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Old 17 March 2011, 21:05   #1
Eamoe
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Caps n' resistors

Hi all,

just found an old 512k RAM trapdoor board for the A500. It doesn't appear to work, i.e. lets the computer work normally but doesn't give it any more RAM.

I thought I might seize the opportunity to solder-train a little and try and replace all caps and resistors on the PCB and then see if it gets any better.

At my local seller, I can't seem to find the exact same values for these parts, though.

CAPACITOR
I have found a 22µF, which is right, but the voltage is 35V where the original shows 16V. Do I expose the hardware to trouble by using the 35V version?

RESISTORS
The color rings on the old ones give me an indication about Ohms, but not about how many Watts (0,1 or 0,25 or 0,5?).

So for the old ones I have found:
- 10 KOhms / ±20% / Watts unknown
- 410 Ohms / ±5% / Watts unknown

And the closest new I have are:
- 10 KOhms / ±5% / 0,1 W
- 470 Ohms / ±5% / 0,25W

Can I ruin the board by using these?
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Old 17 March 2011, 22:21   #2
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but are you sure chips are ok?

I think 35V caps are probably ok, and resistors are usually 0,25W. (you can always use 1W but they take more space).
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Old 17 March 2011, 22:49   #3
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insert a model of ram or a photo
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Old 18 March 2011, 01:31   #4
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Using a capacitor rated for higher voltage will cause no problems whatsoever.

Resistor wattage can easily be determined by looking at the size of the resistor, however unless they look burnt it's very unlikely replacing them is going to be of any benefit.

Personally I think you're going about this the wrong way. A much better way to start troubleshooting the board would be to get a logic probe onto it and verify all the necessary signals are present and doing what they should be.
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Old 18 March 2011, 07:46   #5
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A dead TTL chip* on the trapdoor expansion is the most common culprit on this matter.

Burned memory chips will not let the Amiga boot at all, or the Amiga will crash as soon as the memory get used.

*TTL chips all start with 74, then one or two or three letters (or not), then more two or three digits. Examples:
7400
74LS08
74F244
74245
74ALS84
etc

Also existed the "military spec" TTL family, but those where never used on Amiga boards. The initial number code is 54 instead of 74.


BTW: there is not a jumper or switch on this board? Those are there to enable/disable the board. Switch it on or put a jumper and check again.

Last edited by rkauer; 18 March 2011 at 07:48. Reason: voices in my head told me so
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Old 18 March 2011, 09:19   #6
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Thanks for your advice all of you.

I know that defective chips will either prevent the Amiga from booting or make it randomly crash. That's why I thought the problem could be somewhere else.

I have no idea how to test a 74x logic. I don't have a probe and I wouldn't know how to use it anyway. I guess it's better to just find the same chip and replace it straight away, then. No?

It looks like all the components on the board are going to be replaced as I already have removed the resistors and caps...

Will post pics when I get the chance.
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Old 18 March 2011, 09:47   #7
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Replacing an IC when you don't know if theres anything wrong with it is ridiculous.

Get yourself a cheap logic probe and learn to use it, it's not difficult.
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Old 18 March 2011, 12:33   #8
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Logic probe isn't a bad idea, but the problems may also be with a stuck line or reset, just to name two.

A picture of the board is essential, top and bottom, I have found that bending one row of pins ever so slightly on the trapdoor expansion plug can help in these sorts of situations.

As for the practise on a board, get yourself an old PC PCB, ie. 486 vintage or earlier and practise on those, good boards because they are thick and multilayer, if you can work on those without ripping out barrels or destroying tracks, you're well on your way to making yourself 'solder worthy' TM

'Solder worthy' is used courtesy of Amisynth and is a legal and binding statement whereby use is excluded for...

Friday night funny
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Old 18 March 2011, 20:38   #9
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I don't see any TTL chips on a501, dunno about others..
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Old 18 March 2011, 22:52   #10
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I'm about to post some pics of an A501 as bizarrely I'm doing the same thing!
35v caps are fine, you can even use 50v ones (if they fit). Always a higher voltage and never lower.
The resistors are 0.25w.
EDIT: If you are changing resistors, you may as well change those two diodes as well. I'll pull a leg up and check what they are (this is an electronics term and not pervy).
Quote:
Originally Posted by orange View Post
I don't see any TTL chips on a501, dunno about others..
74F27N
74F86N
74LS163AN



It would be cheaper and easier to actually change the logic chips IMHO.
(Orange I think that's what he means)

Last edited by Eclipse; 18 March 2011 at 23:06.
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Old 18 March 2011, 23:11   #11
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oh, but you got different a501. my broken a501 looks like this:
http://www.amiga-hardware.com/displa.../a501_1_sm.jpg
(only clock and dram chips)
don't know what OP has..
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Old 18 March 2011, 23:15   #12
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Quote:
Originally Posted by orange View Post
oh, but you got different a501. my broken a501 looks like this:
http://www.amiga-hardware.com/displa.../a501_1_sm.jpg
(only clock and dram chips)
don't know what OP has..
Ahh I see, must be logic glue for the memory on mine.
If he has the same as you it only has RAM and the IC for the clock! You are correct, no logic.
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Old 19 March 2011, 02:42   #13
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The battery may well have leaked and done some track damage.
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Old 19 March 2011, 18:26   #14
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Quote:
Originally Posted by Eclipse View Post
I'll pull a leg up and check what they are (this is an electronics term and not pervy).


OK, an early version of the expansion can be seen here:

http://amiga.resource.cx/exp/zydec501

Mine is the Amram-X, and here are two 3.5MB jpegs of it:

front
back

There's no 74x logic on it, but a chip I cannot identify (OKI M62X42B).

- battery, resistors and caps removed
- small leakage oxydized tracks near battery: thoroughly scraped and cleaned tracks then applied nail polish where open
- checked for other visible leak damage: none
- left all button caps, two diodes, the unident OKI chip and these three narrow, black plastic wrapped... things

The switch was soldered directly onto the board and looked really bad (bent, dirty and greasy). Continuity test showed it was ON in both positions. Removed it, cleaned the gooey mess and soldered temporary leads for testing.
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Old 19 March 2011, 19:30   #15
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OKI is clock chip.
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Old 19 March 2011, 23:37   #16
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you need insert a switch or jumper for use the ram (3 pin near ON)

http://www.amiga-hardware.com/downlo...ram500plus.jpg
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Old 20 March 2011, 03:17   #17
Eamoe
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Quote:
Originally Posted by cpiac64 View Post
you need insert a switch or jumper for use the ram (3 pin near ON)
and my reply is

Quote:
Originally Posted by Eamoe View Post
The switch was soldered directly onto the board and looked really bad (bent, dirty and greasy). Continuity test showed it was ON in both positions. Removed it, cleaned the gooey mess and soldered temporary leads for testing.
But cipiac64, thanks for the very instructive picture, it tells me a lot about this expansion: it can work without any other component but the RAM chips and one capacitor. Testing will now be way, way easier.

Looks like this "Issue 3" was made for the A500+ which didn't need any clock as it already had its own, while my version "Issue 2" was built to give the regular A500 a clock (and apparently nearly any component on the expansion PCB is there to enable this clock function).

Despite this, removing these resistors and caps wasn't a waste all the way, because it helped me to better clean the board. Thanks for your advice folks, I have learnt a few things here!

Does anyone know what the three narrow black components are?
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Old 20 March 2011, 03:44   #18
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Quote:
Originally Posted by Eamoe View Post
Does anyone know what the three narrow black components are?
Resistor networks. Here's a pic of one without a packaging around it, I haven't pasted it here because it's huge.

http://www.sv-components.com/Photos/...20NW%20SIP.jpg
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Old 20 March 2011, 04:07   #19
Eamoe
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There's nothing like a good picture! Thanks
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Old 20 March 2011, 10:19   #20
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have you test a card with switch on or not?
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