Saa7220 sparks from ground pin (killed a saa7220 ?)

[ash_dac]

Hi,

I built a separate supply for the saa7220 but stupidly didn't heatsink the 5V regulator. The regulator got very hot, and failed I guess!

I put the Saa7220 power supply back to how it was originally but now the thing does the spinning like it's going to fly thing.

Now I have a fantastic spinning machine.

I know where the problem is:-

-the clock was damaged

and/or

-the saa7220 has died.

I thought i'd measure with a multi-meter the voltage at the saa7220 vdd pin to ground to check the voltage.

As soon as I touched the ground pin with one probe of the multimeter I could see a spark ! (between the ground, and one multimeter probe). I tested during the hyper spin up.

Is this just the current that the saa7220 dumps onto the ground rail!!!!

[hembrow]

The SAA7220 doesn't draw so much current that it would instantly incinerate a 5 volt regulator such as a 7805. My 7805 only has a very small heatsink on it and runs at a very reasonable temperature.

So, I think you've done something else wrong. Did you test for shorts before switching on ?

What input voltage do you have to the regulator ? They usually need to drop about 3 V or so (i.e. 8 V input). Any extra above that simply produces extra heat and at some point you exceed the voltage that the regulator can cope with.

Did you test that the regulator output really was 5 V before connecting it up ?

David.

[ash_dac]

Originally posted by hembrow
The SAA7220 doesn't draw so much current that it would instantly incinerate a 5 volt regulator such as a 7805. My 7805 only has a very small heatsink on it and runs at a very reasonable temperature.

So, I think you've done something else wrong. Did you test for shorts before switching on ?

What input voltage do you have to the regulator ? They usually need to drop about 3 V or so (i.e. 8 V input). Any extra above that simply produces extra heat and at some point you exceed the voltage that the regulator can cope with.

Did you test that the regulator output really was 5 V before connecting it up ?

David.

shorts: possibly on the regulator

I measured 12V input into the regulator. Quite alot to drop so heat!

Upon testing the 5V regulator this evening it wasn't working.

Well the player started playing music, and then stopped!

I have tested the clock today, and it doesn't work.

[jean-paul]

That SAA7220 is dead. Do you have a spare one ?

Please measure the new supply with SAA7220-like load before going on with any further changes.

[ash_dac]

Originally posted by jean-paul
That SAA7220 is dead. Do you have a spare one ?

Please measure the new supply before going on with any further changes.

I have a spare one but it's soldered to a board.

I will have a look tonight.

[hembrow]

Originally posted by ash_dac


shorts: possibly on the regulator

I measured 12V input into the regulator. Quite alot to drop so heat!

What is the regulator ? 12V is OK for a 7805 input so long as you're not drawing loads of current and so long as you have enough heatsink.

Upon testing the 5V regulator this evening it wasn't working.

Well the player started playing music, and then stopped!

This is confusing. If the player ever manages to make music then the SAA7220 is still working. When you say the regulator wasn't working, do you actually mean that it worked for a bit until it overheated ?

I have tested the clock today, and it doesn't work.

But you said music came out of the player. That's not going to happen if the clock isn't working.

From reading between the lines of what you said, it appears that:

When you tried it before, the player worked for a bit and then stopped. You noticed at that time that the regulator was very hot.

Today it worked for a bit again and then stopped.

To me, this suggests that everything is fine except that your regulator is overheating and shutting down.

As the player needs a working SAA7220 and clock to make any sounds at all, this must still be working when it's got a good power supply. However, after the power supply goes out when you overheat the regulator, you're not going to be able to see anything much around the SAA7220 - including the clock. So, if I were you, I wouldn't bother butchering the board to replace the chip until I'd got a stable power supply.

Have you tried simply bolting a lump of aluminium to the regulator ? If your regulator is a TO220 package 7805, the heatsink will be connected to the centre pin which is 0V, so make sure it doesn't short against anything else.

If your regulator is not a 7805, what is it ? A 78L05 (tiny package without heatsink) is probably not up to the job.

So far as heatsink size is concerned, you shouldn't need much and that makes me wonder if your 5V rail is being pulled down by something else. Back in 1989 I installed a 7805 with about 5 cm^2 of aluminium attached as a heatsink in my CD player to supply the SAA7220. This worked just fine up until last year when other parts of that player finally gave up the ghost. My current CD player has a similar arrangement.

[ash_dac]

Hi,


I have just soldered the crystal back into the player instead of my clock, and took out the regulator (thanks for tips).

The player now reads a disc, and sound is ok.

My thoughts were that it was a clock problem probably a dry soldering joint or failed regulator on my clock module causing the player to go warp speed!

I still don't understand why there was a spark!

Does the warp speed cause the saa7220 to spark ?

Any ideas ?

[hembrow]

Originally posted by ash_dac
Hi,
I have just soldered the crystal back into the player instead of my clock, and took out the regulator (thanks for tips).

The player now reads a disc, and sound is ok.

Good. No harm done, then.

My thoughts were that it was a clock problem probably a dry soldering joint or failed regulator on my clock module causing the player to go warp speed!

When I had a player spin like that it was due to IDC cable connectors having become less conductive than they ought to be. I doubt it's actually anything to do with the SAA7220 itself as that's just the oversampling filter. Some people take that chip out altogether and still have a working player. So, you'd upset something else, quite possibly due to the clock not working.

I still don't understand why there was a spark!

Does the warp speed cause the saa7220 to spark ?

No chip would store enough energy to create a spark. For that you need a capacitor. Almost certainly you shorted out a largish power supply capacitor. Quite possibly your probe shorted the 5V to 0V, or something similar.

You have still not described just what you had connected to the player. It's not really possible to say much without that information.

[ash_dac]

Originally posted by hembrow
[B]

Good. No harm done, then.

[B]

When I had a player spin like that it was due to IDC cable connectors having become less conductive than they ought to be. I doubt it's actually anything to do with the SAA7220 itself as that's just the oversampling filter. Some people take that chip out altogether and still have a working player. So, you'd upset something else, quite possibly due to the clock not working.



No chip would store enough energy to create a spark. For that you need a capacitor. Almost certainly you shorted out a largish power supply capacitor. Quite possibly your probe shorted the 5V to 0V, or something similar.

You have still not described just what you had connected to the player. It's not really possible to say much without that information.

Hi,

It was a 5V regulator with a 11.2896Mhz canned oscillator on a DIL socket. I was going to upgrade the oscillator to a low jitter type.

[Guido Tent]

Originally posted by ash_dac
Hi,

I still don't understand why there was a spark!

Any ideas ?

very likely ESD, you and your multimeter where at different potential than the SAA

I always touch the cabinet first, with my hands or soldering tip or multimeter

best