Merlin's Glassblower / total newbie

[NorthCoast]

The -25 and -40 are references to the amount of gain the transistor has. You can find it in the datasheet. I don't know what the other letters mean.

But, like djaaz said, it doesn't matter much here.

One thing to check, is the voltage rating on C5 and C6.

Also, you could pull both trannies and the IC and check voltages on the sockets. Post them here.

Also, this:

I would say in the meanwhile, check your wiring. Check the obvious thing that should not be connected to each other (no continuity between 9v & ground for instance). Make a visual inspection of the different components.

And: I looked at your pic and C5 and C6 look like they are physically over the top of some resistors, you might want to check that they are not touching those components. For example, is C5 touching R8 and R9? That's the first thing I'd check.

[jimilee]

Is there any solder on lug 3 of your potentiometer?


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[NorthCoast]

And: I looked at your pic and C5 and C6 look like they are physically over the top of some resistors, you might want to check that they are not touching those components. For example, is C5 touching R8 and R9? That's the first thing I'd check.

Just looked at the other pic and C5 doesn't look like it is touching R8 and R9 at all. Hmm.

[dicey_reilly]

Thank you all so much for your patience and help here. 

@jimilee: do you mean where the wire from lug 3 goes to on the circuit board?  from the top side, there isn't much solder there, alright, but from underneath it looks good.  i checked for continuity between the top side of those pads and the lugs on the potentiometer itself, and all have continuity. 

@NorthCoast: 
- Thanks re the transistor naming sleuthing!
- re voltage rating on C5 and C6, apologies again here for the newbie question, but do you mean de-solder them from the board first and then check the voltage (as described here, for instance: https://www.electrical4u.com/how-to-test-capacitors/)?  Or visually verify what's written on the capacitor? (in which case, it's 35V)
- i had a look at any potential physical contact between C5/C6 and neighbouring resistors, but there's clear daylight between everything.

- i've measured those voltages (to be clear, all measured without any of the IC, Q1 or Q2 in the circuit, rather than removing a single component at a time):

Q1:
Left: 2.9V
Middle: 0.91V
Right: 8.9V

Q2:
Left: 2.9V
Middle: 0.86V
Right: 0V

IC: (anti-clockwise, starting from bottom right, as per this graphic: https://startingelectronics.org/beginners/components/IC/8-pin-IC-outline.png)
1: 0.82V
2: 0.8V
3: 4.47V
4: 0V
5: 0.78V
6: 0.01V
7: 0.01V
8: 8.9V


I've also realised initially mixed up the placement of Q1 and Q2.  I thought they were identical components, but obviously one is a PNP and the other a NPN.  If I'm very fortunate, maybe nothing else will have blown, and getting replacement transistors in the *correct* place might solve the issue.  I'll order a spare TL072 while I'm at it, in case I've fried the IC too.

I can't thank you all enough for your help with this.  Good to be learning on this learning curve!

[djaaz]

Q1:
Left: 2.9V
Middle: 0.91V
Right: 8.9V

All your transistor legs are to the right of something. Look at the schematic, look at the transistor, google your transistor datasheet or at least pinout


And then post something like:
E: 2.9V
B: 0.86V
C: 0V

Before powering up, in case of doubts, better remove all active components that you socketed and make voltage measurement.
Once you are confident that you have the correct voltage per the schematic (9v on the collector of Q1  and 0V on the collector of Q2 for instance), socket and power up.

[NorthCoast]

Or visually verify what's written on the capacitor? (in which case, it's 35V)

Yes, that is what I was wondering. 35v is plenty.

I've also realised initially mixed up the placement of Q1 and Q2.  I thought they were identical components, but obviously one is a PNP and the other a NPN.  If I'm very fortunate, maybe nothing else will have blown, and getting replacement transistors in the *correct* place might solve the issue.  I'll order a spare TL072 while I'm at it, in case I've fried the IC too.

Yeah, that makes a big difference. Those transistors are apples and oranges.

[dicey_reilly]

Thanks, djaaz, that would be much clearer alright - apologies.

So, I think it should be:

Q1:
E: 2.9V
B: 0.91V
C: 8.9V

Q2:
E: 2.9V
B: 0.86V
C: 0V


And thanks NorthCoast, I'll get replacements for those transistors and post an update here one way or the other.  Thank you for all the help!

[dicey_reilly]

An update here: I had blown the BC337 transistor, so with that replaced, I am at least getting audio through the circuit now.  Woohoo!

However rather than boosting the signal, it's giving me a signal that's quieter than the original. 

The pot does work, and turning clockwise increases the signal from inaudible to a max of just very quiet. 

I've tried replacing the BC327 transistor and the IC chip too, but no improvement, still the same behaviour. 

Any ideas on what I've done wrong?

[djaaz]

You really need to post that in the right section, now, to get the attention it deserves.
As of now, you need to look at the schematic and understand the different sections of it even if you don't need to understand everything about it.
Retake the voltage and post them.

Also, you might need to build a few very simple tool to help you troubleshoot further.

[dicey_reilly]

o.k., much appreciated djazz, i'll do that.  once again, thanks to yourself and all here for the help getting me this far.

[djaaz]

You're very much welcome. Someone far more knowledged than i am will probably pickup the ball in the THcustom support section.