Converting the video signal from a Mac Classic logic board?

[zeppelin87]

Hello everybody!

I have a Macintosh Classic with a dead powerboard, and I'm not going to go thru the trouble of fixing it, instead, I was wondering if it is possible to convert the TTL signal (HSYNC, VSYNC, VidOUT) to a standard composite video (for connection to a television) or to a standard VGA?

It would be fun to connect the logic board to a television like a Commodore 64

Any ideas?

// Peter

 

[porter]

Yes, there is a classic little circuit I used to use, the exactness is lost in the mists of time. Basically you need an XOR gate to combine the HSYNC and VSYNC to one signal.

Then using a couple of digital transistors and a few resistors you need to create a 1v peak to peak signal such that...

The video signal pulls the signal from 1v down to 0.3v ( white to black)

The sync signal pulls the signal from 0.3v to 0v ( blacker than black )

It's easier if you have the transistors combine the signals at about 3v peak to peak then you a couple of resistors to step this down to the 1v peak to peak signal, remembering the 75ohm standard impedance for composite video equipment.

 

[zeppelin87]

Thnx, for your reply!

I figured that it would be something like that, but sadly I'm not a electronics designer, so I'm not really good at designing circuits Soldering is all I do

Would be appreciated if you could find some more info on the circuit you used, or could draw me a diagram?

Maybe someone else has other ideas?

Thanks!

 

[Charlieman]

The Power R MC-10 (which was designed specifically for the Classic) did something like this. Can somebody confirm my suspicion that it was still TTL output? Would it work with a Classic II?

 

[tomlee59]

Those adapters provided TTL-compatible signals for use with TTL video monitors of the day. Electrically, it ought to work with a Classic II, but some cable rewiring might be involved.

To the OP: There is no simple circuit to produce TV-compatible composite video from the signals inside a Mac. The bandwidths are quadruple that of TV; the video is progressive (TV is interlaced); and the horizontal sync frequency is too far off. Don't even try.

There are adapters that produce TV-compatible output from, e.g., VGA (within the resolution limits of TV), but they are well beyond what a casual hobbyist is going to be able to reproduce.

Building a circuit that will drive a VGA monitor (at least some VGA monitors, that is) is less doomed. You can start with the circuitry described in http://68kmla.org/files/classicmac2.pdf (see section toward the end, about how to adapt PC hardware for use with Macs). You'll need to add some simple buffers to drive the RGB signals together with the one video signal from a monochrome Mac. Beyond that, it should work.

 

[Mac128]

The Power R MC-10 (which was designed specifically for the Classic) did something like this. Can somebody confirm my suspicion that it was still TTL output? Would it work with a Classic II?

This thread was where we discussed the PowerR adapters ...

And yes, it did provide a standard TTL output:

Power R Model MC-10 Video Adapter for Macintosh Classic and Classic II
Allows you to view your Macintosh display on LCD panels, LCD projectors, external monitors and scan converters. Installs in minutes with no cutting, drilling or soldering.

Specifications:

Resolution........................................512 x 342

Horizontal scan rate..........................22.3KHZ, (4µs on, 40µs off)

Vertical scan rate .............................60HZ (180µs on, 16.4ms off)

Output level (all lines) .....................TTL Positive

Video Connector Pins (pin out)........1-2 gnd; 3-4-5 RGB; 8 Horz; 9 Vert.

Power Consumption .......................0.25 watts from Mac power supply

Price..........$99.00

 

[zeppelin87]

Thank you all so far

The impression I now have is that i should forget about the composite video output, and instead try to convert an existing monitor for use with the Classics logic board. This does not only require a special monitor, or some experience of fiddeling with an existing one, something that I'm not quite capable of doing

Sadly, this is not really what I had in mind...

Is there really no circuit known to man, that could convert the video out to a standard VGA signal? I will forget about composite video, but please help me get VGA

 

[equill]

Then again, looking through the large end of the telescope, you could be one dry joint away from a working analogue/power-sweep board.

As near as dammit, most of the advice that you have received points towards rescue of the board—which is designed to drive the Classic's CRT—as the first course. There will be more-rewarding machines on which to experiment, later.

Best of luck!

de

 

[tomlee59]

I think you should consider equill's (always) wise words of advice. Although you declared in your original post that you don't want to fix your analog board (presumably because of some assumed complexity of doing so), all of the alternatives involve much more fuss and bother.

Equill's reference to a "dry joint" is about a commonly-encountered fault in the compacts -- bad solder joints in the yoke connector cause many display problems (including no display). The fix is simple and quick. So, too, is a replacement of a particular capacitor on the analog board that is notorious for failing.

The document I linked to earlier has a collection of troubleshooting tips for compacts in general. Many of these old machines can be resurrected with very little effort.

 

[Bunsen]

A "dry joint" is one where the solder has lifted from one or more of the points it was meant to connect. These will often cause intermittent symptoms, or ones that come on or go away when the system has had time to warm up.

Thermal expansion and flexing can make or break the "dry" contact. Service folk (at least those who still do component level work) will narrow down the location of the dry joint with a can of coolant spray - hit the bad joint, it cools, contracts and breaks contact. Then visual ID with a magnifier.

A two second touch up with a decent iron, perhaps a smidgelet of more solder and/or flux, and Bob is your mother's sister's husband again.