DSOTM Phaser Rate

[Matt]

After building a couple (working) DSOTM Triple combos for others I’m finally building one to keep one for myself.

Everything seems to be working fine other than the phaser. The effect is passing signal but it isn’t phasing. When looking at the LED it just remains on and doesn’t pulse. All controls seem to effect the signal appropriately with the exception of the Rate knob. Upon reviewing the schematic and poking around with a multimeter I believe the problem exists around R20 which seems to have continuity with everything it should with the exception of C10.

Since this issue is seems to exist not in the guitar’s signal path I can’t do my usual “stab it with the audio probe until I find the problem” method that has gotten me through the last year or so of the hobby. I’ve reflowed solder and double checked values, looking for a little guidance to get me looking for the right thing.

Some pictures below of the build and the hi-lighted part of the schematic that I believe is the problem location.

As always any and all help is greatly appreciated!

[Barry]

If it were me I would make sure you have firm contact in those MPSA18 sockets by adding a few tiny bends to each of the leads and reinsert them into the sockets for firm contact.

If that doesn’t help I would try exchanging them for new ones if I had them available as well.

The LED must be pulsing and Q1 & Q2 are right there.

Also be careful bending the pots back to reflow too many times or you will need to reflow those as well. They can also break from stress eventually.

 

Feel free to jump in if you have any thoughts.

[Matt]

Swapped for new transistors and same issue.

if it’s helpful at all the LED immediately lights up (without strobing) as soon as Q2 is inserted, regardless of the status of Q1.

Removed sockets and soldered transistors directly to the board with the same results.

[Billy]

The problem seems to be your LFO check the voltage oscillates on Q1 collector

[Matt]

Voltage not oscillating on any transistor legs.

Tested another build of the same pedal I was doing simultaneously to the same issue, i’m wagering it’s a component/orientation issue. Will check everything again and circle back.

[Matt]

Checked components on both boards and can’t find anything wrong, only consistent thing is the Tayda transistors.
Planning to try more from a different supplier and see if that is where my issue lies.

[Matt]

Update here: Replaced the transistors with no change in performance.
Went back through checking continuity and found a cut trace between +C8 and -C9 but repairing that didn’t solve the problem.
I checked components with a multi meter and visually against a picture of a working build of the DSOTM that I did last year. The only difference is I used 25v electrolytics for C11 and C12 as opposed to 50v in this build.
Another thing I’ve noticed, I don’t think the Intensity knob is affecting the led at all (which makes sense if I can’t get it to oscillate).
Typically I am able to audio probe my way through troubleshooting a build but this one has got me stumped as there is no instrument signal in this side of the circuit. I think part of the problem is I’m not totally sure what I am supposed to be looking for from an electrical perspective (aside from voltage oscillation makes the LED strobe).
Is there some stupidly basic, troubleshooting step I’ve missed by over-analyzing the problem?

As always, any help is greatly appreciated.

[Billy]

We need a clear well lit image of the whole component side of the pcb so that we can clearly see all values and orientation

It’s fairly difficult to see anything in your pics and very easy to miss something even after checking several times  I’ve done it many times

As you’ll know C8, 9 and 10 charge and discharge in combination with the rate pot to adjust oscillation make sure the solder joints are good

[Barry]

I agree there are several dull flat solder joints in exactly that location. This could also explain not getting continuity. I would suggest a reflow and have some extra solder on hand for when it is needed but be very careful with repeatedly pulling back the pot to get at them. You will likely want to reflow the pot lugs when you are done as well.

And yes better pics so we can see the top of the board well.

[Matt]

Had some time to dig further into this build today.

Re-flowed all joints on the phaser and replaced all capacitors, both transistors, and a couple resistors. After reflowing I put a little rubbing alcohol on a toothbrush and brushed around to clean up some of the flux and any possible bits of stuff that could bridge pads.

I have left the pots bent so they aren’t getting moved back and forth

Below are the voltages for the transistors:

Q1

• .4v
• .3v
• 0v

Q2

• -6.9v
• -4.4v
• -3.78v

Here are some better pictures of the build, my phone’s camera isn’t great but hopefully these can get you a better idea.

[Billy]

I’m taking it voltages are collector base emitter top to bottom are Q1 voltages 0.3v etc and Q2 aren’t negative voltages are they

I wouldn’t change any component until I know they’re a problem it only increases the potential for lifting pads etc

I realise it’s frustrating but I wouldn’t do it

[Barry]

I highly recommend always posting voltage readings and clear photos (front and back) before replacing components, as excessive rework can risk damaging PCB pads and solder joints.

From your voltage report, it’s clear that Q1 is not receiving voltage. However, I don’t believe the transistor itself is faulty, since Q2 has proper readings.

• After reviewing your updated photo, I noticed that the resistor pad for R20 does not appear to be properly filled, which suggests a dry solder joint. Additionally, the circled PCB pad at R20 is directly connected to C11 and the collector of Q1 (square pad), which could be causing the issue.
• Also equally important is the C11 + and – pads (right beside the circled pad) look as though they may be bridged. Re-flow and wipe excess solder away to remove any potential bridge.

 

Update upon further examination: If that C11 + pad (circled) is burnt out (looks like it is) you will need to create a bridge instead. That will be a more delicate operation but it is very doable. If it were me I would remove R20 and replace with the same value but do not solder, or cut the lead with the burnt pad. Then bend the excess lead over and cut precisely to connect with the C11 + pad (square pad). Then solder it. You will still have to make sure that there is no bridge between the + and – pads of C11.

If that is the case use this image to create a bridge from the new R20 resistor to the C11+ pad (Blue) and then make sure there is no bridge where the Green Arrow points to.

Also, if you’re using only the LED as verification that your build is working be sure to set the Rate and Intensity controls to the midpoint or slightly lower for easier visual verification.

 

[Author]

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