help diagnosing - Fender stage 112 SE guitar amp

[Peter Blair]

Ooops! Excuse me while I wipe this egg off my face. I totally mixed up the AC, DC, capacitor thing. I must have been tired or something. I know I wasn't drunk, 'cause I don't drink! Please excuse the pun, but I got my wires crossed!!!!!!
You are right about R75, 76. (I think you might have mis-read or typo-ed on the value. They are 47ohm, not 4.7ohm. One of them is reading 23.8 out-of-circuit. The other one reads OK (also out-of-circuit). It just looks really bad. I had already looked up literature on "zobel network". . . WAY over MY head! I'm just a hobbyist, mostly whistling in the dark. I just took this up recently. So far I occasionally have success. I don't have the benefit of decades of education, training, and field experience. I'm just trying to see if I can get lucky and maybe with a little help, find a solution. Again, I apologize if my lack of knowledge is tedious to you. I'm not here to try to offend anyone. I'm more than happy to admit what I don't know. Perhaps I should re-evaluate whether or not I have the right to be on this forum.
My apologies to anyone who feels that they were wasting their time trying to help me out. I did try my best to follow everyone's instructions. Sorry if I let you down, or if you feel insulted by my ignorance.
The best to you all.
God bless,
Pete

[J M Fahey]

Oh, don't worry.

Just try to follow instructions as close as possible and test whatever's asked to, you are our hands and eyes on the spot.

In general we suggest exactly what we would do or check if it were open on our own benches.

[DrGonz78]

Pete, people like youself have every right to be on a forum like this one. Humility is a strength that a lot of people don't possess. You learn and learn and learn. That is all it is about and then you eventually learn by helping other's to learn. Don't be so hard on yourself. We all make mistakes and we all have blunders.

[Peter Blair]

Thank you Mr. Fahey, and you as well, DrGonz78.
I'll report back with how everything is functioning after I receive and replace the resistors that I guess I could get away with leaving in, and get everything back in one piece and hopefully working again.
Thank you for your generosity.
Pete

Just as a point of amusement, here's a picture of the damage to the PCB directly under R75/76, with the suspect resistors in the foreground. You have to admit, this at least, doesn't LOOK good. Maybe this will help explain my apparently misguided concern for this area/these components.

[Enzo]

Peter, I hope you didn't take my description of the basics as criticism, it was not intended as such.  I've been in electronics 60 years, and believe me, I am still learning stuff today.  I have been training technicians for forty years, so my efforts are mainly to instruct, that's just how I naturally approach things.

Yes, those resistors burnt up, and should be replaced regardless of reading.

[phatt]

Pete, people like youself have every right to be on a forum like this one. Humility is a strength that a lot of people don't possess. You learn and learn and learn. That is all it is about and then you eventually learn by helping other's to learn. Don't be so hard on yourself. We all make mistakes and we all have blunders.

Obviously spoken by A man of great Wisdom.

Pure gold,, :dbtu: :dbtu: :dbtu:

Phil.

[Peter Blair]

phatt ,
Thank you for your expression of support. I've always been very comfortable with humility. My aspiration is complete absence of ego. There is one thing a lot worse than making a mistake, and that's sticking to it. Ego encourages the latter. (My "words of wisdom" for the day, acquired through life experience.)

Enzo,
No, I did not interpret your outline as criticism at all. It sound very much like the words of an experience instructor. Nothing in there sounds like criticism. Just good information.

OK, one comment on topic. Now I don't feel so much like I'm replacing parts for no reason, (thanks to your comment Enzo). Excuse me for repeating myself, but; I will report on function again, once everything is back in one piece and I can fire this puppy up again. (To put you at ease, I will be using my test lamp.)
I'm hopping the Solid-State gods will be smiling on me.

Thank you all for your display of compassion and kindness. Both attributes indicate a good human being.
(not to get too philosophical)

Pete

[Peter Blair]

Time for an update. Parts came in a few days ago, but I've been quite busy until tonight. I replaced the two obviously burnt out resistors, R75/76 and C49 based on sheer proximity. I apologize if that violates approved procedure. I couldn't help myself.

I know that I made no changes in the very poorly functioning tone stack for the "drive" channel. (The original reason for all this investigation.) Not surprisingly, the symptoms have made no change either. Gain and Volume both work fine. Contour behaves like a bass pot (somewhat). Tone pot and Bass pot do almost nothing, or nothing at all. It's a pretty tough call. If either one IS doing anything, it almost defies detection. To review, I've checked values on every resistor in the tone stack. Just to report exact numbers, this is what I got. I know these numbers warrant no concern, but I thought I'd post them just so you KNOW I did the work.

Resistor       value      as read value
R39             47k          46.1k
R41             47k          46.1k
R44             47k          46.7k
R45             22k          21.6k
R47             47k          46.7k

R40  Contour Pot - Working fine, I think. Behaves like a bass pot, more-or-less.
R42  Volume Pot - Working fine.
R43  Treble Pot - I did not write down the value, but I know it was close. Nice smooth sweep, yet unresponsive
R46  Bass pot - Right on the money, nice smooth sweep from 0 to 250k, but again, unresponsive.

Just as a point of comparison, the tone stack on the normal channel works great! Very responsive to any adjustment, so I don't think this is just a characteristic of the amp. EVERYTHING else on the amp is working as it should. If anyone has any idea on checking anything else, I sure would appreciate hearing from you.

Thank you,
Pete

[J M Fahey]

Well, you'll have to feed the amp some specific tones, from low to high or viceversa and measure at some specific points.
Start by downloading into an MP3 player or equivalent:
http://www.mediacollege.com/audio/tone/download/
100/250/440/1k/10k Hz tones, best are the 30 second MP3
You should set your player to "Loop/Repeat One" so tone does not stop (it will mute for a very brief time every 30 seconds, not big deal) .

You also need a multimeter with a good AC range, with a 200mV scale available.

If not, try to get a Germanium diode (1N34 , 1N60 , etc)  and I'll post the schematic of a peak reading probe, the idea is feeding different frequencies, measuring them just before the tone control - after RC nets - at top of potentiometers - at their wipers - at pot and caps ground (yes, finding audio where there should be silence is also a problem) - at the mixing resistors - at the following stage.

*Somewhere*  along that path sound is lost ... we'll find the leak  :trouble

If not suitable multimeter nor Germanium diode are available, I can post an active rectifier which can read down to a few millivolts of audio, but it uses an Op Amp and a couple standard diodes , and you'll have to Protoboard it.

Of course the real luxury would be to have a Scope and an Audio Millivoltmeter available

[Peter Blair]

Thank you Mr. Fahey,
More than I can handle tonight, but I'll be back to you as soon as I can. I do have to occasionally earn some $$$$. I usually do this by repairing antique clocks. Electronics may not be my strong suit, but mechanically, I stand undefeated! It's the one gift God chose to bless me with. I don't know what I'd do without it. So as soon as I can follow through with that download, and figure out a way to feed it through the amp, I'll be back to you again. I can't thank you enough for your generosity, and your tenacity.
I'll be in touch soon,
Pete

Sometimes I'll take a repair of stringed instruments too. This is my latest endeavor. It's almost done now. Some may find it interesting.
http://s1085.photobucket.com/user/peterblair1/media/A1%20as%20found_zps5ymcxy7h.jpg.html?sort=9&o=1

[phatt]

I just simulated the tone circuit and my hunch was right,, those controls don't do much. xP
The curves might look pretty but if you note from 100hZ up to 5khZ not much change happens.

See pic,

The contour filter has only about 9dB effect.
Bass can only create a 3dB difference from off too full
Treble is only about 7~8dB at 3~5khZ.
So yes the tone section may well be working as built,,, just a pathetic idea.

hopefully the plot graph can be read,, the control pots are full up then full off.
10 means low and 90 means high.
The contour pot might be backwards but it gives an idea.

If I had a dollar for every hot channel Amp that was a disappointment I'd be quite well off. :lmao:

I've lost count of the amount of guitar players who have a 2 (or more) channel amp they paid a fortune for but just use the clean channel and do all the Dirt sound with pedals.

One of my mates has a Fender pro 1000 and the hot channel is never used.
He bought it cause it had a Valve inside the hot channel,,,durh.

That Valve is not even in the audio,, it's just used as a diode for fuzz.
Go figure??
Phil.

[J M Fahey]

Hi Phatt, thanks for taking the time for simulating

Thjat said, 2 comments:
1) yes, it does not do much , compared to normal controls
2) it's often done on purpose: distorted signals have TONS of harmonics  compared to clean signals, and tone controls are usually overkill, go from extremely buzzy unbearable ice pick to as muddy as the Ganges , so often designers get a basic EQ curve (easy to see on your graph, a mid scooped fixed bass and treble boost ) and allow tone controls only a very limited control over them.
FWIW classic Marshall controls do much less than the original Fender ones they were derived from (basically by doubling treble cap from 250pF to 500pF and halving bass cap from .047 to .022uF so they "meet in the middle" ) , classic comment on them was : "bass and mid do very little, only reasonable one is treble, but presence is stronger and anyway moving normal and bright volume controls has stronger effect on tone"  :duh

People also said: "Marshalls are one tone, one volume (loud) amps"

Go figure.

FWIW Pappo Napolitano, our local guitar god , always said: "I only use Marshalls, I sweep my hands left to right on knob tops so they are all set to 10 and rip away" .

[Peter Blair]

Yes!!!!, Thank you Phatt, for taking the time, and probably saving me a bunch of time. Mr. Fahey, I was hoping you'd weigh in on the simulation. After I read the post from Phatt, I got to thinking exactly what you describe. The absence of responsiveness is probably no accident. Probably for the best. So all this leads me to being one happy camper. Although I do have to say, I'd love to squeak a little more responsiveness from those two pots. If it was just a matter of changing out a couple caps, and/or a couple resistors, I'd be game for that. What do you think Phatt? Would you be willing to plug it a couple different numbers and see about configuring a MODEST improvement? I think anything more than that might be asking for trouble. I'd be delighted if I could at least be able to tell which way each knob was rotated. If not, thanks just the same. Once again, this forum has been very helpful. I hope I can pay it forward somehow, someday, some-way.

Thank you,
Pete

[J M Fahey]

I've been thinking that to dispel all doubts, you might build an external, unmounted tone control, (very simple, 2 pots , 2 caps and 1 resistor) and connect it to that circuit.
I can suggest one which *does*  work
If you like it, you can cut a couple tracks in your amp and incorporate it.

[Peter Blair]

Hello again Mr. Fahey,
I find the idea VERY intriguing. If it not a lot of work for you to draw up, I'd love to see it. If I think I can handle it, it would be so fun to give it a try!
Pete