Piggyback 4164 to Diagnose Memory Fault

[t0nyt]

I don't know if these pictures of bare boards will be of any help viewtopic.php?p=24839#p24839

If you blow them up, you can just about follow the tracks.

Thanks. Will finish replacing IC19 & IC20 and then use the images to trace/check the IC8 top of board connections

[Martin_Head]

I know I keep harping on about that solder blob on the 2nd pin in your picture.

I had a quick look at the circuit diagram (issue 5). And I think that is pin 7 of the RAM chip, and that large track is most likely either ground, or 5Volts.

Pin 7 is the DA1 line which goes to IC19, (which you've been looking at) via a resistor. But it goes directly to the ZX8301 IC22, which must be output lines to drive the RAM address lines. Now if pin 7 of IC11 was tied to ground or 5Volt by a solder blob, then ZX8301 would being trying to drive a power line. And it probably would not like it very much, So you might have blown up the ZX8301, unless you've already tried it.

I would try measuring the resistance of the DAx lines to ground on the ZX8301, looks like pins 13,17,18,20,22,24,27,28 (drawings a bit bad) especially pin 17 the DA1 line. If any look a bit different to the others, try removing the ZX8301 and trying again.

[t0nyt]

Latest update is I’ve now changed all the logic ICs but it still reports IC8 as the problem

With regard to 2nd pin down there’s definitely not a short and never has been, I’ve always checked for shorts before powering up

The other problem is I only have a soldering iron and a simple multimeter and don’t have the knowledge (even if the meter can do it) to make the tests you suggest. I wish I did. Sorry.

Am going to try some tests on IC8 based on the top of board images you linked, many thanks, but if that throws up nothing I’ll cannibalise the board parts

I really appreciate all the advise/help but this is now beyond me

Many thanks
Tony

[t0nyt]

IC8 connections all check out, so I''m at a brick wall now unfortunately

(Just to be clear I don't even know what a DA line is or how to measure resistance)

I may track down a replacement ZX8301 to try in it before I cannibalise the board though

Many thanks all

[Chris_68]

I would try measuring the resistance of the DAx lines to ground on the ZX8301, looks like pins 13,17,18,20,22,24,27,28 (drawings a bit bad) especially pin 17 the DA1 line. If any look a bit different to the others, try removing the ZX8301 and trying again.

Tony, Martin has a point, the multimeter is what you need to measure. Of course you do what you want with your PCB, if you start to dissect it will be beyond repair. Maybe there's members living nearby who can have a look.

I can take a photo how to measure but in principle,
Set you multimeter to measure resistance, usually marked with an omega
Take black probe and attach to GND, either pin 6 of IC22 or the metal part on 7805.
Then take the red probe and measure each pin (13,17,18,20,22,24,27,28) on IC22 they should have similar resistance.

Let us know the value so we can compare

[t0nyt]

I would try measuring the resistance of the DAx lines to ground on the ZX8301, looks like pins 13,17,18,20,22,24,27,28 (drawings a bit bad) especially pin 17 the DA1 line. If any look a bit different to the others, try removing the ZX8301 and trying again.

Tony, Martin has a point, the multimeter is what you need to measure. Of course you do what you want with your PCB, if you start to dissect it will be beyond repair. Maybe there's members living nearby who can have a look.

I can take a photo how to measure but in principle,
Set you multimeter to measure resistance, usually marked with an omega
Take black probe and attach to GND, either pin 6 of IC22 or the metal part on 7805.
Then take the red probe and measure each pin (13,17,18,20,22,24,27,28) on IC22 they should have similar resistance.

Let us know the value so we can compare

Hi Chris,

I think I can manage that

The meter has different settings for resistance (200, 2000, 20k, 200k, 20m, 200m), which one should I use please?

Should the QL be powered up when I do this please? or does the meter supply the current?

I did say I know nothing about electronics!

Many thanks

[t0nyt]

Trying, without any power to QL, and a setting of 2000, I get 632 to 635 on the listed pins (black probe was on 7805)

EDIT: to add insult to injury it's now also suddenly saying IC16 is faulty

Thanks
Tony

[Chris_68]

Hi,

Wait for someone else to reply, but when I measure I get 2M Ohm.

At least good that you get same on all the pins.

[t0nyt]

Hi,

Wait for someone else to reply, but when I measure I get 2M Ohm.

At least good that you get same on all the pins.

Ok, thanks

I just re-tested with the meter set to 200M and I get readings between 0.9 and 1.1 (mainly 1.0 to 1.1) with the black probe on pin 6 (with 2000 setting and pin 6 I still get 632 to 625)

Many thanks

[t0nyt]

Hi,

Wait for someone else to reply, but when I measure I get 2M Ohm.

At least good that you get same on all the pins.

Well I managed to gain access to my working QL (stripped screw) so I ran the same tests on the zx8301 in that and I got the same results (632 to 635 & 0.9 to 1.1)

Had been planning to put the suspect zx8301 in the working QL but I can’t safely extract it from the working QL (I always bend pins and quintupler is in the way of me accessing one end, and I don’t want to risk extracting the quinupler as I had a lot of problems fitting it the board socket was so tight)