SOLVED: HM-2 clone, high pitched shriek when Lo is dimed

[matopotato]

I just finished a build of HM-2 clone based on the first/upper layout from here: http://tagboardeffects.blogspot.com/2014/05/boss-hm-2-heavy-metal.html.
A first check and all seems to be working except when I turn the Lo (B10k) pot all the way CW, there is a very loud shriek kind of noise.
I asked in the blog, but my question is either lost or being checked, so I took my chances to ask here too.
I planned for it to be a b-day gift on Tuesday for a friend, so that is the reason for my impatience.

[ElectricDruid]

Ok, so I'm assuming that this is the schematic:

https://www.hobby-hour.com/electronics/s/hm2-heavy-metal.php

A shriek when the "Lo" is dimed suggests a problem in the 3B op-amp, which acts as a gyrator producing the low frequency response. If the result is a high-pitch squeak, perhaps some component in that section is wrong? It's easy to put 10K where 100K should be, or similar - 10n where 1n should be, etc. After all. the "Low" control should be giving you a *low* frequency control, not a *high* frequency squeak! 

You'd have to find the relevant components on the stripboard layout. WhyTagboardEffect don't post schematics, I don't understand. It's not like it's illegal, and pretty much everything they post as a stripboard layout is from a schematic which is available somewhere else on the web. So why not post a link to the original schematic?


<edit>link fixed

[matopotato]

Thanks!
I did not find any working schematics from that site.
I edited away the trailing "c" from your link, and it goes to what seems to be the "original" traced HM-2. With 10? Transistors. Mine had 3, so not the same schematics.
I found this thread in here though
https://www.diystompboxes.com/smfforum/index.php?topic=123329.0 and it seems more like it.
But I agree that the Lo was not very "bassy" in its nature. So I might go over that part to spot any mistakes I could gave done.
The layout I used was verified by several so it ought to be ok.
I used other transistors and they all had different pinouts from the layout. And the internet was not consistent. But I have a meter for it and I  used pinouts from vendor's site and it makes sound so I am guessing the Qs are correct.
Again thanks for pointing me in a direction.

[bluebunny]

WhyTagboardEffect don't post schematics, I don't understand. It's not like it's illegal, and pretty much everything they post as a stripboard layout is from a schematic which is available somewhere else on the web. So why not post a link to the original schematic?

Agree wholeheartedly, Tom.  The site is a great resource, but this^^ a thousand times over!

[matopotato]

WhyTagboardEffect don't post schematics, I don't understand. It's not like it's illegal, and pretty much everything they post as a stripboard layout is from a schematic which is available somewhere else on the web. So why not post a link to the original schematic?

Agree wholeheartedly, Tom.  The site is a great resource, but this^^ a thousand times over!

From my limited experience, I'd say there is often a link to schematics in the blog/chat below. But I agree that there is a sense between the lines to keep low key and not rouse any feathers. Sort of.
Like it a lot and so many nice builds are available plus good discussions about alternatives etc. And the way the moderator listens and adjusts the layout.
Will make a walk-through of the build in my case. Need to flip a few caps to "the other side" to make everything fit.
That seems to be a fine art in itself. Mostly the circuits are discussed there and here, but more often than not the boxing is a bit of a challenge each time with new issues popping up.

[Elektrojänis]

At a quick glance it seems like the tagboard effects version has excluded the FET-swiching and the output buffer.

Shriek from the tone controls could also have something to do with the feedback components on op-amp 3a. If I understood the circuit right the tone controls at minimum ground the respective part of the frequency spectrum (at least partially) at the input of 3a when set to minimum. When set to maximum it grounds the feedback on the op-amp 3a for that same frequency range. If you ground too much of the feedback, the gain will go up and it may lead to oscillation.

If the shrieking only happends with the bass pot, I'd inspect stuff around op-amp 3b first too, but if you don't find anything there, check out the parts around 3a too.

[duck_arse]

is there any reason we can't see what you've built? maybe post some photos, eh. also, what part numbers did you use for your transistors/fet?

[matopotato]

is there any reason we can't see what you've built? maybe post some photos, eh. also, what part numbers did you use for your transistors/fet?

So sorry, no reason at all. Just felt it would not be very helpful. An I naïevely thought that "shriek at Lo pot dimes" would be met with some "you probably put C12 polarity the wrong way. Happens all the time." Like a newbie thing.

[matopotato]

I made it for my Swedish-Turkish friend. He said he couldn't get any metal tones, and I heard HM-2 would be better ata that than the MT-2 for instance. It's gis B'day tomorrow, but with the shrieky thing I am galling behind schedule a bit.
The text is direct translates except input output which are more of jokes.

I used LM358s for ICs after recommendation on the blogsite.
Ge diodes are D9K as prescribed 1N34A were out for me.
The others 1N4148 and 1N5817 as prescribed.
Q1 is the FET K30A-Y (got a -GR just in case)
Q2 is B549B instead (have 2N5088 in case)
and Q3 is BC559C (with 2N5087 in reserve)
My pre-studies suggest that these options should wotk ok at least. The ICs might even be better than M5216L as the original indicated.

[matopotato]

At a quick glance it seems like the tagboard effects version has excluded the FET-swiching and the output buffer.

Not sure how to tell this, but you think this might be the issue? I tried following the schematics I found in another thread in this forum, and that part has me lost. I find no 10k after the IN on the tagboard. And the 9 volts to 4,5 volts has me lost on the tagboard as well...

Shriek from the tone controls could also have something to do with the feedback components on op-amp 3a. If I understood the circuit right the tone controls at minimum ground the respective part of the frequency spectrum (at least partially) at the input of 3a when set to minimum. When set to maximum it grounds the feedback on the op-amp 3a for that same frequency range. If you ground too much of the feedback, the gain will go up and it may lead to oscillation.

OK, I think I at least understand the conclusion. But is there any remedial action I can do in the circuit to counter this?

If the shrieking only happends with the bass pot, I'd inspect stuff around op-amp 3b first too, but if you don't find anything there, check out the parts around 3a too.

I compared the schematics mentioned with the tagboard layout and apart from the ICs numbering and selection of op-amp in respective IC, they seem to match.
I will go through the wiring I made and check for continuity etc.
Or should I probe around to try to detect where the shriek happens?

Thanks

[duck_arse]

well, here's three bits of bad news. looking at your photo, I can't seee too much detail, but - K30A fet is gate centre pin, layout is for end gate. the two other transistors in the layout are pinned for end base - your BCxxx parts are centre pin base. unless you've done some leg twistings, all three are in wrong.

Like a newbie thing.

no sir. the newbie thing is not banging up the photos and the layout and the circuit dia every time you start a thread, the other stuff is just the usual day to day. don't sweat it.

[matopotato]

The other side. I try to make a habit of checking each leg to not conduct to the neighbour copperlines, but only to its own. Tedious but found a few overflows that way.

[matopotato]

well, here's three bits of bad news. looking at your photo, I can't seee too much detail, but - K30A fet is gate centre pin, layout is for end gate. the two other transistors in the layout are pinned for end base - your BCxxx parts are centre pin base. unless you've done some leg twistings, all three are in wrong.

Like a newbie thing.

no sir. the newbie thing is not banging up the photos and the layout and the circuit dia every time you start a thread, the other stuff is just the usual day to day. don't sweat it.

Not easy to see, but I did twist and isolated all three. Gate/Base are the colored ones

[matopotato]

I tried probing, but felt the shriek was everywhere sort of.
But observed that Dist. needed to be quite high for the Lo pot to shriek. Seemed Hi pot was not without contribution either but it only happens when almost diming the Lo and high "enough" on the Dist.

[idy]

Those kind of high gain circuits on strip board with lots of wires are notoriously sensitive, but usually it is the treble pot that sets them shrieking. Have you tried moving the pots and wires, around, separating them from the board and each other?
Even the footswitch can be a site for mischief. Better to test before you wire those up!

[idy]

[idy]

Note that the two opamps are "swapped" from the schematic and layout. In the Schematic the low gyrator uses pins 5,6,7, in the layout it's 1,2,3.

The components you should check if the lows gyrator is the problem:
1.5uf electro attached to low 2
330 ohm R between 1.5uf cap and IC pin 1 and 2
68nf cap between 330 ohm and pin 3
100k between pin 3 and VB.

And hey, you haven't posted voltages yet.
did you get the "cut" between that cap and the IC? It is hiding under that 100k R next to it. There are two cuts under that R but that's the treble control side...probably not your problem.

[matopotato]

Thanks, yes that's the one I compare tagboard with

[matopotato]

Seems the posted schematics and tagboard order match with 1 further up on the board an 3 connected to Level 1. I noticed that 1,2,3 vs 5,6,7 are swapped in some place(s).
So I will number as they are on the board, so possibly not the same numbering as the schematic.
Here goes
IC1
4.42 4.42 4.39 0.0
4.11 4.12 4.12 8.9

IC2
4.42 4.42 4.42 0.0
4.39 4.42 4.42 8.9

IC3
4.43 4.43 4.22 0.0
4.38 4.40 4.43 8.9

[matopotato]

Those kind of high gain circuits on strip board with lots of wires are notoriously sensitive, but usually it is the treble pot that sets them shrieking. Have you tried moving the pots and wires, around, separating them from the board and each other?
Even the footswitch can be a site for mischief. Better to test before you wire those up!

Thanks, yes enough cable to keep them away from each other.
And yes I agree, it sparkles when you touch wires and move about so quite sensitive.