Spectrum Computing

Update: I finally found the cause of the short...a tiny piece of solder that had wedged itself between two traces under a RAM socket...I discovered this after desoldering several sockets. That is the good news... The bad is that my Toastrack has now devolved into a standard 48k Spectrum! https://i.p...

Yes, the 200 Ohm is shared with the conductivity option. The meter registers 0.03 when testing between pin 2 on IC18 & 19.

No other work has been done other than the socketed RAM ICs that I am aware of. If it helps, I have removed IC32 and IC31, the short on pins 7 & 8 of the Z80 remains.

I definitely have continuity between pin 2 on IC18 & pin 2 on IC19. With resistance set at 2k on my meter, the reading very briefly shows a value around 0.5 and drops to 0.0. There is also conductivity between pins 7 & 8 on the Z80. IC32: there is conductivity between pins 7 and 8 of the Z80...

Thank you Mark. I checked the connections between all the RAM ICs and they are as expected. I did however find connectivity between IC18 & IC19 on pin 2 (blue dots) : https://i.postimg.cc/gXDS59Z9/IMG-20230817-081424-edit-20230817082012.jpg Those vias terminate under a group text of resistors an...

That's good as I have connectivity between those on pins 8 & 16. I know this is a proverbial "stab in the dark" but I am attempting to determine if my socketing of all the RAM ICs have caused my 128 to stop booting. All the ICs have connectivity to the IC on the right with their corres...

Hello,

Could some kind soul explain which pins on IC6 should be connected to IC17 (the 4164 to its immediate right). I have had a look at the schematic but it is not clear to me.



Thanks.

So...I purchased two Z80s, removed the existing Z80, soldered a socket and tried a different Z80. The pattern on startup is different. The screen fills with small squares and then flashes to the same screen but with half of the previous one patched on to the right. https://i.postimg.cc/w7Z6RT6K/IMG-...

Based upon an earlier post to determine the state of the Z80, I checked the following pins: Pin : my Z80 state with multimeter or logic probe 6 clock : Hi/Lo active when powered on 16 /INT : Hi continous 17 /NMI (should be high, > 3V) : 5V 18 /HALT (should be high, > 3V) :5V 19 /MREQ : Hi/Low active...

The ROM removed and the diagnostic cartridge inserted:

When I push the reset button, single line white pixels briefly appear like a cursor at different points on the screen. But, the machine does not reset (anymore).

Full of the delights of summer, I awoke this morning determined to revive my Toastrack with all the useful feedback I have received...suffice to say that delight was transformed to dread when I booted the machine: https://i.postimg.cc/QHx06XRG/IMG-20230802-081551.jpg https://i.postimg.cc/v19z1D69/IM...

RF.

Can I quickly add... The machine was "working" prior to this episode with a presumed faulty ULA (after a period of time it would crash hence I sourced another ULA). The machine was then left alone. A few weeks later I obtained another ULA and swapped them over. Turned the machine on and no...

Yes, you are correct. I have never seen capacitors like those before. I will now attempt to test each of them.

My board has resistors in those positions which I have checked. Interestingly, they all read 196 except the one I have circled in blue, that reads 1.

MustardTiger wrote: ↑Tue Aug 01, 2023 8:42 am I'm not an electronics expert but could it be the capacitor for the ram chip for bit7 that's faulty?

Which capacitor are you referring to? My board is revision 6U.

I suppose I could test each capacitor without having to remove them...

This morning I removed all the RAM ICs and put them back in different positions, same result. Correct me if I am wrong, but does that not prove that it is not a RAM issue, unless of course a faulty RAM IC had been put in the position of a previous bad IC?

No.
So...I have now removed all the ICs - and managed to break two in the process - tested them all, soldered new sockets...replaced the broken two ICs, the same...

I have now checked five RAM ICs and just tested the machine.

The good news is, it is no worse than before. The bad news is mentioned in the previous sentence.

Should I continue and check the rest?

Seven.FFF wrote: ↑Mon Jul 31, 2023 3:27 pm Why would bad 4164s cause the whole display to have inverted colours, though?

Ah, bad 4116s would cause the stripe, and maybe force set the flash bit. Is the display alternating at the usual flash rate? What does PRINT PEEK 22528 give you?

PRINT PEEK 22528 gives me 56.

But IC13 is one the 5 I have checked, and it passed with flying colours...

Correct...I have now removed, tested and socketed 5 ICs...never have I wanted to find a bad RAM IC as much...

The first thing I would do is open the machine and very carefully disconnect the keyboard membrane. Power the machine back on and you will know if the keyboard was causing the problem. Keyboard membranes are notorious for corroding over time so you will probably need to replace it. But a quick visua...

OK...I tried the old piggyback technique and found that placing a 4164 on the back of IC19 & IC20 produced the following results in my diagnostic cartridge: https://i.postimg.cc/D8xdbcBP/IMG-20230731-122910.jpg https://i.postimg.cc/gXGnDC0X/IMG-20230731-122750.jpg So I decided to quickly remove,...