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CBM PET 3032 STRANGE BOOT

daver2 said:
Hmm, you shouldn't be getting that noise, especially if you have replaced the 191. But why did you replace the 191 in the first place?
Click to expand...
maybe the ls191 is bad again...logic tester tell me this!!
 
I'm sorry, I read it wrong...in the meantime, I ordered two more new 191s because according to the tester this one is also faulty...and I'm waiting for the 165 to arrive..
 
In the meantime we have to work out what (if anything) is (or was) wrong with the 191.

Out of interest, was the original a 74191 or an 74LS191?

Dave
 
Some of these logic IC testers which are around seem less than perfect, I know someone who has one which fails every 74(LS)45 tried in it, even when they are known to be working. Clearly the truth table / definition for 7445 is faulty in that particular tester. If Desperado's tester says the second replacement 191 is faulty as well, I would begin to suspect the tester is not working at least with respect to this particular IC type.
 
Can I suggest another little test with your oscilloscope?

Set your oscilloscope PROBE to the X10 setting.

We are going to read TTL-level signals - so you will need to set your oscilloscope Y channel to approximately 0.1 V/div - to give 1 V/div actual (you have a X10 probe connected now that will reduce the voltage seen by the oscilloscope by a factor of 10 - so 5.0 V will appear at the oscilloscope as 0.5 V)

Probe all of the pins of G5 (191).

If you observe a DC level - what voltage is it?

If you observe a signal - what frequency is it?

If the machine squeals, take the probe off quickly and state 'SQEAL' when you post that pin detail.

I am trying to see whether our problem is related to one particular pin on G5 or more than one pin. If more than one pin, which pins.

Dave
 
I suggested going around all of the IC output pins in post #617 with the x10 probe, to try to identify if that loading affected things. If it does, there is something definitely wrong with the IC's output stage feeding that pin. Though, if one IC appeared to be particularly problematic, , just to be sure I, would make sure that the IC was properly bypassed on its supply pins by adding a 0.1uF capacitor across these and check for a solid +5V to the IC's power pin.

As mentioned, though it is more of a cmos than a TTL issue, IC's can behave really oddly if they get disconnected from their + supply voltage, could be due to a fault inside the IC or even a broken track or bad soldering leading to the supply pin on the IC. They can still "work" sometimes because the die can source power from input pins when they are high, due to the internal clamping diodes in the substrate. Until that is, for a moment, all input pins go low and the IC's function crashes.

The other thing that can give the game away with a logic IC, is to check the actual levels on the scope (scope on DC coupling) that the output voltage swings between, on the output pins, it should be for TTL very close to ground when low and in the order of 3 to 3.5V when high, unless there is a pull-up resistor when it can get close to 5V..
 
Yes, I remember, but I am afraid @Desperado may not have, hence the repeat here.

My fear is, with two intervening posts, he may miss the critical detail I am asking him to perform in post #646!

Dave
 
daver2 said:
Can I suggest another little test with your oscilloscope?

Set your oscilloscope PROBE to the X10 setting.

We are going to read TTL-level signals - so you will need to set your oscilloscope Y channel to approximately 0.1 V/div - to give 1 V/div actual (you have a X10 probe connected now that will reduce the voltage seen by the oscilloscope by a factor of 10 - so 5.0 V will appear at the oscilloscope as 0.5 V)

Probe all of the pins of G5 (191).

If you observe a DC level - what voltage is it?

If you observe a signal - what frequency is it?

If the machine squeals, take the probe off quickly and state 'SQEAL' when you post that pin detail.

I am trying to see whether our problem is related to one particular pin on G5 or more than one pin. If more than one pin, which pins.

Dave
Click to expand...
Can i try this test with ic on socket or not?
 
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