Atom just acquired not working

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John Ferguson
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Atom just acquired not working

Post by John Ferguson »

I have purchased an Atom and connected to a 5volt supply the voltage regulators having been bypassed with links 6 & 7.

I show a photo of the display achieved so far. Clearly more work has to be done. I loosened the main chips to get this far. Pressing break does nothing.

I would appreciate some advice on what to do next. For example if the legs of the chips need cleaning what is the best way to do this.

The chips are warm to the touch which suggests the power is getting through.

Only half the display memory chips are present.

John
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The current display.
The current display.
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IanS
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Re: Atom just acquired not working

Post by IanS »

John Ferguson wrote:
Wed Apr 01, 2020 4:17 pm
Only half the display memory chips are present.
Which half? My guess is that the wrong half has been fitted/removed. It looks like the top half of the screen has no ram fitted, but the bottom half does.

Got any pictures of the motherboard?
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roland
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Re: Atom just acquired not working

Post by roland »

And if there is no RAM at the lower memory space the Atom cannot start at all if there is no RAM at #8000 - #83FF. Does CTRL+G ring a bell?
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Re: Atom just acquired not working

Post by John Ferguson »

Memory chips 32 to 37 have been fitted. Pressing break does nothing and nothing from control g afterwards.

Photo of board enclosed. Thanks for the responses.

John
DC112451-E4B3-4960-9FEC-0ECEF173489B.jpeg
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Re: Atom just acquired not working

Post by John Ferguson »

The reason for all the wiring is that the Atom has a replacement keyboard.

John
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Re: Atom just acquired not working

Post by IanS »

John Ferguson wrote:
Wed Apr 01, 2020 5:10 pm
Memory chips 32 to 37 have been fitted. Pressing break does nothing and nothing from control g afterwards.

Photo of board enclosed. Thanks for the responses.

JohnDC112451-E4B3-4960-9FEC-0ECEF173489B.jpeg
The video ram should be filled from the other side.
From the Atom TRM
atom_video.PNG
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Re: Atom just acquired not working

Post by John Ferguson »

I can see that I need to relocate those memory chips.

John
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oss003
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Re: Atom just acquired not working

Post by oss003 »

Hi John,

you can find some info to start with minimal configuration here if the Atom still doesn't start: viewtopic.php?f=44&t=6318&p=60505&hilit ... ion#p60505

Removing and reseating the chips a few times also helps and if the legs are corroded, please clean then.

Greetings
Kees
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Re: Atom just acquired not working

Post by John Ferguson »

Hi Kees

Just the information I needed. Thank you.

On cleaning the pins how would go about this as I fear to touch them?

Regards

John
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Re: Atom just acquired not working

Post by 1024MAK »

John Ferguson wrote:
Wed Apr 01, 2020 9:58 pm
On cleaning the pins how would go about this as I fear to touch them?
The best tool for this is a fibre glass cleaning pencil.

Mark
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Re: Atom just acquired not working

Post by John Ferguson »

By the way it may be of interest I have connected my 5 volt DC power supply to the DC power connector of the Atom by enlarging the entry hole.

Thanks Mark for the advice about cleaning.

John
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Re: Atom just acquired not working

Post by roland »

John Ferguson wrote:
Wed Apr 01, 2020 9:58 pm

On cleaning the pins how would go about this as I fear to touch them?
Don’t fear.... they won’t bite you :lol:
The IC’s in the Atom are NMOS chips and I have never destroyed one by static electricity. So if you touch an earthed metal object (water tap, central heating or something like that, nothing will break. And if it does ... we have plenty spares for you.
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Re: Atom just acquired not working

Post by janvb »

i dit some of my chips with sandpaper.
Works great :D.
Today again troubles with screen going blanc.
Ic 23 replaced with new 74ls138. (thought at first of bad connection).
Now all works fine.

Jan
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Re: Atom just acquired not working

Post by John Ferguson »

I feel much more confidence about cleaning the chips now. Tomorrow I will have a go and report back.

John
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Re: Atom just acquired not working

Post by John Ferguson »

I have now completed the cleaning of the chips as recommended and only have ram in 10, 11, 42, & 43 plus block zero ram.

Control G now works so checked that basic is loaded.

The problem is that the screen will not clear so it only shows the random characters from the 6847 video chip.

I am hoping that I can be advised where to look for a solution. If the view is that a particular chip is the problem I do have some spares available.

John
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Re: Atom just acquired not working

Post by 1024MAK »

If you are getting a picture, the 6847 is likely fine.

The problem could be the address or data buffers between the 6502 MPU and the video RAM.

IC29 (DP8304) is the data bus buffer, IC27 and IC28 (both 81LS95) are the address buffers. And IC30 (74LS138) is the address decoder for the 2114 SRAM chips.

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Re: Atom just acquired not working

Post by hoglet »

It seems the Video RAM writes are not happening, but everything else is fine.

1. the NWDS is not getting through to the video RAMs (it passes thrigh IC27).

2. the VDG signal is stuck high (inactive). This comes from IC8, which connects to IC23.

You could try swapping IC27 and IC28 over.

Or you could try replacing IC8 and then IC23.

Do you have a scope or a logic probe?

Dave
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Re: Atom just acquired not working

Post by oss003 »

If IC's 2/3/4 were in, you can exchange them with IC's 27/28/29.

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Kees
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roland
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Re: Atom just acquired not working

Post by roland »

On the picture they are missing. If you need them, I have plenty in stock :)
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Re: Atom just acquired not working

Post by John Ferguson »

Many thanks for the advice. I can’t wait to try out what has been suggested.

I do not have Scope and friends who have one are to remote from me. I am one of those who are in self isolation so that option is not available.

John

John
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Re: Atom just acquired not working

Post by Kazzie »

John Ferguson wrote:
Sun Apr 05, 2020 3:04 pm
I do not have Scope and friends who have one are to remote from me. I am one of those who are in self isolation so that option is not available.
Another option would be a multimeter (continuity tests and DC voltage averages can be useful measurements in a pinch), or alternatively you could buy a logic probe online and have it shipped to you. The likes of eBay will sell you a basic one for ₤10-15, or even under ₤5 if you fancy a solder-it-yourself kit.
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Re: Atom just acquired not working

Post by John Ferguson »

I have tried all the suggestions for substituting and swapping chips but without success.

One test which did fail was that the RST signals test by pressing break so that pin 35 of IC25 was 0V and not +5V

Kazzie

The logic probe is affordable but what would it tell me lacking any knowledge of electronics?

I do have a good quality digital multi-meter so I will check the VCC and VEE for all the chips.

John
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Re: Atom just acquired not working

Post by roland »

If you press break, pin 35 of IC25 should be ± 5V because the reset signal is inverted by IC9; the 8255 needs a high level for reset. The 65xx however need a low level for reset.
So if you release break, the level of pin 35 must go low (± 0.2V).

If the reset level of that pin is stuck in a way that the 8255 "does not start" than you can't hear a beeb when you press CTRL+G.
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Re: Atom just acquired not working

Post by hoglet »

I assume you have found an Atom schematic, and have some idea how the video RAM works. If not, you need to do a bit of research and ask more questions.

When break is released, you would expect to see a burst of activity on NWDS and VGD as the screen in cleared. A logic probe would allow you to confirm this is happening.

In your case, one or other of these signals is not getting through to the RAM.

Once you know which one is problematic, you then work backwards with the logic probe to find the cause.

The cause is usually:
1. one (or multiple) failed ICs
2. a broken connection, either due to a dodgy IC socket, a bent IC pin, a fractured PCB trace, or bad soldering
3. a shorted connection, due to bad solderinging

I think you have eliminated (1), but it would be useful to hear exactly what steps you took. For example, did you swap certain chips between this Atom and a known-working Atom?

The IC sockets in at Atom are rubbish; that's where my money is....

Dave
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Re: Atom just acquired not working

Post by Kazzie »

John Ferguson wrote:
Sun Apr 05, 2020 7:07 pm
Kazzie

The logic probe is affordable but what would it tell me lacking any knowledge of electronics?

I do have a good quality digital multi-meter so I will check the VCC and VEE for all the chips.

John
The digital multimeter can give very useful readings for voltages that are fixed, but it's of limited use with fast-changing signals, such as clocks, data lines, and address lines. It will give an average voltage reading, but there's no easy way to tell whether a 2.5V reading is because you're measuring a square wave signal going back and forth from 0V to 5V, or because the voltage really is fixed at 2.5V (or several other possibilities).

To put it another way, a DC average reading can positively identify some faults (if the reading isn't what's expected), but it can't guarantee that a signal is definitely fine (because several signals can give the same reading).

A logic probe does a limited amount of signal processing, and will tell you, via lights and/or tones, if the voltage is a low logic level (~0V), high (~3-5V), alternating between the two, or floating somewhere in the middle. In this way, it can disambiguate some of the uncertainties you may have if you're only using a DC voltage reading.

If one were to choose to only have one of the two tools, I would choose the multimeter myself, as voltage readings + continuity checks are very useful for diagnosis purposes. But I say that as a man who's owned a good meter for years, had access to a scope (and recently acquired an old one from work), and never actually owned a logic probe. :)
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Re: Atom just acquired not working

Post by John Ferguson »

Thank you Kazzie for the advice about logic probes and multi-meters. I will purchase a logic probe.

Dave

I do have the circuit diagram and can see the critical connections. I used your references to the critical chips to swop and change them round to no effect.

I do have another working Atom so I will do as you suggest and install the critical chips to see what difference this will make. I will also look at the reverse of the pcb to see if I can spot any faults. Then I will use the logic probe to see what is happening.

Thank you Kazzie, Dave, and Roland

John
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Re: Atom just acquired not working

Post by hoglet »

John,

It's also worth just using a multimeter to confirm there is pin-pin connectivity on the relevant ICs.

For example:
- IC23 pin 14 to IC48 pin 5
- IC23 pin 15 to IC48 pin 4
- IC48 pin 6 to IC8 pin 1
- IC9 pin 6 to IC8 pin 2
- IC8 pin 3 to IC27 pin 1, IC28 pin 1 and IC29 pin 9
- IC49 pin 11 to IC27 pin 14
- IC27 pin 13 to IC42 pin 10 and IC43 pin 10

You should add IC48 onto the list of chips to try swapping over.

And also try a different pair of 2114L SRAM chips, in case the RAM itself is faulty (though it's unlikely that both have exactly the same fault).

Dave
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Re: Atom just acquired not working

Post by John Ferguson »

Roland

I have been taking readings of pin 35 of IC8255. Putting the negative probe of the multi-meter on the negative point of the DC input connector and the positive probe on pin 35 I get zero volts when pressing break both on the working Atom and the one I am working on.

This appears to be different to your experience. Control G makes the beep as usual.

I wonder what the explanation is?

Current tasks are checking connections and swapping known good chips with suspected faulty ones.

John
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Re: Atom just acquired not working

Post by roland »

John Ferguson wrote:
Tue Apr 07, 2020 7:52 am
I have been taking readings of pin 35 of IC8255. Putting the negative probe of the multi-meter on the negative point of the DC input connector and the positive probe on pin 35 I get zero volts when pressing break both on the working Atom and the one I am working on.
Are you sure you have the correct pin?
Schermafbeelding 2020-04-07 om 09.10.58.png
When you look at the diagram you see the break key in the lower right bottom. If the key is not pressed then pin 13 of IC9 is at 5V and pin 12 is at 0V. Pin 12 is connected to pin 35 of IC25. If you press break then pin 13 of IC9 is at 0V and pin 12 is at 5V and so is pin 35 of the 8255. Pin 4 of IC9 should be the same as pin 13 (and the break key) except that it has a 10-20ns delay.
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Re: Atom just acquired not working

Post by hoglet »

Just to confirm what Roland is saying....

On my Atom:
- BREAK pressed: 8255 pin 35 measures 3.79V to ground
- BREAK released: 8255 pin 35 measues 0.15V to ground

If I completely disconnect pin 35 by bending the pin out (to simulate a broken track), then it measure 0.00V, and the Atom still boots.

John, what's the exact voltage reading, to two decimal places?

With the Atom powered off, can you check the resistance between IC9 pin 12 and IC25 pin 35.

(It's very unlikely both your Atoms have the same fault)

Dave
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