Diefet problem - low volume and gain, sounds thin

[marcus262]

you mean the cap listed on the schematic as C17?
Why that cap?

How do I check jfets for continuity, in reference to what?

I will check cuts again.

[motterpaul]

I think he means to make sure the JFet pins are connected to the vero (through the sockets).

[motterpaul]

By the way - I completely understand how frustrating it is - but sometimes it is best to look away for a day or so and start fresh later. Often the problem is something you are currently thinking you are sure you checked. Starting over your trouble-shooting goes a lot faster.

One thing I have done is had missing parts, but I didn't realize it because I already only checked that once, missed it, and then assumed all was well.

Another thing i have messed up a few times is the pot lug numbers (which side is 1) - easy to do.

In most cases it is human error, not part failure. But in my last build the problem was just a couple of strands of wire touching the wrong lug on a pot (and it was already boxed, so hard to see).

[marcus262]

checked GSD pins of all jfets and there is continuity with the next component that they are supossed to connect to.
But still cannot confirm that caps are working properly, my shitty meter doesnt do that. So maybe it is some of the caps.

very frustrating, if it didnt work at I would at least know that something is very wrong.
As it stands it seems there is no major error but still doesnt work like it is supposed to, and wasting your precious time. Sorry about that.

as soon as i check a few more things again ill take a break.
gonna check again pots, cuts, lines...

Is there anything else I could maybe measure, to get some references?

[motterpaul]

I'll tell you, if you could upload higher resolution pictures it would help. I had a hard time seeing the ones you posted.

I am really sorry, but I know the feeling I have probably three or four builds right now where I swear they should be working. I get some sound, but not enough gain. Those things are very hard to troubleshoot.

When I get to the point where I am replacing caps & diodes and still have no luck I figure it is best to just set it aside and revisit later.

Another recent fault I had was a bad batch of TL072s. I had five of them in a sealed tube that must have been fakes. Once I found a good one (and it had a different orientation marker) the thing worked.

So - it could be a bad component, or it could be user error.

Is your DMM autoranging? If not it is actually easier to test caps. Do a google on how to test caps with a regular DMM.

Do check your pot connections, make sure you know which one is "1". Plug the circuit in and wiggle things. If you find a crackle you might have short. Double checking ground is always good, too.

I looked for a schematic for you that showed the inline voltages but I could not find it. One came up in Google listings but the actual image link was not good. If you can find one then you can check inline DC  voltages (in milliamps - you need to open the circuit and put the meter into the open spot and read the voltage crossing at a given point).

[Frank_NH]

"you mean the cap listed on the schematic as C17?  Why that cap?"

Yes, C17.  That capacitor is known as a JFET source resistor bypass capacitor.  Essentially, it allows AC signals to be amplified more than if you just used the source resistor (R11) alone (usually by a factor of two or more).  It might be worth replacing that capacitor just to be sure.  Check to make sure the leg connected to ground is actually seeing ground (use your continuity tester on your multimeter to see if there is continuity between that leg of the resistor and the ground wire).

And, yes, it can be hard to debug complex circuits like this one.  Often it can be something very small like a bit of copper causing a bridge over cut in the vero track.  Or a tiny blob of solder unintentionally connecting two adjacent tracks together.  Keep trying, but if all else fails, take some notes, put it aside for a while and come back to it later.

[marcus262]

i will google it.

Is there any other way to check tl072 besides voltages that I gave?

I used audio probe to listen to DSG of the jfets,
D is always the loudest on q1 - q2 - q4, but on Q3 S is the same volume as D (loud) and more distorted.
Is it normal?

[marcus262]

i will also check offboard wiring, remove 3dpt,
check input output jacks

[motterpaul]

Just fyi - once you find the area of the problem (which you have, it seems) you can be pretty certain it is one of the involved components there.

Last night I picked up a similar circuit (AMT B1) which I had been using for days. But it was dead, but I knew the last thing i had done was try to add more resistance to one of the source JFets (by soldering another resistor inline with a resistor onboard). That didn't work so I just restored the original resistor.

The old resistor I replaced appeared connected, but somehow one leg was too short - so I had a cold solder joint even though it held in place. The resistor even tested fine so I took it out & reset it and now it works. So, looks can be deceiving.

[motterpaul]

I was just looking at your pictures. I see a wire going into A2 (second hole down in first vertical row). I am think it must be treb 2 which is supposed to go into hole 3. The diagram does not have a wire in that space 2. It is hard to see everything there because the greenie is pretty big, but....

Yeah - now that I look again it looks like an optical illusion. Sorry.

[marcus262]

Q3 doesnt have a capacitor, maybe thats why the S is same volume as D and more distorted,
i dont know if that is a error.

Removed switch and all pots exept gain, still the same.
remelted again all connections, cheched continuity, cuts... resistor values.

So is q5  the final level of distortion, meaning I should be getting the same level at the output,
nothing distorts the signal more after q5?
If thats the case then the problem is before q5

Checked that all 1uF caps have connection to ground and to S.
Gonna try removing them and changing order, so if one is dead or isnt properly soldered i should hear difference in level of q1 to q4.

[motterpaul]

I assume you did use all J201s? I have heard there are fakes out there. Did you buy them recently?

If you did use something else that would be fine, but you have to check pinouts, J201s tend to be "backwards" from many.  (I had a circuit sitting for weeks that did not work, but I had heard mpf201 transistors are good subs for what was called for. Finally got them today and it did not work - until I double-check pinout and found it is different than what is shown in the diagram).

Also - what is the part # on your 9v1 zener diode? It looks different from the ones I have. Is there a reason you socketed it? Diodes also tend to fail - you might try changing that one out and/or remove the sockets since the leads on that diode look really big.

[marcus262]

yes, all are j201 bought from banzai shop.

it is a 9.1 v 5w zener.
I read that zener protects ic charge pump to not get more than 9v.
I removed charge pump because of the high pitch background noise  i mentioned in the first post, so i am feeding directly with 17 v, so i removed zener.

[motterpaul]

 I am not sure of the reason it is there, so I am not sure of the ramifications of removing it. Most charge pumps dan't have a zener for that, but in this case maybe it was needed(?). But in any case, did you put a jumper link in place of the zener you removed? I assume that is what the sockets are for?

I wouldn't worry about a high pitch squeal until I get the circuit working, personally. Often they go away when its boxed up. Right now I also have one cheap power supply that puts a squeal into everything (so I know what the source is).

[marcus262]

ok, so I desoldered, removed from circut, cleaned with desoldering wire,
checked and make sure i get good connection on vero,
all components from q1 gate to q4 gate, swapped 1uf caps, still the same.

So I give up, havent got a clue what to do next.

If you think of something let me now.

Thanks you all for trying to help.

[marcus262]

hi all,

I cannot believe this.

Built Dr. Boogey with all new components exept j201, fet sockets, dc jack, input/output jack, trimers and one 100uf electrolytic cap.
And the problem remains the same (although 1M pots on volume and gain vs 100k on diefet help a little.) Have to keep the gain on full, but still in crunch

Used no 3pdt switch, biased normaly to 4.5 to 5V on 9v circuit.

So the problem must be the fets, right?

But if I swap all  the fets for example on Q3 they all sound the same with the same breakup.

Jfets are F BJ13 J201 - these are normal? (ordered from banzai shop).

thanks

[motterpaul]

Actually, I am not surprised at all since after everything you tried I started thinking there was a very good chance it was the J201s. I was going to recommend you build the JFET testers on this site - here is the best one I think:

 

[marcus262]

Thanks for the info.

But before I build it I still have to order new j201 from abroad and from another shop obviously.

[motterpaul]

Mine have the Fairchild F followed by

F0625
J201

[marcus262]

thanks again,

ordered new ones, will report back.