YJM 308 oscillation

[dosmun]

I finished my YJM 308 and when the Gain is maxed it squeals when the volume gets over 1/2 way up.  Any ideas,  possibly a Ground problem?
I used Torchys layout for the build.

[MartyMart]

I think Torchy updated the layout  
I didn't have the problem, as I had an "early" layout and wired up
differently.

Marty.

[dosmun]

The updated layout just removes one of the trace cuts.  In the Layout you sent me the Diodes and volume were not connected to ground.

Did you use the old layout Marty?  I don't think my problem is with the layout.

[MartyMart]

Sorry mate,
I had an "original" one, with no "R8" and some errors,so mine is a bit
of a Frankenstein !! ie: has three extra wire jumpers.
I'm pretty sure the latest one is fine, Torchy put it up today from another similar post.
I jumpered the diodes to ground, cut the pin4/5 op-amp trace, joined and
isolated pin 5 to pin 7 and grounded pin 6, moving the top of R5 over one
hole . . . . . . . phew !
Just looked at the site and his is the same as mine now ??
Marty.

EDIT: I noticed that I have a much larger "power filter" cap, its 100uf rather than 47uf . . . . .?
Could even go to 220uf  

[dosmun]

I still can't get rid of the squeals.  Does anyone have any suggestions?  This thing is driving me nuts. :?

[petemoore]

Opamp pin voltages...?
 Audio probe..

[dosmun]

Pin voltages on the opamp (4558) are:
1: 4.2v
2: 4.3v
3: 4
4: 0
5: 7.9v
6: 0
7: 7.9v
8: 8.5v

Should I be getting any voltage on pins 5 and 7?

[MartyMart]

Pin voltages on the opamp (4558) are:
1: 4.2v
2: 4.3v
3: 4
4: 0
5: 7.9v
6: 0
7: 7.9v
8: 8.5v

Should I be getting any voltage on pins 5 and 7?

Thats about the same as my voltages, mine are a little higher as the battery is 9.05v so pin 8 has 9.05v 1&2 have 4.5v the rest look normal
My guess is a bad solder joint/bridge where it shoudn't be.
They can be "miniature" on vero, or a cut copper trace can have a very fine "link" where you dont want it ... mag glass ??

Put a knife/small screwdriver, carefully down all the spaces between copper tracks and rub off all the "goo" thats there
Just to make sure.... I always do that and it helps a LOT !

Cheers,
Marty.

[Torchy]

HINT ...

Buy yourself a small wire brush (the kind used for suede). Mine cost 99p  :oops:  After soldering lightly scrub the board with the brush. This shows up any dodgy joints, if there are whiskers across the trace cuts it shows them up by brightening the copper, and cleans out the crud between tracks. Then a once-over with a small magnifying glass.

[dosmun]

I have went over the circuit with a magifying glass quite a few times and touched up most of the solder connections.  I pretty much make it common practice to check connections with a magniying glass as I go on tight builds although I still miss stuff :roll:   I agree it still is probably a bad connection somewhere.  It is not intermittant at all even when I wiggle all of the components which usually helps me find bad connections.  For such a simple build this one sure is giving me fits. It actually seems to work fine except when I get the levels cranked up.

Another thing is I have the circuit run through a loop box so I can bypass it ( it is not in an enclosure yet) and when the circuit is bypassed I can still hear the oscillation or squealing through my amp only it is not as loud.  This makes me wonder if it is something to do with the ground.

[dosmun]

Thanks again Marty for all of your tips.

[Banjotaz]

How about a larger capacitor on the negative feedback loop?   This usually stops oscillation and rolls off abit more squealing treble. :lol:

[dosmun]

I have tried increasing the the 25pF cap on the and it does help a little but doesn't totally eliminate the problem.

[petemoore]

Maybe the bypass looper has something to do with it, that and no shielded box, this is a high gain circuit, probably able to oscillate if everything isn't just so...could be something else entirely ya know how these things are...
 Looks like the opamp is setup for good bias conditions  
 Since there's only one stage, audio probing would probably only tell you it's happening after the 1rst stage... :roll:
 If the looper has buffer, this can increase the high end maybe, and the 308's got alot of that going on already, maybe between that, no box, and how long are the cct wires...

[dosmun]

The loop box is just a passive 2 loop 1 switch box that I made.  The wires to the pots are about 3" long.  Maybe I will just put it in an enclosure tonight and see if that helps.

[MartyMart]

The loop box is just a passive 2 loop 1 switch box that I made.  The wires to the pots are about 3" long.  Maybe I will just put it in an enclosure tonight and see if that helps.

Dosmun, did you try a much bigger PF cap ?
Thats the only difference I can see to mine.... I went with a 220uf in the end and mine is NOT boxed up yet ! zero noise/hums ..

Marty.

[Paul Marossy]

Hmm... I built mine on a piece of scrap perf, and I have had no problems with oscillation. It appears that no one has suggested lead dress as a possibility...

[dosmun]

I went with a 100uF PF cap and it made no difference.

[Paul Marossy]

Wires, wires, wires... Are all of your wires as short as possible? And where they cross, are they at right angles to eachother? DO you have "long" lengths of wire running parallel to eachother?

[petemoore]

Wires....
 Short, next to a ground plane is nice, I try to lay out high gainer [esp[ so thatthe SP signal path wires are like this.
 Signal path wires also get going along ground wires sometimes in my boxes.
 I used to shield an input wire, run that farther away from the switch, then use the circuit board distance to work my way back, thereby getting unshielded SP wires very short, but I don't do shielded wiring unless it's a long, double CCt box. [ie long SP wires].
 I try to keep the SP wires also not near V+ wires, but I don't know if this matters.
 Well the opamp seems to have happy voltages, the only other things are caps which are 'generally reliable, and resistors which are 'very reliable and seem ITC to be doing their jobs...and the connections...I like to 'see' the heat...taking into account inconsistancies in tip to X thermal mass...'heavier' solder masses take longer to heat up...all solid surfaces need to be X' temperature [where you see the solder 'grab and relax']...I haven't had cold solders for a good long time.
 I just figure whatever it's doing, it's doing for a reason...finding the problem is easy, finding the solution may require more 'digging'...it's a drag, sometimes, but that's the way she works.