Mac Plus Display Problems

[codepoet]

I've acquired a plus that would make a happy boot chime, but show no video -- although occasionally the screen would flicker if the case was smacked. Following Dead Mac Scrolls, I re-flowed the solder on J4, and following a tip found elsewhere online, I re-flowed J1 as well. Now it comes up to only this. Adjusting contrast does not show any other pictures. The Mac boots happily from a FloppyEmu, but the screen doesn't change at all. Reducing voltage dims the image, but does not change it -- nor do any of the other screen adjustments cause an image to be visible.

Any advice? I'm obviously going to re-cap, but I want to make sure this thing is a survivor before I lay down the cash for the caps.

 

[unity]

I have never had to re-cap a Plus board... yet. Usually the problems are cold solder joints. I always reflow the flyback and every connector including the motherboard. Before replacing components, look things over again. I have also found cold joints at the power connect, battery, etc.

 

[codepoet]

Thanks for responding, I concur that all the caps look good. The video problem looks like an extreme version of this guy's SE issues, except he gets a faded picture, and I get none.

I replaced the logic board, to rule out anything on that side, and got the same result. Its got to be something on the analog board...

 

[codepoet]

This was caused by damaged to a chip on the analog board, caused by discharging the CRT to the frame. My discharge cable did not include a resistor, because I read somewhere they wouldn't work. Turns out leaving it out is a bad idea.

The damaged chip was a Motorola SN74LS38NDS, which I replaced with the same number but from a different brand, from a spare Plus that was more dead. The picture is now sharp and clear, without any lines.

If an expert is someone who has made all the mistakes, I'm well on my way to becoming an expert

 

[unity]

Glad you figured it out. Kinda surprised that a CRT discharge cause it. I really never discharge CRTs,  just push the high voltage cable cap to one side, tilt and lift then pop it out. On the off chance I do discharge, I do it when plugged in so it uses the ground.

 

[trag]

IIRC, Pina warns about discharging without a resistor and that it will cause that specific problem, somewhere in the text of one of his books.  Perhaps in both The Dead Mac Scrolls and in Upgrade and Repair Secrets.

I recommend against discharging the CRTs because of problems such as this.   Also, I used to do a lot of work on compact Macs, and the **ONLY** time I got shocked by a CRT was in the process of discharging it.  If I had just left the CRT alone, I would have been fine.  Also, the shock was startling, but hardly life threatening.   Perhaps if one has a pace maker...

Anyway, that component is available from Digikey:

https://www.digikey.com/product-detail/en/texas-instruments/SN74LS38N/296-1664-5-ND/277310

 

[codepoet]

There's a lot of conflicting advice online about discharging -- everything from "don't do it" to "do it or you'll die."

I have a copy of Dead Mac Scrolls, but not Pina's full guide. I went middle-of-the-road and set up my discharging kit, but didn't include the resistor -- since many forums suggested any arc would just hop over any resistance. I've probably used this setup 2 dozen times without issue: I only discharge if I'm going to be working near the high voltage gear...which is most of the time, since that's where most problems seem to be. On this occasion, the discharge was fairly shortly after the unit was powered on, and the spark was an enthusiastic one. Clearly I should have had the resistor in place.

I've order a replacement part from Mouser.com for the donor Mac (which is missing other parts as well*) as well as a resistor to add to my gear.

*Speaking of the donor Mac, its also missing the yoke connector -- apparently donated to some other project before I inherited it. Does anyone know where I can find such a connector?

 

[unity]

They removed the connector on the yoke? Odd. You are talking the male 4-pin connector, right? Are the connector ends good? I have dozens of spares if you need one, just pay shipping.

 

[codepoet]

The connector on the analog board is gone -- solder points cleaned bare. I think that's the female end?

The end on the wires is still there, just nothing to plug into!