Trying to fix two Mac plus logic boards

[Retrologist]

Hi, I’m hoping you guys will be able to help me!

I recently bought a Mac Plus for cheap. I enjoy repairing and tinkering with old machines but have never owned an old Mac before.

This machine produces a checkerboard pattern and makes one deep tone upon boot.

I also acquired a second cheap broken machine in an attempt to make one good one. This one actually booted at first. However, now I get the correct tone and just a black screen.

I have two working analogue boards and the problems remain the same when I switch the logic boards. This means it must be the logic boards with faults.



Board 1 (checkerboard)

so far I have replaced the roms, replaced the rom sockets, replaced the ram, contact cleaned the ram sockets, tried two sticks (and moved the resistor), reflowed anything that looked iffy. No luck so far.

board 2 (no image)

This board occasionally works. I think this is when the machine has been turned off for a while. I have removed and replaced the solder along the connector to the motherboard.

I just want to get one of my logic boards working so that I can enjoy getting into the classic Mac!

 

[Iesca]

Regarding board 1, checkerboard is usually indicative of an analogue board problem. Have you tried with another analogue board? You may want to reflow/resolder all of the J connectors on the back of the analogue board.

Regarding board 2, if it beeps normally but the screen remains black, this is definitely an analogue board issue. As with board 1, check for cold joints or cracks and reflow/resolder all of the J connector joints as necessary on the back of the analogue board.

 

[Retrologist]

Hi, thanks for replying. I have two analogue boards and they appear to work fine. Both have been fully reflowed. I have removed the solder from the connectors and re-flowed with both. I have also fully re-capped one of the analogue boards.

The result is the same whichever logic board I use with whichever analogue board. I’m sure the logic boards are at fault.

 

[Retrologist]

Are there any chips or other components on the logic board other than the ram or roms that could result in either of the problems?

 

[bibilit]

Logic boards are pretty reliable, the Achilles heel of those are the Analog boards.

I will start with the logic board giving you a good chime to troubleshoot the other one :

is the brightness knob turned all the way down ?
can you try to give some light taps to the side of the AB to try to have the screen back ?

Black screen is usually a solder joint or a bad Flyback (this last one being pretty unusual)

 

[Iesca]

If you did recap one of the the analogue boards, doublecheck that the polarity is in the correct direction for all the caps you replaced. For C1, which is non-polar, an equivalent 3.9 or 4.7uF, 100V polyester film cap should have been used, which would also be non-polar (if you replaced it).

 

[dochilli]

Did you measure the voltages at the floppy port? Should be 5 and 12 V.

 

[Retrologist]

Hi, yes I fully expected all of my issues to be analogue board related. There were issues on the analogue boards but they both now seem to be rock solid.

There are no disturbances when I hit the sides of the machines or boards.

Whichever analogue board I use to test with gives me a rock solid image (checkerboard on one logic board, sometimes get a boot up from the black screen board).

I’m Pretty confident that flybacks are good.
Brightness controls are working normally.

Voltages are as they should be on both boards using the floppy port. I did read that a checkerboard pattern could be caused by a lack of 12v. Could the 12v be failing to get elsewhere on the board?

Could a checkerboard be caused by a faulty cpu? I have a spare 68000, but it’s a bit of a big job to de-solder the one that’s on the board.

 

[Retrologist]

With the black screen logic board, I have discovered that if it is left a few hours it boots perfectly and gives me the insert disk image. As soon as I power cycle it we’re back to black screen.

This happens when attached to either of my analogue boards. It must be to do with heat on one of the components on the logic board, but I don’t know where to look. I have resoldered all of the connector joints and polished up the connector pins so I don’t think the problem lies here.

Any ideas of areas to target on the board?

 

[Retrologist]

When I say left, I mean left unplugged!

 

[mogs]

I was going to start a new thread, but I think I'll just jump in here since I also have a mac plus with a black screen issue which may be related.

My initial suggestions to your symptoms above.
Chequerboard: remove the RAM and clean the contacts. Also, if both boards have exactly the same RAM configuration try swapping RAM (if not, do some research about Mac Plus RAM configurations before changing anything).
Black screen: check/reflow solder joints with flux at both ends of the AB->LB cable again. But also given my experience below, there is a slight chance it might be related to the ROM sockets, it seems unlikely, but I'm pretty sure mine are not making good contact.

So my Mac Plus...

When I first turned it on a couple of years ago it would chime and present the floppy icon, but it also tripped my RCD(GFCI) symptomatic of dodgy Y caps on the inlet. About a year ago I finally got around to replacing these (and the X cap) and was immediately presented with the floppy icon, and no longer tripping the RCD. I was not able to test it further at this time due to lack of keyboard, mouse or boot disk.

I have since acquired mouse, keyboard, and a ROMinator.
Today I naively installed the ROMinator without testing first, hoping it would just bounce to life after a year on the shelf (I was however super happy to find four identical SIMs already installed, Suggesting 4MB).

Here is a summary of my attempts.
1. with ROMinator freshly installed: clicking sound from speaker, no chime, nothing on the screen.
2. with original ROM: chime + screen starts up only sometimes but with corruption.
3. (original), Removed SIMs, cleaned contacts and put them back: chime to black screen initially, maybe third power cycle screen comes up with floppy icon!
4. (original with mouse): screen working after a couple of tries. Mouse moves cursor, holding button can hear floppy drive clicking (this is important later).
5. (ROMinator with mouse): I notice the ROMinator PCB is not making convincing contact with logic board ROM sockets. Some pins riding up, spongy feeling sockets. Push it in firmly and cross fingers. Power on, "Ping!" (alternative ROMinator chime sound, yay!), screen dark (boo!).
I tried maybe 5-6 times with this configuration but was not able to get the screen to turn on. Interestingly once or twice I saw the screen flash as I turned it off.
Also after the chime if I held the mouse button down I could hear the floppy drive clicking, same as with the original ROM when at the blinking floppy icon. This is a good sign for the health of the system other than the dark screen.

At this point I decided to put it back on the shelf and try again another day.
My next steps were to going to be to post here, and order some machined ROM sockets to replace the spongy originals.

I will have a close look and probably reflow the solder joints on both LB and AB after reading this thread.

Good luck!

 

[mogs]

So update, and success!

I had a close look at the AB with the macro mode on my phone camera and could see where the solder pins to the connector might have been cracked. So I removed most of old solder, applied flux and re-soldered the pins, trying to ensure I was putting more heat in the pin than the pad.

I did the same again on the LB connector, although these looked good.

Reassembled and "Ping!" can boot with the ROMinator. It turns out the four SIMs are just the standard 4x256kb, not 4MB as I was hoping. I will have to look at sourcing some upgraded memory.

So yeah, I would recommend resoldering the connector on your LB as this could definitely be the cause of your black screen issue (I understand if the sync signals are not getting to the AB the screen will remain dark).

 

[Iesca]

Congrats on the fix!

Quick tip: If there are four sims installed and also a resistor present at the RAM selector area of the logic board, then they are 256k each, if there are 4 modules and the resistor is otherwise snipped, lifted, or altogether removed, then they're 1mb sticks!

(Rarely, there can be 2x1MB and 2x256KB for 2.5MB total!) (EDIT: Got this slightly wrong at first)

 

[mogs]

Congrats on the fix!

Quick tip: If there are four sims installed and also a resistor present at the RAM selector area of the logic board, then they are 256k each

Thanks, it's taken till now for me to understand how those resistors are supposed to work.

I have a pair of 1MB modules from a non working Classic to try. 4x1MB on the way.

In the mean time I have the ROMinator and various SCSI gadgets to play with....
Oh, and 30+ years of filth to clean from the plastics.