AB763 Lite Scratch Build not working :-(

[ToneJunkie]

Hey guys its been years since I have built an amp... My new scratch build isn't working.  There is hum when I turn it on and the usual pops when I measure the voltage along the dropping string but no sound when I try and put a signal into it.  I don't really have a process of troubleshooting this thing and wondering if you might help me. I would be grateful for any starting point suggestions?

[ToneJunkie]

Here is the schematic I used...

[ToneJunkie]

Here is a picture of the build...

[eleventeen]

Your build looks pretty carefully done. It's hard to instruct you as to how to proceed because we don't know your skill level and we don't want to encourage newbs to poke around inside amps bearing high voltages that can be lethal. We don't know if you know what a cathode is. Not to be insulting.


Most "complete failure" episodes are something very simple; something so simple that there could be 100 different "very simple" suggestions we could make.


If you google "amp troubleshooting" you'll find numerous websites that have most of this kind of procedural stuff laid out.


http://www.geofex.com/ampdbug/ampdebug.htm <<---here's one.


Nevertheless, here are a few thoughts:

Tube heaters lit up? (duh)

Measure your power supply nodes. Please be careful and keep one hand in your pocket as you measure. If there is no lower-lower-lower succession of voltages, something isn't conducting and thus isn't working.


The cathode of every preamp tube in a Fender sits atop a paired R-C which has this function called "bypass". Without explaining what that is, both halves of the first tube in your circuit should have a small voltage on pins 3 & 8. 1.2 volts, 1.8 volts. Something like that. If those cathodes are at ZERO, that particular tube is NOT WORKING, not conducting, signal will not pass it. If those cathodes have those small voltages, they ARE working--which does not mean the signal is reaching the tube, however. In your amp, V1 cathodes should be at those low voltages and not zero. V2 cathodes, tied together, are at about 90 volts. +/- 5 volts here is nothing to worry about)


Likewise, every plate resistor comes from a power supply "node". (R4, R11, R20, R21) VERY CAREFULLY, you can measure across any of those plate resistors and find 50-80-100 volts difference. They are all 100K except one is 82K. Any place where you find "no" or "almost no" difference across those plate R's, the tube under it is not conducting and thus is not working. 


Can you get speaker buzz/noise touching the grid(s) of V2 with your meter probe? (pins 2 and 7)


I would suggest going to Doug's schematic library and getting a schemo of a Deluxe  AA or AB763. Compare that side by side to your schemo and try to see how they are similar. That schemo will have a few voltage readings on it and you should be able to get a few more voltage-level clues. http://el34world.com/charts/Schematics/files/fender/Fender_Deluxe-AA763-schematic.pdf

[Willabe]

Doug's listing amp works great for tracing the signal through an amp;

http://el34world.com/Hoffman/tools.htm

[sluckey]

In addition to the other suggestions, measure the voltages for all pins of the tubes. Post them. We need to know all voltages even though some will be zero. Zero is an important number too.

I see you used my schematic but decided to use a very different layout. Double check (triple check) your layout as compared to the schematic.

[ToneJunkie]

OK thanks for all the replies guys!  Here are my voltages and Eleventeen I get a huge explosion of sound when I tough the grid of V2 on pin 2 and I do get a regular hum when I touch pin 7. I'm going to trace the circuit around the phase inverter to see if I can find anything...  Oh also on the fender schematic I do not see a voltage on the grids of the phase inverter and I obviously have quite a bit there... Is that right?


     V1
1          243
2          2.7mV
3          2.15
6          244
7          .1 mV
8          2.6
 
V2
1          266
2          39
3          73
6          252
7          44.4
8          73
 
V3
3          419
4          421
5          -43.7
8          30.5mV  1 Ohm Bias resistor
 
V4
3          419
4          420
5          -43.2
8          33.7mV
 

[sluckey]

Oh also on the fender schematic I do not see a voltage on the grids of the phase inverter and I obviously have quite a bit there... Is that right?

That's right.

All your voltages look typical of a working amp. No red flags.

[ToneJunkie]

So tried all known working tubes and a little retracing of the circuit and not finding anything... Is the only option now to make a signal tracer like willabe suggests?

[shooter]

the only option

you could beg borrow or steal a scope.  If you have a good meter, a tone generator, and are good with your meter, you can try tracing with the meter on AC, but if you've never done it, that's gonna be a waste of time.  building the tone tracer will pay off by finding your problem. 

[SILVERGUN]

I'm curious about the connection I highlighted in yellow in the picture below

It appears to be an incorrect path just before the input of the PI   

[SILVERGUN]

Just to clarify...
It appears as though you made an incorrect connection at this location on the schematic.
If I am correct then your signal only sees 100 ohms resistance to ground, and that's where it's going.

Remove that connection and as long as nothing else is mis-wired you will have sound.

[sluckey]

In addition to Silvergun's issue, where is the signal connection between the preamp output cap and the PI input cap?

[Toxophilite]

I was curious about the faint reddish line(wire or drawing on the circuit board? it's hard to tell)) that seems to connect the preamp output cap to the ground lug for the two cathode bypasscaps/resistors. It goes around the top of the board (I can't %$^&% highlight it in iphoto)

[SILVERGUN]

Update?