Scope Probing Corrects Problem Why - Fender Blue Jr Rev3 2003

[Ryteone]

It's a stock 2003 Fender Blue Jr. I first noticed a hum low level (that stays the same volume level regardless of volume or master volume setting) that reminds me of a grounding problem but havn't been able to track it down.  Also when I would hit heavy cords at lower levels the sound would break up get fuzzy like electrical static sound then fad back to sound good as the level dropped.  The same thing would happen when I turned up either the master volume or the channel volume. Here is what I've done so far:

• I changed out all the electrolytic caps (they were stock and know they have issues)
• I resoldered the tube sockets since I know they have issues.
• I swapped out all the tubes to check the chance that a tube was bad.
• I thought it was some kind of oscillation that seems to be happening to the V3-B side of the phase inverter.  I found a link to Bill M's site about this problem. I tried the ribbon cable changes shown and played attention to other wire routing, but it still persisted. 

I decided to scope the amp and see what the signal look like.  The signal along the V3-B path seems to look like the image below but more blurry and not as defined, while the V3-A path is very clean and sharp.  But when I probe R20 (220K) at test point 11 and at R34 at test point 16 on the attached schematic, the signal long the V3-B path becomes sharp and clean also the hum disappears.  What is the probe doing to correct the signal?  I know it can add resistance or capacitance (if that is a word) or Inductive loading but trying to figure out why it helps so I can repair it.


Side note when I run the amp with a resistive 8 ohm load instead of the speaker the signal path is clean and sharp.




Also when I pull V4 the amp sounds great, when I pull V5 the amp sound very weak very low signal and but both times it doesn't have the above breaking up of the signal.




Looks kind of like this but more fuzzy and less defined





Any idea of what it might be.

[mat janssen]

Your scope probe has a capacitive and a resistive "load" for that point.
The measuring point is now "loaded" with 10Mohm to ground in parallel with 10 pF or more.
The circuit could be oscillating without the "load", and stopped oscillating with the "load".

[PRR]

Try 100pFd from "right here" to ground.

[stratele52]

Try cleaning tube socket 's contact, tightened Output tubes socket contact .


Read voltage at Power Supply  when you hit chord if voltage drop. Should not drop.

[Fresh_Start]

If you did all that to a Blues Jr. and this is your only problem, you are luckier than I am.

Pulling one preamp tube at a time to diagnose a problem helps.

Pulling one of two power amp tubes in a fixed bias amp like the Blues Jr. is a bad idea. Swapping in a different output tube or better yet a different pair is a much better idea.

What is the bias voltage at test point 30?

Do you have a way to measure the bias of your power amp tubes?  The Blues Jr. Tends to run output tubes very hot and burn them up quickly. Given what you said about the amp sounding ok with only V5 installed but bad with V4, my bet is that you need new power tubes. My Blues Jr. burned up EL84s on a regular basis until I installed a bias adjustment.

Installing a trim pot to adjust the bias isn't too difficult. Hmmm... My old post here about that mod must have been lost.

Respectfully,

Chip

[Jack1962]

I have worked on a few of these , if you did everything on your list above , there only a few things it could be 1. bias , lower the bias of the amp(I always do ) 2. this amp use's a resistor between the chassis and ground try a jumper in it's place . 3. clean the pots (I do this to every amp that hits my bench , you would be amazed how many amps that's is all that is wrong with them) 4. check all your signal caps , this is most likely your problem or perhaps a bypass cap.

[stratele52]

I have worked on a few of these , if you did everything on your list above , there only a few things it could be 1. bias , lower the bias of the amp(I always do ) 2. this amp use's a resistor between the chassis and ground try a jumper in it's place . 

A resistor between the chassis and the ground.....      


What do you mean ?

[Ryteone]

I had tried swapping out tubes and adjusting the ribbon cable but I failed to read to the end of the BillM page here:


http://billmaudio.com/wp/?page_id=115


It shows to cross over the R30 resistor with a 100pf Ceramic 1Kv cap:





Other items include splitting the ribbon cable and adjusting them to help.


I'm just beginning to learn how to use a scope.  So wasn't sure why the scope would act that way on the circuit.


Thanks....

[Ryteone]

That cap took care of the problem.  Thanks for the replies.

[Fresh_Start]

That cap took care of the problem.  Thanks for the replies

Great!  You solved the problem.

Now, can someone explain to me what might cause oscillation like this and how a plate resistor bypass cap in that spot fixes it?  The OP stated that the blurry signal was on V3-b, but that cap bypasses the plate resistor of V3-a. V3-b feeds V4 power tube.  When he pulled that tube the amp sounded fine.

What I trying to say is that he added a high pass filter to ground on the side of the PI which looked clean. Why did that fix the problem?

Respectfully,

Chip

[stratele52]

Fresh_Start ,


Click on the link ( BilmAudio)  where is the amp circuits photo , there is some explanaition

[stratele52]

I have worked on a few of these , if you did everything on your list above , there only a few things it could be 1. bias , lower the bias of the amp(I always do ) 2. this amp use's a resistor between the chassis and ground try a jumper in it's place . 

A resistor between the chassis and the ground.....      


What do you mean ?

I did not read the answer yet