What does a dead ASIC look like? 464 Plus

[Overkill]

The biggest difference at the RGB socket is the LUMI, 1.25v on the 464+ and 1.57 on the 6128+.


I was wondering if it was just a video fault, so I left the BASIC ROM cartridge in to see if I got the Burning Rubber sound, but nothing.

[Overkill]

Just to add, I do seem to get a very high pinched low volume noise when plugged into the TV.

[gerald]

They are identical, a 5v square wave.

NSYNC is not supposed to be a square wave or we do not have the same definition of a square wave 
For me a square wave is a a signal that alternate 0 and 5V with a duty cycle of 50% : half time low, half time high, repeat.
NSYNC , on a CPC, is a digital signal that should look like this
http://kudelsko.free.fr/analy_logic/utilisation/LM1881.gif

[Overkill]

Yes, it wasn't square, it was 5v high but dropped to 0v a bit like the composite sync in that gif.


I still wonder if I'm on to something with some of the pins on IC15 being at a steady 5v without any wave form.  I was thinking of removing IC15 and then testing those points again.

[Overkill]

Testing the extra diodes still in circuit (seen in the picture below), I get a very low resistance.  Testing the same on my working 6128+, I get expected readings. 

[Overkill]

I'm getting 72 ohms across 3 of the 4 diodes in that circuit.

[Bryce]

Testing the extra diodes still in circuit (seen in the picture below), I get a very low resistance.  Testing the same on my working 6128+, I get expected readings. 

Measuring resistance on a diode is pointless. It tells you almost nothing and is dependent on the direction you connected the meters leads. Voltage drop is all that's important, so measure the voltage drop and you'll know if they are ok. Also, if you measured them in-circuit, you have no way of knowing whether the 72R is from the diode or some other part that's in parallel to the diodes.

Bryce.

[Overkill]

OK, work this out   ?? 


I put a meter across a lot of the components in that circuit, then I went back and tested across those same diodes, the readings were NOW OK and what I'd expect for diodes in circuit.  So I thought I might have accidently fixed a short, so I plugged it in and still got the same grey screen.  I now test the diodes for a third time, and they are back to the low ohms   

[Overkill]

Measuring resistance on a diode is pointless. It tells you almost nothing and is dependent on the direction you connected the meters leads. Voltage drop is all that's important, so measure the voltage drop and you'll know if they are ok. Also, if you measured them in-circuit, you have no way of knowing whether the 72R is from the diode or some other part that's in parallel to the diodes.

Bryce.

Thanks Bryce, I do understand the problems you mention but I am able  to compere readings with a working machine.

[Overkill]

Ignore all that, it was the difference between have the RGB cable plugged in or not DOH! 

[Bryce]

As I said, too many other things are being measured at the same time. The 72R was most likely the terminating 75R of the video lines.

Bryce.

[Overkill]

I'm running out of ideas, its looking more like IC1 is at fault. I think I'll shelve this repair for a while.  I've just ordered a cheap IC tester from China, so I can test IC15 and IC16.

[Bryce]

Can you use the names/part No.s of the IC's rather than IC15 / IC16. Even I have to check the schematics to know what you're talking about! 

Bryce.

[Overkill]

Sorry, they were named in posts above


[font=]SN74LS27N IC16[/font]
[font=]40464 DAC IC15[/font]

[Bryce]

What sort of tester did you buy to test them? A 74LS27 cost 50c, you don't test them, you swap them. As for the 40464, I doubt any Chinese tester will be able to test this.

Bryce.

[Overkill]

Yes, I was wondering if it would test the 40464, I didn't know how custom that chip is.


My logic is, I'd rather know an IC is faulty than just replace them.  The IC tester was £25 and I'm sure it will be a handy tool for me



https://www.youtube.com/watch?v=66hPg_r4IOw

[Bryce]

I prefer to manually test chips. Those testers may say the chip is good and it still doesn't work in the circuit.

Bryce.

[Overkill]

I think this tester, although cheap is very good. It will only identify an IC if all the functions pass the test.

[robcfg]

If it's a custom part and there's interest, I can send one for decapping.

[Overkill]

If it's a custom part and there's interest, I can send one for decapping.

I think it's only IC1 Gate Array custom chip that we could do with, but I can't see anyone manufacturing them again

[robcfg]

You can get the decapped ASIC pictures by following the links on this post.


Regarding 40464, if there's no info about it, it may be worth to decap it just out of curiosity. Besides the ASIC, ACID and 40464, there are no custom chips there.

[Overkill]

Gerald, have you ever tried lying a ASIC chip on the contacts?  I was thinking of removing IC1 from a working GX4000 and seeing if I could test the IC1 chip from this 464+ by just placing it on the board and adding some weight, instead of soldering the chip in.    Is this possible, do you think?

[Bryce]

Gerald, have you ever tried lying a ASIC chip on the contacts?  I was thinking of removing IC1 from a working GX4000 and seeing if I could test the IC1 chip from this 464+ by just placing it on the board and adding some weight, instead of soldering the chip in.    Is this possible, do you think?

You have about a 1% chance of that working. Because the pads won't ever be 100% smooth, the chances of getting every pin to make sufficient contact is extremely low.

Bryce.

[Overkill]

You have about a 1% chance of that working. Because the pads won't ever be 100% smooth, the chances of getting every pin to make sufficient contact is extremely low.

Bryce.

I'd make a jig to press all the contacts down, I don't know if it would work, but I might give it a go.

[Bryce]

By doing that you will bend the pins to different levels to force it to fit the uneven pads. It will make the IC very difficult to solder in afterwards or may even damage the part.

Bryce.