Surf PI Burnout help!

[Metalmatt]

With the current lockdown i've been looking at an old project that i started a couple of years.
I bought a Surf PI complete board from an Ebay member in turkey already populated. The gentlemen in question had very good feedback on his work and the unit worked no problem a few years back, (although not tuned).
Now ive boxed the unit made a nice coil housing etc but i wanted to use lithium batteries but in my error connected the polarity the wrong way around. A little smoke issued forth!
Ive attached images to outline the prob.
Would anyone be able to point me in the right direction? i see the main culprit as component 78L05, just because of the charring on the joints.
Very limited experience and any information would be great.

Thanks in advance

[Xtrem]

If you are lucky , then only this diode is burning

[Metalmatt]

Ah ok, thats good information. Thanks

[waltr]

If you are lucky , then only this diode is burning

[ATTACH]49840[/ATTACH]

I agree since that diode is across the Battery and is there to protect the circuit from reversed battery connection.
When battery is connected in reverse this diode Conducts and is a SHORT across the battery.
Measure it with the diode scale on your DMM.

[6666]

That parallel diode is ment to blow an inline fuse when the battery is reversed, if you are not going to use a fuse then the diode should be in series and conducting when the battery is connected correctly, and non conducting if the battery is reversed. You loose .7 of a volt but your circuit is protected.

[Metalmatt]

Thanks for everyones input. Im going to get another diode. What voltage diode do i need? I see on ebay ranging from 50v 1000v.
Thanks all

[Qiaozhi]

Thanks for everyones input. Im going to get another diode. What voltage diode do i need? I see on ebay ranging from 50v 1000v.
Thanks all

It's highly likely that the diode is not the only problem. Once the diode overheated and went open-circuit, there was nothing to stop damage to other parts of the circuit. I would suggest removing the diode and then checking if the circuit is working. If not (which I suspect will be the case) you will still have some faultfinding to do.

[Metalmatt]

Thanks Qiaozhi. I may just buy the diode. If it needs more fault finding i suspect its out of my range of skills. Which volatage do i need.
Many thanks

[Qiaozhi]

Thanks Qiaozhi. I may just buy the diode. If it needs more fault finding i suspect its out of my range of skills. Which volatage do i need.
Many thanks

I would still remove the diode before you order a new one, then power-on the circuit and see if it works. If it doesn't work, you've saved time and money ordering unnecessary components. You don't need the diode to test it.

[MartyJ1963]

Hiya MetalMatt,,,,,,Good to see you opened a thread for the problem you wrote me about,,,,you'll get loads of help on here and don't worry that you think you lack the skills to repair it, when I first started building detectors I was always nervous of removing faulty parts in order to replace them cos' I was fearful of making it worse!,,Lol. Geotech and its members have taught and helped me build detectors accurately now and I can't thank them enough so you'll get all the help on here you need. You'll probably get addicted to building your own eventually like the rest of us!!,,Lol,,,Hope you find the problem soon and I'm sure you will,,,Regards, Marty.

[Koala]

the advise to remove and test is sound advise

standard 1N4001 and cost about 2p

[6666]

the advise to remove and test is sound advise

standard 1N4001 and cost about 2p

Yep and buy a fuse

[Metalmatt]

Offending diode

Upon reconnecting power i now have a steady low hum through the phones with no difference in tone....
Ive ordered the following on peoples advise, testing any further is going to be a bit of a challenge.... Any further ideas?

Diode 1N4001
78L05
ICL76605

[Qiaozhi]

[ATTACH]49855[/ATTACH][ATTACH]49856[/ATTACH]

Offending diode

Upon reconnecting power i now have a steady low hum through the phones with no difference in tone....
Ive ordered the following on peoples advise, testing any further is going to be a bit of a challenge.... Any further ideas?

Diode 1N4001
78L05
ICL76605

The most likely components to have failed (apart from the diode) are the 78L05 and ICL555.

[Koala]

how many volts on the 78L05 out ?