128K analog trouble after bad discharge

[Elemenoh]

I discharged a 128K Mac without a bleeder resistor and manage to damage it. At first I got a chime but no raster at all. Per Macintosh Repair & Upgrade Secrets, I replaced the LAG chip on the logic board and got raster lines but a dark screen. I then replaced U2 on the analog board, but the issue persisted. I had previously reflowed all of the solder joints and swap tested with another J4/J7 cable assembly. Nothing on the analog board is visibly burnt.

Any tips on what to check next?

Here's the part of the book I referenced. I think he's got the symptoms backwards though. The logic board works fine and even if I install another known good logic board, I get a bong and screen with raster, but no floppy icon. So the issue is with the analog board, but not U2 since it was just replaced with a new part.

No Raster, Completely Dark
If the set bongs on startup but there's no raster, check for cracked solder joints on pin-1 and pin-3 of the J4 connector. If that's not the problem, check the
white and orange wires of the analog board to logicboard cable for continuity. Table 5-7 gives the color code and function of each of the wires.

While the harness is out, check for tarnished pins inside the J4 and J7 connectors. Sometimes you don't find anything wrong, but after you disconnect and reconnect the harness, everything works again. What that suggests is a high resistance connection similar to the fluctuating +5.0-V problem described in Chapter 4. The only permanent way to fix this is to replace the harness and treat the connector pins inside J4 and J7 with stabilant. This material is a lot like heat sink grease in that it fills voids and imperfections in the metal. It's used to retard corrosion and lower electrical resistance.You'll be amazed at how well the stuff works. Referto Chapter 4 for more information.

If everything in that area checks OK, both the LAG chip on the logic board and U2, the SN74LS38N IC on the analog board, may be bad. These chips often blow in tandem when you d ischarge the CRT without using a l0-meg bleeder resistor. If you did that, and you got an exceptionally big spark, you probably blew the chips. Now, you've got problems on both boards. If you replace the LAG but not U2, you'll momentarily see a vertical line, then the display will collapse to nothing. If you replace U2, but not the LAG, you'll get an overly bright raster but no blinking floppy disk icon. If you have a spare set of boards (from another Mac) you can try swapping boards to see whether or not your symptoms match those just given. Swapping the logic board essentially gives you a new LAG. If you see a momentary white line with the new logic board, then nothing, you know the LAG on the original logic board is bad. Swapping the analog board but not the suspect logic board essentially gives you a new U2 (SN74LS38N). Now you should get an overly bright raster but no blinking floppy disk icon. The solution is to install two new
chips, a new LAG on the logic board, and a new SN74LS38N on the analog board. For details on how to replace chips, refer to Chapter 10.

 

[Elemenoh]

Update: +12V, -12V and +5V are all testing okay at the floppy port. Next I'm going to try to check components downstream rom U2 on the analog board.

 

[Elemenoh]

Nearby resistors all tested okay. There is a 2N5485 (Q5) off of U2 pin 3 that shows resistance between source and gate in both directions. I'm not sure if this is because my meter isn't suited for it or if the transistor failed, but I've ordered some replacements to try.

 

[Elemenoh]

Update: Replacing Q7 changed the symptom to a vertical line (collapsed horizontal deflection). Q6 tested okay. Will test Q3 next.

 

[Patrick]

I discharged a 128K Mac without a bleeder resistor and manage to damage it.

that was not something that occurred to me could happen. i don't think i have any good advice. but i'll be following along to see where you end up.

 

[pfuentes69]

Wow... I wasn't neither aware this can happen... now it's scary...

 

[Gregor]

Wow... I wasn't neither aware this can happen... now it's scary...

Hi Everyone,

The same think happened to me (after discharge). I was not using 10 meg resistor.. I am investigating the board.
Do you have any measurements of the voltage that goes to the T1 flyback pins (1-8).
I measured them all and I cannot see -12 V and I cannot see -12V on 9 pin of J2 (yellow cable). So I am suspecting that there is somewhere failure. I got schematics and checking all.

 

[Elemenoh]

Wouldn’t -12V just be needed for audio and maybe serial? I’d be curious to know if it’s required for basic operation.

 

[Elemenoh]

I found a pad for C1 had come loose from all of the handling of the board. I reattached it and now get a chime, no video and then lose voltage (flup flup flup noises). The rectifier at CR20 seems okay, so maybe I have a bad cap somewhere else now. It's weird that the symptom keeps changing.

 

[jajan547]

Does this help?

 

[Gregor]

Does this help?

Hello, thank you. I will try to check it. From my first checks I cannot see +30V from L4 nor +30V from 8 pin of T1.

 

[Gregor]

Do you have any tips how to check Flyback transformer (T1)?
I cannot see +30V from L4 nor +30V from 8 pin of T1, so don't know what is the cause of this.

 

[68kPlus]

You did discharge to the ground screw/lug right?

 

[Gregor]

You did discharge to the ground screw/lug right?

No. Only to the chasis. I got Spark.

 

[68kPlus]

That could be why it happened. Apparently you have to discharge to the ground lug otherwise you will send that voltage to the logic board and the analogue board (I think). Maybe only a little, but it might still happen

 

[68kPlus]

I discharged a 128K Mac without a bleeder resistor and manage to damage it. At first I got a chime but no raster at all. Per Macintosh Repair & Upgrade Secrets, I replaced the LAG chip on the logic board and got raster lines but a dark screen. I then replaced U2 on the analog board, but the issue persisted. I had previously reflowed all of the solder joints and swap tested with another J4/J7 cable assembly. Nothing on the analog board is visibly burnt.

Any tips on what to check next?

Here's the part of the book I referenced. I think he's got the symptoms backwards though. The logic board works fine and even if I install another known good logic board, I get a bong and screen with raster, but no floppy icon. So the issue is with the analog board, but not U2 since it was just replaced with a new part.



View attachment 8104

Did you discharge to the chassis or the ground lug?

 

[Elemenoh]

Did you discharge to the chassis or the ground lug?

Chassis, without a resistor. I'm still chasing down the problem. I found one open capacitor so far at C39 but replacing it didn't change the symptom.

 

[68kPlus]

Chassis, without a resistor. I'm still chasing down the problem. I found one open capacitor so far at C39 but replacing it didn't change the symptom.

Theres your issue. It MUST go to the ground screw, otherwise you fry either the logic board, analogue board or both.
It looks as though your analogue board is toast. Sorry.

 

[KingDingus]

Just to be clear and for my piece of mind, is the ground screw the one that attaches the CRT to the front bezel? That's what I have seen on YouTube and used on my project recently. Working on the full recap now so have not reached the point of knowing if CRT discharge was done incorrectly.

Thanks.

 

[pfuentes69]

Just to be clear and for my piece of mind, is the ground screw the one that attaches the CRT to the front bezel? That's what I have seen on YouTube and used on my project recently. Working on the full recap now so have not reached the point of knowing if CRT discharge was done incorrectly.

Thanks.

Yes, that's it. The right place is always the mounting screw of the CRT, where the ground cable also goes.

The discussion that is going around is that discharging the CRT using other ground point such as the chassis could fry components.