4.compuserve.com> you write:
>Well, I’ve done it again. I bought another 1541 drive that doesn’t
>work. The guy told me it was „out of alignment“. Needless to say, that
>was an understatement. So, I need some help trying to fix it.
>
>Here are the symptoms. When I power it on, the red and green LED’s
>both flicker on for about 1/4 of a second, then stay off. The drive
>motor spins constantly when powered on, and will not respond to
>commands. Do you why that is?
>
>Here’s what I’ve tried to find out where the problem is. I swapped the
>main processor board with a working drive, and it seemed to power up
>OK. So I put the original, faulty board back in and tried swapping out
>chips. I replaced the 901229-05, 6502, both 6522’s, and the tiny
>EL7407-0284 chip with no posititve effect. No other chips in this
>drive have the chip sockets for easy replacement.
>
>901229-05 6502
> 6522 EL407-0284
> 6522
>
>If you know what the problem is or have some suggestions on what I
>could do to diagnose it, PLEASE! Thanks.
>
>——-
>Cris „PC-Geek“ Berneburg <74171.2136@compuserve.com>
>The Basic Bombardier, Manager of Project 64
>http://ourworld.compuserve.com/homepages/pcgeek/
>PGP public key A1CE4355 available on keyservers
>

The LED flicker tells you immediately that there is a problem with
the power — they aren’t connected to anything else. Most likely
you have a blown rectifier. There are two of them, at the back
of the drive, and they look like this:

	  _____
	 /    |
	/     |
	|     |
	-------
	| | | |

That is, a notched square with four pins coming out of the bottom.
After you power up the drive, at least one of them (the smaller one)
will probably be hotter than blazes.

The fix: go to Radio Shack and buy a new, heftier one for $1 or so,
and solder it in. These are full-wave rectifiers of course.

-Steve

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