Apple III without any signs of life??

[BEU]

I have an Apple III,  A3S1 no 57901  motherboard  820-0043-00E  5V memory board. When I got it the power supply was dead. I´m trying to restore it but I have tried with ATX PSU unit that delivers -12, -5, +5 and +12V. Put in a new keyboard lamp.

When I turn it on there no sound, no video, no reaction from the floppy drive or keyboard.

The internal LED CR7 has a faint light. Otherwise no signs of life.

I have tried two different floppy drives.

It is obvious that someone has replaced some capacitors. The electrolytic caps C60, 61, 62, 63 and tantalum C1, C10, C12, C13, C71 looks new.

I have re-seated all IC without any help.

I have tried with two working ATX PSU and the 5V output goes down when I turn it on. The PSU can deliver 5V, 30 Amps.

 

PSU   On       Amp         

-12   -12         20mA       

-5      -6           0             

+5      + 2.9    1.5A         

12      +10.8   0.5A         

 

How to proceed? Shall I replace all caps ? Is there a short somewhere?

Thanks for any help or advice.

 

[bibilit]

Probably a short in the 5 volt rail.

 

[BEU]

Probably a short in the 5 volt rail.

Yes, probably.  But its strange (at least for me) that the resistance between 0 and +5 on the AIII is 215ohm and that implies that there is not a constant short. Maybe a capacitor thats defect??

 

 

[techknight]

Or the power supply is weak and is unable to maintain regulation on the 5V circuit. 

if you had an adjustable bench power supply, you could feed in 5V to substitute that particular rail, and see if the rest of the power supply voltages come up, or if its still being dragged down. If its dragged down, then you need to feel around to see whats getting so hot you cant touch it, as thats the likely issue. 

But, if all the rails stabilize, and the machine comes to life, its your power supply. 

Edit: Just saw at the end of your post, you substituted the power. Well, if the voltage is getting dragged down, you either need an infrared flir, or, feel around to see whats getting hot. Something is getting VERY hot. 

If I am going to hazard a guess, it could be a multi-layer ceramic, or a tantalum capacitor shorted. 

 

[BEU]

if you had an adjustable bench power supply, you could feed in 5V to substitute that particular rail, and see if the rest of the power supply voltages come up, or if its still being dragged down.

Thank You techknight for that suggestion. I have an adjustable bench power supply but it can only deliver two voltages. Maybe I can combine with several PSU and all " 0 Volt" line connected.  Is it dangerous not to feed the AIII with -5V or -12V?  Can I test the motherboard without the memoryboard to see where its getting hot?

 

[techknight]

No, you need an adjustable regulated bench power supply that can feed in 5V with a specified current into the rail. Youll make it worse by experimenting otherwise... 

Anyways, You already tried ATX power supplies so it kinda renders things moot. You need to power the machine up with a KNOWN GOOD power supply, monitor the rails correctly, and then attempt to find out whats shorted by whats heating up. Something is shedding off alot of current based on your numbers that you provided, and thats the easiest way to find out what... 

 

[ScutBoy]

Spend the money for the ReactiveMicro power supply with the Apple /// cable, and you can remove power problems from your troubleshooting equations.

And, if your /// is a goner, you can get the other cable and use the same power supply in a Apple ][

 

[BEU]

I tested with a Velleman  programmable DC Lab Power supply. When set to 5.0 V it drains 2.0 A. Nothing is really hot on the board, the only thing thats warmer is the SY6522 ic at B6 thats 36 degr C. Also tested with a working PSU from a Apple II.  All four voltages. Same results as the tests with ATX PSU, 5.0 V goes down to 2.9 V and no booting.  AII PSU tested OK afterwards.

 

[techknight]

Remove the SY6522 and see if your consumption goes down. 

 

[BEU]

I tested the motherboard (connected with memoryboard, keyboard and monitor) with different voltages from Velleman . Only 5 V line. The current is displayed on the PS. Resistance calculated from that.

Output V       Measured Amps                 Calculated resistance

0.5                       11mA                                              45 ohm

1                           120mA                                            8

2                           518 mA                                          3.8

3                           997 mA                                          3.0

4                           1.46 A                                             2.7

5                           1.96 A                                             2.5

Tested drainage with 5 V with and without SY6522  but no difference in Power consumption. The temp on the two SY6522  after 20 min  was 40 degr C. I dont know if thats OK or not.

Spend the money for the ReactiveMicro power supply

I Think ScutBoy is right, I will get a new working PSU with all four voltages. Report later after that.

 

[BEU]

The Apple III is alive - partially

The reason for it to wake up was a little suprise for me. I measured the 5V line at the motherboard when using the ATX PSU, it was 2.9 V but when I measured it at the cable at the PSU it was 5.0 V.  The reason for the voltage drop was that I used thin, bad and cheap cables from the PSU to AIII.
When I used new good cables and soldered them to the PSU cables. The voltages at the motherboard then was -12.7, -5.2, +5.3, and +12.4 V.
When I turned it on, there was one beep from the speaker and this on the monitor. Its a blinking cursor at the left top position


No light from the keyboard lamp, no light from the floppy drive. If i pressed the reset on the keyboard it restarted with one beep and the same picture on the monitor.

I have learned from this mistake not o use cheap, thin and bad wires when connect a PSU.  

Its a good thing to learn from your mistakes, even better if someone else can learn from my misstakes.

Thank you all who had comments about this AIII, You were all right , bad PSU and voltage drop.

I turned it on efter disconnected the keyboard. One beep, then the floppy drive reacted with the red lamp. monitor as this picture.



after a while this picture



Im glad that there is some reaction from this AIII. I will see if the fault is in the keyboard or in the motherboard.

No light from keyboard lamp. The voltage at the lamp is 0.12V . according to Apple III service manual it could be a defect Q9 (MPS U51) on the motherboard. 

 

[BEU]

The Q9 was OK but I noticed a bad trace to the Q9. After reparing that the AIII is booting ( Apple III system utilities) and the keyboard is working, but there is a videoproblem.



No text, only vertical lines.  I have replaced the RAM on the 5V memory board but the same results.

According to Apple III service reference manual it can be defects in several IC on the motherboard.

Any suggestions Before I order a lot of new ICs?