Macintosh 128k issues

[techknight]

Judging by the wavy picture, I would say your main input filter is wasted. Or at least some of the output capacitor filters. 

Flubbing/pulsing is caused by not going into proper regulation. Need to check the opto-isolator or whatever other parts might be in the feedback loop, As well as checking the over-current sense circuit. Both will cause this. 

The way to determine which is to scope the output to the sweep circuits. If you see a nice ramp with severe overshoot before the "shutdown pulse" then its going out of regulation. If you see a swoopy line before shutdown, you have an overcurrent problem. 

 

[Carboy7]

thermal camera? Omg, is that necessary? Haha, just kidding.

 

[EagleTG]

Nope, but that doesn't mean it isn't AWESOME.  Hahah.

 

[Ferrix97]

The Flir One thermal camera is one of the most useful tools I've bough. It is great for troubleshooting Apple logic boards (the new ones).

If you have a shorted power rail on a MacBook you stick some voltage on it and with the thermal camera you immediately see where the damaged component is located. It is not uncommon to find a shorted ceramic or tantalum capacitor. I was also able to fix a shorted HD6970M MXM card from a 2011 iMac by locating a bad Mosfet IC.

If you deal with broken computers all the time, it is a really useful tool and will pay itself with all the time you'll save.

Also, it lets you see in the dark!

As for the Macintosh:

I started isolating the various rails by desoldering the inductors, the problem still persist.

I also tested the feedback circuit and the protection SCR and both seem to work (the SCR triggers at about 13.10V).

I think there is still something wrong on the primary (where they connected a 110V mac to a 220V line), maybe a failed ceramic or polyester capacitor that makes the circuit less stable therefore creating voltage spikes on the secondary.

 

[andy2k68kMLA]

What resolution are those thermal imaging pictures? I was looking at a Flir One attachment for the iPhone. It's substantially less money than an all in one unit and the pictures looked decent. Anyone have any experience with an iPhone or Android Flir attachment? Pretty good reviews on it.

 

[Ferrix97]

I do have a first generarition Flir ONE (I took the photos at the Analog Board), the resolution is not that high (something like 80x60), but it uses a regular (VGA resolution) camera to increase the quality by blending the two images, this is the main difference between the Flir and the Seek camera

 

[andy2k68kMLA]

Cool Well I guess that seals the deal. You can get one new for about $235. Clearly it works well.

 

[Ferrix97]

I got mine used for 150€, right after the iPhone 6 launch (since it was compatibile with the 5 and 5S only)

 

[EagleTG]

Oooh, that's a great tip, and a great use for an older iPhone!!!

 

[Ferrix97]

Progress has been made:

I swapped the 2N5064 SCR with an original E0102 from my working Macintosh Plus ED, the Mac 128k started working perfectly!

So I guess the 2N5064 is more sensitive than the original E0102.

What is a good replacement for the E0102? (or the other similar ones found on Larry Pina's book, all of them unobtanium)

 

[Ferrix97]

Well, so far I've tried a bunch of "should be compatible" SCRs and they all do the same thing, like the BT169D, MCR100-8, MCR22-8 and another 2N5064.

I've ordered a few P0102s as they are the closest thing to the original I could find locally at a reasonable price

I there a way to make the SCR less sensitive? maybe a resistor in series with the gate?

 

[Ferrix97]

This thing is really driving me crazy...

The only way to make it (sort of) cooperate is by lowering the resistance of R25. By dropping it down at 220Ohm, I can get it to stay on up to 93Vac.

I also tried a couple more SCRs with no difference what so ever, even a P0102, which should be the closest replacement to the original.

The only other reason I can find is in the main switching transistor, I used a 2SC2739 while the original one is a 2SC3039, but on paper they are practically identical.

I can obtain a new 2SC3039, but I don't think this is going to solve the issue I'm having.

 

[techknight]

Ok, I am lost on whats going on and what your trynig to achieve? Three posts ago you said you changed an SCR and everything is fine. Now its not. 

what??

 

[Ferrix97]

I borrowed the SCR from my Mac Plus ED just to see if the problem was the SCR or something else, since I didn't (and still don't) have the correct SCR replacement on hand.

The mac works fine with the SCR from the Plus ED, but with a modern replacement the crowbar kicks in at about 60Vac, the original one is blown and shorted (cannot read what's on it)

 

[techknight]

That would be a shitty design if the diode drops between older and newer SCRs affect the circuit that much... 

The direct replacement for the E0102 is the BRX49, but comparing that datasheet against what you used, such as the 2N5064 the trigger current and voltage, plus forward channel voltage is the same. So it should trigger identically against the E0102. 

However, the BRX49 matches the correct pinout and polarity to the original. 2N5064 does not. 

 

[Ferrix97]

If a datasheet is available, I cannot find it. And if there is one around in the inernet, it would probably a low res scan taken fron an 80's book or magazine with diagrams drawn by hand and waveforms acquiried with film cameras.

I have older books about semiconductor replacements, but they don't show specs.

 

[techknight]

Take your 2N5064 and flip it around. Backwards. 

 

[techknight]

Pins 1 and 3 are swapped on the E0102 vs 2N5064. pin 2 (center) is the same. 

 

[Ferrix97]

Really? so essentially I was using the noise in the pulses to drive the main transistor?

 

[techknight]

Yep, looking at the datasheets right in front of me. the E0102 is not the same. its pins are swapped. 

So you have to take the 2N5064 and rotate it in your fingers, place it back in. Opposite of the silkscreen. Itll work.