• Please review our updated Terms and Rules here

Mac 512 brightness control

PhilipA

Experienced Member
Joined
Aug 4, 2013
Messages
476
Location
Larose, LA, USA
Howdy.

I just finished recapping my 512 and due to years of bad storage the brightness control pot (R57, 1M Ohm) has failed internally.

It's got really long legs that look like they've been bent from a horizontal mount to vertical and the support bracket on the back added as an afterthought.

Does anybody know if it was a specific Apple part or has anyone found a suitable replacement? I don't mind a bit of homebrew hacking to assemble something that fits.

Thanks


Phil
 
I don't think Apple's analog board parts are even manufactured by Apple. Only some of the chips on the motherboard.

On my old Apple II monitor, there are 100KOhm control pots for brightness, contrast, and screen height. I was able to replace them with newer versions of the same type, with some light modification. (i.e. I had to bend the legs a certain way to fit them into their places on the board) The dial stems don't look the same, but they work well enough.

You should be able to replace the pot with a generic version of the same type of part. That is, if you are able to find a way to remove it from the bracket. (I wouldn't know, because I only opened my 512k to relubricate the mechanical parts of the floppy drive.)
 
Upon working on it, looks to be moderately standard kit all round. I've seen a couple likely candidates on Mouser, I'll get them ordered and see if I can't get this all back together again soon.

Phil
 
I don't think Apple's analog board parts are even manufactured by Apple.

Besides the early Apple I, Apple has never manufactured any of their hardware in-house, it's always been done by a third party. Their current go-to is Foxconn because Foxconn makes everything.

Upon working on it, looks to be moderately standard kit all round. I've seen a couple likely candidates on Mouser, I'll get them ordered and see if I can't get this all back together again soon.

Phil

Make sure you get the exact same part, or an equivalent replacement. Even if you know what the ohms range of the pot is, there are logarithmic and linear types that behave very differently from each other.
 
You got it backwards.

A logarithmic pot has a taper, either the width of the track, or the thickness of the material.

A linear pot has a uniform track all around, which seems to be what yours it. Though with the significant wear pattern on that pot, it probably acts more like a log pot now.
 
Well, this appears to have been successful, reading the numbers on the multimeter.

Ordered the pot that I linked above. The board, overall is the same size with some minor differences.

20171030_121018.jpg

Pulled apart carefully with a set of cutters to get the right leverage:

20171030_121054.jpg

The build style is slightly different. Cut away the crimped section in the center to allow it to be split into its component pieces:

20171030_121242.jpg

My fingers were covered in grease so no photos here but the two differences were the mounting ears on this had 2, the original 1; the gap they sat in was identical so I used a nail file to remove the new ears. The center hole is a larger diameter and the center sweeper on the old shaft was slightly misaligned, so that was bent inward slightly to make contact with the center ring (on this one metal so better than the older carbon style one).

Crimped it all back together again, tested good with my multimeter throughout the range.

20171030_123450.jpg

I'll solder that back in tonight and see how it works.

Phil
 
20171030_163601.jpg

Well worth doing. Brightness minimum set, 5 volts tweaked to 5.00, left 12 at 12.2 so that's good.

Brightness now adjusts smoothly from very dim to very bright.

Phil
 
Well, that was short-lived. Periodically, randomly, I'll lose image on the screen (it was too light to see if there was raster still) but giving the case a gentle slap brings the image back. Wiggling the brightness control has no effect.

I'm guessing I've made a dry joint somewhere on the board. Flyback connections perhaps? I'm going to take it to bits again and reflow all the usual culprits.

Phil
 
Well, that was short-lived. Periodically, randomly, I'll lose image on the screen (it was too light to see if there was raster still) but giving the case a gentle slap brings the image back. Wiggling the brightness control has no effect.

I'm guessing I've made a dry joint somewhere on the board. Flyback connections perhaps? I'm going to take it to bits again and reflow all the usual culprits.

Phil

Make sure you check all the joints on the plug-in connectors as well. I had a similar problem on my Mac Plus and I at first I just re-flowed all the solder joints on the flyback, but the problem actually turned out to be one of the solder joints on one of the connectors that the deflection yoke plugs into.
 
Thanks, all of the connectors have been disturbed because I had the board out to solder all new caps in.

I'll certainly check those, would make sense if the grids aren't connected also.


--Phil
 
The cable between the logic and the analog board is a common problem.
Pay careful attention to the plugs on each end and look for evidence of discoloration due to heat.
 
I had cleaned the pins up after reading this wise advice previously. There is no discoloration of the plastic thankfully, it's all still just that pale white color.

I'm going to check them again, considering they were apart and back together again a few times. Voltages measured OK on the floppy connector so it's not got much of a volts drop through the power pins, at least.

Plan is to check the flyback, the power connector because the signal does go that way, and the yoke connector.

--Phil
 
That was short lived. The capacitor I put in (4.7uF, 100v non-pol, 105°c) position C1 failed. Guess it couldn't handle the horizontal frequency.

So, I put in 4 ceramic caps, rated 50V 1uF in parallel.

Powered it up and now the horizontal is all screwed up. The interlace doesn't line up. Adjusting the width coil core does still have an effect and the screen is just about legible.

I'm trying to figure what either doesn't like that set of ceramic caps (should it matter?) or if something has failed due to C1 failing. Edit: I was looking at the schematic last night and the input side of the circuit, if loaded down with essentially a dead short through C1 shouldn't do anything bad.. I will check to see that any of the primaries on the flyback haven't failed- either that or part of the feedback circuit is not good.

20171102_212137.jpg

This is what I put in:

20171102_211447.jpg

Thanks

Phil
 
Last edited:
That was short lived. The capacitor I put in (4.7uF, 100v non-pol, 105°c) position C1 failed. Guess it couldn't handle the horizontal frequency.

So, I put in 4 ceramic caps, rated 50V 1uF in parallel.

Powered it up and now the horizontal is all screwed up. The interlace doesn't line up. Adjusting the width coil core does still have an effect and the screen is just about legible.

I'm trying to figure what either doesn't like that set of ceramic caps (should it matter?) or if something has failed due to C1 failing.

View attachment 41728

This is what I put in:

View attachment 41729

Thanks

Phil

I have replaced C1 in the past with a gang of five 1uf monolithic caps. My reasoning for five (about 4.5uF) was because ceramics are typically about 10% under the rated capacitance value, electrolytic caps tend to increase in value with age so I figured the circuit would be more tolerant of a higher value than a lower one and using five also lowers the ESR a little more.
 
At 14kHz their original range of 3.6-4.7uF should be fine.. I put a stack of 4 in and that got it back up and working, kinda. I'm trying to figure out what would cause the horizontal to get all out of sync on the interlace.

--Phil
 
Looking at the image you posted the sync looks to be stable because the horizontal start and end locations all line up vertically. The problem looks like over scan (too wide) so my money is on the value of C1.
 
Fair, I'm going to pull it back out and measure it to make sure it's within spec.

I'm surprised that the 100V, 4.7uF non-polarized cap that was in there went pop. Makes me wonder if something else isn't right and upsetting things.


--Phil
 
Back
Top