Mac SE/30 suddenly strange screen. [see photo] Startup Chime good.

isokinetic

New member
I‘ve recently acquired a recapped SE/30 that was working. I had it running three hours when suddenly the screen did not longer work properly. I had to restart using programmer switch. It does chime but the screen only shows a strange image. IMG_2577.jpeg I swapped the analog board with a working 1/40 (both used C type) and used another ROM from a SE/30 but to no avail: Chime but same screen.

Can you point me what to do next please?

Kind regards!
 

djc6

Well-known member
I had a similar experience recently on my first classic mac (an SE/30). I recapped, it worked for a while, then a black screen. Turns out chip UG8 went bad - I think the leaking capacitor electrolyte got to it.

I was able to figure it out by purchasing a cheap oscilloscope from Aliexpress and an ATX power supply extension cable, so I could run the machine with the logic board outside the case. I had to learn about reading schematics and using an oscilloscope, and surface mount desoldering/soldering - but in the end I was able to narrow down the one bad chip and replace it, and I have a working machine now!

Oscilloscope I bought - Zoyi ZT-703S with two probes: https://www.aliexpress.us/item/3256...st_main.5.2ab91802zciZ35&gatewayAdapt=glo2usa

There are other inexpensive, handheld oscilloscopes from Hantek and OWON. I picked this one because many retrocomputing youtubers say its decent for a first oscilloscope, the company seems to offer firmware upgrades for it, and its easy to take screenshots to post on forums (you use usb cable to transfer them to PC). It also has one of the biggest if not the biggest LCD of various handheld scopes.

ATX board extension cable I bought: https://www.amazon.com/gp/product/B09N9BZRD9/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

This cable lets you extend the connector at J12 to the analog board, so you can position the logic board outside the case.

Here are threads on my recent adventure:


To my untrained eye that has only ever worked on one SE/30 and with minimal retro repair experience, it looks like you have VSYNC and HSYNC (vertical and horizontal sync) but you are missing the "Video Out" signal. I would be curious if an oscilloscope shows working VSYNC (pin 14 on UG6) and working HSYNC (pin 13 on UG7) but MISSING VIDOUT signal (pin 13 on UG6). If you are missing VIDOUT then I would suspect chip UF8 the binary counter surface mount chip is bad, which seems like a common chip to fail due to its proximity next to leaking capacitor C7.

Depending on the revision SE/30 motherboard you have, you may have to modify the ATX extension cable a bit as shown in this video:

Video shows a 24-pin atx extension which is really wide, I purchased a 20-pin atx extension since its less wide - it fits without modification on my later revision SE/30 where the CPU is not socketed but instead soldered to the board.
 
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bibilit

Well-known member
Your issue is related to UE8, part number is 74LS166 (don’t use the HC one) near a leaky capacitor, common issue goes bad frequently.
 

isokinetic

New member
Many thanks for your many and kind replies. The legs/joints on UE8 look a tiny bit corroded, but the label on the part is gone (burnt?). [see image] This built my resolve to go a little more serious with this hobby. I don't own an soldering iron, isolating transformer, or oscilloscope. all of which I will buy now. thanks for your suggestions. img2591.jpg
 

bibilit

Well-known member
yes this is the one, UE8 (74LS166D)


You will need a soldering iron at least, but probably easier with hot air.
 

finkmac

NORTHERN TELECOM
if it's gonna be replaced, might as well just cut the leggs off the old one with some flush cutters
 

imactheknife

Well-known member
A friends board looks just like that and we replaced ue8 and no change. So not sure what next. Unfortunately this board has some rotten vias.
 
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