HM-2 Clone - AION Graviton - Gain issue

[octave]

Hi everyone,

I recently finished to assemble a BOSS HM-2 Clone with AION Graviton pcb. It is working but the distortion/gain is very mild compared to the original, even we all potentiometers to the max, it doesn't feel hi-gain at all, it only starts distorting when playing chords heavily. I'm wondering if someone would have a hint on where to start troubleshooting it. All voltages are pretty much the same as the build guide recommends (slightly lower (0.5v)). Here are the schematics, bom, build guide and photo of my build.

Build guide (includes schematics): https://www.dropbox.com/s/21sum89nkusjejp/graviton_documentation.pdf?dl=0
BOM (exact parts I used): https://docs.google.com/spreadsheets/d/1_uNdOw8qGX1zT6J1YDXLcT1U0zCnxSlMV5cHOIpgHPc/edit?usp=sharing

Here is an analysis of the original pedal: https://atomiumamps.tumblr.com/post/139197356031/boss-hm-2-analysis

Thank you!

Octave

[Slowpoke101]

From your picture I can see that your build quality is excellent.

I suggest that you make certain that the potentiometers are not shorting to the PCB and that there are no solder splashes shorting anything.
Next thing to do is to get out your audio probe (or build one first ) and start to check where your signal is getting lost. Set all pots to maximum and start at the Gate of Q1 (note how loud the signal is ) and then check the signal at the Collector of Q2 - Signal should be much louder. Then check that the signal at the Emitter of Q3 is there.
See what you find and let us know.
One thing to consider is that Q1 is a JFET. Sometimes these items have different pinouts to what the documentation shows. Your problem may just be that the JFET needs to be rotated.

[octave]

Thank you for your reply!

I will try with a probe! Q1 (2N5457) pinout seems to be in the right place (DSG). Right now the volume gets pretty loud at max but it stays mostly clean, which makes it a clean boost HM-2, no distorted/clipped sound, just more level/gain.

Should I measure in AC voltage too? So it's more accurate than my ear?

Thanks again, I will keep you updated,

Octave

[octave]

Allright so I had parts to build a second one (photo attached), I built it with exact same parts and sure enough it sounds exactly the same as the first one So I'm thinking that it should be a component issue. But even after reviewing the parts with Kevin from AION and layout we couldn't find anything wrong. So as suggested by Kevin here are the dc voltages for all transistors and ics in case someone has an idea!

Q1 (2N5457)
D 8.57v
S 5v
G 3.9v

Q2 (2N5088)
E ~0v
B 0.6v
C 4v

Q3 (2N3906)
E 7.9v
B 7.22v
C 3.4v

Q4 (2N5088)
E 3.2v
B 3.7v
C 8.57v

IC1
3.4v
3.4v
3.4v
0v
4.2v
4.3v
4.3v
8.57v

IC2/IC3
4.3v
4.3v
4.2v
0v
4.3v
4.3v
4.3v
8.57v

Both pedals have pretty much exacts same voltage readings. Q2 seems weird right?

[willienillie]

Probably unrelated, but that 4558 at the bottom isn't fully seated in the socket.  Can't tell on the others.

I'm not smart enough to calculate what the Q2 voltages should be, but they look reasonable to me.  You subbed a 2N3906 for a 2N5087 in Q3, which would be a lower gain transistor, possibly much lower, but that part of the circuit is strange to me, and I don't know how much difference it could make.

[octave]

Probably unrelated, but that 4558 at the bottom isn't fully seated in the socket.  Can't tell on the others.

You are right! But yeah that didn't change anything...

Kevin at AION told me that the 2N3906 is closer in specs to the original one (2SA970-GR) used in the HM-2 than a 2N5087. Do you think that could be the source of the problem?

[willienillie]

Regarding Q3, scratch what I said earlier.

From the build doc:

Q32N5087BJT transistor, PNP, TO-92The HM-2 uses 2SA970-GR (a Japanese equivalent).

Datasheet says -GR hFE 200~400:

http://njsemi.com/datasheets/2SA970.pdf

The (measured) 2N3906s I have are all in the 300s, so yours is probably fine.

I was relying on faulty memory, I thought I had measured them down below 100, but I must be thinking of a different type.

[Brain.Tentacle]

Hey everyone,

I hate to bump this but I am having almost the exact same issue so I was hoping it's okay to contribute.

I just built the Graviton and while I wouldn't say I have low-gain, the pedal sounds a little weak so I decided to probe the IC's and transistors as the guide recommends and I get this:

IC1
3.45
3.45
3.44
0
4.05
4.35
4.35
8.73

IC2/IC3
4.34
4.35
3.93
0
4.33
4.35
4.35
8.73

Q1
D: 8.73
S: 4.62
G: 2.15

Q2
E: 0.01
B: 0.58
C: 3.88

Q3
E: 8.01
B: 7.11
C: 3.45

Q4
E: 3.38
B: 3.38
C: 8.73

Source Voltage: 8.99
VA: 8.73
VB: 4.32

Q1 in particular seems way out-of-spec. S calls for 7.49V and G for 4.36V.

I was just wondering if octave or anybody else had found the problem. Thanks!

[Slowpoke101]

Welcome to the forum.
Don't worry about bumping this old thread. It reminded me that I didn't get back to it when it was current.

Not to worry. Now for your problem.

Check Q4. There may be a short between the Base and Emitter. Having exactly the same voltage reading on both those legs is very odd.

The voltage that you are reading on the Gate is correct if your meter is loading the applied voltage. The Gate is supplied via a 1M resistor and if your meter has a 1M input impedance you will have formed a voltage divider which will read exactly half the supplied voltage, which it does. Also the Source leg is at 4.6 volts (which is correct by the way ) so the JFET is working correctly.

[Brain.Tentacle]

Hey, thank you and thanks for the reply.

Okay I checked Q4 and there is no shorts I can see, no continuity between the base and emitter. I checked voltage again and got 3.38V at the emitter and 2.77V at the base. I think I must've accidentally probed the wrong one 

I think I follow what you're saying about the gate. So maybe I don't have any issues?

It's not so much that I thought the pedal *sounded* wrong (It's my first HM-2 so that could say a lot), it's more that I wanted to confirm everything I was seeing is correct, which the manual lists some different voltages for these points.