• Please review our updated Terms and Rules here

List of Capacitors for Macintosh Classic

G

Glen M

Experienced Member
Joined
Apr 27, 2020
Messages
57
Location
Belfast, Northern Ireland
I picked up a Macintosh Classic today but it badly needs a recap. Thing is I can't for the life of me find a capacitor list for the main board.

Would anyone happen to have a list?

I really didn't expect this to be so hard to find.
 
7x 47 µF, 16V
1x 1 µF, 50V

You should do the sweep/power board as well. While it has no SMD caps, on mine most did leak anyway. I ordered caps for it yesterday. Let me know if you need a list.
 
Timo W. said:
7x 47 µF, 16V
1x 1 µF, 50V

You should do the sweep/power board as well. While it has no SMD caps, on mine most did leak anyway. I ordered caps for it yesterday. Let me know if you need a list.
Click to expand...

Many thanks for this. I've got the caps ordered.

From a visual inspection the power board looks fine but as you say it probably needs a recap too. If you have the list, I'd be grateful if you could share.

I was going to clean and recap the motherboard then take a look at voltages before making a call on the sweep/power board. Thing is I've never done any work so close to a CRT and that HV. I've watched practically all of Adrians videos (Adrians Digital Basement) so I'm fairly confident and it seems that the HV should discharge itself but I do need to get something to check this. I do know a TV repair specialist so I might ask if he has any old HV probes I could borrow.
 
For the power board (rev. A) you need a total of 25 caps:

5x 1 µF, 50V
1x 4.7 µF, 250V
1x 10 µF, 25V
1x 10 µF, 50V
2x 47 µF, 25V
1x 220 µF, 16V
1x 220 µF, 35V
2x 220 µF, 400V
3x 470 µF, 25V
1x 1000 µF, 6.3V
1x 1000 µF, 10V
2x 1000 µF, 16V
1x 1000 µF, 35V
2x 2200 µF, 10V
1x 2200 µF, 16V

Since you can always use higher-voltage ones, you can combine those with the same capacity to some extent. E.g. just order 5x 1000 µF, 35V and 3x 2200 µF, 16V.

The Classic has a bleeder, so the CRT is normally discharged after some hours with no power. HOWEVER, that is only a safty fallback and does not free you from discharging the tube. You have to do this anyway, especially since the bleeder resistor often fails. Follow this video and nothing bad should happen: https://www.youtube.com/watch?v=x1DeMOl_nK4
 
Last edited:
After watching the video again, I need to make an addition that he does not mention:

The Mac he uses to show how to discharge the tube has a simple neck plug. Everything he does is 100% correct. However, for compact Macs with a neck board (like the Classic) instead of a simple plug, you need to ground the screwdriver to the metal chassis, not to the upper left lug of the CRT. Otherwise you risk frying the neck board. Likewise, if you use the metal chassis on compact Macs with no neck board, you risk frying the main board (if it is still connected, that is).

So for the Classic, ground the screwdriver to the metal chassis.
 
Timo W. said:
Since you can always use higher-voltage ones, you can combine those with the same capacity to some extent. E.g. just order 5x 1000 µF, 35V and 3x 2200 µF, 16V.
Click to expand...

Higher voltage caps are usually internally different than lower voltage ones, so drifting too far in voltage can cause problems with ESR. Like a 50v cap will have a different and sometimes much higher ESR than say a 10v cap.
 
Link to some serious site stating and explaining this, because this contradics with 99.9% of what even people deep into electronics say. In fact, this is the first time I ever saw someone stating that at all. Now, I would certainly agree that it makes no sense to replace an electrolytic cap rated 6.3V with one rated 250V, because that's quite an extreme anyway - and that's why I clearly said: "to some extent". But for e.g. 35V or 50V instead of 10V, it makes no difference. When comparing data sheets, you'll notice that those with a higher voltage rating can take higher ripple current, which is an advantage, too.

Besides, ESR won't be an issue in this case. These days, you only seem to get low-ESR caps anyway and the Classic uses ordinary caps. So even in the worst case, the ESR will still be better than that of the old caps used back then.

Could it be that you are thinking of ceramic caps? Because there, this is partly true. Among other things like temperature, the capacitance of ceramic caps also depends on how close the working voltage is to the rated voltage, so you must use the correct ones. But we are not dealing with ceramic caps here.
 
Last edited:
I successfully managed to discharge the CRT, well, I tried it but there was no spark. I can only assume the internal bleed resistor is still working.

Just waiting on the postman now to hurry up and deliver the replacement caps I ordered!

The leaking electrolyte is extensive on this board but at least it looks cosmetic only, hope so anyway. Washed it all off and there's just some tarnished pads but the fibreglass pen should make short work of those. I haven't ordered caps for the power board yet. Want to test it before jumping in there. No bulging or signs of leaking so may be OK.
 
Glen M said:
Want to test it before jumping in there. No bulging or signs of leaking so may be OK.
Click to expand...
On my power board, there were no signs of leakage either and the Classic still worked, too. But knowing the caps were 30+ years old, I wanted to renew them anyway. As soon as I had desoldered the first one, that fishy smell came up and the cap was wet underneath. That large cluster of caps at the speaker was particularly bad with 6 of 9 leaked.
 
Timo W. said:
On my power board, there were no signs of leakage either and the Classic still worked, too. But knowing the caps were 30+ years old, I wanted to renew them anyway. As soon as I had desoldered the first one, that fishy smell came up and the cap was wet underneath. That large cluster of caps at the speaker was particularly bad with 6 of 9 leaked.
Click to expand...

I thought I'd take your advice and at least have a look before powering up and well its not good news. I suspect most of the caps on adjacent to the motherboard power lead have leaked, 1 rifa has cracked but more so there is signs of a short on the primary side of the transformer. The board is well and truly cooked, black and crumbling.

Few pics







I can certainly replace and clean up around the caps but the damage at the transformer is another thing. If that's gone there's probably not a lot I can do. Any advice for testing a transformer for a fault, can I measure it on the multi-meter?
 
Last edited:
Wow, that looks bad (but fixable). Give it a good clean with IPA to see how bad the damage actually is.

No idea about the transformer, but since it's supposed to work with AC, a multi-meter won't help you much. It will most likely measure near 0 ohms.

What makes you think there's a short on the primary side?

btw, your power board is slightly different than the one in my Classic (see here). It only uses one 220 µF, 400V cap instead of two and I can spot at least one cap that has a different capacitance value. Check my list of caps against your board before ordering the caps.
 
Last edited:
Timo W. said:
Wow, that looks bad (but fixable). Give it a good clean with IPA to see how bad the damage actually is.

No idea about the transformer, but since it's supposed to work with AC, a multi-meter won't help you much. It will most likely measure near 0 ohms.

What makes you think there's a short on the primary side?

btw, your power board is slightly different than the one in my Classic (see here). It only uses one 220 µF, 400V cap instead of two and I can spot at least one cap that has a different capacitance value. Check my list of caps against your board before ordering the caps.
Click to expand...

Where that is turned to charcoal is that not the primary side of the transformer? I removed it for a better look and it actually looks ok. Hoping it was just a short caused by the electrolyte.

I've also noted down all the caps and there position across the board. There is only the 1 big cap on the 220v PSU with a jumper link in place of the second one.
 
Glen M said:
There is only the 1 big cap on the 220v PSU with a jumper link in place of the second one.
Click to expand...
And a 470 µF, 50V one where mine had a 1000 µF one. Wonder why that is. Don't you have 230/240V mains in Ireland as well?
 
Timo W. said:
And a 470 µF, 50V one where mine had a 1000 µF one. Wonder why that is. Don't you have 230/240V mains in Ireland as well?
Click to expand...

Yeah it's 230v here. The board didn't look to have any work done to it previously so can only assume its always had the 470 cap.

Maybe a different revision of the board?
 
Most likely Rev. B then, because the service manual mentions that you can not mix revision A and B power boards and the according tubes, and your power board has a different connector for the tube's collar.

Still suspicious that the second large cap is missing and has a jumper wire installed instead. That would normally indicate it's for 120V mains.
 
Glen M said:
Where that is turned to charcoal is that not the primary side of the transformer? I removed it for a better look and it actually looks ok.
Click to expand...

When the PCB burns like that, it becomes conductive. You'll need to remove the transformer and test for resistance between the burned area. If there is resistance, you'll need to start digging into the board to get all of the burned contamination out. If it goes too far into the board, you may have to resort to drilling a small hole through the entire board and then using a tiny file or saw blade to cut a slot between the two traces to get rid of the path between them.

To prevent it from happening again, you can insert plastic wafers through the slot and secure them to form a wall between the traces. You may also want to apply more conformal coating to the PCB because it looks like it's flaking off in several areas. When the copper is exposed to the air, it starts to tarnish/corrode and its electrical properties change. It also allows high voltage to more easily "escape" and potentially arc over to something nearby. Clear fingernail varnish works decent enough and is cheap if you can't find the purpose made stuff.
 
Last edited:
I've cleaned the effected areas and as you can see a fair bit of the solder mask has come away. I will be coating this with some green nail varnish after I'm finished the recap.



Problem though. Were the PCB was burnt at the transformer 1 pad just fell off as I desoldered. From looking at other pictures this pad has no traces from it so is it just there to help secure the transformer in place? I do note this leg of the transformer has continuity with the leg above it (looking at the picture), that continuity within the transformer itself when measured removed from the board. What should I do here, just leave it floating or try to plug the hole with solder?

 
If there is no trace going to the pad, it was just there for mechanical support. It should be fine just floating.

As for the copper, it looks like it has tarnished significantly, I would do something to neutralize the oxide before you put any sort of conformal coating on it. Krud Kutter works if you soak the end of a cotton swab with it and rub it on the bad areas. It usually only takes a minute or so to work of light scrubbing, then rub it dry with the other end of the cotton swab.

https://www.walmart.com/ip/Krud-Kut...619958?wmlspartner=wlpa&selectedSellerId=1194
 
Wonder what made that analog board so unhappy as to burn like that, is that a common problem when caps go bad? If so my preventative maintenance spiel may be happening sooner rather than later on my Plus.
 
You must log in or register to reply here.
Back
Top