Laser 128ex....how to revive

[OleLila]

Probes here: on CPU pin 3 steady 5 volts, CPU pin 39 cannot find active signal, CPU pin 37 cannot find active signal, CPU pin 40 no delay and immediately jumps to 5 volts

 

[OleLila]

These are the probes....can I even pickup/measure a 1mHz with these?

 

[IlikeTech]

Yep, those are max bandwidth numbers.  You might try to see if you  can find a schematic.  I would also recommend building something like a 555 astable circuit to check the scope with!

 

[bibilit]

So apparently no reset and no clock signal either ?

can you have a look at the MMU and clock signal (pin 64) looking for a 14 MHz wave ? This is the master clock signal. 

Also the outputs signals (66,67,73,74,76)

 

[bibilit]

Also, looking again at the schematics, the reset line is generated by a 556 timer (U34) if i am correct. (pin 10 and 4)

 

[OleLila]

On U34, pin 10 and pin 3 both read as steady 5 volts. I DO NOT have keyboard or internal floppy disk drive connected because it will be much more difficult to access the board.

Even with magnifing glasses its difficult to hit the pins on MMU and then get a good look at oscilloscope, but  not getting anything from MMU 64, 76,74,73.

 

[bibilit]

Bad crystal oscillator ? XTAL1 

14.318 MHz 

 

[OleLila]

So the manual lists as part of U26, but XTAL1  looks pretty well labelled, big and silver just below U26 on the board and it gives me the only patterned oscillations I've found on the board, one pin a pretty smooth sin wave and the other regular and a little more stepped pattern......14.35 mhz  on both pins, peak to peak voltage about 6 volts.

 

[bibilit]

So crystal looks ok. 

Why are you not getting anything on the other side (MMU) ? (Pin 64) 

something in between ??

 

[bibilit]

View attachment 128EX.pdf

See file

 

[OleLila]

Learning and measuring here. I think we've narrowed it down. Sorry about the JPEG format on this post, but moving between computers and file types.

 

[bibilit]

Ok, think you have got something.

As SM14 is the master clock, and is lost, the Laser won't do anything.

U26 (74LS04) is a logic inverter, out of the 14 pins, where 7 is Ground and 14 is VCC (5 volts) the other pins are inverters ( 6 pair ) 

So if you input a Low signal at one pin, the output will be High, and the other way round...

https://electronzap.com/7404-ic-hex-inverter-not-gate-74hc04-74ls04/

I will not jumper or bypass anything for safety reasons, the chip is probably fried and cheap anyway, so you can cut legs of the old one to help the process of chip removal.

Concerning the nearby chip U13, if you have a look at Appendix D-7 concerning the external drive (this chip is related to drive failure)

As you had issues at the beginning with the external drive, this chip is probably bad as well and may had cause the failure of U26.

So i will replace both chips as a matter of fact, U13 is 74HCT244

Concerning the last question, you can probably tweak the crystal afterwards using VC1 (see your picture above) to the correct value.

 

[OleLila]

parts on the way.....will be several days; at least U13 is socketed; work to pick up so may be a while; just hope it doesn't uncover more issues; my concerns are the "designed" chips for the video, MMU and disk control. Thank you. 

 

[OleLila]

Well, took some work but finally got U26 off and replaced.....same behavior, no S14M signal, still stops between pins 9 and 8. Replaced U26 first then U13, still the same.

 

[bibilit]

According to Datasheet for the two chips, and if i am wrong anyone can correct me, this is what you should get :

Pin 1 of U13 should be Low (if not is OFF) 

http://pdf.datasheetcatalog.com/datasheet/NXP_Semiconductors/74HC_HCT241_CNV.pdf

Then pin 2 and 18 are both Low or High (see logic table above)

On the U26, as you have two gates of a same inverter, if Pin 18 is High, then pin 9 of U26 will be High as well and pin 8 will be Low (and the other way round)

So, what feedback can you give me back on those ?

At some point you said a capacitor was shorted ? (C286) 

Just my two cents, but i will more than happy to help you get your Laser working again.

 

[OleLila]

When I turn the machine on and measure first with voltmeter then with oscilloscope..........pin 1 of U13 shows no voltage and no frequency detected, at the same time PIN 2 of U13 shows 1.34 volts and frequency 14.35;  PIN 8 of U13 shows 4.76 volts and no frequency detected, Pin 9 of U26 shows 14.35 frequency, PIN 8 of U26 shows nothing.

One time briefly (maybe over the course of 2 minutes)  when I measured PIN 8 of U13, I got a 14.35 frequency however I quickly plugged in the screen (nothing on screen)  and when I got back to measure other pins, it had reverted to the 3.74 volts again and all as above.

To be transparent, when I measure these pins, there is no doubt that as I put the probe on a particular pin and look to the oscilloscope, I have moved the probe and shorted between the pins. If this is ruinous then its all out the window....not much room for error at all and hard to imagine getting through all of these measurements, esp on the smaller chips, and not eventually getting between 2 pins.

I ordered replacement part for U13 (74HCT244) but received 74HCT244N which I did plug in and try  but showed no change in activity.

So when I get a chance my plan is to go ahead a replace shorted C286 (68 pF) and crystal XTAL1.

Have to balance this with the day job and activity but hope to get to this in the next couple of days. Very much appreciate your looking bibilit.

 

[bibilit]

You say pin 8 of U13, rather pin 18 ?

the strange thing is the missing frequency between those two pins and why you get it D14M

Maybe you can use a camera/phone to record your oscilloscope while focusing at the pins ?

 

[OleLila]

I did measure pin 8 above,  not pin 18. These pictures are of pin 18 of U13....14.35 frequency is there.

 

[bibilit]

So Xtal and U13 are ok. 

As you replaced U26 already, this is not the problem. 

So something in the neighborhood, what about other pins of U26 ? VCC and ground present ? 

 

[OleLila]

Pin 7 is ground and pin 14 is at 5.02 volts.

Will go to find a 68 pF cap to replace C266...with U13 and D14M checking out, is this cap bringing down everything still a possibility? 

What about the reset circuit?