Schaller Tremolo not working

[kabi]

Hi all,

I just built the Schaller Tremolo using the below layout from Topopiccione:

http://topopiccione.atspace.com/PJ20SchallerTremolo.html

I have signal when the effect is both bypassed and engaged but i have no tremolo effect at all. The 'Frequency' pot causes a slight volume drop but that's pretty much it, 'Depth' pot does nothing. I already triple-checked all connections and fixed a couple of spots that looked suspicious but it didn't fix it either. Audio probing showed that signal is flowing normally from input to output, BUT it goes silent from the base of Q1 onwards (which i assume is the part that all the tremolo 'work' happens)

I also played around with the orientation and type of the transistors (i used BC547s, BC549s and BC550s) and also reversed the connections of the 'Depth' pot, just in case, still no dice. By the way i have connected the pots as following (looking at the layout file, from top to bottom): 1, 2, 3.

At this point i can't for the life of me figure out what's wrong, so i need some extra help

Any suggestions would be extremely appreciated. Thanks in advance

http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/Guts_01.jpg.html
http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/Guts_02.jpg.html

[GibsonGM]

Hmmm.   It looks to me like everything behind Q1 is an oscillator that turns Q1 into a switch, thereby giving you your trem, so you wouldn't expect signal there.

You say you get audio right thru Q2?  If so that means that section is OK.    Trouble is over to the left.   Pinouts, dead transistors, solder bridges....

Take a good close look at Q3, Q4.   Too bad you can't scope it to see if that section shows oscillation...post voltages here, ok?  (EBC)
You are looking to see a varying signal on the base of Q1.     Have you messed with P1, P2?
   
It's probably not a major thing, you just need to find the mistake/bad part.

[kabi]

Sounds promising, thanks 

I do get audio through Q2, yes. It goes silent after Q1 and for the rest of the circuit. I'm pretty sure i have replaced all transistors at least once. I will report transistor voltages when i get home, hopefully later today

The only thing i messed with P1 and P2 is that i reversed P1's connections on the PCB the other way around, just in case. Haven't messed with P2 at all. Can you verify which way the pots should be connected? Maybe i have them connected the other way round? Also it kind of surprised me that at least one lug of each pot is not connected to anything, is that normal?

[GibsonGM]

Focus in that area, Kabi )       The pots would be connected via 1 leg and wiper.  They must be installed so they are variable.   Using one leg only, and the wiper (which can rotate around, giving varying levels of resistance - think of a volume control) lets you vary how the circuit operates.a  So yes, one leg omitted is ok (altho many connect it to the wiper for reliability, don't worry about that right now tho).

https://en.wikipedia.org/wiki/Potentiometer

Something is up with the oscillator part of your circuit....Q3, Q4 and the associated capacitors/resistors.  They make a control voltage go 'up and down', if you will, and 'turn the volume up and down' in your signal path, in turn.    Rough way to explain it, but there ya go.

There is something wrong in there.   Posting voltages will help us see if we can spot exactly where.    Bad solder joints or solder bridges are very common causes of these problems, as is overheating transistors when they're installed, or getting them backwards. 

[kabi]

ok transistor voltages time:

Q1:
E: 0.00
B: 0.00
C: 0.00

Q2:
E: 2.87
B: 2.35
C: 1.59

Q3:
E: 9.43
B: 1.13-1.31
C: 0.59-0.70

Q4:
E: 1.83-7.36
B: 0.59-0.70
C: 0.00

Q1-Q3 are BC550C, Q4 is BC549B, so i have them placed exactly the same way as on the layout

[GibsonGM]

Oh boy...those voltages are WAY STRANGE, my friend!!!!   I totally think you  may have gotten the pinouts wrong on those transistors.

I would expect the collectors of Q4 to read what you have as the emitters (they look reversed to me).  Emitters are the leg with the arrow...   Both collectors should be on the high side of things, near 9V. 

The base of Q1, I'd expect to be close to 4.5V after you adjust trimmer P1....see the 9V connection there, to its left thru R7?   

1) Double-check that your trimmers are in right and are not shorted, and then please go back and make SURE those Q's are oriented the right way - if they're backwards, this will never work...and make sure all your connections to 9V are there.  I know it seems like basic hum-drum sorta 'make work' stuff, but something isn't right there - and the only way to find it is to look, look, look...and look again.

2)  You have a DMM?   Use it to follow the 9V connections to the nearest trannies...you should have a 'voltage trail' from 9V to the collectors of Q2, 3 and 4 and the base of Q1.  Set both trimmers in the middle of their travel before you go measuring.   You may be able to find a sweet spot with those trimmers that gets things going, if you know what I mean....they have to be dialed in properly. 

[kabi]

No trimmers on the circuit, so i guess you mean the pots (which act as voltage dividers), correct? i placed both in the middle position before measuring

Ok i placed the transistors correctly following the schematic, these are the voltages i'm getting now:

Q1:
E: 0.00
B: 0.31 (goes from 0.00 to 0.44v depending on P1's position)
C: 0.00

Q2:
E: 0.39
B: 0.94
C: 7.79

Q3:
E: 0.83
B: 1.50
C: 9.43

Q4:
E: 0.00
B: 0.82
C: 0.03

[kabi]

Actually it just came to life 

I swapped some transistors and at some point it started working so i guess i got a couple of faulties maybe? In addition it appears that i had a certain resistor shorting somewhere, causing the effect speed to drop but i fixed that as well, so i finally got myself a working stompbox. Sweet little tremolo, can't wait to hear it on rehearsal levels tomorrow. The only thing i noticed is a slight volume drop when the effect is on, but i assumed there is a way to minimize this somehow, try different transistors perhaps?

Thanks for all the help GibsonGM, chances are i would had quit and let it aside at some point, if it wasn't for your suggestions 

[GibsonGM]

Glad to hear it, Kabi! I'm also glad I was able to help a little, but YOU fixed it!  These can be tricky, because unlike something like a distortion, there are parts 'doing things' that don't have an apparent audio purpose.   You may have had a bad transistor, or placed one in backwards, overloaded it (?), hard to say.   The trims should be used, I am assuming, to make the bases be about .5v or so above the emitters, I am assuming (don't know this circuit, but given what you listed for voltages, that's what I'd shoot for).  

I bet there are a few things we could do to Q2 to increase your output a little, but for now, let's enjoy the effect and see what it sounds like!!  )   Q2 is just a gain stage, and as such, you can tweak resistor values to make it amplify more.    Maybe ask that question in a new post later, after you'd made certain she's running ok....
rock on!  

[GibsonGM]

Just for grins, I looked at your gain section there, simmed it up...you SHOULD have something more than unity when you are using the effect...but I can't tell how much Q1 is loading the signal (making it quieter, in essence).   

If you need it to be louder, and it seems to work well otherwise, you can play with the value of R4.  Maybe make it 1K, see if it's louder (lower value R = more gain, to a point).  I wouldn't go much below 680R, really, and you shouldn't need to.   The more you tweak that, the more you'll be affecting bias and things could begin to sound cruddy.   But it's something you could certainly mess with!

[MrStab]

Actually it just came to life  

congrats, Kabi!

Actually it just came to life  

I swapped some transistors and at some point it started working

i remember killing both the transistors in the  EA Trem i made back when i was a bit less experienced, and once when building my current Heartthrob (modded Schaller trem). by the time i'd got round to the second one, i just had an instinctive feeling it was the NPNs. it was as though this circuit is more prone to breaking them, or the effect is so dependent on them that the slightest damage can prevent them doing this particular job. who knows!

maybe your build has better values than mine and this wasn't an issue for you, but with my Heatthrob (and i think to some extent with the EA) there are some pot extremes which, when engaged, don't allow the LFO to cut the wave at all. i've minimised this hugely via. a resistor(s) - could get rid of it completely with another 1k or so maybe - but it's always something to consider when debugging. just for future reference in case it helps anyone.

enjoy!

[duck_arse]

kabi,

can you post your voltages, now that it's working?

[kabi]

It's boxed and on my pedalboard right now, hopefully i will make some time to measure voltages later tonight

Didn't had the chance to test it on rehearsal yet though