Dead 512k, rapid clicking?

[tattar8]

I'm working on a dead 512k that's making a rapid clicking noise when turned on.  I took an audio recording and uploaded it here:

https://drive.google.com/file/d/12m4cIr_g4gwdeV6ooGvvR8RU5sfe1Vt7/view?usp=sharing

I get no chime and no video.  Probing the 5v and 12v lines going to the logic board, I get varying numbers, all below 0.5V.  What could be the culprit?  I'd like to minimize the number of orders that I place to save on shipping costs, so I want to identify as many likely culprits beforehand as possible.  I've already replaced the R39 thermistor since that measured as an open circuit and just fell apart when I touched it, and I've removed the C33 and C37 filter caps for now since they also looked suspect and I read on another thread that they aren't absolutely needed to for the board to work.  I also tested the four diodes in the bridge rectifier and found that they seem to be OK.

 

[bibilit]

Probably the optoisolator, fairly common issue. 

Ilrc position U3

 

[tattar8]

Do you know the appropriate replacement part number?

 

[bibilit]

This one works well

https://fr.farnell.com/vishay/cny17-3/optocoupleur-phototransistor-5000vrms/dp/1652511

 

[tattar8]

I got one of those optocouplers and installed it, and also recapped the analog board.  No change.  Are there any other likely culprits?  

 

[tattar8]

One more thing:  Following Dead Mac Scrolls, I tested the resistance across CR21 in-circuit and found that it seems to be a dead short.  It says to replace it in that case.  I'll do that, but after taking it out, I don't measure a short across CR21, but still measure a short across its pads.  Does that mean there's a short somewhere between the +12v and Gnd rails?  What would be likely culprits? The resistance across CR5 is normal, which would suggest that the flyback isn't the culprit.

I'd like to know what parts I'll likely be needing to replace so I can minimize the number of orders (and thus the shipping costs). 

 

[tattar8]

Sorry for chain-posting but I have an update, and I can't seem to be able to edit my posts here.  The culprit was the floppy drive; there's apparently a short between +12v and ground somewhere in it.  If I disconnect the drive, I'm greeted with a distorted bong, and a distorted Sad Mac on the screen; this seems like a logic board problem, so it looks like the analog board is now functioning.

 

[mdeverhart]

Nice, that’s good progress! Now you can at least start diagnosing the problems.

Have you replaced the caps on the logic board yet?

Can you read the code under the Sad Mac? If so, what is it?

 

[tattar8]

I have not replaced the caps on the logic board yet -- when I ordered the analog board caps I forgot that there were any on the logic board at all.  I'll order them soon.

In the meantime, I'd like to try to get it working.  After giving the board a good wash, I now get a good bong, but a distorted Sad Mac along with vertical white lines on the screen.  I can only read parts of the error code, as seen in the following image:  https://imgur.com/0tITiiy 

Is it likely that there are dead RAM chips?

 

[blusnowkitty]

I have not replaced the caps on the logic board yet -- when I ordered the analog board caps I forgot that there were any on the logic board at all.  I'll order them soon.

In the meantime, I'd like to try to get it working.  After giving the board a good wash, I now get a good bong, but a distorted Sad Mac along with vertical white lines on the screen.  I can only read parts of the error code, as seen in the following image:  https://imgur.com/0tITiiy 

Is it likely that there are dead RAM chips?

https://udcf.gla.ac.uk/~gwm1h/Error_Codes/Sad_Mac_Codes.html

I'd hazard a guess that the error code is 020004. DRAM ICs are pretty cheap on eBay so I'd pick up a bunch as well as some sockets and start swapping things out.

 

[tattar8]

What would be the correct DRAM IC I would need?

 

[blusnowkitty]

What would be the correct DRAM IC I would need?

Pull your logic board and find out; the RAM is in F/G 5-12 on the board, just below the 68000. It looks like the 512k uses 4256 DRAM ICs (which is a 256Kx1 chip) with 15ns refresh. Couldn't tell you if a faster part will work with slower parts, but you don't want to put a slower part in place of a faster part.

 

[bibilit]

Yes ram issue. 

As the code is not complete, hard to locate the faulty one (F7 or G7) piggybacking always worked for me. 

 

[tattar8]

Yes ram issue. 

As the code is not complete, hard to locate the faulty one (F7 or G7) piggybacking always worked for me. 

From the code it seems like the last digit is 4.  Wouldn't that narrow it down to F7, since all the G chips have a 0 for the last digit?

 

[bibilit]

Problem is some information is missing from the screen, so a 4 is present for sure, we can only guess the rest. 

Once again piggybacking is the way to go, worked a 100% for me. 

My guess is that maybe more than one chip is bad. 

 

[tattar8]

Can you describe that method for fixing bad chips?  I know about piggybacking to add more RAM, but I've never heard about it in this context.

 

[tattar8]

So I replaced both F7 and G7, but the same error persists.  Should I continue replacing chips or try something else?

 

[bibilit]

Piggybacking a new ram on top of a faulty one should solve the issue. 

If not, the fault code should change enough to give a clue. 

 

[tattar8]

I tried piggybacking on top of all four chips that can cause an error code with a 4 digit, and nothing changed.  I then replaced all the chips with sockets+chips, still no change, although the chime no longer sounds.  Could the fault be in the muxes or buffers?