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Tried repairing mac classic analogue board but now its blowing fuses

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Glen M

Experienced Member
Joined
Apr 27, 2020
Messages
57
Location
Belfast, Northern Ireland
My mac classic analogue board I'm fairly sure now is beyond repair, certainly beyond my knowledge so for one last go I'm hoping someone out there will be able to help.

This started a while back when I purchased a mac classic locally for £30, talk about to good to be true! It needed recapped and I was happy to do that but when powered up with all new caps it gave a wobbly checker board. After some discussion that turned out to most likely be caused by electrolytic still on the board causing shorts. I took the decision to remove all components so I could properly wash the board. Its all through hole and single sided (obviously) so easy enough to do. All components were removed, labelled and stored for reuse except diodes DP5, 6, 7 that were crusty so got replaced along with IP1 and QP1. I tested all components I could before reinstalling and all checked out fine.

The board got a soak in soapy water then a bath in IPA to try and remove all the leaky gunk. It was thoroughly dried before reassembly.

Now however when powered up the fuse goes instantly with a bright light so must be some serious current running through it. Power supplies aren't really my thing and I'm starting to think removing all the components was a bad idea (all be it I got plenty of soldering practise!) Worth noting there is no obvious sign of damage to any other components.

To be honest I've more or less given up on it now but if anyone has any wild ideas as to whats happening please let me know. Failing that does anyone in the UK fancy taking a look at it? I'll warn you though the underside of it isn't pretty. The damage from leaked electrolytic was extensive and unfortunately various pads and traces lifted on me while desoldering. About 10 in total and all around the power section of the board. I've repaired it as best I can but I openly admit part of the cause was more than likely me being to heavy handed with the heat on the desoldering station.

Its a real shame as I had hoped this would be a cheap project box, just a recap and it would be fine. Never seems to work out that way.

Sorry for the long winded post!
 
Hate to ask, but are you absolutely sure all components are installed back on their correct place and in correct orientation?
 
If the fuse is instantly blowing, then you have a direct short between hot and neutral. I'm going to guess that you either put a component where it shouldn't go, or you put something in backwards. Chances are that said component is going to be dead.

Use a multimeter in continuity mode and check between hot and neutral past the fuse on the analog board, you'll almost certainly find a dead short. Start inspecting components for being in the wrong place or wrong orientation. If you can't find a smoking gun, start tracing the circuit and pulling components off the board until the short clears. A direct short across the fuse is going to be somewhere on the primary side of the supply, if it was on the secondary side, the supply just wouldn't start up properly, or have weird voltages.

In the future, you need to take detailed pictures of the board, because it sounds like you just made a list of what component goes to a silk screen label on the PCB, which isn't really enough detail.
 
GiGaBiTe said:
If the fuse is instantly blowing, then you have a direct short between hot and neutral. I'm going to guess that you either put a component where it shouldn't go, or you put something in backwards. Chances are that said component is going to be dead.

Use a multimeter in continuity mode and check between hot and neutral past the fuse on the analog board, you'll almost certainly find a dead short. Start inspecting components for being in the wrong place or wrong orientation. If you can't find a smoking gun, start tracing the circuit and pulling components off the board until the short clears. A direct short across the fuse is going to be somewhere on the primary side of the supply, if it was on the secondary side, the supply just wouldn't start up properly, or have weird voltages.

In the future, you need to take detailed pictures of the board, because it sounds like you just made a list of what component goes to a silk screen label on the PCB, which isn't really enough detail.
Click to expand...

I can never understand why people want to make such a mess of this stuff. Gigabite is right. You most likely got a component upside down or a blob of solder where it doesn't belong.
Dwight
 
Firstly just let me clarify. I took high res detailed photos of everything and as best as I can see everything is in its correct place and orientation. I've triple checked the soldering there are no blobs as you put it.

Primary section before


Primary section after


Doing some further testing this evening there is no straight short but with the meter set to 2K I get a reading of about 1400ohms between live and neutral. Weirdly with the meter set to 20k and 200k I get nothing then with it set to 2M I get a reading of 450k ohms between live and neutral.

I'm wondering though if DP9 could be at fault here. It is without doubt installed in the correct orientation but it reads short between + and - in both directions. Doesn't seem correct but then again I admit I'm not sure what is correct for this component.

EDIT: nope testing the diode out of circuit it seems fine. More hunting required.
 
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After some more testing I think I might have found the culprits.

DP2 is shorted as are the middle and bottom pins (looking at the images) of QP2. Edit - checking a data sheet this is pins 2 and 3.

With DP2 removed the short between + and - on DP9 still exists. With QP2 also removed this short is gone.

But... as I say I'm by no means an expert with such things, so in your opinion is this likely to be the cause of the problem?

Surely a good starting place anyway.

Also, out of interest and just to confirm, can I power this analogue board out of the computer by just connecting say an old HDD for a bit of load? Am I correct in saying that with the motherboard disconnected the HV side of things is dead?

In my initial power on the analogue board wasn't connected to the motherboard but I did make all connections to the CRT. It would certainly be handier for testing if I could run it out of the machine and not have to worry about the HV side of things.
 
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Glen M said:
Also, out of interest and just to confirm, can I power this analogue board out of the computer by just connecting say an old HDD for a bit of load? Am I correct in saying that with the motherboard disconnected the HV side of things is dead?
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Yes, that is correct. The HV stuff is driven by the mainboard.
 
I've order replacements for DP2 and QP2. I can't help but wonder why they failed. Possibly another component is bad or perhaps it was just their time to go with the stress of desoldering / resoldering causing them to fail. I'm fairly sure from memory that I checked the diode before reinstalling and it was fine though.

Reviewing the schematic

https://imgur.com/a/Vfq2biw

with DP2 and QP2 shorted that is more or less a direct + to - fault. RP30 and RP15 test fine. I'll check the other various components around this also and I suppose there is potential that IP1 might be damaged.
 
There is a difference on the schematic not shown for 220V compared to 110V. The 220V would use a capacitor rated for about 450V while the 110V would be two capacitors of 250V as shown in the schematic.
You are running 220V is my first question? If your line is 110V it will blow the switcher transistor. Also, make sure the line voltage does not drop too low. When I was in the Azores, at night the line voltage would drop to about 160V. That much under voltage will often blow the switching transistor. These power supplies are not happy with under voltage.
I'm looking at your two original pictures. The optical isolator looks different. It is hard to tell if it is just a lighting difference or possibly it is upside down.
Dwight
 
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Dwight, I'm in the UK so its 230v. Our voltages are stable (well generally run about 236 up to about 248 dependant on region). Interesting comment on Azores with voltage fluctuating so wildly down to 160V, I assume their normal domestic supply is 230. Don't suppose you know how they generate electricity at Azores being in the middle of the Atlantic, Solar or Hydro? I'm sure it would be a fantastic place to visit though.

I replaced the optical isolator when reassembling the board. I had read its one component liable to failure from spilt electrolytic so thought it would be worth doing while at it. Its an equal package according to the site I ordered it from. I still have the old part, perhaps I should just reuse it.
 
Glen M said:
Dwight, I'm in the UK so its 230v. Our voltages are stable (well generally run about 236 up to about 248 dependant on region). Interesting comment on Azores with voltage fluctuating so wildly down to 160V, I assume their normal domestic supply is 230. Don't suppose you know how they generate electricity at Azores being in the middle of the Atlantic, Solar or Hydro? I'm sure it would be a fantastic place to visit though.

I replaced the optical isolator when reassembling the board. I had read its one component liable to failure from spilt electrolytic so thought it would be worth doing while at it. Its an equal package according to the site I ordered it from. I still have the old part, perhaps I should just reuse it.
Click to expand...

I was there in the early 1970s. I'm sure things have changed since then. I orders a couple 12V CT transformers and used them as boost auto transformers. I then used a Variac to fine tune. It worked well with my HiFi. I was in the Navy while there. It was a little crazy. The air force ran most of the ground logistics, the navy flew most of the planes and the army ran the boats in the habor.
Dwight
 
Well the new components arrived today (DP2 and QP2) fitted switched on and... its working 100%. Nice solid 5V and 11.7v which I suppose might be a bit low but that could be the lack of load on the 12v rail.

Time to get it in the machine and fingers crossed it boots.

Much thanks for the help guys.
 
desiv said:
....
And?????

Enquiring minds want to know!!!! ;-)
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Sorry, I should have updated this.

Yep she fired up straight away and booted off the old HDD all be it, its not making nice noises and occasionally throws an error so I think its on its way out. The drive is full of the last owners documents some of which are dated 2018 so it wasn't that long ago that this system was someones presumed main computer.
 
Set it up to boot from ROM and forget about the hard disk. Most games you can run on it will fit a single floppy disk anyway. :)
 
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