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

Good idea..... I also plan to put a socket in there too

Hi Andy,

RX1 referred to a resistance setting of Resistance times 1 which means that the numbers on the dial of the analog meter will be actual. If the needle points to 12 the meter is reading 12 ohms. This setting will give you the best indication of shorts and continuity and also uses the highest testing current the meter is capable of.

NOTE: I concur with Michael that you should not apply power until the 741 is replaced. I would add that I would not connect the regulator to the rest of the circuit until I had verified that it was working and putting out the 12 volts that it is suppose to.

John

Hi Andy,

The failure of the BC309 indicates that it had a very high current flowing through it. This is not a good sign for the rest of your circuit. Our best hope is that the 33 uF electrolytic cap shorted out and took the brunt of the current. Plan to replace the 33 uF cap if you can not check it. I know that the current was Emitter Collector on the BC309 as the 100K resistor on the base would have limited the base current and not allowed the transistor to heat enough to explode. This says that there was a short further down the line. Either the C12 33 uF cap or one of the other components. Especially vulnerable would have been the LM339, 4013, SAD1024, and the 4558 as they are hooked with no current limitation across the power rails. Once the BC309 exploded the current path was removed and any component not already damaged would probably be OK. If I get the time later I will build the power supply on a bread board and try to simulate your failure to see how the regulator powers the rails under reverse voltage conditions and just how long it takes for the BC309 to fail and open the circuit.

John

I just noticed that C9 100 uF on the reverse side of the circuit board would also have seen a reverse polarity directly and could also have suffered the same trauma as the C12 33 uF cap.

John

Should you get the circuit up and running you should put a diode in the circuit as indicated to prevent reverse polarity problems in the future. If you use a Schottky diode it will only take 0.2 to 0.4 volts from the supply and will not affect operation.

Will, that's a good idea.

I've had a rifle through all the IC's i've got lying around and I have hit gold...  I've got a UA741CPUA741CP chip, I expect the CP doesn't matter?? The original is a UA741TC

Hi John, that's not good news if the SAD1024 could be fried... I hope it isn't.

I'll start in a methodical way, i'll sort all the voltage regulator section out first, test then if there's no life i'll move on to the other electrolytic caps and start replacing those that are exposed directly to ground as they likely would have got the full 18v on the ground rail. It won't hurt to replace a lot of them anyway as they're all at least 43 years old and are possibly suffering leakage anyway.

CP and TC are package codes.CP and TP are both plastic DIP 8 through hole, should fit...

Hi Andy,

I wanted to understand the voltage regulator a little better so I built one on the bread board. My original intention was to also hook it up backwards to see how it failed. I have decided not to waste the parts as the way your BC 309 failed is excellent evidence that reversed voltage got past the regulator to your circuit. Here is my set up:

Output voltage of the regulator circuit

Load current on the regulator circuit.

The voltage input to the regulator above is 18.3 Volts and the Output voltage is 13.35. It is under a load of 42 mA. I found that this was all the load that I could put on the regulator before it started to pull the voltage down. To me this indicates that your circuit that is being powered by the regulator can be expected to take something less than 40 mA. I mention this as you should be able to try to power the 13 Volt rail with a current limited power supply. As long as the current draw is below the 40 mA things may be OK. If you see any sign that the circuit tries to draw more than 40 mA you will have to continue to search for problems and damaged components.

John

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??