Macintosh Plus problems

[Ike]

I recently bought a fully working macintosh plus.

with that, another macintosh plus came in ok cosmetic condition but the owner said it wouldn't boot...

when i plugged it in and flicked the switch, it chimed normally, showed the memory check diagonal lines and then:

it showed the blinking disk icon... but

1. it was not blinking... just the question mark standing there (Cpu freeze?)

2. the background was solid black instead of dotted (Bad ram?!)

Opened it up, examined everything inside... nothing special, looked quite clean, original caps were there.

Cleaned the ram slots and SIMMS and reseated and cleaned the HI en LO rom chips.

Then the second problem ocurred... when i turned it on it would first beep fully... and then really reapidly start to reboot itself...

sometimes when gently knocking the analog board side of the case would make that stop and allowed it to fully boot... (loose connection in the power to the logic board?)

also when it got to the screen with the steady disk icon i lifted up one side and put it down again with a little thump, just to see what shaking did to it.

the CRT image started to wobble, did it again, CRT went black and it started to boot loop again.

maybe also useful: when it would boot loop you can hear the crt power up *blip* *blip* *blip*

Any idea's where to start with this?

my own suggestion would be to investigate the analog board... but i very scared of the high voltage part.

 

[krye]

Yup. Bad caps on the analog board or a dry solder joint. In any case, it's the analog board. Maybe touch a soldering iron to the connections to refresh the joints. You also might need to swap out some caps.

 

[nvdeynde]

Measure your voltages +5 +12 and -12 on the external FDD connector and post them.

The +5 volts will probably be much too low.

How does the plastic of the P1 connector look ? Refresh the solder at the back of the PCB.

Replace the Ram modules with others as the vertical or horizontal lines point to a ram problem.

Is the Ram jumper set in the correct position for the amount of Ram installed ( or are the right resistors cut on the logic board ) ?

 

[Ike]

Suspected a ram issue as well.

The ram resistor was the first thing to catch my attention. The resistor was cut, then soldered back to the lead.

Then checked the ram which was indeed 2x 256k + 2x 256k = 1mb.

i want to reflow and measure the analog board as well as suggested.

but as i said i'm quite scared of the high voltages on the board.

some people suggest to leave it plugged out for a few days but even then the caps can still hold a charge.

I'm willing to try to discharge the CRT, but i want to know: does this also remove any remaining charge of for example the big filter capacitors?

if that is not the case, can someone tell me how to properly discharge the complete analog board.

already lots of thanks for your suggestions, i will measure and post the +12v and +5v lines.

by being able to touch and remove the crt as well as the analog board i'm able to fix all my classic macs since most of them have an analog problem or two!

 

[nvdeynde]

Discharging he CRT does not remove remaining charge of the other capacitors on the Analog board.

However, it's unlikely that after a few minutes there's much if any charge left in them.

A simply solution to discharge them all at once: plug in the power cord, power on the Mac and then pull the power cord without turning the power switch off first.

You can work on the Analog board without discharging the CRT as long as you don't touch under the Anode cap ( the suction cap with red wire from flyback to CRT ).

For 4x 256 Kb ram modules, the resistor for 256 k-bit should be there, the other bellow saying 2x bank should be cut or removed. If you upgrade to 4x 1Mb modules, which I recommand, then you need to remove both resistors.

Measure you voltages first on the external FDD connector: this will tell us something about your analog board. The logic board doesn't need to be booting to test these.

 

[Ike]

Results of the measurements at the FDD connector:

pin 5 -12.23

6 4.97

7 12.29

8 12.29

Grounded at pin 1.

look perfectly fine to me. off course this is without anything actually connected to the port.

logic board was connected, chimed.

so the restarting really only looks like a bad connection to me... which is good and should be fixable.

 

[nvdeynde]

Resolder the connector on the Analog board that holds the wire harnass to the logic board, better refresh the solder on all 3x connectors ( Analog to CRT board, to logic board, to deflection yoke )

I think it's either a bad solder joint there, the cable between analog and logic board is also notorious for bad connections, faulty ram or a faulty logic board.

The capacitors on the Analog board are definately degraded given they are almost 25 years old, however not so bad since the voltages are still in spec.

Since you have another working plus: I would swap the logic board and see what happens. It's a faster way to isolate the problem when lacking spare parts.

 

[Ike]

I'm going to swap the logic boards, that's a sensible idea.

maybe even swap ram SIMMS and setting the '256k'-memory resistor in the right value (soldered or clipped) to see if the system freeze is caused by bad ram.

Another problems i have seen on my working plus that i would love to have fixed... seen the symptom somewhere here in the forums but can't seem to find it anymore:

The brightness control works... but as soon as you turn it up all the way... the screen becomes horribly out of focus to the part where text is almost unreadable...

turn the brightness down a tad and it becomes pin sharp again.

my guess: analog board caps?

Or is this something more serious?

Again many thanks for your input, support and help fixing my first serious Macintosh hardware problems!

 

[James1095]

The power switch is on the primary side, so pulling out the power cord is exactly the same as turning off the power switch. No need to fear the high voltage, it's not necessary mess with the anode connection to the tube to work on the analog board unless you have to completely remove the board. The 160V B+ on the bulk filter capacitor will discharge itself within a short time, so shut it off, unplug the power and let it sit for a few minutes and you'll be fine. If you want to be extra careful, it's not a bad idea to short across the capacitor to make sure it's fully discharged. I like to use a 10 ohm power resistor attached to a pair of probes, it makes discharging a charged capacitor far less dramatic.

 

[nvdeynde]

Another problems i have seen on my working plus that i would love to have fixed... seen the symptom somewhere here in the forums but can't seem to find it anymore:The brightness control works... but as soon as you turn it up all the way... the screen becomes horribly out of focus to the part where text is almost unreadable...

Adjust the Brightness pot on the Analog board: set the contrast control maximal first and then adjust the Brightness pot so that the screen is stable. You will probably see lines now when it's full open ? Also adjust the Focus pot on the Analog board when necessary.

If you don't have the test disk with the screen pattern, then you'll have to adjust it on sight trying to get the best focus for the full screen.

 

[trag]

Test pattern generator:

http://www.prismnet.com/~trag/Classic%20Mac/TestPatternGenerator.img.bin

 

[Ike]

Problem is that the focus really changes according to the brightness levels... it is not a matter of not being able to see that its not in focus,

its really quite a drastic change...

0-90% of the brightness potentiometer = pin sharp.

90-100% = blurry mess.

Quite weird right?

Thanks for the tips working on the analog board. I'm going to make me a capacitor discharge tool and short the bulk filter caps.

Then i'm going to reflow the connections to the logic board. Somewhere i read that you really had to do this with the cable still attached otherwise you might melt the plastic right?

 

[James1095]

Loss of focus at high brightness is a common symptom of a worn out tube, although if it's that rapid of a change I would look elsewhere first. Your HV may be low, or you may have a problem with the focus divider.

 

[Ike]

i suppose without discharginng the CRT first i cant resolder the yoke connector and connector to the video board right?

 

[Ike]

Guys! Goooood news!

Finally decided to take it easy and started working on the analog board of the 'broken' plus.

resoldered some connections that looked very bad, resoldered the power connector, the switch and the total connection to the logic board.

*bong* -> black screen with solid disk icon, Nice! One problem fixed.

then swapped the 4mb ram with my working logic board (keeping the 256k resistor in place)

grey screen with solid disk icon... then i saw that i was able to press the interrupt button and upon restart just move the mouse...

the machine was not frozen at all... it just displayed the solid [ ? ] icon because the floppy disk drive was not connected well.

up to this point i thought the original ram was bad (thought it was weird i did not see any sad mac code for a specific bad ram chip or SIMM)... until i saw the solder connection with the resistor and the logic board was horribly done:

when i measured it it made contact and it looked like it was nicely attached... in fact is was a totally broken connection that sometimes made contact.

re-flowed the solder so it made contact.... swapped back to the original ram: same thing, black screen with blinking disk icon.

removed all the solder and really disconnected the 256k resistor... and VOILA.

it booted it made a bong and showed the gray background with blinking disk icon.

Thanks for all your help and tips on working on the analog board and the ram issue!

 

[trag]

Problems with focus adjustment are often caused by one plain 1/8 or 1/4 watt resistor (can't remember which) on the Analog board which fails open. Also can't remember which resistor darn it. I want to say R36, but I think that's the Cache disabling resistor on the PM9500. Messy wetware memory.

Anyway, it's mentioned in Pina's books and probably findable on the web. If I had Pina's book here, I'd look it up.

 

[James1095]

i suppose without discharginng the CRT first i cant resolder the yoke connector and connector to the video board right?

Discharging the CRT is really nothing to be scared of. I use a clip lead with one end clipped to the metal part of a small screwdriver and the other end clipped to the metal chassis around the tube. Poke the screwdriver under the suction cup looking thing and you may get a small static pop as it discharges. The charge on such a small tube is really not that big, it's about like static electricity zaps you can get around the house on a cold dry day.

That said, the way the analog board is positioned, you can do virtually any necessary repairs without removing the board or messing with the anode.

 

[Ike]

James or any other person who can answer this:

Where does the HV on the anode cap come from? Is this mainly the flyback transformer -and thus are the pins on the flyback containing any of this high voltage or not?

i'm willing to try to discharge the CRT when i have a proper insulated long screwdriver with a wire clip on it.

I'm asking all this because i really want to know how the analog board works in general. I've also been reading some PDF's about classic mac repairing. And while one of them goes into nice detail about re-soldering the Yoke connector, it does not mention anything about whether it is required for the CRT to be discharged first.

The trick of setting the CRT to full brightness, then unplugging the device while it is running seems like a good idea because you can really see the CRT die out because of the lack of power... should get quite some of the voltage away.

I'll keep you updated on one of the pluses problems with stripey video... been told it's because of the bad solder joints on the Yoke connector to the analog board.

 

[Ike]

Allright, just 'discharged' the CRT... there was actually nothing to discharge. I did the 'yank the cord' method first, then opened it up and tried the official method using a wire clip and a long insulated screwdriver. no spark, no pop, nothing properly discharged it seems.

now lets re flow those connections and see if that makes the video any more reliable.