HELP!!!.... Blown Electric Mistress pedal

Ok chaps, a small update.

i have now snipped the old 741 IC out of the circuit, and removed the legs form the holes. 1 of the solder pads fell off from the substrate when I was heating the solder up but fortunately it's not connected to anything, it's just there to solder 1 of the legs to for stability. All the others were OK, albeit very fragile. the new socket is now soldered in place and the UA741CP  chip is in place.

I've started desoldering some of the electrolytics and have so far replaced C9, C10, C11 and C12. When i tested the old capacitors after removal they tested fine albeit out of range. C9 should be 100uf but tested at 116uf, C10 should be 10uf but measured 14uf, C11 should be 10uf but tested as 14uf and C12 should be 33uf but measured 47uf.

i'm waiting on a BC309 as I haven't got one in my stock of transistors so can't fire up the pedal just yet to see if these replacements have sorted the issue.

I'm hoping that the news that these electrolytics weren't blown could be a good sign that the 309  took the brunt of the charge........ any thoughts??

You can measure the resistance between a V+ point and ground with your meter in Resistor mode while waiting for the transistor. With unpowered circuit. There should be no shortcut.

Leave the probes on the circuit until you have a stable reading. It will be unstable until capacitors are charged.

If you want to use diode mode of your meter: https://electronics.stackexchange.com/questions/366143/using-diode-mode-on-my-multimeter-to-debug-circuits-a-good-idea

I might add to Jan's comment that you can take a reading with the meter and then switch the probes and take the reading again. The pins of C-12 33uF would be a good point to get V+ and Ground. Since we are all curious post the readings that you get. The current that passed through BC309 and destroyed it had to have a path to ground. We are trying to determine if that path is still there in the form of a shorted IC.

John

It looks like I need a known good board to use diode mode, which I haven't got.... I will however take some measurements and post the results.

John, what readings would you like.... Any specific readings??

Hi Andy,

I just wanted the resistance readings across C-12, the 33 uF Cap. Since this is across the ground to power a resistance reading could tell us if you have other things that are shorted out or not. As Jan requested allow the meter to stay connected so the capacitor can charge and settle to a stable reading. It would also be advisable to switch the meter probes around and take the reading again (in the opposite polarity) once again waiting for the ohms reading to stabilize. If your circuit is working properly I would expect this reading to be above 1000 ohms.

John

Hi john, with my DMM set to 20k, its reading approx. 1,709. If I swap the leads around it shows as out of range... to get a reading with the leads swapped i have to adjust the DMM to 200k range, then it reads 21.5

Looks good.

Jan is right. While this doesn't tell us if any of the chips are damaged OPEN at least they do not look shorted. Putting power (of the proper polarity) on the unit and seeing if it works is the next step. If it doesn't work you will have to continue replacing chips until it does.

John

thats next on the agenda.... im now waiting for delivery of the transistor so I can replace that, then test..... i feel tempted to remove the SAD1024 chip when I test..... if any of the other chips blow I can live with that... but if that 1024 blows, its goodnight Vienna

thats next on the agenda.... im now waiting for delivery of the transistor so I can replace that, then test..... i feel tempted to remove the SAD1024 chip when I test..... if any of the other chips blow I can live with that... but if that 1024 blows, its goodnight Vienna

Hi Andy,

Unless it is damaged already from the reverse polarity it is very unlikely that any damage to the other chips could affect the SAD Chip. You are more likely to damage it by removing it than leaving it in the circuit.

John

its in a socket.... so not an issue to remove  it