A600 woes

[alie]

Hello,
I decided to recap my smellyfish Amiga 600 r1.5 June Bug. It went on with a few issues and a couple of broken pads, the first was the C324 (worst placement ever designed) and C235 (same old very very bad position).


A few days ago, I finally finished soldering the last cap, I have been pretty slow, taking my time in the evening. So I tested it with my monitor to find out that it does not properly boot anymore.


I suspected the infamous reset, but the cap seems to be sending a signal and it's handled by the 555 IC.


The screen is outputting good refresh rates, but it seems that is not actually proceding after the powerup.


ICs are warm but not hot.


I actually acquired another A600 with a completely different issue, it had his caps replaced but who soldered it didn't actually try to fix the lifted pads in C304 and C303 (added wire to the hole, soldered the wire and the lifted pad after cleaning it up) and he left a good amount of flux on the board. The issue is that after boot the U8 will become very, very warm and the picture fades to a red-ish color eating all other channels.




Of these two I would just like to have a working one at least




Photos of course I can make more please ask!

https://imgur.com/a/yqzDw37

(also added a shot of my cheap oscillator on the output pin of NE555)

[Pollock]

caps need replacing.


smelly fish is a dead giveaway.

[alie]

Originally Posted by Pollock

caps need replacing.

That's what I did, replaced ALL the caps with new ones from amigastore.eu the second A600 has new caps aswell.


The A600 was actually working fine before I put my pawns on it with a soldering iron (and hot air). The smell was barely noticeable and only after some time it was on. The audio channels were bad, the pad there was corroded.

[psoma]

Is your Caps Lock key stuck on? That’s usually a dead giveaway of a 555 issue...

[alie]

Yes, caps is on. Is there a good way to diagnose it without having to desolder and replace?

I do read “something” here https://imgur.com/a/yqzDw37

[EzdineG]

Alie, get that board under a magnifying glass and check for solder blobs that may have landed on parts of the board unrelated to the caps.

I have seen this leave me with a black screen too many times before.

Good luck!

[alie]

no luck there, unless I didn't see it I cleaned it up completely, even where I didn't pass with the iron.

On the other hand the other amiga (the very hot U8 one) had to be cleaned up to remove flux residues and solder blobs between caps, for this one I'm thinking there might be some soldering paste residue making contact (is it possible?)

[Lemaru]

Originally Posted by Pollock

caps need replacing.


smelly fish is a dead giveaway.

Did you not read the post?

[jlin_au]

The A600 rev 1.5 schematics (http://www.amigawiki.org/dnl/schematics/A600_R1.5.pdf) are available on webpage http://www.amigawiki.org/doku.php?id...ice:schematics)

TIP for any future replacements of C324 and C325
================================================
If you have to replace electrolytic capacitors C324 and C325 on another A600 in the future, desolder and remove the keyboard connector CN13 beforehand.

This will let you:
1) clean up any leaked "conductive" capacitor electrolyte from under CN13,
2) give you much better access to remove and clean under both capacitors C324 and C325.
3) prevent melt damage to CN13.

555 Power on reset issue
========================
For 555 output traces, set your Digital Storage Oscilloscope to DC mode instead of AC mode. We're expecting a digital type signal output on 555 U14 pin 3 where the output is a low voltage nearly gnd (a logical 0) for most of the time and near Vcc (1) for about 0.5 seconds when the 555 monostable is triggered.

The 555 (IC U14) based power reset circuit (on page 10 in the schematic) is dependent on:
1) capacitors C611, C612 (the 10 micro farad electrolytic you would have replaced), and C613.
2) resistors R611, R612, R623, R624 (0 ohm), R514, and R515
3) U36 (74F27) pin 12 via 2N3904 transistor Q622 for trigering a soft reset from the keyboard connector CN13 pins 10, 11, and 14.
4) Keyboard Controller 6570 U13 pin 34 via 0 ohm resistor R624 above to 555 U14 pin 2.

Check for continuity/resistance across each specified capacitor's or resistor's contact pads and also the connectivity from that component to other connected component pads or powerline connections in the circuit as per the schematic.

You're looking for "open circuit" resistors, "shorted" capacitors, and broken circuit traces or vias.

Capacitors should show a "low" resistance initially between their contacts that increases to open circuit as they charge up under "resistance" measurement. If they show a constant resistance after about 30 seconds then I'd suggest you clean around the capacitor with isopropyl alcohol to remove any old conductive electrolyte residue.
You may need to desolder and dismount the capacitor and recheck its resistance while it's out of circuit and plus the resistance between its mounting pads on the board with the component removed.

The resistors should have resistances "near to" their specified values in the schematics.

Second A600
===========
A) 8520 U8 (even CIA) getting hot
Check for shorted pins on this CIA.

This even CIA (U8) is identical to the odd CIA and is used for Floppy drive control, the RS232 serial port, and the parallel port.
The odd CIA (U7) handles the keyboard, Mouse/Joystick, Floppy drive control, audio filtering, and the parallel port.
Both have timers used by the system.

B) video fading
I'd suspect an issue near Denise (8373 U4) or the two associated 74HCT244 ics U31 or U32 is the cause.

[jlin_au]

Also, on the first A600 does a Soft reset appear to work?

[alie]

Thank you very much for all the info!

Next time I'll handle the infamous caps near the CN13 I'll desolder it first.

I can tell the vias for the _RST and KBRESET signals are connected, I did check the mV values and these seems to check 22mV~12mV (depending on the probe), and the oscillator (set on DC ... I was fiddling with it these days ... me dumb) check with the same values.

I'll check the soft reset, honestly I didn't attach his keyboard yet.

If I can't find the issue I'll yield looking for someone with experience in my zone.


Thanks again!

[suppah]

jlin__au, great stuff, thanks for the long post! Useful information for any amigian.

[jlin_au]

The values of the resistor capacitor pair R612 and C612 actually determine the length of the monostable logic 1 pulse on pin 3 of the 555 IC used in the power up reset circuit. The generated logic 1 pulse on pin 3 should have a duration of about 0.5 seconds.

Check the voltages seen on 555 IC U14 pin 3 relative to the GND or 0 volt line on U14 pin 1 using your the DSO in DC mode. VCC (+5 volts) should be on Pin 8 of U14. When the 555 in monostable state is triggered the signal from U14 Pin 3 should go from nearly 0 Volts to a steady positive voltage above 3 volts for nearly 0.5 seconds and then return back to nearly 0 volts again.

The output of pin 3 should be near 0 Volts in the untriggered logic 0 state and above 3 volts when in the triggered monostable pulse logic 1 state.
This positive pulse is then inverted via transistor Q511 to generate the _KB_RESET signal pulse down to 0 Volts that then resets the Amiga 600. The _KB_RESET signal is normally held above 3 volts preventing a reset of the Amiga 600.

[psoma]

Wow, that's some quality info and diagnostic detail jlin_au. Nice one!

[alie]

It is indeed the NE555 circuit, the voltage on the output pin remains high, stable. While the second board actually sends a signal up for 0.5s as you perfectly described (in the meanwhile the second board was almost burning, I left it on for a few seconds to get the measurement and the U8 have become scolding hot, I almost hurt my finger when touching it ... but that's going to be another story).

After some thinking about it I checked the keyboard input for the reset pins 10 11 14 and these are showing up as high on the chip.

Right now I'm de-soldering the connector, I think I'll go further and replace it with a different type of connector. My membrane is losing connection with it, when I first got this A600 running I had to trim it a little bit to actually use the keyboard, the contacts were almost gone, now there are barely contacts on the exposed side.

I'll report back when I'm finished.

Thanks again for the detailed post.