PT80 getting hot and starting to give off burning smell - plz help

[znowman]

hi,
im in the process of debugging a PT80 i just built for a friend (its the newest version of the project, from GGG with the charge pump)

this is what ive found so far:
im getting a normal guitar signal when both bypassed and engaged, but no delay.

the circuit started to get hot, and i was getting a burning smell  (at which point i turned the unit off).
also, the led looks too bright.

my immediate thought was that it must be a short, and after checking through everything, i did indeed find one (between 2 tracks), but after correcting it, the pedal is stil acting the same.

next i went through the resistor values, and they were all ok, except there is a 4.7k listed on the bill of materials, but its not on the circuit layout?, in fact, there are 30 resistors listed on the bill of materials, but i can only count 28 on the layout.

ive triple checked polarity on the electrolytics and diodes, orientation of the IC's and all offboard wiring is as it should be.

i cant take any voltages, as im afraid of doing some damage (if i havent already  ), so if anyone has any ideas, you would be helping me greatly

[O]

Is it boxed up? Pictures might help in case you've missed something; a second set of eyes is always good.

[znowman]

just taken some piccies, but i can find the data transfer cable     i'll upload em when/if it turns up...

thanks for looking btw

[znowman]

my brother has borrowed the cable, so i'll put some pics up tomorrow

[znowman]

ok, heres some pics, because the case is already painted/labelled, i had to do some cross wiring (to get the pots in the right order) i hope you can follow it

















im still wondering why there are more resistors on the bill of materials than are on the layout..... 

[stumper1]

What part of the circuit got hot?

Try pulling all IC's except the 1044 and checking for proper voltage at the 1044 and each regulator.

[znowman]

because there was no delay effect, i lifted the board out of the enclosure, and felt the solder (around the 1044 chip) getting hot, i turned it off pretty much straight after that, so im not sure if anything else was heating up.

i'll try what you said with the IC's (when i get chance later today) and check volts at the 1044 (i assume correct voltages will be on the data sheet?)

[Pushtone]

I think that extra resistor in the BOM your talking about is the current limiting resistor for the LED which off board if I remember right.


I just finished one of these and it's working fine. Let me know if theres any voltage readings I can compare for you.


Dave

[znowman]

with only the max1044 in its socket, the board is not getting hot and there is no smell !    i also mentioned in my first post that the led looked too bright, its still the same, so i think that was just my imagination.

voltages im reading on the 1044 are:
pin 1 - 9.16v
     2 - 8.14v
     3 - 7.79v
     4 - 4.30v
     5 - 3.30v
     6 - 7.55v
     7 - 6.82v
     8 - 9.16v

voltages at the 2n5089 are:
     c - 2.524v
     b - 3.065v
     e - 6.285v

I think that extra resistor in the BOM your talking about is the current limiting resistor for the LED which off board if I remember right.

when i made the old version of the PT80 (without the charge pump) that had a 4.7k at the led, i wonder why its not on the new layout

[Pushtone]

Voltages for a working GGG PT-80 delay.
I got the PCB from JD in March 2008 and it matches the current project files on his site.


MAX1044:
1. 9.5V
2. 4.6V
3. 0.01V
4. 0.05V
5. 0.33V
6. 4.7V
7. 4.2V
8. 9.5V


PT-2399
1. 5.0V
2. 2.5V
3. 0
4. 0
5. 2.7V
6. 2.5V
7. 0.69V
8. 0.75V
9. 2.5V
10. 2.5V
11. 2.5V
12. 2.5V
13. 2.5V
14. 2.5V
15. 2.5V
16. 2.5V


SA57
1. 0.09V
2. 1.7V
3. 1.7V
4. 0
5. 1.7V
6. 6.5V
7. 6.5V
8. 1.7V
9. 1.7V
10. 6.3V
11. 1.7V
12. 1.7V
13. 12V
14. 1.7V
15. 1.7V
16. 1.1V



TL072
1. 6V
2. 6V
3. 5.9V
4. 0
5. 3V
6. 6V
7. 6V
8. 12V




78L05
12V
0
16.4V



78L12
5V
0
16.4



I put the 1k LED resistor on the board but I didn't use it or the "L" pad.

[Pushtone]

I just noticed the part of your message that says "with all the chips out except the max1044".


I I popped out all the ICs (except the MAX1044, 5088, and 78L05/12) and metered the MAX1044 again.


MAX1044 with all other ICs out
1. 9.5V
2. 4.7V
3. 0
4. 0.1V
5. 0.3V
6. 4.7V
7. 4.3V
8. 9.5V



78L05
5V
0
17V



78L12
11.9V
0
17V




Looks like you've narrowed it down to an issue around the MAX1044 and/or power supply.
You need to meter and post the voltages on the regulator pins of both 78L05/12.

Do you know what was burning?
I'm thinking it was the 78L regulators because you are driving them with 8V from the MAX1044 and they are trying to regulate that down to 5V for the PT2399 chip.
That would make them run hot. check the data sheet for their max voltage input but regs are tough little guys as long as you pulled the plug at the first whiff of burning silicone.

[znowman]

i put all the chips back in and tried it again briefly, the 1044 itself was definately getting hot, not sure about the regulators, i pulled the power quicker (before i got a burning smell)

then i removed the chips again, except the 1044, and checked the regulators:

78L05
output -  5.046v
ground - 0
input -    7.7v

78L12
output -  6.274v
ground - 0
input -    7.7v

thanks for the voltages Pushtone, they will be a big help to me, and what you say about the regulators running hot makes a lot of sense.

ive been reading a lot about electronics lately, and im starting to understand things a bit more, i will have to read about voltage regulators, and how the 1044 works next i think.

[znowman]

ive just been reading about the 1044 chip here:
http://www.geofex.com/circuits/+9_to_33.htm

im wondering whether it could be one of the diodes thats not working correctly and allowing current to flow both ways (im thinking thats why only 7.7v is getting to the input of the 78L05) - this is a beginners 'guessed' theory

other than that, could one of the electrolytics be at fault ?

[stumper1]

You're definately on the right track.  Double and triple check the trace side of the PCB all around the 1044/caps/diodes for solder bridges.  Then to be safe, check again .

What's the voltage rating on the 2 10uf caps connected to the 1044?  They should be 25v or higher.

[znowman]

the 10uf (first back from the 78L05) is reading 7.7v, the other one is constantly alternating between -0.2v and +0.2v   

just had another quick check and cant find any solder bridges, ill go through the whole board again tomorrow, marking each joint as i go

[znowman]

damn this thing is getting me down now....

anyway, there are definately no solder bridges on the board,
ive changed the 3 diodes for new ones,
ive also changed the 2 10uf electrolytics,
and ive tried a different 1044 chip

but im still getting the same voltages on the regulators (the readings ive given are with the black lead on negative at the dc socket, and red lead probing)

i really dont know what to try next - maybe i should try new regulators ?

[Papa_lazerous]

Are you 100% sure that you have all the orientation correct on the caps, IC's, diodes and regs?  is it possible you have something round the wrong.  Triple check every one!!

Next I would be tempted to check every single resistor is in the correct place, its not viable to remove each one and check resistance so check with the coloured band if you are nto sure there are plenty calculators on line that you type in a value and it shows you the colours, check EVERY one and tick off as you do.  Just in case

Then remove ALL off board wiring and start again assuming nothing was correct so dont leave something as it "had" to be right.  Tick off each connection as you make it.

I suggest these things as you have a definate problem and you are sure you have no solder bridges, so logic suggests if you do all of the above you will be ok.

[znowman]

diodes and electrolytics are all correct way round, capacitors are non polarised, and the IC's are ok. 

the only thing im a little unsure of is the transistors, i have read the data sheets and i think they are ok, the PT80 photo on the GGG website also looks the same as mine.
the reason i have a little doubt is because i dont know the manufacturer of my transistors, and in the past, ive seen different pinouts for the same model (even in the same case style)

i know the resistors are all fine, ive been through those 3 or 4 times (although i still dont understand why there are more resistors on the bill of materials, than there are on the actual layout), but i will try the other things you have suggested, im still thinking the problem must be before the voltage regulators, as the correct voltage is not getting to them, but im probably wrong because i dont know much at all

thanks again for all your advice, im still determined to get it going...

[head_spaz]

Re-read the pdf on the neovibe. Might I suggest that you re-read it with the sole intent of COMPREHENDING what you're reading this time? Simple deduction indicates that the extra resistor values are provided as revisional options, for tayloring the circuit to your particular taste.

If you want to fix the volume drop and use true bypass, change resistor R9 to 3.6K and resistor R8 to 910 ohms.
This increases the gain of the initial preamp to make up for some of the losses.

I don't see a battery in your pics... are you using an external powersupply. Are you SURE that its output is DC and not AC?
Why are you using nonpolar capacitors in the charge pump circuit?
Something is not right with your readings on the MAX1044. Pin 3 should be zero volts... it's supposed to be at ground level.
Have you verified the polarity of your power input?


When in doubt.... RTFM!!!

[znowman]

im not building the neovibe...