Macintosh Plus screen slowly drifting horizontally to the left

[pipkato]

I have a vintage Macintosh Plus recently removed from long-term storage. It seems to be working well and I added a BlueSCSI emulator which boots fine into Systems 6 and 7.

I have one minor problem I noticed when it was running for a while. The screen, which on boot-up was nicely crisp, bright, and centred, began to move horizontally ever so slightly left of centre after about an hour of use.

I know there are controls on the Analog board to adjust the CRT picture and one of those controls manages centring the desktop image. So I’m guessing some part of that circuit is drifting with time and temperature. It’s still acceptable after an hour but I’m not sure if running longer will cause the screen to move even further left. Although I’m unlikely to have it running for such a long time I’d like, if only for aesthetic reasons, to have it centred and stable. I did the 'Diode Mod' on the logic board to power the BlueSCSI from the SCSI port and took a look at as much of the analog board I could see at the time and the capacitors seemed in pretty good shape, with no obvious bulging or leaking.

I’m not a techie, or an electronic engineer, but I have a common-sense approach to troubleshooting and problem solving. Can anyone advise on how I might go about fixing this problem? Or point me at a link about the circuitry controlling horizontal centring of the Macintosh Plus display? And, if there’s a better forum to post this question in please let me know. Thanks!

 

[Phipli]

I have a vintage Macintosh Plus recently removed from long-term storage. It seems to be working well and I added a BlueSCSI emulator which boots fine into Systems 6 and 7.

I have one minor problem I noticed when it was running for a while. The screen, which on boot-up was nicely crisp, bright, and centred, began to move horizontally ever so slightly left of centre after about an hour of use.

I know there are controls on the Analog board to adjust the CRT picture and one of those controls manages centring the desktop image. So I’m guessing some part of that circuit is drifting with time and temperature. It’s still acceptable after an hour but I’m not sure if running longer will cause the screen to move even further left. Although I’m unlikely to have it running for such a long time I’d like, if only for aesthetic reasons, to have it centred and stable. I did the 'Diode Mod' on the logic board to power the BlueSCSI from the SCSI port and took a look at as much of the analog board I could see at the time and the capacitors seemed in pretty good shape, with no obvious bulging or leaking.

I’m not a techie, or an electronic engineer, but I have a common-sense approach to troubleshooting and problem solving. Can anyone advise on how I might go about fixing this problem? Or point me at a link about the circuitry controlling horizontal centring of the Macintosh Plus display? And, if there’s a better forum to post this question in please let me know. Thanks!

My first assumption would be a failing capacitor, but I haven't seen this specific issue in a Mac. I have seen it on (someone else's) oscilloscope, but didn't have a chance to troubleshoot that.

I would start by inspecting the analogue board for signs of cap leakage or bulging like you said you have. I then would check that the various voltages are stable (carefully) over time. The various voltages are available from the external floppy port, so you can check them without having the case off.

 

[Phipli]

I’m not a techie, or an electronic engineer, but I have a common-sense approach to troubleshooting and problem solving. Can anyone advise on how I might go about fixing this problem? Or point me at a link about the circuitry controlling horizontal centring of the Macintosh Plus display? And, if there’s a better forum to post this question in please let me know. Thanks!

This might not be massively helpful, but here is some information that might interest you :

Vintage Apple - Gamba - Mac Schematics & Logic Board Layouts

This book describes how the plus works, but doesn't go into the component level detail.

https://vintageapple.org/inside_o/pdf/Guide_to_Macintosh_Family_Hardware_2nd_Edition_1990.pdf

 

[pipkato]

Thanks very much Phipli for your quick reply. I'll have a look again at the analog board when I get a chance. I've viewed various online videos about analog board issues but none described the problem I'm experiencing. Never knew voltages could be checked from the external floppy port. Do you know where I can find more detailed info on this?

 

[pipkato]

Thanks again, Phipli. I just saw that you've already answered my questions with those links. As I said, I'm not an electronics engineer but it should be useful nonetheless.

 

[Phipli]

Thanks again, Phipli. I just saw that you've already answered my questions with those links. As I said, I'm not an electronics engineer but it should be useful nonetheless.

The floppy port pinout is on PDF page 378 (printed pages 339).

 

[joshc]

This could easily be caused by a weak solder joint, which is going to be affected by heat. Plus analog boards are notorious for bad solder joints - I would start by reflowing the common suspects - the joints for the yoke connector are the main ones to look at.

 

[pipkato]

Thanks very much Phipli and joshc.

 

[alexGS]

I have a vintage Macintosh Plus recently removed from long-term storage. It seems to be working well and I added a BlueSCSI emulator which boots fine into Systems 6 and 7.

I have one minor problem I noticed when it was running for a while. The screen, which on boot-up was nicely crisp, bright, and centred, began to move horizontally ever so slightly left of centre after about an hour of use.

Yes - I’ve noticed the same on the two Mac Plus machines I have on the bench at the moment. We’re only talking a small shift of a few millimetres to the left, and I think it happens within about 20 minutes.

The solution is to adjust the display position after it’s warmed up, which has always been the Service Source advice for later models.

I think the cause could be the horizontal output transistor (the BU406) just changing its characteristics as it warms up.

I replaced a BU406 with a new one and that seemed to make the shift with temperature more noticeable; also, with the new BU406, the image had a tiny amount of wrap-around at the right edge which was very much more annoying I put the original back in. I also tried changing the infamous C1 - the non-polarised electrolytic capacitor beside the width control - but it was already the modern type and made no difference.

 

[pipkato]

Thanks alexGS for that really interesting and useful information. I haven't had my old MacPlus on for long enough to decide if it continues to drift to the left. So it may be that, as you suggest, it will stabilise after a short while. I'll give it a try and see if it stays put in that position. Sorry to hear about your unsuccessful attempts to resolve it with a new BU406. I did go over the board and redid all the solder joints or most of them, although I could only see a possible split joint on one. Didn't make any difference. I put my MacPlus aside for a couple of months powering it up now and then and it seemed fine but still drifting slowly to the left. Then one day my display disappeared completely although I had the chime. A slight tap on the side of the case restored my display, and it's been that way for a while. So I probably need to double check solder joints again.

BTW I found what seems to be a solution for a replacement Macintosh Plus PRAM battery, at least for a short time. I ordered the correct replacement and even tried the multiple LR44 solution. But I decided to try these 14500 3.7V Lithium Ion rechargeable batteries. Got them from Aliexpress here https://www.aliexpress.com/item/100...der_detail.order_detail_item.3.7d9df19c88gsIc. Bought a basic charger from Amazon here - https://www.amazon.co.uk/dp/B09ZQRZZ1D?psc=1&ref=ppx_yo2ov_dt_b_product_details. Wrapped some cardboard around the battery to 'fatten' it to the approximate size of the original MacPlus one and it's been working fine for the past few months. I can't remember the voltage of the original battery but it was close enough to the 3.7V 14500 batteries and I imagine they will eventually drop below the required voltage for the Mac. But I can have charged replacements standing by and can continue to rotate and recharge the Aliexpress set.

 

[alexGS]

A slight tap on the side of the case restored my display, and it's been that way for a while. So I probably need to double check solder joints again.

BTW I found what seems to be a solution for a replacement Macintosh Plus PRAM battery, at least for a short time. I ordered the correct replacement and even tried the multiple LR44 solution. But I decided to try these 14500 3.7V Lithium Ion rechargeable batteries. Got them from Aliexpress here https://www.aliexpress.com/item/100...der_detail.order_detail_item.3.7d9df19c88gsIc. Bought a basic charger from Amazon here - https://www.amazon.co.uk/dp/B09ZQRZZ1D?psc=1&ref=ppx_yo2ov_dt_b_product_details. Wrapped some cardboard around the battery to 'fatten' it to the approximate size of the original MacPlus one and it's been working fine for the past few months. I can't remember the voltage of the original battery but it was close enough to the 3.7V 14500 batteries and I imagine they will eventually drop below the required voltage for the Mac. But I can have charged replacements standing by and can continue to rotate and recharge the Aliexpress set.

Nice work With regard to the video loss (I had that recently too), it really is often caused by the pins of the analog board connector (for the logic board cable) as people say - one of the pins stands separate from the others, is often missed in resoldering, and that’s the video signal. I suggest using a desoldering tool to get rid of the old solder; adding new solder isn’t always effective unless the old solder is removed.

The battery solution sounds great to me, I’m sure it will last for a long time. 14500 is AA-battery sized, I thought that was a bit short to reach the terminals, but you’ve obviously proved it works! I have another idea involving replacement of the entire battery compartment, since its terminals are often rotted away, but I haven’t got to that idea yet.

 

[pipkato]

Thanks very much alexGS for that tip about the video pin. Next time I'm working on the Macintosh Plus I'll certainly check that out and take your advice to completely de-solder the pin before resoldering. My batteries fit perfectly and didn't really need the 'fattening' I mentioned once positioned correctly. But for better physical stability within the battery compartment I added the cardboard so now it fits snugly, more or less like the original.