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Ideas on repairing Quadra 840av

fegos

Member
Hi all, first post here. I rescued an abandoned Quadra840av. When you press the power button, the LED turns on, power supply comes up, but no chime.

I went ahead and recapped it with tantalums, found leakage in most electroytics, but not too bad. One trace was corroded and i bridged it accordingly. Still same symptoms: no chime on power on, no video.

Here is what i've done so far:
  • Checked PSU voltages, seem ok (12V and 5V)
  • I am testing it with floppy and scsi disconnected form the motherboard
  • I can power on from the keyboard as well not just he power button
  • I replaced the battery with a new one - no leakage on the area around it
  • I ordered a new SIMM from ebay, maybe it's the memory.
Any other ideas anyone? i have good soldering and electronics instrumentation equipment.. Not sure if these symptoms bring ideas from anyone. Thanks, i hope i can revive this beast one day...
 

beachycove

Well-known member
How well was the board cleaned? I had leakage near where the molex from the ps connects to the logic board that caused identical symptoms. It wasn’t especially close to the caps I had worked on/ cleaned/ replaced, so I had missed it.
 

fegos

Member
i'll take another close look, and post some pictures. What would be the best method to clean the whole board? i see people dipping the boards in ultrasonic cleaners and other "submerging" methods, any recommedations? i cleaned it with 99% isopropyl and a toothbrush on specific spots
 

joshc

Well-known member
An isopropyl soak would do the trick for a full clean.

Some pictures of your board might be good in case anyone can spot anything obviously amiss.

The 840AV schematics are available here: https://archive.org/details/Macintosh68kSchematics

Might give you some clues of what to look for, you can start checking component values and checking for shorts.
 

keehn

Member
An isopropyl soak would do the trick for a full clean.

Some pictures of your board might be good in case anyone can spot anything obviously amiss.

The 840AV schematics are available here: https://archive.org/details/Macintosh68kSchematics

Might give you some clues of what to look for, you can start checking component values and checking for shorts.
Thank you for the archive link! Do you happen to know if any Quadra 950 schematics exist?
 

fegos

Member
An isopropyl soak would do the trick for a full clean.

Some pictures of your board might be good in case anyone can spot anything obviously amiss.

The 840AV schematics are available here: https://archive.org/details/Macintosh68kSchematics

Might give you some clues of what to look for, you can start checking component values and checking for shorts.
this is great. thanks for the link, it is a great starting point, i will start some probing and will upload some pics of the board, will do an isopropyl cleaning as well and report back
 

keehn

Member
I've looked before and didn't find any. What problem are you trying to sort on your Q950? (might be worth starting your own thread).
It's actually a workgroup server 95 mac (Quadra 950 board). I'm just curious if there's any publicly-available schematics in existence. I have a separate thread on this forum detailing my work on it.
 

fegos

Member
this is great. thanks for the link, it is a great starting point, i will start some probing and will upload some pics of the board, will do an isopropyl cleaning as well and report back
well, finally today i got some time to spend with the machine. I did some good isopropyl cleaning. no changes on symptoms. Here is what i tried and have found so far by following the schematics:

  1. All the on board regulators seem ok, i can measure directly on the regulators the +5 and -5V power outputs. The +12V and -12V power rails are ok too. Checked on several ICs, all well fed with +5V pretty much everywhere. Also checked the caps that i replaced and they are all good, in parallel with their respective VCC measured at the cap.
  2. I also put a scope in the clock generator chips in the board, i was able to see the 16, 20, 25, 40 and 66Mhz clock outputs on the scope. A little distorted with a passive probe, but those frequencies are just too high to see clean on a scope without an active probe.
  3. I also tried with a different SIMM to the same result. Powers on but no chime.
Any other ideas to check...? i also notive the 68040 getting "warm" i took an IR picture with my FLIR camera (attached) nothing obvious there, some hotspots on the CPU and around the VRs.


20220129T133423.JPG

well some more questions...
  1. Does this system has memory on-board? maybe that could be the culprit..
  2. What's the easiest path to connect a regular ATX power supply to this machine? PSU seems ok, but maybe power on is noisy or some drooping, if easy, i have a few of those and could try this
  3. What type of issues may cause a "no chime" at power on scenario? which is what i see
Thanks in advance
 

Siliconinsider

Well-known member
I have two 840av motherboard that do not chime, and it is something that seems common. There's likely a common point of failure for these and hopefully someone can figure it out, but it will probably require some significant time and test equipment (maybe a logic analyser) to see what the CPU is doing after power up.
 

demik

Well-known member
Please check -12V. Mine need that or it will behave like your board.

Long explanation: -12V is used in the sound circuit. Sound circuit is used to generate RESET in most Macs of that era.

Also, check if your board has a ROM. Early ones don't have one, you should have a SIMM slot for the ROM if it's the case.

Edit: nevermind, you already tested -12V.
 

fegos

Member
Here are some pictures of the board. My board does not have a removable ROM, you can see the unpopulated footprint for that connector

IMG_9597.jpg
IMG_9596.jpg
 

fegos

Member
did some more work on VRs and oscillators, all seems to be working as expected. I am thinking on replacing the on-board VRAM. Does anyone has a mac with no onboard VRAM, does anyone knows if a failure on VRAM will result in no chime/no video ?
 

fegos

Member
VRAM will not prevent the board from not chiming/booting if corrupted or absent.
Do you know what was the typical POST sequence of a 68k mac? Another thing i want to try is the reset circuit and the timing of the RST pins on the ICs that can be identified, all based on the BOMARC schematics, maybe it is a dead ASIC and that will be the sad end here
 

Siliconinsider

Well-known member
Unfortunately I do not know further easy ways to debug. Having a logic analyser to look at the bus activity during boot would greatly help I'd guess, to see if the CPU does something, and where it would hang. But I have no idea about the boot sequence, this would have to be reverse engineered.
Also, since many 840av board have the same failure, it'd be very helpful to figure it all out.
 

fegos

Member
Unfortunately I do not know further easy ways to debug. Having a logic analyser to look at the bus activity during boot would greatly help I'd guess, to see if the CPU does something, and where it would hang. But I have no idea about the boot sequence, this would have to be reverse engineered.
Also, since many 840av board have the same failure, it'd be very helpful to figure it all out.
I could probably do that. Which will require some work to setup the address and data buses into a big LA that i could probably use at work. But what do we know about memory maps for the board architecture. I guess you could see if the CPU comes out of reset and if fetching data from somewhere...? Likely this is failing post for some reason... are you sure that VRAM is not part of POST?
 
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