SRV Special debug - EH?

[digideus]

Hi all.

I did my first build after breadboarding a few things with mixed results.  I built the SRV special tubescreamer on Vero over the last few days and now im stumped.  The layout is as follows...



The build seems to pass a signal through the circuit, but its distorted and hums a lot. and there is no definition.  the guitar makes a horrible farting sound that cuts out unless you play quite hard, so it seems that its cutting the signal somewhere.  Volume and gain seem to have an effect on the signal.  Cant tell if tone does from teh noise its making

At the moment, the board has all controls wired to it, a battery and 9v dc jack, stereo input jack and mono output jack.  I wired it correctly as far as I can tell and Ive checked continuity of the jumpers which is all ok.

Testing the voltages has revealed that the battery supplies 9.2 volts to the board

- Q1 is receiving 8.99v at the C, 1.97v at the B and 2.17v at the E
- Q2 is receiving 8.99v at the C, 8.91v at the B and 8.29 at the E
- All Caps seem to have higher voltages on the + terminal

IC1 is a RC455HC and is oriented to the south as in the layout  its wired into an IC holder.  Voltages as follows...

0v        -  8  4 -   0v
7.72v    - 7  3 -   2.24v
8.15v    - 6  2 -   2.24v
8.96v    - 5  1 -   2.12v

(facing south)

Confusing?

Ive seen a few people list voltages and mine seems way off to most.  Im pretty sure the trans are oriented correctly, although they aren't wired directly to the board.  they are sitting in an IC holder cut down to accept the pins from the transistors.  They seem a bit flimsy to be honest.

Any help would be appreicated.  Just an idea of what to try next.

[Govmnt_Lacky]

RC455HC 

Are you sure it is not an RC4558D?

[digideus]

FACEPALM!

I checked the IC.  its an RC455HP.  ARRGGGHHH! 

i need to lie down

[mthibeau]

I built that layout and was able to get it to work.

I would just check your part orientations and trace cuts, I think when I build that one I had C1 or C2 oriented wrong and it sounded similar to what you describe. But once I figured that out it was a pretty sweet overdrive. I think I used a TL072 instead of a 4558.

[digideus]

Well, that stumped me!

After removing the IC holders that the transistors were plugged into and soldering them directly to the Veroboard, pin 8 of the IC suddenly lit up to around 8 volts.

I tested again and got zero singal through the board!  Grrrrr

I tested the voltages again and I noticed that Pin 4 of the IC (top right on the layout) remained at 0 volts, so I traced the whole circuit back from that point to locate the break in the path and lo and behold... I found a dead 10k resistor at R15.

Strange!  after all that, it turns out to be a simple resistor!  

I shall pick up a replacement tomorrow and retest the board once thats in place.

[Govmnt_Lacky]

Well, that stumped me!

After removing the IC holders that the transistors were plugged into and soldering them directly to the Veroboard, pin 5 of the IC suddenly lit up to around 8 volts.

I tested again and got zero singal through the board!  Grrrrr

I tested the voltages again and I noticed that Pin 4 of the IC (top right on the layout) remained at 0 volts, so I traced the whole circuit back from that point to locate the break in the path and lo and behold... I found a dead 10k resistor at R15.

Strange!  after all that, it turns out to be a simple resistor! 

I shall pick up a replacement tomorrow and retest the board once thats in place.

Pin 4 should be 0 volts! It is tied to ground.

Did you retest with the proper dual op amp? Meaning NOT the RC455HP. Try a JRC4558D or even a TL072   

[digideus]

Not yet, although there are others who have built this with the same Op-amp.

I think the fact that i found a component that refuses to pass a signal is a sure sign that its naff and needs changing.  Everything up to that point has voltage, and following the schematic, it needs to pass that signal to make any noise!

Ill replace that one first and see what happens. 

[digideus]

OK scratch that... replaced the 10k resistor with a new one and the same thing is happening.  There is no voltage going through R14.  It comes in via the E gate of Q2  and it seems to be going into the resistor, but doesnt come out the other side!  That seems to be where the output is, which is why im guessing that I have no sound! 

Orientation of the caps seems to be accurate.  short of ordering new componants and rebuilding the entire circuit, im not sure what to try next.  ideas?

Oh anf Op-Amp is a 4558P, not a HP as I said before

[slacker]

The top of R14 is connected to ground so there shouldn't be any sound there. The signal on the emitter goes through c6 to the output. If you have signal on the emitter then the problem is somewhere around c6 or r15 or between the output of the board and the jack.

[Mustachio]

Ive built this from the same layout . Had problems with mine as well but, it was my own mistake.

Double check all your trace cuts and go over between the tracks with a razor and then a soldering iron to make sure no bridges. that was my problem.

I would also take the layout and flip it horizontally in photoshop or some other image editor. That will help you check the cuts a lil better.

My problem was that i put the cuts around the clipping diodes on the wrong side.

I would stare at this board for hours over a few weeks. Would check everything and finally one day it all clicked and i spotted my mistake! I have no idea how I missed it, But now a days I check everything flipped .

Double check them cuts! and the orientation of the trannies and electro caps. Oh also on this layout Note the orientation of the opamp Its upside down compared to most layouts. good luck.

Also this circuit sounds awesome!

[Eb7+9]

Also this circuit sounds awesome!

[twabelljr]

[quote0v        -  8  4 -   0v
7.72v    - 7  3 -   2.24v
8.15v    - 6  2 -   2.24v
8.96v    - 5  1 -   2.12v][/quote]

Should the pins 5-8 be reversed in these measurements?
Like this:
0v      - 5     4 -  0v
7.72v  - 6    3 -   2.24v
8.15v  - 7    2 -   2.24v
8.96v  - 8    1 -   2.12v

That would make more sense. Power and ground are correct to I.C. that way, Bias is out of range.

[roseblood11]

This is similar but a lot more flexible:

[digideus]

Ok, i decided to rebuild the circuit on breadboard to see exactly whats going on.

This time, I have voltage through the circuit, but no sound from the guitar UNTIL I UNPLUG THE BATTERY, and which point I can hear the guitar signal as the capacators power down.

This tells me 2 things.

1 - I have a guitar signal working through the circuit
2 - its still not working correctly!

Given that an un-powered circuit seems to work.  That would seem to suggest that there is something stopping the live signal getting through even though there is voltage to wach point of the circuit.

Although im enjoying this as a learning experience, it is frustrating.  can anyone suggest an approach to try and work out whats going on?

[digideus]

I HAVE SOUND!!!  

Now I just need to work out why the gain control seems to be permanently on.

Also, ive been comparing the layout to the schematic...



Forgive me if im wrong here... but R6 is supposed to go to ground, yet it clearly doesnt!  The schematic says that the signel should go through Q1 and out through R6 to the GND via the "E" gate.  From there it is connected  pin 3 of the opamp and through the amp stage to exit via pin 2 and 1 (to route to the gain knob via the 2 diodes).  From there it will pass back into the opamp to go through the output stage to be passed to output via the volume control.

In actual fact, it looks like the reverse is happening on the layout.  Q1 to R6 via the "E" gate is passed through C4 to pin 5 of the op-amp (2IN+).  it also bridges to pin 4 of the op-amp (Vcc-) which I understand is ground.  

So, where exactly does it go through the input stage of the op-amp and out via the gain control?  It doesnt seem to follow the schematic that closely

Also, according to the schematic (NOT THE LAYOUT) R5 connects to the "B" Gate of Q1 and *SHOULD* go to the bias via the volume control, yet the vero layout points to the same path past R16 and connecting to pin 4 and 5 of the opamp.

its all quite confusing!  some explanation would be helpfel of where I have got it wrong (i reckon its my fault 85% of the time!)

Right back to the circuit to find why its playing silly games on me!

[roseblood11]

The layout is 100% correct. You just can't read schematics... You mixed up the audio signal path with the ground connections.
I've marked the ground in green here, maybe it's a bit easier to understand:


btw: I've never seen a tubescreamer layout with so many jumpers...

regards, Immo

[digideus]

That is actually REALLY helpful - thanks

[Seven64]

i built that layout.  didnt work, found a solder bridge, and then it sounded great!