• Please review our updated Terms and Rules here

Pet video board 320064-02 bad

Hi guys!
Ok, i've checked all pins:

H5:
PIN 13=HIGH
PIN 11=HIGH AND LOW
PIN 12=HIGH
PIN 9=HIGH
PIN 10=HIGH AND LOW

G6:
PIN 3=HIGH AND LOW

F4:
PIN 5=HIGH AND LOW
PIN 9=HIGH AND LOW
PIN 2=HIGH AND LOW
PIN 12=HIGH AND LOW

F2:
PIN 5=HIGHN AND LOW
PIN 9=HIGH
PIN 2=LOW
PIN 12=HIGH

Thanks so much!!
 
I have managed to catch up with the thread now I am back from my business trip (fault finding on an intelligent communications card).

I see you have made good strides in identifying the area of the video circuity that is likely to be causing problems.

I was just reviewing the last post and would like you to just recheck a couple of things please - because I am seeing a slight inconsistencies.

Can you recheck H5 pins 11, 12 and 13 please.

Can you also check ALL of the pins on F2 please.

Regards,

Dave
 
daver2 said:
I have managed to catch up with the thread now I am back from my business trip (fault finding on an intelligent communications card).

I see you have made good strides in identifying the area of the video circuity that is likely to be causing problems.

I was just reviewing the last post and would like you to just recheck a couple of things please - because I am seeing a slight inconsistencies.

Can you recheck H5 pins 11, 12 and 13 please.

Can you also check ALL of the pins on F2 please.

Regards,

Dave
Click to expand...

Welcome back Dave!
on now i check!
Thanks
 
Thanks.

I'll let dave_m run with the problem to prevent distracting his train of thought - it was just that I noted some potential inconsistencies in the results that I would like clarified.

Dave
 
H5:
11:h&l
12:h
13:h&little light green (low)

f2:
1:h&l
2:l
3:no signal
4:no signal
5:h&l
6:h&l
7:l
8:h&l
9:h
10:h
11:h
12:h
13:h
14:h
 
ilpaninaro said:
f2:
1:h&l
2:l
3:no signal
4:no signal
5:h&l
6:h&l
7:l
8:h&l
9:h
10:h
11:h
12:h
13:h
14:h
Click to expand...

Thanks Daver2, you put your finger on the questionable counter outputs that are not pulsing.

With the re-measurement of H5, it is fine.

But the 74177 counter F2, outputs Qb (pin 9), Qc (pin2) and Qd (pin12) are not counting. The input clock is pulsing and the count/load is pulsing which is correct. The /Clear signal (pin13) is high telling the counter to count. Note that the schematic has a typo. Pin 13 is the Clear signal not a clock input. So all the counter outputs should be pulsing until the 200th count is reached to enable the next state, but it never get there and so sequencer hangs up.

So it appears that chip F2 is malfunctioning. Il Paninaro, Remove and replace the 74177 counter chip at F2!
-Dave
 
ilpaninaro said:
H5:
11:h&l
12:h
13:h&little light green (low)

f2:
1:h&l
2:l
3:no signal
4:no signal
5:h&l
6:h&l
7:l
8:h&l
9:h
10:h
11:h
12:h
13:h
14:h
Click to expand...

Hi,

I agree with Dave_M that likely F2 has malfunctioned, however, there could be something else. In theory at least there should be, using your logic probe, no such thing as "no signal" as the logic states should be high or low or pulsing between those states. Perhaps this indicates that pins 3 and 4 have an anomalous voltage on them between logic high and low and if that were the case, then I would also suspect G3 the 74LS373, which in theory on its outputs should give normal logic states or maybe G2's input loading it down. So just in case its worth ordering those IC's.

Hugo.
 
Hugo Holden said:
In theory at least there should be, using your logic probe, no such thing as "no signal" as the logic states should be high or low or pulsing between those states. Perhaps this indicates that pins 3 and 4 have an anomalous voltage on them between logic high and low and if that were the case, then I would also suspect G3 the 74LS373, which in theory on its outputs should give normal logic states or maybe G2's input loading it down. So just in case its worth ordering those IC's.

Hugo.
Click to expand...

Hugo,
Good catch.

Il Paninaro, Please re-check chip F2-Pins 3 and 4. If they show no green or red light, then check at outputs at G3-Pins 2 and 5. If the same 'no lights', then use a voltmeter set to measure DC Volts.
 
The 74LS273 Latch (G3) along with the character address counters F4 and F2 form an interesting circuit. The latch sequentially holds the starting row address for all 25 rows (40 characters per row). That is address 0, then address 40, then 80, 120, etc. until all 1000 addresses of the screen are displayed. The reason those starting row addresses need to be latched is that they are needed to load the counters eight times per character row as there are eight horizontal scan lines per character row. A clever circuit, but I can see why Commodore went to the 6545 CRT Controller in the next version of the PET to let it handle the address bookkeeping rather than most of the circuits on Schematic Sheets 6 and 7.
 
Thanks for that Hugo. I was going to mention it also in my post but forgot - it was too late at night for thinking and posting obviously!

Yes, this circuit is reminiscent of the NASCOM video circuit made from TTL chips. When it went wrong (and the video circuit is always something that goes wrong on a NASCOM) you knew you were in for the long haul!

With all the feedback paths you can very easily get lost in the logic maze!

But I agree 100% - there is no such thing as 'no signal' - it is the logic probe's way of telling you that it is not a logic high or low - but somewhere in between, which smacks of a faulty output stage on the device driving these pins. The alternative is a bad connection from the probe to the IC pin you are probing.

With my Apple IIe I had a fault where some of the IC pins had been 'eaten' through so they looked (visually) as though they were a bit tarnished, but electrically there was no connection from the IC pin itself to the PCB track and the pin was effectively floating! When I removed the IC from the socket (SN74LS74 I think) two or three pins were actually left in the socket itself!

Probably worth checking the state of the IC pins, PCB tracking and solder joints around these ICs as well...

Dave
 
ilpaninaro said:
I don't understand very well....must i change F2 and G3??
Click to expand...

You don't have to do anything in particular.

The discussion simply highlights that with these sorts of logic circuits, you need to keep an open mind.

You might for example come a cross an IC that seems like in testing that it is not working. So it is very tempting to blame that IC. However it could be malfunctioning because of another IC driving its inputs is faulty, or another IC loading its outputs is faulty.

If the IC's are in sockets, then it is all pretty easy, especially since 74LS*.* IC's are easy to get and cheap, you can just try replacements.

However, in cases where the IC's are soldered onto the pcb, then a lot of patience, thought and testing (usually with a scope) is required, or if that is not done IC's get removed from the pcb unnecessarily, often with pcb damage, unless the technician is very experienced and has a good temperature controlled soldering iron & solder suckers & solder wick.

So buy some IC's in that region of your malfunctioning circuit, F2,F3,G2 & G3 so you have them on hand to try out.

When you get more experienced you will be able to fault find these sorts of circuits with a scope and you will get to the bottom of where the fault resides more quickly with less uncertainties. Also, it pays to look up the function of each IC in the data book so you can be sure to know how it is expected to behave and what all its inputs and outputs do.
 
Hugo Holden said:
You don't have to do anything in particular.

The discussion simply highlights that with these sorts of logic circuits, you need to keep an open mind.

You might for example come a cross an IC that seems like in testing that it is not working. So it is very tempting to blame that IC. However it could be malfunctioning because of another IC driving its inputs is faulty, or another IC loading its outputs is faulty.

If the IC's are in sockets, then it is all pretty easy, especially since 74LS*.* IC's are easy to get and cheap, you can just try replacements.

However, in cases where the IC's are soldered onto the pcb, then a lot of patience, thought and testing (usually with a scope) is required, or if that is not done IC's get removed from the pcb unnecessarily, often with pcb damage, unless the technician is very experienced and has a good temperature controlled soldering iron & solder suckers & solder wick.

So buy some IC's in that region of your malfunctioning circuit, F2,F3,G2 & G3 so you have them on hand to try out.

When you get more experienced you will be able to fault find these sorts of circuits with a scope and you will get to the bottom of where the fault resides more quickly with less uncertainties. Also, it pays to look up the function of each IC in the data book so you can be sure to know how it is expected to behave and what all its inputs and outputs do.
Click to expand...

Ok thanks, these logic chips are soldered but there isnt a problem cause i have a desolder station...
I have G3 ic 373 but i don't have F2 (74177)....can i found this in shop or these ic are too older?Thanks!
 
74177's are on ebay here:

https://www.ebay.com/sch/i.html?_fr...11.TRC1.A0.H0.X74177.TRS0&_nkw=74177&_sacat=0

Also, if you find yourself stuck trying to find a 74 or 74LS series TTL IC, search for 54 prefix instead, these are the mil spec variants which are very good quality, most are in ceramic packages and they are very (completely) reliable. It used to be said by people in the electronics industry in the 1970's that 1 in 100 of the 74 TTL's could be faulty from new (a remark from an Atari employee). I have been testing hundreds of 54 series TTL IC's for many years and I still cannot find a single faulty new one. I think you can trust these with your life.

https://www.ebay.com/itm/3-PCS-5417...070293?hash=item335a39cfd5:g:PzQAAOSwWTRWykzD
 
Hugo Holden said:
74177's are on ebay here:

https://www.ebay.com/sch/i.html?_fr...11.TRC1.A0.H0.X74177.TRS0&_nkw=74177&_sacat=0

Also, if you find yourself stuck trying to find a 74 or 74LS series TTL IC, search for 54 prefix instead, these are the mil spec variants which are very good quality, most are in ceramic packages and they are very (completely) reliable. It used to be said by people in the electronics industry in the 1970's that 1 in 100 of the 74 TTL's could be faulty from new (a remark from an Atari employee). I have been testing hundreds of 54 series TTL IC's for many years and I still cannot find a single faulty new one. I think you can trust these with your life.

https://www.ebay.com/itm/3-PCS-5417...070293?hash=item335a39cfd5:g:PzQAAOSwWTRWykzD
Click to expand...

Ok thank i've just ordered!!
I can wait next week for repairing.... :(
See you soon ;)
 
You must log in or register to reply here.
Back
Top