Need support/guidance to repair my A1200.

[JeanYves]

Hi,

My youthful Amiga 1200 stopped working after not using it for 8 years. I'm a little bit tech-savvy and I can do electronics. But I am not an expert.

My problem is similar to this thread.
https://eab.abime.net/showthread.php?t=98896

Now, here some information.

Specs;
- Amiga 1200, motherboard REV 1D, pretty clean, NTSC (see pictures)
- I changed all capacitors, excepts for 2. The ones without the tape on it.
- New ROMS 3.1
- Video composite is connected
- I have an oscilloscope for testing and a multimeter

Symptoms;
- Not booting - just a quick white flash when I turn the power on, then it goes black.
- Not entering the color cycling

Measurements done;
- The power supply returns great power voltage.
- The keyboard claps lock does one flash when powering the Amiga
- The keyboard power led is solid green
- No drives are connected.
- I tried soft resets and the 2 mouse bottons when powering.
- Based on Solarmon chart (see hyperlink) i probed;
- U49 - leg 2&3 (VCC & GRD) -> 5 volts stable
- U49 - leg 1 (power test) -> 5 volts steady - is this normal or it should be an impulsion?
- R116 (_RST) - 5 volts steady, is this normal?
- U32 (RESET - PIN 18) - 5 volts steady, is this normal? should it be an impultion?

NB: Sometimes (rarely), when I am probing the pin 1 of U49, the Amiga goes what I beleive solid Red and stays there. But this is very rare.

From this point, I am suspecting a reset signal problem, but I am not completly sure. I don't know what to do as the next step and I am kindly seeking advice for the steps to preserve my A1200. Which pins and chips would indicate without doubts that the reset signal is working (or not)?

Any help would be appreciated,
Thanks!

[JeanYves]

[Krashan]

Check 28 MHz clock signal from quartz generator.

[solarmon]

How are you probing the pins? The reset signals are best checked with a logic probe, or an oscilloscope if you have one. The reset (and halt) signal should pulse low very briefly and then go and stay high.

[JeanYves]

Thank you both,

Yes, I have a logic probe and an oscilloscope. I hope that I am using them well lol.

1. I tested the crystal (X1) and it returned a 1.25 MHZ and 1.65 volts. I am not sure where is the best place to test it, so I went behind the board and test directly the pins for a signal. I also tested E131R and I am getting the same result.

2. U49 - leg 1 (power test) -> Yes, the signal pulses low very briefly and then goes and stay at 5v.

[Krashan]

Originally Posted by JeanYves

I tested the crystal (X1) and it returned a 1.25 MHZ and 1.65 volts.

Weird. For PAL Amigas it should be 28.37516 MHz, more or less square wave with TTL levels. The best place to test are lower pads of R100 and E131R, located to the left of the generator. Signal should be referenced to GND.

[solarmon]

Originally Posted by JeanYves

Thank you both,

Yes, I have a logic probe and an oscilloscope. I hope that I am using them well lol.

1. I tested the crystal (X1) and it returned a 1.25 MHZ and 1.65 volts. I am not sure where is the best place to test it, so I went behind the board and test directly the pins for a signal. I also tested E131R and I am getting the same result.

2. U49 - leg 1 (power test) -> Yes, the signal pulses low very briefly and then goes and stay at 5v.

I would check/follow the other reset (and halt) signals. The latest reset signal diagram can be found at (although, this is based on a rev 2B A1200) :

https://sites.google.com/one-n.co.uk...-reset-signals

[JeanYves]

I have a cheap OSCILLOSCOPE; EM125.

http://www.all-sun.com/EN/d.aspx?pht=1184

It says that the bandwidth is 25mhz.

Does this mean that it can't measure a frequence above 25mhz? I guess so...

[JeanYves]

Is there another place I can check the clock frequency, is it slowed down elsewhere? The Motorola 680EC20 CPU clock supposed to be at 14 MHz. Is this clock coming from the crystal?

[whaka]

Originally Posted by JeanYves

Does this mean that it can't measure a frequence above 25mhz? I guess so...

dependig of the probes too. but above specified bandwith, they can give something somewhat very attenuated.


you can check 7Mhz and 3,55Mhz. they are derivated from the 28Mhz.
so, if they're fine, 28 is fine

[Krashan]

Originally Posted by JeanYves

Is there another place I can check the clock frequency, is it slowed down elsewhere? The Motorola 680EC20 CPU clock supposed to be at 14 MHz. Is this clock coming from the crystal?

Crystal is connected to Budgie chip. Then it produces 14 MHz CPU clock on pin 90, CCK signal (for Alice, Gayle) on pin 36, also 7 MHz clock on pin 92.

[JeanYves]

thanks for the tips!

I will check the clock signals at those pins and various reset signals tonight.

[JeanYves]

Here are my results based on your requested homeworks;

1st - Clock tests on BUDGIE

- Pin 90 -> 5 MHZ (unstable)
- Pin 92 -> 2.5 MHZ (unstable)

If those results are true, I guess I need a new crystal. (I ordered one few days ago).

2nd - Reset signals

- U13, P38, U49, TP1(p9), U32 (92, P18), and U1 (P6, P22) -> 1 logic (5V) stable for all

NB:
- Sometimes after a while the screen turns yellow for about 5 seconds, then goes black again.
- No component are too hot, just little warm or cold, eveything seems normal here
- I will try my oscilloscope on a working C64 to validate it.

Any thoughts?

[Krashan]

Originally Posted by JeanYves

I will try my oscillator on a working C64 to validate it.

I doubt C64 can be overclocked to 28 MHz...

[JeanYves]

Krashan, sorry for the confusion. I don't want to overclock my C64. I wanted to verify(validate) that my oscilloscope is working since it is lock(limited) at 25MHz and I can't directly check the Amiga crystal with it. Indeed, my oscilloscope returns me 1MHz on a C64. So my oscilloscope seems to work well (under 25MHz).

Now, for the Amiga, is it possible that a faulty crystal returns 5MHZ instead of 14MHz (for PIN90 on Budgie)? If so, than my Amiga crystal is bad. I mean, I have no idea what is the behavior of a faulty crystal. Could it return no clock at all? Or could it return only half of the frequency?

I ordered a new NTSC crystal from UK, but the shipping will take some time to arrive to Canada. (I can't wait to replace it, fingers crossed)

[Krashan]

Originally Posted by JeanYves

Krashan, sorry for the confusion. I don't want to overclock my C64. I wanted to verify(validate) that my oscillator is working since it is lock(limited) at 25MHz

Well, you have mixed "oscilloscope" with "oscillator", so I've got confused. Anyway, failed cristal (which is in fact a complete square wave generator) usually does not work at all, so there is no clock signal. Your unstable frequency readouts are probably caused by measuring signals above the oscilloscope bandwidth.

[JeanYves]

Yes, you are right, I meant "oscilloscope". I am using square wave for the measurement, and I have tried my best to measure things properly.

Not sure why I am getting on Pin 90 -> 5 MHZ (unstable) on my Budgie. I might produce a video on youtube for you to see.

I will replace my crystal and I should receive the new one hopfully in 1 week or 2. More to follow, thank you all.

[JeanYves]

Ok, some development, I bought a second Amiga 1200 motherboard (functional) that I will use for testing and I received my crystal. I haven't replace it yet.

Some new results;

- I calibrated my oscilloscope and tested my faulty board. All clock pins and crystal frequency are good.

- One thing, I noticed a behavior of the power led; first it turns solid green, than half green intensity, than solid green again and stays that way. This appends quick, but easilly noticable.

Any thouths?

[solarmon]

Originally Posted by JeanYves

- One thing, I noticed a behavior of the power led; first it turns solid green, than half green intensity, than solid green again and stays that way. This appends quick, but easilly noticable.

Any thouths?

The POWER LED changing brightness from solid to dim is normal and expected during a normal/good boot. However, I'm not sure about the part where it changes back from dim to solid. Normally, the POWER LED dimming is when the audio filter is turned on (which is enabled by default).

If you connect a floppy drive, do you hear it clicking/ticking? This is a sign that it has booted up OK and waiting for a disk to be inserted.

[JeanYves]

Yes, I do. There is a small impulsion on the motor.