another peppermill thread! (problem)

[alderbody]

i have a problem with my recently built peppermill.

It works fine and plays great, but after 5 minutes things change...

There is a significant loss in volume and crackles and pops are all i get from the output!

I measured the pin voltages on both transistors and the J201 had the correct values,
but the BS170 had much higher values. (VD=8.20V VG=4.91V VS=3.68V) (Vcc=8.97V)
The circuit is 100% stock, and built on perfboard. (checked and verified)

The strange thing is that these values are indicated REGARDLESS the mosfet,
which means that even if i replace the BS170 with another, the voltages remain the same!

I suspect that there must be something wrong with the other components in the "area". (4K7, 1M, LED...)

If i unplug the pedal for some minutes (no power), it gets back to playable condition,
and the pin voltages for the mosfet get back to "expected" values.
But after 5 minutes i get the same effect...

Has anyone had a similar problem?

Any assumptions on what to check, before i start unsoldering the whole thing?

Btw, regarding the recommended value for the J201's Vs being around 115mV,
i tried all my J201 stock and found only one that gave 156mV...
All others were in the 250-350mV range.

Am i supposed to be close with that value or not? 
What's the "tolerance" for the ideal value?

[billings]

Are the crackles and pops coming from the playing?  Does it sound like it's just fuzzing out at some point?

[George Giblet]

First the J201's I think your values are OK.  JFET voltage will vary.   The runoffgoove value actually looks a bit low to me.  That doesn't imply there is something wrong with either your or their circuit or parts, it's just a tolerance issue.  The average over all JFETs is probably more like 0.2V so the 0.115 is a low number and your 0.35 is high.

As far as the MOSFET goes there' s something weird going on:
1) The Vd voltage is not consistent with the Vs voltage.

    Vd implies the MOSFET current Id = (8.97-8.2)/4.7k  = 0.16mA
    Vs implies the MOSFET current Id = 3.68/1k            =  3.68mA

Given that there is only high value resistors around here there's no other current paths to make the currents different - these values just don't add up!

The extra current in the source resistor means there must be a short on the source track.  With such a simple circuit you should be able to measure 3.68V somewhere else in the circuit, but none of the other voltages seem to match this.

So where does that leave us?   perhaps wrong resistors values on ?

2) The gate voltage isn't consistent with the 1M divider, the gate voltage should be a little less than halve Vd ie. 4.1V but your measurements are higher.  This is weird.  Maybe you MOSFET gate is shorted to the JFETS drain?

[alderbody]

Are the crackles and pops coming from the playing?  Does it sound like it's just fuzzing out at some point?

no, they are there all the time. No need to strum or play in order to get them.

[alderbody]

Maybe your MOSFET gate is shorted to the JFETS drain?

I'll have to check once more, but not till i get home...

The resistor values were checked before soldering. Maybe i ran into a bad resistor after all...

I guess i'll have to desolder and replace all the relative components just to make sure.


Thanks anyway!

[Ucho]

Btw, regarding the recommended value for the J201's Vs being around 115mV,
i tried all my J201 stock and found only one that gave 156mV...
All others were in the 250-350mV range.

Just for reference...
I've tried 10 differents j201s.
8 of them were in the 200-250mV range.
Other two in 170-190mV.

Regarding you problem, I'm sorry but I can't help you.

[alderbody]

...any other opinions/similar experiences?


(OK, it's a bump!...)

 

[George Giblet]

My suggestion is you bread board the MOSFET part, if you don't have a bread board solder the parts in air.  Leave off the caps and the connectors.  Just have the MOSFET, resistors and a power supply connected directly to the circuit.  Measure the DC bias, if it doesn't work then you have a very small circuit to debug.  If that works try adding the LED.  If that works pull the input cap and pot on your original board and wire the power directly to the board - this should be the same circuit you breadboarded.  if that works add the input cap,  check again, then connect the pot back.  Go in small steps and work out where it's breaking.

[burnt fingers]

I had a similar problem with a mosfet distortion I've been working on.  I had everything socketed so I could tweek it.  Once I settled on most of the things I soldered them in.  It worked for a few days but then I started running into the same problem.  Voltages just started getting wierd.  I replaced several of the componants but never got it figured out.  I assumed that there was something microscopic wrong with the board that I just wasn't seeing.

I scrapped that board since it was so trashed from the soldering and desoldering of sockets and componants. 

I know this is not a lot of help, but you're not alone
Scott

[Meanderthal]

 Not having seen the schematic the one thing that comes to mind would be a bad aluminum electrolytic cap. I had that problem with a baa-baa: snap crackle and pop with a touch of white noise. Sometimes worked fine for a couple minutes first. No mosfets there...Drove me nuts 'til I figured it out.

[alderbody]

My suggestion is you bread board the MOSFET part, if you don't have a bread board solder the parts in air.  Leave off the caps and the connectors.  Just have the MOSFET, resistors and a power supply connected directly to the circuit.  Measure the DC bias, if it doesn't work then you have a very small circuit to debug.  If that works try adding the LED.  If that works pull the input cap and pot on your original board and wire the power directly to the board - this should be the same circuit you breadboarded.  if that works add the input cap,  check again, then connect the pot back.  Go in small steps and work out where it's breaking.

thanks for the debugging tips!

That's a procedure i had on mind, and i guess i'll try something like that.

[alderbody]

I know this is not a lot of help, but you're not alone
Scott

Thanks anyway, Scott!...   

[alderbody]

Not having seen the schematic the one thing that comes to mind would be a bad aluminum electrolytic cap. I had that problem with a baa-baa: snap crackle and pop with a touch of white noise. Sometimes worked fine for a couple minutes first. No mosfets there...Drove me nuts 'til I figured it out.

That's what i initially thought about checking, before descovering that it was the mosfet that causes the problem.

Unfortunately, there is no electro in the mosfet part of the circuit, but only in the JFET.

I think i'll replace that one too, just in case...

thanks anyway!

btw, here's the link to the project: http://www.runoffgroove.com/peppermill.html