A601 Battery+RTC Problem

[dimgr]

Hello Amiga fans,

I was an old Amiga user and have resumed my old hobby.
I have an Amiga 600(HD) and recently got the A601 expansion board with 1Meg of ram.

The extra 1MB works fine but the RTC (real time clock) does not! I opened the metal casing and discovered, surprise, battery leak. I removed the old 3/V30R flat battery pack and after a bit of cleaning up it looks like the picture in the following photo album:

A601 battery leak board picture

Apologies for the non crisp quality but that's as good as my tiny camera will do.

By looking at the board I thought that the corrosion has not affected the R913 or C9 or C913 next to it.

Unfortunately I was not able to find a proper schematic of the A601 but I have looked in the A501 schematic (a relevant picture of which is also found on the web album link posted above).

After replacing the battery with a new one fully charged the RTC would still not work.

So I took my trusty multimeter and checked a few things:

1. The line going from the positive pole of the battery delivers power just fine to the left side of the R913 (resistance 0)
2. The negative pole of the battery gets connected properly to the ground circuit.
3. The negative battery pole and pin 12 (GND) of the M6242B connection is fine (resistance = 0)
4. Without the battery on the circuit, checking between the + and - receptors I get a 1512 Ohm resistance reading
5. Same reading I get when checking between the pin 24 (VDD) of the M6242B and the + battery pin on the board (1512Ohm) :-)
6. Finally, because I only have high school knowledge about electronics, I also checked the resistance between the two pins of the C9 small capacitor and the C913 barrel-shaped capacitor. In both cases I get a reading of 21 Ohm.
Of course I realise how a capacitor works and checking it's resistance does not really make sense but I don't have any other tool.
7. Pin 24 (VDD) of the M6242B and any point in the ground circuit shows up 21Ohm !!!!

Summing up, the issue is that if I check the resistance between the + and - receptors for the battery on the board I get a 1533Ohm (notice, it is equal to 1512+21) reading. That, with the battery connected, drains it overnight.

Any idea as to which component has failed, where is the short circuit or what else to check?

P.S.

Forgot to mention that WITH or WITHOUT the battery connected setclock reports battery backedup clock not found

[dimgr]

Thanks for the answer.

You are of course right in that there are components in parallel.

In fact, as can be seen from the similar A501 diagram that I had posted above, what's happenning is, in ascii art:

Code: [Select]

[b][color=0000ff][font=Courier]
+12v --- R911 --|-------Diode 911---> +5V
                |
               ----
               \  / Diode 912
                \/
      R913      |
 |---/\  /---------------------------------------
 |     \/       |              | + Pin 24       |
---            ----(+)       -----              |
BATTERY        C913 .1uF     | R |             ---
3.6V           ----          | T |M6242B        C9 4.7uF
 -              |            | C |             ---
 |              |            ----- - Pin12      |
 |-----------------------------|-----------------
 |
 |
-----
 ---
  - GND
[/font][/color][/b]
So, the battery and the resistor, the capacitor C913, the +/- pins of the RTC Chip itself and the C9 capacitor are all in parallel.

All connections have checked ok using the multimeter, but of course I can not verify the status of the two capacitors.

I wonder if somehow due to a damaged component in the diagram above the chip does not get it's + power supply even when the computer is on (as it reports battery backedup time not found)

The components closer to the leaked battery are the R913 resistor and the C9.

[dimgr]

it isnt matter of battery, when detecting RTC - amiga want to read time bits from RTC circuitry, which can be done without battery connected of course. It runs on battery only at power off state.

So if you want to find where it woes, look at those eight data lines from miggy to RTC chip, not on a battery side.

Thanks for the answer.
You are right I believe about the existence or not of battery regarding the RTC detection.
However, I checked the data lines D0-3, A0-3 and they are all ok and far away from the corroded part of the circuit anyway.

Additionally, if you look at the Real Time power schematic in my first link to the webalbum and the ASCII art version of it, in my previous post, it seems to me that the RTC voltage pins are in parallel to the other battery components.

So I think that if something is not working there, perhaps the RTC does not get juice to operate?

Perhaps I should try carefully measuring the voltage of the RTC while the Amiga is powered on (a bit difficult though)

[dimgr]

Try measure the VDD pin of the RTC chip when the machine is on (need to open the 600 case - be carefull to not break the back grips) and measure the VDD in the battery + pin and in the RTC chip ones.

Hello and thanks for the answer.

I measured the VDD while power is on. It read 0.97V I guess that is much less than it should. Wasn't it so that the threshold for being able to keep the time with the battery, let alone operate as a proper RTC, is 2V?

And pull out the diode 912 (the echivalent one of A501) and measure it both ways with contuinity test. In one hand will be measure 0.3 or 0.7 (ok) in other they will looks like open.

I will try that as next. This diode is a bit far from the corrosion area, but of course I fully understand what you said about it breaking easily due to a short circuit.

Any idea about how to check the capacitors as well?

[dimgr]

Hello all and thanks once more for the great and insightful comments. I managed to fix the RTC circuit of the A601 eventually.
setclock works ok and time gets set and saved now.

Castellen, you were right about the similarity of the A501 board diagram and that's why I have been using it all along from the start as a basis for my work.

The traces were ok. The capacitor C9 was broken.
The diode D912 that was suggested earlier as a possible broken component checked ok.
So, I was looking at the RTC circuit again and again and as the resistor R913 looked ok as well, I thought that there are two possibilities either the C9 .1uF (tiny SMD) or C913 (barrel looking SMD) 4.7uF capacitor broken.

After removing the small C9 it checked wrong (short circuited!) and I luckily I found a replacement .1uF SMD capacitor in an electronics store.

Because I don't have a hot-air tool or solder paste I devised a technique of my own about replacing the tiny C9 capacitor using my soldering iron.

There should be 5V on pin 18 (Vcc) of the RTC when the computer is running. Pin 9 is ground. About 3.6V when it's off and the RTC is running from battery.

I've written a guide on repairing the Amiga RTC which is mainly directed towards the A4000D, but might be of some use.

Now I checked the board again and when the Amiga is on there is indeed +5V between pin 24 and pin 12 of the MS6242B (I checked the schematic of MS6242B and pin18 in NC so maybe you are referring to the RTC of the Amiga4000D?)

Castellen, I have already dug up your *very* useful guide about repairing the RTC and have read many many times your guide about soldering SMD components before starting to post here

If anyone is interested I can describe how I managed to unsolder and most importantly solder this tiny C9 SMD (two legged) capacitor using a normal soldering iron without solder paste -- just very thin solder.

Thanks again to all