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YACFAB Yet Another Classic Faulty AB: continuous restart, multiple reboot

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I'm not really lucky with Classic AB :(

Story short:the Mac is rebooting continuously.


Longer story;:

I'm restoring a Classic AB. the low voltage caps were severely leaking.

I replaced them all, so do the MOSFET which was blowing the fuse.

Given the many recommendations I found in other threads I also replaced the TDA 4605.

In the picture I marked in red the replaced parts, in green the one which tested ok (de-soldered and tested).

The voltage are very stable V5.05 BUT when I connect the MB and the CRT it behave as it follows:

initially, when cold, the screen flickers for a few seconds, than I hear the happy chime

it keeps playing the chime several times, every second or so, but less and less frequently until I'm able to see the happy face and even the desktop, but inevitably it restarts.


- The digital multimeter shows no drop in the voltage when it restart but I might not be fast enough: I do not have an oscilloscope for a proper signal analysis.

- I noticed that it is less prone to restart if I use an external power supply for the SCSI2SD.

Some component are unable to provide the required current load, but which one?

Where should I turn my attention?




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  • 4 weeks later...
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Dear pals,

I've replaced ALL the caps in the Analog Board.

So do the MOSFET which was blowing the fuse, the TDA 4605, the optocoupler and the DP14 zener, see the pic below.

The Mac starts just fine, it gets to the System, then the image flickers quickly a couple of time before it restarts.

It happened once that the image flickered rapidly without restarting, then I turned it off, to avoid damages.

I can not tell if the issue is on the PSU side or the in CRT AB.

The Voltages measured at the HD molex are stable at +5 .05 and +11,95, even during the restart, but it might be that the multimeter is not fast enough to show the voltage drop.

It's so close to be fixed, it's frustrating :(

Time to replace the two IRF740?

Any advice?



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Could you describe the flickering in a little more detail? Does the screen geometry stay stable? Is it an "on/off" type of flicker or more of a wobble? 


Does the issue persist if you instantly turn the machine back on after shutting it down due to the wobbling? If so, this could hint at a temperature-related issue. 


What wiring harness looks extremely tired compared to mine. I would do a continuity test on all your wiring. 


You should check the solder connections on your filter capacitors and possibly replace them again. They are extremely easy to damage while soldering them. 

Edited by PowerMac_G4
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I recently had a Classic II that was constantly rebooting, very similar to what you’re describing. The problem was on the logic board, the 68030 chip had a bent pin shorting to a neighboring pin. I must have bent it when recapping. 

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@psa425 thanks, but this is not the case, I tried with another working MB but it keeps rebooting, so it's about the AB.


@PowerMac_G4, the screen has a small quick contraction, mostly on the horizontal axis, barely noticeable before rebotting. I seems to improve when warmer, because at the beginning it restarts during RAM test or even just after the chime. Sometimes I get to the system for a minute or so.

In one occasion, as I said, the screen was wobbling intensively.


To test for weak soldering joints or ground issues I poked a bit with a plastic tool on the back of the LB and the flyback, but I did not noticed any relationship with the rebooting .

I'll inspect the harness but I'm more in favor of a faulty components, the point is ... which one? :D


Thanks for supporting me on this thread

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3 hours ago, ironborn65 said:

I'll inspect the harness but I'm more in favor of a faulty components, the point is ... which one?


I know that some SE owners have had to replace wiring to restore full functionality so it's not out of the question that that's where the fault lies. Either way, it's a useful troubleshooting step. 


Was this a preemptive recap or did the old caps leak? 


Are you 100% certain that all caps were replaced like-for-like? (i.e. Did you test capacitances before fitting? Are the tolerance ratings the same? Temperature ratings?) 


As I said before, you should take another look at your filter capacitors. They are ultra-sensitive to both heat during the soldering process and to not having perfect solder connections. When they act up they cause all sorts of weird issues. 


I'll have a look at the schematics later and see if I can figure out what might be causing your trouble based on your descriptions. 

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@PowerMac_G4, I got the AB with severe leaking caps, I replaced them all from the low voltage section after cleaning in the washing machine, and with cotton swab with IPA, no soaking in IPA I must admit. Then the fuse got blowed up, so I replaced the IRFEC40, the TDA4605-3 and the CNYG17-3. I tested DP1 DP4 DP3, which were ok (no shorted).

I tested randomly the other capacitors which were in the expected range for loss and ESR.

At this point it stared rebooting as it does now.

So I kept replacing other parts. the two filter capacitors (one was cracked) and  blindly ALL the remaining capacitors , DP5 and DP7 with higher Amps and the DP6 from a donating AB even if they tested ok. The voltage of the new capacitors are sometimes higher of about 10% to 20%.

I did not took care of temperature rating, I was successful my past salvages with the set I'm using.


The back of the AB does not look perfect, but I tested for continuity in the critical pads.





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DP3 and DP4 are known for being « leaky » so tests are ok with a multimeter, but are bad most of the time. 


One is directly connected to the TDA and CNY, so in the regulation side. 


I have had a lot of issues with this two diodes, from completely dead AB to unstable video. 


I will replace both of them first. 

Edited by bibilit
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@bibilit done, I replaced the DP3 and DP4, but I got no improvements, at the beginning it was even worst ...


@PowerMac_G4 I have other details:

at the first boot I had about 5 large and faint vertical bars and no chime at all

then they start wobbling

I kept having this behavior for a couple of more boots then it played the chine and the screen was wobbling on right and left side (like a vertical moving sinusoidal), no bars, well you could say that the bar was just one as large as the screen :)

this effect was less and less evident over time

After about 3 minutes it showed the "usual" behavior, chime and boot in sequence every to 10 to 30 seconds, and yes to goes to the system

Does a bell ring? I can not figure out which component is faulty, it affect the horizontal axis and it improves with the heat

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