My Microdrive sounds wierd

[SeneX225]

Hi! I've built this Tim Escobedo's Microdrive, using this layout:



I didn't have a 3904 at hand (the only one I found was promptly dead) so I replaced it with BC548C (and also found out that transistor shown in the layout is placed incorrectly, because according to this schematic the emitter should go in the ground).


However, the finished Microdrive sounds... off. Here's a sample with a sinuous curve of picking strength:

I expected an overdrive but this is more like a very aggressive bitcrusher. Is it supposed to be like this or have I messed something up? Maybe it's because the transistor isn't a suitable replacement for 3904?

[duck_arse]

show us what you have built. from your looking at the datasheets, you will KNOW that the BCxxx transistors are opposite pinout to the 2Nxxxx - which means that layout dia is correct for a BCxxx, but wrong for a 2Nxxxx.


photos. always photos. always.

[SeneX225]

photos. always photos. always.

Sorry! Here goes. Please don't laugh at my soldering.







As you can see, it was thrown together very haphazardly while I was figuring out this transistor buisness.

from your looking at the datasheets, you will KNOW that the BCxxx transistors are opposite pinout to the 2Nxxxx

I never noticed that, thank you!

[GibsonGM]

I forget where I got it, but at one time there was a graphic going around that had a little pic of transistors and their pinouts, and under each BJT is a list of which ones go to the pinout.  I added opamps, jfets, mosfets and things like 386's to it, as well as stompbox switch wiring and the like.   It has been extremely helpful over the years!  It hangs in a place of honor in front of my workbench.


Do the LEDs blink when you play?

[SeneX225]

Do the LEDs blink when you play?

Yes, they do. For all effects and purposes it works, I'm simply not sure if it does so as it supposed to.

I forget where I got it, but at one time there was a graphic going around that had a little pic of transistors and their pinouts, and under each BJT is a list of which ones go to the pinout.  I added opamps, jfets, mosfets and things like 386's to it, as well as stompbox switch wiring and the like.   It has been extremely helpful over the years!  It hangs in a place of honor in front of my workbench.

Something like this?


It actually is a very good idea, thank you! I think I'm going to make one for myself, probably based on the one above.

Sidenote: Until a few minutes ago I had no idea SOT103's were a thing! They look something straight out of 80's sci-fi movie.

[antonis]

Those 400V metallized polypropylene film capacitors of 10mm spacing are the perfect choice for a 9V circuit on a 200cm² board..!!

[SeneX225]

Those 400V metallized polypropylene film capacitors of 10mm spacing are the perfect choice for a 9V circuit on a 200cm² board..!!

They were all I had at hand and it's a long way to any local radioshack. Could they be too big for this schematic and affect the sound somehow?

[antonis]

Could they be too big for this schematic and affect the sound somehow?

Not at all..

P.S.
I'd place a 1 to 10nF across Volume 3 and 1 lugs, with not high expectations..

[SeneX225]

I'd place a 1 to 10nF across Volume 3 and 1 lugs, with not high expectations..

It does make a sound more "stable", meaning there's less of a jump between signal being overdriven and not. However, the effect sounds still very crunchy. Oh well, I guess it's just how it is.

One thing I noticed, though: Gain pot is nearly pointless, because it changes actual gain only at the very end of rotation. So, can I somehow replace this pot with a regular resistor (a pair of them?) so that the effect would always stay at max gain, therefore turning it into one knob signal muncher?

[antonis]

If you like to let it in max gain, just omit gain pot..
(or just replace it with a signle 100k resistor..)

[SeneX225]

If you like to let it in max gain, just omit gain pot..

This I can comprehend, just plug Gain 2 directly to Input hot and Gain 1 into ground. It's the replacing part I'm having trouble, so please bear with me.

As far a I understand (I measured resistances while pot was still in place, although for reasons unknown a 100k pot read 1.8k), there should be zero resistance between Gain 2 and Input hot and a resistor (100k or 1.8k?) between Gain 2 and ground. Is that correct?

[antonis]

As far a I understand (I measured resistances while pot was still in place, although for reasons unknown a 100k pot read 1.8k), there should be zero resistance between Gain 2 and Input hot and a resistor (100k or 1.8k?) between Gain 2 and ground. Is that correct?

First of all, take pot of the circuit, or better, just disconnect pot's wiper from input cap..
(although, for resistance measurement it shouldn't be essential..)

Measure resistance between pot's outer lugs (3 & 1)..


P.S.
What's your pot marking..??

[SeneX225]

As far a I understand (I measured resistances while pot was still in place, although for reasons unknown a 100k pot read 1.8k), there should be zero resistance between Gain 2 and Input hot and a resistor (100k or 1.8k?) between Gain 2 and ground. Is that correct?

First of all, take pot of the circuit, or better, just disconnect pot's wiper from input cap..
(although, for resistance measurement it shouldn't be essential..)

Measure resistance between pot's outer lugs (3 & 1)..


P.S.
What's your pot marking..??

It's an A100k pot. Neither schematic nor layout specified which pot to use, so I used logs.

It reads 104k between lugs 1&3 and same between lugs 1&2 and 3&2 when turned accordingly. It works as it should, which is the main reason why I am confused about that 1.8k reading. As it turns out, I took the reading wrong somehow. Now after reinstallation that 1.8k is nowhere to be found and the pot reads the same as it does off-board, as it should. My level of incompetence is over 9000!

[antonis]

1.8k measurement could be due to 100nF charging, via transistor B-E diode..

You can try with a 100k linear pot (if availiable) to see if gain setting is made more "uniform"..
(I strongly think so..)

[SeneX225]

Also, now the effect is really, really quiet. I've done nothing else but soldered the pot out and soldered it right back in. What kind of evil sorcery is this?

[antonis]

What kind of evil sorcery is this?

Could that evil witchery be captured in some pics..??

[SeneX225]

Doubt it (it's magics we're talking about, after all), yet here they are anyway:






Wires were replaced for convenience sake and jacks ground were switched with power ground for the same reason. LED's do light up, and a very faint crinkling comes out, but volume pot (on the right on the left that is, thanks camera!) is completely unresponsive.

[antonis]

I think there is a short between BJT Collector and Base..

[SeneX225]

I think there is a short between BJT Collector and Base..

Cleaned all the contacts of extraneous tin, but it didn't help, unfortunately. However, volume pot now behaves appropriately.

Are you completely sure it isn't witchcraft?

[antonis]

Are you completely sure it isn't witchcraft?

Can't say for sure..

Now, where are we..??
What is working, what isn't and what is partially OK..??