Some trouble with Mojotone FX loop

[Yoda]

Well I followed Mojotone instructions and swapped the 1.5K with the 47K resistors on the effects loop board and I am now getting 265v of B+ on the FX loop board itself and 35v of voltage drop across that 10k/2w resistor.

This fixed the problems I was having with voltages/power supply but with the loop engaged the volume level is much lower than when bypassed and I cannot hear any effects in the loop, only a thinner version of the dry signal. Not sure what’s going on now. I get 133v at Q1 and 0.63v at Q2 now.

[R.G.]

Well I followed Mojotone instructions and swapped the 1.5K with the 47K resistors on the effects loop board and I am now getting 265v of B+ on the FX loop board itself and 35v of voltage drop across that 10k/2w resistor.

This fixed the problems I was having with voltages/power supply but with the loop engaged the volume level is much lower than when bypassed and I cannot hear any effects in the loop, only a thinner version of the dry signal. Not sure what’s going on now. I get 133v at Q1 and 0.63v at Q2 now.

Good news! It was an incorrectly assembled board.

The 35V across the 10K shows 35V / 10,000 = 3.5ma of current flowing. 265V of B+ is in the expected range.

Which pins are you measuring for the transistor voltages? If they are the source pins, those look about right. the 133V is pretty close to half the 265 B+ as expected, and the 0.63 is OK for the source of Q2. The 133V, if it's at the source pin of Q1, shows 133V/47K = 2.8ma of current in Q1. If you measured the source pin of Q2, it's flowing 0.63/1500 = 420uA. That would leave 420uA * 150000 = 63V across the 150K resistor and 265 - 63 = 202(ish) at the drain of Q2.

Do those check out on the board?

[Guy77]

Well I followed Mojotone instructions and swapped the 1.5K with the 47K resistors on the effects loop board and I am now getting 265v of B+ on the FX loop board itself and 35v of voltage drop across that 10k/2w resistor.

This fixed the problems I was having with voltages/power supply but with the loop engaged the volume level is much lower than when bypassed and I cannot hear any effects in the loop, only a thinner version of the dry signal. Not sure what’s going on now. I get 133v at Q1 and 0.63v at Q2 now.

Have you turned up the Return level volume with the boards blue Trimmer pot? Also I like to run the B+ a little higher closer to 300v.

Cheers
Guy

[Ten Over]

In this setup, if the power supply is suddenly connected, the gate goes to the mid-power-supply voltage. Current flows in the channel and the source voltage rises until it gets to a point where it starts cutting back on the ability of the channel to conduct by backing down the Vgs. We know that if the source gets somewhere between -1 and -3V, depending on which LND150 we got, the channel current will drop to zero, so the source voltage will not rise to more than 1 to 3V higher than the gate, or it will turn off the current making it rise.
The chart only shows Vgs for -1.2V to +1.4V, it being a typical device. the source voltage will rise until it hits a gate voltage between -1.2V and +1.4V, because -1.2V causes 0 drain current. For this one device, it won't need to get higher than +1.4V because that gets you about 5ma of current. As an aside, it probably gets the device really near it's power maximum, too, that hyperbola in the chart.
Given that the gate is sitting at half the power supply, the source must rise to that same voltage, give or take a volt or two. So you're going to get about half the 200-ish to 350-ish power supply at the source, as long as the LND150 can conduct that much current and not burn up.

First of all, I would like to point out that you in no way disputed what I said.

In this setup, the least amount of current flows in the channel when the power supply first begins charging. More channel current flows as the power supply voltage rises. Once the power supply is fully charged, the maximum channel current flows. Ohm's law precludes the possibility that rising Source voltage will cut back on the ability of the channel to conduct.

[Ten Over]

The cutoff voltage is important in this case because it sets a hard limit on how low the source resistor's voltage can go.

I stared at this one for a couple of minutes. Taken literally, it makes no sense. If you mean it sets a limit on how low the the Gate-to-Source voltage can go, it doesn't matter in this case because we are operating on the other side of 0V.

[Yoda]

Well I followed Mojotone instructions and swapped the 1.5K with the 47K resistors on the effects loop board and I am now getting 265v of B+ on the FX loop board itself and 35v of voltage drop across that 10k/2w resistor.

This fixed the problems I was having with voltages/power supply but with the loop engaged the volume level is much lower than when bypassed and I cannot hear any effects in the loop, only a thinner version of the dry signal. Not sure what’s going on now. I get 133v at Q1 and 0.63v at Q2 now.

Have you turned up the Return level volume with the boards blue Trimmer pot? Also I like to run the B+ a little higher closer to 300v.

Cheers
Guy

I did but unfortunately the loop just passes the thin, low-volume audio when the level trimpot is around the center of its range but when I turn it all the way clockwise or counter-clockwise the audio just drops out all the way. Still can’t hear any effects in the loop.

[Ten Over]

I did but unfortunately the loop just passes the thin, low-volume audio when the level trimpot is around the center of its range but when I turn it all the way clockwise or counter-clockwise the audio just drops out all the way. Still can’t hear any effects in the loop.

The bypass switch works, correct? (Toggle up when standing upright viewing the top os the board)

Have you tried running a guitar cable from the Send jack to the Recv jack?

Here is a roadmap with numbered components so that you can easily tell people any location you might be talking about.

Mojotone FX Lp v 4 PCB.jpg

[Yoda]

Yes I was referring to the voltages on the source pins, man you guys are good, I had no idea that documentation even existed for the loop! Very nice.

Also I moved the B+ supply to the PI power supply node and it bumped up the B+ on the effects loop board to 311v. Much better but still no audio.

Yes the bypass which seems to work because when I bypass the loop the amp is significantly louder. When I bypass the loop on my 1987X there is hardly a change in volume unless the amp is being overdriven hard.

[R.G.]

Also I moved the B+ supply to the PI power supply node and it bumped up the B+ on the effects loop board to 311v. Much better but still no audio.

Looking at the schematic;
Q1 gets the input signal, amplifies it by a gain of just under 1, then divides it by a ratio of 470 / (10k + 470), or a factor of about 22. This goes out the "send" jack to the loop.
Q2 gets any signal from the receive jack, and then amplifies it by a factor of about ten (150K/1.5K), sends this out through C5 to be divided down by 0.8125, and the output level lets you have 0 to all of the 0.8125. Total out the output jack is 0 to 8.125 times what came in the receive jack.

With the output level pot full up, and everything working as it should, and a guitar cable between the send and receive jacks should see a Q1 "gain" of about 1/22, then a Q2 gain of 8.125, or an overall gain of 0.365, a level drop of 8.75dB. There is also a high frequency rolloff of at least 100pF and 150K in the drain of about 10kHz, immaterial to guitar signals.

It would be instructive to use an audio probe to listen to the signal at the input jack, and at the source lead of Q1. These two should be about equal. The signal at the send jack should be noticeably quieter.
With a guitar cord between the send and receive jacks, the signal at the send and receive jacks should be quiet, but equal. The signal at the top of R10 should be noticeably louder than the receive jack signal, and a little quieter at the top of the level pot.

[Snicksound]

Yes I was referring to the voltages on the source pins, man you guys are good, I had no idea that documentation even existed for the loop! Very nice.

Also I moved the B+ supply to the PI power supply node and it bumped up the B+ on the effects loop board to 311v. Much better but still no audio.

Yes the bypass which seems to work because when I bypass the loop the amp is significantly louder. When I bypass the loop on my 1987X there is hardly a change in volume unless the amp is being overdriven hard.

Just thinking here... Sounds like the incorrect resistors you had at first pulled way too much current through Q1... It may have been damaged.

LND150s are cheap, maybe try and replace them?

[R.G.]

I agree.
I was hoping that the listening test would detect that.

[Ten Over]

I get a gain of 49.15 to 61.4 for Q2 using .001 S to .002 S for the forward transconductance.

LND150 pg 4.png

I get 0.7692 for the voltage divider after C5.

[Ten Over]

If the guitar cable from the Send jack to the Recv jack results in performance similar to having an effects device in the loop, then I was thinking of plugging a guitar straight into the Recv jack and see what happens.

[R.G.]

I get a gain of 49.15 to 61.4 for Q2 using .001 S to .002 S for the forward transconductance.
LND150 pg 4.png

I get 0.7692 for the voltage divider after C5.

That's what I get for just doing stuff in my head while I'm on the road. I keep missing stuff.

Yeah. First I divided 150K by 15K to get gain of ten, not by 1500 to get raw gain of 100. Then I didn't account for the low transconductance or the loading of the output divider.

The output divider is two 1M resistors, ignoring loading on the wiper, which may or may not be a valid thing to do, but we can't know what will be plugged in there. It could be as benign as a 1M tube amp input. But even that would parallel another 1M resistor with the two already there if the output level pot is all the way up.

So - a 1M pot and a 1M resistor in parallel give 500K in series with R10, 150K. If the output level pot is all the way up and feeding a 1M tube amp input, this would give a 150K/330K divider when the output pot is full up. So the divider gets to being 333K/(150K+333K) = 0.689. This varies with the exactly load plugged into the jack, of course.

This load appears in parallel with the 150K drain resistor for Q2, so Q2 sees 150K paralleled by something between 150K + 500K (no load on the pot) and 150K + 333K (1M input on the wiper and pot full up). That could be as low as 150K paralleled with 483K, or 114K. The ideal, infinite transconductance, gain at Q2 would be limited to 114k/1.5k or 76. Limited transconductance and a flotilla of equations will lower that some. Yep, 55-is sounds reasonable.

[Guy77]

If the guitar cable from the Send jack to the Recv jack results in performance similar to having an effects device in the loop, then I was thinking of plugging a guitar straight into the Recv jack and see what happens.

I would request a replacement from the dealer.
Let us know how it goes.

Cheers
Guy