### Author Topic: Calculating missing capacitor value  (Read 1572 times)

0 Members and 1 Guest are viewing this topic.

#### haggar

• Contributor
• Posts: 26
• Country:
##### Calculating missing capacitor value
« on: October 25, 2017, 08:25:16 am »
Hey all,

I'm pretty new to electronics, but what you're looking at is the main board from an Atari Flashback 3 (sorry about the bad quality).

Shouldn't take too long to see what's wrong with it.  So without a schematic or board view; what's the best way of determining the value of the blown capacitor?

The two remaining electrolytics are both 100µF 10V, however I'm not sure if they are in series or parallel, as it's pretty hard to see the traces.  The board is powered with 12V 830mA.

Based on my limited electronics knowledge, and their positions on the board around what I think are mosfets?  I'd say they are for smoothing purposes?  I could be widly off though.

Any input or advice would be super helpful.

Ta

#### Zero999

• Super Contributor
• Posts: 14927
• Country:
• 0999
##### Re: Calculating missing capacitor value
« Reply #1 on: October 25, 2017, 09:09:47 am »
The devices near the capacitors are most likely voltage regulator ICs. The U designator is normally used for ICs. Transistors would usually be labelled with a Q or Tr.

I suspect the blown capacitor is on the input of the voltage regulator IC. It will be a smoothing capacitor, so the actual value will not be critical. Try  47µF 16V.

#### wraper

• Supporter
• Posts: 12652
• Country:
##### Re: Calculating missing capacitor value
« Reply #2 on: October 25, 2017, 09:20:33 am »
Most likely it was the same as capacitor nearby. Exact value shouldn't be critical anyway. Nearby voltage regulator IC looks blown and probably destroyed with high voltage what it was powering as well. So don't be too optimistic to think that replacing capacitor and vreg IC will fix the issue.

#### wraper

• Supporter
• Posts: 12652
• Country:
##### Re: Calculating missing capacitor value
« Reply #3 on: October 25, 2017, 09:26:35 am »
Also considering that both capacitor and Vreg are blown, most likely power supply with opposite polarity was plugged in, or with much higher output voltage.

#### haggar

• Contributor
• Posts: 26
• Country:
##### Re: Calculating missing capacitor value
« Reply #4 on: October 25, 2017, 10:48:47 am »
Thanks all for the input.  I hadn't initially noticed that the regulator was blown too, I was too focussed on the capacitor, good spot!

Well, even if it never works again, it's a good learning opportunity.

So if it's a smoothing capacitor used for feeding into a voltage regulator, it doesn't necessarily matter what the capacitance is, just as long as the voltage is >=?

So thanks!

#### Zero999

• Super Contributor
• Posts: 14927
• Country:
• 0999
##### Re: Calculating missing capacitor value
« Reply #5 on: October 26, 2017, 07:52:18 am »
Does the blown IC have a part number written on it?

It's possible the destruction of the IC and capacitor are linked. If the voltage regulator stared outputting too higher voltage, it will destroy the capacitor, as well as other components. Another scenario is the capacitor failed short circuit, taking out the regulator, but they're usually protected against that.

The voltage rating of the decoupling capacitor is the most important thing. In fact it might even work without the capacitor at all. It's just good practise to include one.

#### haggar

• Contributor
• Posts: 26
• Country:
##### Re: Calculating missing capacitor value
« Reply #6 on: October 26, 2017, 08:21:26 am »
Unfortunately the writing on the IC is gone.

The remaining IC is a AMS1117 which is stamped as 1.8.  Which after a quick Google tells me it's a 1.8V low dropout voltage regulator.

Not sure what the value of the broken one should be, I guess I'll have to follow the traces and see what components it goes to, and then figure it out, somehow.

#### wraper

• Supporter
• Posts: 12652
• Country:
##### Re: Calculating missing capacitor value
« Reply #7 on: October 26, 2017, 01:00:35 pm »
As flash IC should be powered by 3.3V, blown regulator should be 3.3V version of 1117 as well.

#### haggar

• Contributor
• Posts: 26
• Country:
##### Re: Calculating missing capacitor value
« Reply #8 on: October 26, 2017, 05:45:07 pm »
Well some 3.3V 1117's have been ordered.  I shall report back in a few weeks once they arrive

Thanks for the info!

Smf