Macintosh 128k No Chime and Strange Sounds. Need Help to Restore.

Joe_D

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Original poster
Jun 24, 2020
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I am the original owner of a 128k Mac from my first year at Drexel University in 1985. I am trying to restore it back to operating condition. When I powered it up I got the Chime and then the Sad Mac Icon with code 040820 was displayed with a good quality screen picture. So I replaced RAM chips G8 and F10. After that I powered it up and got the Chime and then the Sad Mac Icon with code 020040 was displayed, also with a good quality screen picture. So I replaced RAM chip F11. Now when I power it up I no longer get a chime, the screen comes on with basically a white/black static display, and I get random static sounds out of the speaker. See two links to videos of the current symptoms. Any idea what might have gone wrong with my second RAM chip replacement, or other ideas of what could be wrong now?


 
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MacTech68

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Mar 16, 2008
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Check continuity from chip to chip of each pin in each Row

ie, all RAM chips in row Fxx should have pin to pin continuity
AND all RAM chips in Row Gxx should have pin to pin continuity

It's likely that when you replaced F10, one of the data-lines was severed, and didn't reach F11 (the next error after repair).

IIRC, one pin should not have continuity within a row. However, that pin should have continuity to the corresponding column chip below it. If I'm correct, that should be pin 3.
 
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Joe_D

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Original poster
Jun 24, 2020
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Thanks for the feedback! I rechecked pin to pin continuity with one probe on F5 and the other probe on the corresponding pin of F12. Went through all pins on both sides of the RAM and then repeated for row G. All pins checked out good with the exception of what you mentioned. Pin 3 on one side and pin 2 on the other side of the RAM chip are not connected across the row for either row F or G. That seems to be normal. I also checked continuity between adjacent pins on F11 and there were no shorted pins. So unfortunately, the pin continuity through the traces on the PCB does not seem to be a problem. I also had another new RAM chip so i changed F11 with a different new RAM chip, but still have the same issue on power up.
 

Joe_D

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Original poster
Jun 24, 2020
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I read +11V, +11V, +5V, and -12V on the three you identified plus the one to the right of the 5V.
 

MacTech68

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I read +11V, +11V, +5V, and -12V on the three you identified plus the one to the right of the 5V.
Disconnect the floppy drive (when powered off) - then try reading the voltages again.

Also, check CR5 (diode mounted to the heatsink) at the top of the board. It's resistance in-circuit should read 3.6k in one polarity and infinite resistance in the other polarity.
 

Joe_D

macrumors newbie
Original poster
Jun 24, 2020
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The voltage measurements came out exactly the same with the floppy drive disconnected.

The diode measured over-limit in one direction, and 659 ohms in the other direction. I repeated a couple of times to ensure the 659 Ohms was the reading.
 

MacTech68

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Mar 16, 2008
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The voltage measurements came out exactly the same with the floppy drive disconnected.

The diode measured over-limit in one direction, and 659 ohms in the other direction. I repeated a couple of times to ensure the 659 Ohms was the reading.
According to Larry Pina that's a bad flyback transformer. :(
 

Joe_D

macrumors newbie
Original poster
Jun 24, 2020
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I do have that doc from Larry so i will have to look closer. I am just hesitant about a power supply issue since there seemed to be no analog issues before I changed F11.
 

MacTech68

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Mar 16, 2008
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Is one of the heat sinks at the top of the board discoulred a blue/purple or brown?

Some pics of the analog board may also help.

I agree it would be unfortunate if it is the FBT. Though Larry's quick CR5 check does seem to indicate it.
 

Joe_D

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Original poster
Jun 24, 2020
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Some analog board pics attached. I replaced Q3 once about 15 years ago. It was getting so hot that it almost melted the solder. Everything worked good after replacement. Just felt it now and it is fairly hot to the touch after only a couple of minutes of on-time.
 

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MacTech68

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Do you recall if the heatsink was that color all those years ago ?

My experience is that the heatsink being that dirty color, it has experienced a bad or partially shorted FBT. The Question is when.

However, if the heatsink is too hot to touch after a short time, the FBT is probably toast.

Options are to buy a used MacPlus that at least produces a picture, or a replacement FBT.

A new BU406D is also recommended.

from https://forums.macrumors.com/threads/flyback-transformer-for-mac-plus-1mb.2241496/ :

The only two VERY EXPENSIVE source I've found is DalbaniCorp in the USA and Donberg - below link is NOT an endorsement - Buyer beware.


 

Joe_D

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Original poster
Jun 24, 2020
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I am pretty sure the heat-sink was already that color from the first time I replaced Q3. But it does get fairly hot to touch quickly now. What do you think about those gaps in the top of C15, C16, C17? I can't recall if those were there before I had the issue.
 

Joe_D

macrumors newbie
Original poster
Jun 24, 2020
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If the Flyback transformer is shorted, would there be any picture at all on the screen? I still get the white/black snowy screen. No bong/chime on power up now though.

Today I used a higher resolution multimeter to do the readings. The 5V was at 4.69V so turned the POT to bring it up to 5.00V. No change to the issue though. With this meter CR5 reads 10M in one direction and 21k ohms in the other. So it seems it is ok. Other readings I took which seem normal:
CR19 6.08V
CR18 12.22V
R46 22.5 ohms
R47 0.8 ohms
R48 100.5 ohms
R49 0.8 ohms
R50 1.6 ohms
R51 15 ohms
R52 10.21k ohms

CR1 4.6M/21.8k ohms
CR5 10M/21k ohms
CR20 116/180 ohms
CR21 38/38 ohms

Only other items that Larry Pima points out to test would be the TSM chip on logic board, Q1 chip on logic board, and Q3 chip on analog board. All require desolder and test out of circuit. I might start with Q1.
 
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Joe_D

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Original poster
Jun 24, 2020
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Pulled Q1 on the logic board and did the out of circuit test:
K-O 11.7k 11.15k ohms
I-O Inf 3.65M ohms
K-I 3.6M Inf ohms

The I-O and K-I readings don't match the expected but are also not the typical bad reading noted by Larry. But this seems suspect. Cheap enough to replace so may pick up new part and solder in.
 
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MacTech68

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If the Flyback transformer is shorted, would there be any picture at all on the screen?
Sad to say, yes it can. If a winding is shorted early on itself, it's possible.

At this point, other than a logic board swap as a diagnosis measure, I'm lost for suggestions.
 

Joe_D

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Original poster
Jun 24, 2020
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Thanks for the help so far. Note I did install sockets on the three new RAM chips but I don't think that should be an issue. Unfortunately I do not have a spare logic board to swap in currently. So I will try the Q1 and TSA chips on the logic board after I receive the new parts.
 
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MacTech68

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Hmmm - I'm not sure what the difference may be. A quick glance over two pdfs doesn't reveal much difference (other than minor dissipation).

EDIT: I've never used these as a substitute myself, however they do appear to be used in place on C64s


EDIT 2: I suspect the 6665 may have been Motorola's own part, before the 4164 naming convention took hold.

The 1984 Motorola Memory Data Book shows a 4116 & the 6665 but not a 4164.

 

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Joe_D

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Original poster
Jun 24, 2020
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Replaced the Q1 Linear Regulator on the logic board today. No change. The measurements on the new Q1 out of circuit were about the same as the old one so didn't expect any difference with this. The TSA chip seems like a bigger job to replace but that would be the last item on the debug list. I will dig more into the data sheets on the Memory before doing anything else but it seems that either 4164 or 4264 should work.
 
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