El Troubleshooting El Guapo

[taeagan]

I did an El Guapo build and it has a pretty nasty squeal when the circuit is engaged. It becomes less severe and more like a normal fuzz when I back the guitar volume WAY off but still not quite. I triple checked component values, polarity, solder bridges, etc... Any ideas?

When  I search the formm on "guapo" nothing comes up. Is this a new board? Anybody else built it with success?

Thanks!!!!!

[somnif]

I believe it only hit the store a month or two ago with a bunch of other 1590A's, so yeah it's a recent one.

Any chance you could provide a picture of the board? Perhaps some voltages?

[taeagan]

Had to use a couple of 1/2W resistors but there's enough room for them. Sockets for the transistors and clipping diodes. Thanks for your reply!

[somnif]

Ok, two quick questions:

What's the red wire coming off the LED+ attached to? (just curious)

and what voltage are you getting at the bias test point? Running Q2 too hot can cause noise in the circuit if I remember correctly. (or the voltages in general really, may help ID issues)

[taeagan]

Red wire is running to the A sized 3PDT #2 board.

So I think I'm seeing the problem. Q1 voltages are C=6V, B=0.65V, E=0. Q2 voltages are C=3V, B=0.53V, E=0. When I adjust the trimmer the collector voltage on Q2 goes between 7V and 3V. Can't drop it lower than 3 and the document is recommending to run it under 1V.

Voltage drop across R5 is from 8.3 to 7.6. Do I need to increase the size of R5?

[somnif]

Ok yeah that is most likely the source of the noise.

Just to double check, you do have a 10k trimmer in there right? You can check the resistance between the test point and the "bottom" of R5 to see what it looks like.

Past that, any chance something is shorting somewhere?

[taeagan]

So this wasn't the source of the noise. I socketed R5 and dropped a 2.4k resistor in there. Got the voltage on Q2 down to just under 1V. There's still a really awful noise coming out of the circuit.

I have triple checked for solder bridges, loose wires, component values and orientation, etc... Could Q2 be bad? Not sure what else to look for at this point.

My Harbinger 2 build came together easier than this!

[taeagan]

I hooked up the test rig and poked around. There's a high pitch squeal that seems to be present right at the input and is then getting amplified throughout the circuit.

To be clear - I have the input and output wires on the board connected to nothing. I have power going to the board. When I touch the probe from the test rig to the output from the board or the volume pot I can hear the noise really loud. It gets progressively quieter as I trace back through the circuit to Q2 and Q1. But, I can still hear it even when I'm touching the probe to the IN pad on the board.

Weird. Is one of the Caps or components in this circuit there to filter out gross frequencies like this? Like the 47pF? Any ideas where to look or what to try next?

[taeagan]

Swapped the 47pF out for a 560pF.... Same. Grrrrr

[somnif]

Ok, stepping back a bit, is it being powered with a wall adapter or battery? Does the squeal change with power source?

Are both the input and output jacks of your test rig properly grounded?

[taeagan]

So I made some progress. I swapped out the transistors with a different pair and it's more stable. I can play through it with the guitar volume at 10 and there's no squeal. I was able to put in a 220pF pulldown cap - 100pF or 47pF and I get noise when not playing.

However, it's still very glitchy - gated and staticky.  Is this what it's supposed to be?

Same result with battery or power supply, with test rig or wired up in the enclosure.