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Macintosh Classic issue

demonlg

Member
Situation, blowned again irfbc40, blowned tda4605, short on rectifier diode on 5 volt output, for now! All after power up and working this night for any minutes. I have checked all replaced component if i miss polarization but all are correct.I don't know!
 

demonlg

Member
Now, replaced again irfbc40, replaced tda 4605, reinsert original output shotky diode on 12v and 5v output rail, reinstalled original rifa caps and original first line filter big black capacitors, controlled all new electrolitics caps and all zener 1n4148.

Turned on first time i have a little udible chirp chirp and very low voltage output. Second time turned on after few seconds, i don't have chirp but voltage on 5volt rail are 0.5volt and on 12volt rail are 1,9 volt!

Rechecking again the output area now i have found short on 12v rail shotky diode!

I don't have any other idea, and i have lost all sperance! CNY seems to be ok, but i don't know why the diodes is going shorted first on 5v and then on 12v.
 

demonlg

Member
Baaahhhh! This mac have a demon into it! Now i have returned it on for 30 seconds and it boot correctly with bong sound, voltage 5,10 volt and 12,1 volt... video ok, the shotky diode do not in short but have a low resistence on pins, in both sense of testing...now i try to power up for more time with a sperance of not blownig up again!
 

demonlg

Member
I give up! Now computer start correctly, chimes on boot ok and try to boot from floppy, 5v and 12v are ok, but the screen not powering up! The tube is completely off, heater orange light on tube is not present, i don't know if the HV is present or not, and i don't have enough skill to check on flyback.
 

demonlg

Member
I'm not give up, sorry for my continuosly posting, but i have to save my work. In precedent post i have wrote a low resistance on shotky diode in output section, tracing this line going in area of wire for crt nek board. I have checked component in this area and on nekboard and i have found two 1n4148 shorted on it, with this new the low resistance on shotky diode is gone to normal value. Now i have replaced it with new one and the display now starts but with high luminance and raster line. I don't have moved the cut off trimmer but i will try to regulate it. The important note is for the flyback is not blowned!
 

demonlg

Member
Ok, on the dead mac scroll manual i have found the same problem and it report to a broken 2n3904 on neckboard, and yes it is faulty, one of its pins going to one of shorted zener diode of last post. I have tested with multimeter and with transistor tester, the first sign value in all sense but the PNP normally sings in only one sens, and transistor tester give me a broken parts!

Now i have to find it.

The problem is started for and error during replacing shotky diode, in the schematics it is a BYT03-300 and other one is a EGP30D, i have replaced it with a 1n5408 but this have caused a recently blowup in primary power supply section, i don't know why if the value of diode is in a range of original, now i have reinstalled original good diode.
 

demonlg

Member
Ok, i have temporaly replaced the 2n3904 with a bc547 for rapid testing but with 180 degrees rotation for differencies on it pinout, the bc547 have emitter and collector inverted from 2n3904. Now boots up perfectly and screen is ok. I have ordered the original parts, because the bc547 have a little differencies in Ia value and working voltage, but for short testing its ok.

I'm ready for another war with this mac. -:)
 

demonlg

Member
All ok after two days. I have replaced the 2n3904 with new one and now system working. ( just for info with a bc547 rotated the screen image have a distortion to right side of screen).
I have found a scsi hdd with 136mb of space and seems to working, spin and head check is done.
Now I have to check keyboard, and find a mode for create floppy disk and format hard disk, i tryed from rom boot but tools is not present.
Have tryed with fluxengine software and greaseweazle hardware, now compatible with fluxengine, but probably my 1.44 pc drive B connected to greaseweazle is not working perfecly. I have tried to write a normal pc disk image but its failed too at track 0, same if i try to write a commodore 1581 image, but if i try to write a commodore 1541 image on 5"1/4 drive A its working with write and read, and the disk then correctly readed on very 1541 drive.

For now, thanks for support and patient for my excessive posts.

Emanuel
 

AwkwardPotato

Well-known member
Glad to see you got this Classic running again, there really must've been a demon in it!

The problem is started for and error during replacing shotky diode, in the schematics it is a BYT03-300 and other one is a EGP30D, i have replaced it with a 1n5408 but this have caused a recently blowup in primary power supply section, i don't know why if the value of diode is in a range of original, now i have reinstalled original good diode.

The BYT03/EGP30D are fast recovery diodes with a reverse recovery time on the order of a few tens of nanoseconds, but standard silicon diodes like the 1N5408 have a reverse recovery time at least an order of magnitude greater. As a rule of thumb, it's important to have the former in switching power supplies like these.
 

demonlg

Member
Hi, tomorrow i send photo of my classic, yesterday i have installed the hdd, i have found a 1gb conner scsi disk from germany, formatted with patched hd tools and then I have installed system 7.1.1. But now i have a problem with stuffit 5.5 tools and decompressing game or other software on this machine, when launch stuffit system give me a message "unimplemented trap" and only way is restart system.

Then, after hardware restoring, and after my diodes error the monitor have an abnormal simpthom, when i turn on classic after few days i have a orizontal retrace line on screen, after about 2-3 minutes, the lines slowely going out and the image is normal, but i have noted, when hot, the contrast going slow to lowest level, after about 30 minutes the contrast stays stable but very darking screen, comprensible and usable but dark.I have regolated brightness and cut off in reaar of machine to have a good screen, but then if i turn on again after 2-3 days have a same problem, retrace lines and then when hot its going to the value setted with trimmers.

Why this slowdown of contrast and when hot stays ok? Another component damaged with short caused for wrong diode? I have see schematics and power to the flyback is from the lm317, i jave replaced it before my power output diode short, is possible damaged? I don't thing is the crt tube, because before the diode short not presented this simpthom, and the image is very good and clean.
 
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