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gavo

128k Analog Board Repair

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Hi,

 

I'm finally getting around to taking a look at my non-working analog board. I thought I'd start by simply replacing the bits recommended by Larry Pina in "Mac repair and upgrade secrets". Specifically I thought I'd go for the "heavy duty" upgrade.

 

I have a couple of questions tho.

 

1. If I replace CR20 and CR21 with a MBR1045, what are the installation implications given its in a TO-220 package as opposed to the existing axial package?

 

2. I'm having trouble locating either a GI854 or MR824 barrel diode for CR1 and CR5 - I can locate a MR856, is this even suitable? Its a 3A 600V rectifier diode.

 

3. I've had zero luck in tracking down 3.9uF 100V capacitor for C1, any suggestions?

 

Cheers,

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Hi,

 

I'm finally getting around to taking a look at my non-working analog board. I thought I'd start by simply replacing the bits recommended by Larry Pina in "Mac repair and upgrade secrets". Specifically I thought I'd go for the "heavy duty" upgrade.

 

I have a couple of questions tho.

 

1. If I replace CR20 and CR21 with a MBR1045, what are the installation implications given its in a TO-220 package as opposed to the existing axial package?

 

2. I'm having trouble locating either a GI854 or MR824 barrel diode for CR1 and CR5 - I can locate a MR856, is this even suitable? Its a 3A 600V rectifier diode.

 

3. I've had zero luck in tracking down 3.9uF 100V capacitor for C1, any suggestions?

 

Cheers,

 

Don't worry too much about the package difference. The 1045 should be fine electrically, and its better package will only help longevity. You'll just have a small mechanical challenge to overcome.

 

CR1 and CR5 are "fast recovery" rectifiers. You *must* replace them with same. Current and voltage ratings are only the minimum things to look out for. You need a reverse recovery time of 150ns or less.

 

Replacements for C1 can be found at Digikey. However, the design is flawed, and that capacitor is simply the wrong type. I recommend taking four 1uF 50V ceramic caps and wiring them in parallel. Use that combination in place of C1. The combination should outlast the rest of the mac.

 

Ok, that said, I would not recommend just replacing components randomly to solve a specific problem. What is not working? What are the symptoms? Remember that the three most common problems are:

 

1) Bad C1 (causes either a vertical line or no raster at all). Usually easy to spot, as it will fail by shorting, then overheating. In response, the pressure will rise, and you'll get a bulge on the top as the pressure-relief slots do their work. The top of the cap should be ruler-flat. If it bulges, it's dead. Period.

 

2) Bad solder joints on yoke connector at the analog board. Just freshen up the joints with new solder.

 

3) Bad solder joints on flyback. Freshen those up, too.

 

A fourth, less common, problem is easily solved:

 

4) Misadjusted or dirty pots. Note their initial positions, then rotate them back and forth a few times with the unit powered off. Return to initial positions, try again.

 

For more details, see http://68kmla.org/files/classicmac2.pdf

 

Good luck!

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Thanks for that great advice!

 

I also hadnt seen your Classic Mac Repair notes before (I'm assuming thats you who wrote them:) ), and although I havent read right through them yet, they look to be extremely detailed, easy to read and therefore extremely useful - so thanks!

 

As far as my board, the problem is no video. Powering on the Mac gives the normal bong and the drive reads, so I was hoping it was likely just to be dry joints. I just figured that if I was going to go to the trouble (for me anyway), of pulling it to bits, I might as well give it a service at the same time.

 

Cheers!

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The fact that you get a reassuring bong at startup tells you that a lot of things are working. The main power supply certainly is functioning well, so you can leave the power rectifiers CR20, 21 alone for now.

 

The common causes of the "no video" problem are fairly easy to fix, so you might get lucky. Just resolder the connections mentioned, and replace C1 if needed. You may be pleasantly surprised.

 

Good luck!

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LOL, after all that C1 had already been replaced by a 4.7uF cap!

 

Anyway, turned out to be a (fairly obvious) dry joint on one of the flyback pins. I didnt end up replacing anything, although there was some shrinkage of the plastic around some of the caps (but the caps themselves hadnt bubbled up on top) - so maybe a repair for me at some later date. I did however freshen up all the joints you mentioned.

 

Cheers,

 

-Gav.

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Anyway, turned out to be a (fairly obvious) dry joint on one of the flyback pins. I didnt end up replacing anything, although there was some shrinkage of the plastic around some of the caps (but the caps themselves hadnt bubbled up on top) - so maybe a repair for me at some later date. I did however freshen up all the joints you mentioned.

 

Cheers,

 

-Gav.

 

Glad to hear that it was one of the three standard -- and easy to fix -- problems. Good job saving another compact from the landfill!

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