Trouble with Supro style veroboard build

[erock8303]

I've been fascinated with Supro amps since forever so I thought I would build a Supro style pedal from tagboardeffects. This isn't my first build, but I'm really struggling with it.

I have ZERO signal out of the board. I do have 18v on the 18v rail so I don't think my issue is power. Maybe it's my transistors...maybe.

Not sure if it's kosher to post the vero layout, but I wanted to get some other eyes on it and some of my substitutions. Please remove the link if this shouldn't be linked to... https://tagboardeffects.blogspot.com/2014/03/jhs-superbolt.html

Here's a link to a schematic I've used to troubleshoot too: http://www.coda-effects.com/2015/07/jhs-superbolt-clone-rog-supreaux.html

I couldn't find J201s or the LT1054 IC so I did some research and substituted a TC1044 for the LT1054 and I used 2N5457 in place of the J201s. Could my alternatives be giving me trouble?

I can't find any solder bridges or weird connections on the board, but still no guitar signal. I've checked connectivity throughout the board components with my multimeter and it all seems to check out correct.

Does anyone have any tips or suggestions on what my issue might be? Is something I'm missing in my troubleshooting steps?

Thanks everyone!

[PRR]

Welcome.

> Is something I'm missing in my troubleshooting steps?

Voltage readings. You can see some but not all bad connections and values. The voltmeter *really* helps narrow the possibilities. And IIRC, this build wants you to trim for specific voltages?

[erock8303]

Welcome.

> Is something I'm missing in my troubleshooting steps?

Voltage readings. You can see some but not all bad connections and values. The voltmeter *really* helps narrow the possibilities. And IIRC, this build wants you to trim for specific voltages?

The trim pots are to trim out for voltages. I'll work through the board with my multimeter over the next week. I'm not 100% sure the best way to go about testing through the board outside of the basics. All of my other builds have worked with little troubleshooting. Guess it's a right of passage. 😂

[erock8303]

Ok. I've ran through the board with an audio probe and resoldered a few components. When testing with a frequency generator I get sound all the way through the board.

The problem I'm having now is when I plug in my guitar to test it...nothing. Absolutely nothing.

All the components, jacks, pots, toggle switch work with the freq gen, but nothing works when I plug up the guitar to rock. I don't change any of the connections outside of removing the freq gen from the input.

Any ideas?

[PRR]

Bad guitar cord. Bad guitar. Input jack mis-wired some way that does not work for guitar plug but did work with temporary sig-gen connection. Knocked something loose. Got In and Out jacks confused.

Since it DID work, it has to be some face-palm thing.

[reddesert]

Welcome.

> Is something I'm missing in my troubleshooting steps?

Voltage readings. You can see some but not all bad connections and values. The voltmeter *really* helps narrow the possibilities. And IIRC, this build wants you to trim for specific voltages?

The trim pots are to trim out for voltages. I'll work through the board with my multimeter over the next week. I'm not 100% sure the best way to go about testing through the board outside of the basics. All of my other builds have worked with little troubleshooting. Guess it's a right of passage. 😂

Voltage readings for debugging is what PRR meant, not specific to trimpots. The classic way to troubleshoot (I think there's a FAQ post?) is to
- measure DC voltages at critical points in the circuit, typically at pins of op-amps and transistors, and compare them to known values (if there's a build doc), or post them so someone can diagnose whether the DC levels are all correct.
- use an audio probe to probe along the signal path through the circuit, testing where an audio signal is present and if it disappears, becomes altered/weak, at some point. This diagnoses the propagation of the AC signal through the circuit.

Since you got signal through with a frequency generator, but not guitar, next you could try the audio probe with the guitar hooked up. The simplest thing would be some bad connection as PRR suggested, but following this debugging procedure will help isolate the problem systematically, and help you the next time a project goes wrong.

[erock8303]

Yep. Definitely a facepalm moment. Apparently I don't know how to wire up input/output jacks correctly.  I fixed up those and actually got some guitar signal through the circuit.

Unfortunately the volume is minimal. I have to have the volume and gain maxed just to get some spittiness out of it. I'll go through the components again tomorrow evening. Would it be any specific pieces or a combo of components that would make the signal barely audible?

I'll post the voltages and whatnot tomorrow.

[erock8303]

Here we go. Voltages for the transistors, IC, and diodes. All measurements with a fresh 9v.

Q1 - 2N5457 (couldn't find any J201's)
C = 0.53
B = 0.00
E = 18.23

Q2 - 2N5457 (couldn't find any J201's)
C= 0.51
B= 0.00
E= 18.24

Q3 - 2N5457
C= 0.45
B= 0.00
E= 18.02

IC1 - TC1044
P1= 9.27
P2= 4.75
P3= 0.00
P4= 0.13
P5= 0.33
P6= 4.42
P7= 5.90
P8= 9.27

D1 - 5817
A= 9.27
K= 13.93

D2 - 5817
A= 13.93
K= 18.36


Hopefully you all can help me figure this out. I'm baffled. Do I have the transistors in backwards? I thought 2N5457's have the same pinout as J201's.

Thanks!

[reddesert]

JFET terminals are called drain, gate, and source, which roughly correspond to collector, base, emitter.  I'm not sure which way around you noted the voltages since you wrote CBE. The pinout of a 2N5457 is the same as a J201, so that part should be ok.

Your JFETs aren't biased correctly. In a circuit like this (and most ROG amp-like JFET circuits), you should adjust the trimpot that is between Vsupply and drain to put the drain DC voltage at about half of Vsupply. 

If you set the trimpot at 0 resistance, the drain will be at Vsupply (about +18 V in this circuit), the gate will be at about 0 v because it is connected to ground through the 1M resistor, and the source will be slightly above 0 V. Current is flowing from the Vsupply through the trimpot, the JFET, and the source resistor (the 1.5K resistor, for Q1).  As you turn up the trimpot, there is an increased voltage drop across the trimpot, which makes the Vdrain go down. At some point it should get to about +9V.

The idea behind these circuits is that each JFET stage amplifies its input audio AC signal, so you have a signal of AC amplitude a few volts oscillating about that +9V DC bias level at the drain.  If the drain is at +18V, you have absolutely no headroom for the signal to go positive; the amplifier can't amplify in that direction (and will probably do poorly in the other direction). That could explain why it sounds faint and bad.

I didn't check the charge pump and diode voltages super carefully but they look ok.

[erock8303]

JFET terminals are called drain, gate, and source, which roughly correspond to collector, base, emitter.  I'm not sure which way around you noted the voltages since you wrote CBE. The pinout of a 2N5457 is the same as a J201, so that part should be ok.

Your JFETs aren't biased correctly. In a circuit like this (and most ROG amp-like JFET circuits), you should adjust the trimpot that is between Vsupply and drain to put the drain DC voltage at about half of Vsupply. 

If you set the trimpot at 0 resistance, the drain will be at Vsupply (about +18 V in this circuit), the gate will be at about 0 v because it is connected to ground through the 1M resistor, and the source will be slightly above 0 V. Current is flowing from the Vsupply through the trimpot, the JFET, and the source resistor (the 1.5K resistor, for Q1).  As you turn up the trimpot, there is an increased voltage drop across the trimpot, which makes the Vdrain go down. At some point it should get to about +9V.

The idea behind these circuits is that each JFET stage amplifies its input audio AC signal, so you have a signal of AC amplitude a few volts oscillating about that +9V DC bias level at the drain.  If the drain is at +18V, you have absolutely no headroom for the signal to go positive; the amplifier can't amplify in that direction (and will probably do poorly in the other direction). That could explain why it sounds faint and bad.

I didn't check the charge pump and diode voltages super carefully but they look ok.

Yes...

C=G
B=S
E=D

So when I adjust the trimpots the voltage values range between 18.40 to 18.28. I never get anything south of 18v. Maybe my charge pump is the cause? I get 18.32v on the 18v rail. I used the correct 50k/100k trimpots.

Audio signal wise when the trimpots are at their minimum I get nothing. The only time I get audio out of the board is when I have the trimpots cranked all the way to the right.

Thanks for all the help and guidance! I feel like we are so close.

[reddesert]

The charge pump isn't the problem. It just needs to provide +18V at the supply rail, and it's doing that.

The JFET has to have gate more negative than source to conduct. If your readings are that the gate is 0.5 V and the source is 0 V, then the JFET won't conduct, so there will be no current flowing in the drain and source resistors and no voltage drop across them, keeping the drain at +18 V and source at 0 V.

I would check - pinout of JFET, that you have the resistors connecting drain, gate, and source connected correctly, and value of source resistor. The 1M resistor from JFET gate to ground should be pulling the gate to 0 V. Lastly, make sure your JFETs are good - if you can, substitute a known good one from another source and check the voltage readings.

[erock8303]

She lives!

I scoured my random bin of parts and found 1 J201. Swapped it in and measured. The pinout is different than the 2N5457s I have.

I moved around the pins in the others and I get a nice sweep of voltages and great output. [emoji2]

Now I can bias to taste. Thanks for all of your help!


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