help diagnosing - Fender stage 112 SE guitar amp

[Peter Blair]

Hi everyone,
Before I start, I just want to say, to anyone who remembers, I am still working on my Masco amp. I just haven't had enough time to get back into in yet. I will report back when I can talk about results.
Now to the issue at hand;

I recently came into possession of an almost free Fender Stage 112 amp. Previous owner could tell me nothing but, "It doesn't work". Well, yes and no. Everything on the normal channel seems to be working fine. On the overdrive channel, the treble and bass controls seem to have almost no affect on the output. It WAS almost free, so I COULD live with that. But I'd love to resolve the issue.

So here's what i found. With some of these resistors, I had to lift one leg to get a proper reading. I will note location mapping on the schematic. I'm not sure that I'm doing that in the correct format, but I'm sure it will help even if I'm not.

R33, R37             
Location approx. 4-D   
I just did these because they were in the neighborhood.
Both of these checked out fine. Very close or right on original values.

R39, R41, R44, R45, VR46, R47, R48
Location approx. 2 to 3-D     
All in the tone stack that seems so unresponsive. I took the bass pot out to test, just because that one seemed the worst, and in circuit, the reading was peaking and then dropping, and non of the readings were close to value. Once isolated, it check out fine.
All of these check out fine also. Again, very close or right on original values.

I do have some suspicions about C20, (location 5-D). It looks like it might have taken a little heat at some point. Not to any great measure, but the shrink wrap at on end is sagging away from the lead a little bit?!

The other obvious thing I found, and I don't think it's related, but if I knew I wouldn't be asking;
R75, and R76 (location 5.5-A) are TOAST!! They Put a pretty good half inch diameter brown dot on the board.
I don't see any obvious signs of trouble with C49. (same location)

I've already ordered replacement components for C49 and the previously mentioned C20, just because I have some doubts about both of those. R75 and R76 are also on the way.
I guess my question is this;

Does anyone see what might be causing the more-or-less absence of response from the treble and bass control?   And;

Would anyone care to speculate about why R75 and R76 went up in flames? What else should I be checking?
Or do you think I should just replace what's obvious, and items of suspicion, hope for the best and see what happens?
(All I have for test equipment is a multimeter.)

Thank you all for indulging me.
Pete

[DrGonz78]

My advice is to replace the obvious burnt up parts and then test the voltages in the amp. Nothing here is a fuse blowing fault, but even then there can be other issues. Perhaps someone input a strong signal into the line out of the amp?

[J M Fahey]

Remember to build a lightbulb current limiter (there's a thread here)  and turn amp ON without a speaker or any load connected, then check that you have +/-V power supply rails (around +/- 40V)  and that you do not have DC voltage (should be < 100 mV DC) at the speaker out terminals.

Everything with no input and all controls set to 0 .

Now that you disconnected the speaker check that it's still alive (measure voice coil resistance and/or test it with  a battery, it should click).

[Peter Blair]

DrGonz78
I had the same idea about something wacky like that with the ext. speaker jack. No way to know for sure, really.

J M Fahey
I have been using the "safety bulb" rig I built last year, but thanks for mentioning it. As I said in the beginning of the post, I've already had the amp running. The normal channel is working fine. This includes the speaker. Fortunately, no damage there. Thanks for the spec's to check. I will do so when I reassemble. "Power supply rails"
I see right off of Q3 and Q8 a +45V and a -45V. Is that where and what I should confirm? And check in the same place for absence of DC current? Do I have that right? I'm very new to all this. I apologize if my lack of knowledge is tedious for you.

So I'm starting to think that maybe someone DID do something screwy with either the line out or the ext. speaker jack. that makes sense to me.

Back to the tone stack in the overdrive channel. Can anyone say whether or not C20 COULD be causing the tone stack issues?, or suggest something else to check?? Everything I've checked so far reads very close to value, except I have no way (or I should say I don't know how to) check that capacitor. Of course, replacing it will answer the question. I just thought someone might know if it is POSSIBLY the culprit or should I be looking elsewhere?

One more question; Can anyone tell me the function of R75, R76, C49? Hanging out there on the way to the speaker. What do they do? Noteworthy to me, those two resistors were meant to take at least a little heat. They are both 2watt. Any help?

Thanks to all in advance again,
Pete

[Enzo]

45v rails or whatever they are should be fine, since the power amp works with the other channel.  They have no way to affect just one channel.

Reading your whole report, I had to think to myself several times the following:

It is all about the circuit, not the parts.

You have tested every part in the tone stack, but does the bottom end of that section have a good ground conection?  The bass control itself might be OK, but if the lower end of it has lost the grounding, then it wil have no effect.  Problems are not always bad parts.

WHy do you think C20 would be involved?

Does the contour control work?

R75,76 are part of your zobel stability network.  They should be replaced if they are cooked.  They will burn up if the power amp goes into high frequency oscillation.

[J M Fahey]

Test that one end of C28/C29/R49 (bass pot)  have one end grounded.
Do not use a nearby ground point (which might be ungrounded itself)  but a far away, basic one, such as ground at the power supply itself.

My guess (hinted at by Enzo, by the way ) is that those 3 suspect points might be joined by a single track, which reaches ground through a longer, maybe zig zagging one ... which might be compromised.

As a side check: are pots soldered straight to the main preamp PCB?
Because at least in some PA amps, for space/layout reasons some pots are often mounted to a small sub-PCB which then connects with a couple wires to the main one.

If its ground path is broken, pots on it won't work.

[Peter Blair]

J M Fahey
Overall condition of PCB is very good. Not dirty or beat-up or scratched. Nobody's been in hear before, no doubt. But I completely get what you're saying about the ground being interrupted somewhere. It does not look likely to me. Pots are in fact, attached to the main PCB. It's all in one package, except for the power supply. The housing of each pot IS well grounded. I DID re-solder the four mounting legs on a couple of these, but coincidentally, non of them were in the channel having problems, except for the bass pot which I removed to test. I'm pretty sure I have a decent ground restored to this pot housing, as well as the other three leads. On we go . . .

C28/ C29 both have continuity to ground, at the end away from the pot. I'm sorry but I can't find R49 to save my soul, on the schematic, or on the PCB. You did note "(the bass pot)", so, if there was a typo, and you meant R44/ 45/ 47/ or 48, none of those have continuity to ground, at either end. If R49 does actually exist somewhere else, would you please tell me where, and I will check there also.

I did my testing with one lead to the -45V at Q8. I repeated the testing with one lead to a ground wire coming from the PCB that would normally be attached to the chassis. Both times I found continuity at each pot, where I THINK it belongs;

Wiper of Contour.
One end of volume.
One end of bass.
Nowhere on treble. (although I see no direct connection there to ground.)

Now, as a separate issue/discovery. During the process of testing, all of a sudden I was getting ground at every terminal on every pot, both channels!! I was even getting ground at the + terminal of the speaker connection! No, I'm not working on a metal table. A little poking around and I noticed a very sharp kink in one of the leads to the reverb tank. A little pushing and bending, and I was able to stop and start the short again several times. So maybe that's a big part of the problem. Two things to note however; the "normal" channel always seemed to work just fine. And I never could get the + speaker terminal to short again. I did check out the extension speaker jack very closely. I didn't see anything suspicious, and after the + speaker terminal returned to normal, so did the ext. jack.

And by-the-way, thank you so much for your time and expertise. I think it is EXTREMELY generous of you both!

Enzo
The reason I have a little issue with C20 is strictly via a visual. I once heard somewhere that a faulty component anywhere in the tone stack, has the potential to cause problems elsewhere in the tone stack. Check out the attached photo. See how the wrapping is sagging away from the lead? (arrow) I thought this might be a sign of excessive heating of that cap at some time in the past.

Again thank you both so much,
Pete

[J M Fahey]

Sorry, it was a typo, I meant R46, a 250k "C" taper pot.
Hope the calling it "the bass pot" made it somewhat clear anyway  .

That said, it might be a bad pot anyway,with a  poor inside contact or something similar ; there's 4 "mechanical" , meaning "non soldered" connections inside a pot: 4 possibilities for poor contact.

To boot, knobs get kicked in transport and the shaft can break havoc inside the pot.

To be 100% certain I'd fully pull treble and bass pots out and install new pots , but since I hate messing with PCBs unless it's absolutey necessary, we can somehow simulate their function without:

1) measure treble pot end to end resistance, then set it so resistance from wiper is the same for each end, say the pot measures 90k (pots are never exact printed value, nombig deal) , then set it so center to each end measures half that, in this case 45k .
We are setting pot to its electrical middle point, which not "5" on the knob.

2) roughly the same with the Bass pot.
Here there's external resistors messing the test, so it probably will show way less than 250k and worse, center to each end will not be half that    but no problem,we set it so center to each end  isn the same, no matter the actual  value which will be less than 125k .

3) the idea is to set each pot to half its actual range.

Now we test how well they function regulating bass and treble.

Play some MP3/CD so your hands and mind are free, and test each control:
So far we set them "flat" now tack solder a small piece of wire to treble pot center lug, touch the other end of the wire alternatively to each pot end: we should have flat/max/min Treble alternatively.
Post results.

Same with Bass pot.

4) if you get control, it's a bad pot situation; if not, the whole network has beeeeg ground problems or is bypassed or shorted or some track or cap is open, even if they look fine.

5)  that said, my blood froze when I read you used -45V as a ground reference.
FWIW I asked for >>>GROUND<<< at the power supply , a [power rail is not so.

And i find somewhat nagging that you use the word "continuity" all the time, making me suspect you rely on the buzzer function of your multimeter, I'd rather be sure you use the word "zero ohms" ... if that's what your meter displays, and that you use the ohms scale, the 200 ohms one to be certain.

Also twisting wires, measuring speaker "continuity", , worrying about the reverb tank, extension jacks, etc. , is in no way related to a malfunctioning tone controls problem, the unrelated data only adds confusion.

Let's tackle this one problem at a time.

So please test that tone control as suggested and post results

As you see, experienced Techs test *function*  first, and only after they have narrowed the suspect area *a lot*  may search for bad parts ...or bad connections ... or lacking/wrong voltages or ground , etc.

Just testing for bad parts and nothing else is tedious and may make you run in circles forever.

[Enzo]

R44/45/47/48 would not be expected to have a ground connection, none is indicated on the diagram.

It appears you did ultimately measure the bass pot to ground.  But be aware we were not ever referring to the grounding of the pot housings.  If they are not well grounded, we might get hum, but no loss of function.

As to getting a grounded reading on everything, I would suspect you may have inadvertently made the readings with the meter set for voltage or current - missed the ohms setting.

C20.  First, I wouldn't consider the plastic sleeving there an issue.  Second, C20 is not in the tone stack.  In fact there is a whole op amp between it and the tone circuits.

I agree with Juan, play some music through the amp.  I think that might be easier to hear tonal differences than just guitar

I don't see that you tested the contour control for function, it is a tone control as well.

[Peter Blair]

First I want to say thank you again to both of you for tolerating my lack of knowledge. I'm doing the best I can, with what I have. I DO see how I've probably generated some confusion. I'm just trying to report EVERYTHING I see, because I'm not sure WHAT might be relevant. I wouldn't want to omit anything that may be important. Sorry for that. That said . . .

Enzo
Well, I do notice on the PCB that the pot housing are actually linked together, so in my ignorance, I thought this might be relevant. Again, just trying to cover all the bases.

Ground readings:
I HAVE been simply using the continuity function on my meter. And to address Mr. Fahay as well; the reason being, the amp is inoperable at the time. I have already removed R75/76, because they were so obviously toast, and C49 also. At the time, taking care of this seemed most important. I thought, while waiting for the parts to come in, there's some chance I could also diagnose, and hopefully take care of the tone stack issue. Sorry. I do see that my approach IS somewhat scattered,, at best! Again, I apologize.

C20. OK, I see now what you mean. It is quite removed from the tone stack. In my defense, it IS mounted on the PCB, RIGHT between the mounting legs of the Gain pot. So, again in my ignorance, the sheer proximity to the tone stack made me give it a suspicious look. Again, thank you for your tolerance.

I was getting some fairly good response from the Contour control (before dis-assembly), when I first checked the amp's functions. I do see the point of playing music though it. Obviously, I don't have that luxury right now.


J M Fahey

I had my suspicions on the typo! I had already removed the bass pot, R46 to test. (Sorry for my misinterpretation of your last post. I thought you were just trying to direct my search of the schematic to that area.)
Anyway, the bass pot, out-of-circuit, tested fine. Right on the money, in fact.

Bass pot in circuit, (and I suspect that these readings don't mean much), with my meter set at 200k, (you'll see why), one lead on ground, one lead on the wiper, the reading will smoothly sweep from -
00.0 far left - to 39.2 midway - then descend smoothly to 27.5 at the far right. These reading are what prompted me to take the pot out of circuit to test it.
Again, these readings are in circuit. Sorry if I'm over-reporting again.

As the tone pot is so isolated, I was able to get good readings on that, in circuit. 90.1ohms. Close enough. Nice smooth sweep also.

You are right about my use of the buzzer on my multi-meter. As stated above, the amp is inoperable at the time. I thought simply testing continuity would suffice. Everything did have continuity where it seems it should. (See my last post.) I will retest everything again using the ohms setting as you prescribe. I thought the (c-word) testing was sufficient. I guess I do see that there could, in fact, be some variation.
OK, so I just re-tested everything using the ohms setting.

C28/C29,
Wiper of Contour,
One end of volume,
One end of bass.
all read 0.1 ohms (meter set at 200)
Again, nothing on the treble pot. (I'll note again, I see no direct connection there to ground.)

I do unquestionably see the logic, and advantage of the approach of a more experienced tech. It's a lesson to me, and I will try to learn from it. Thank you for your words of wisdom.

Maybe at this point I should just wait for the parts to come in, that I KNOW need to be replaced. Reassemble the amp, click my heals together three times, bow to the east, turn it on and see what happens! In the meantime, if anyone sees anything else to check on, I'm all ears!! I so much appreciate your indulgence, and look forward to either hearing more, and/or reporting back once the amp is back in one piece.

Thank you,
Pete

[J M Fahey]

Ok, since you already have the pot out, for now don't put it back (as I said, I want to minimize PCB suffering), solder 3 short cables in the original holes and to the pot legs, so you can operate it and/or put a new one there later if needed.

So now enough of parts testing and start functional testing (injecting music and moving pots end to end, checking for any effect on tone or lack of it)

[Peter Blair]

Thank you Mr. Fahey. As soon as the replacement resistors (R75/ R76) arrive, I will get the amp back in one piece, follow you directions, and report back. Parts were ordered from Mouser on 07-06-15. I don't expect they'll be here for a few more days, but I'll bet back to you a.s.a.p. Thank you again.

And if anyone else wants to chime in with any kind of brain storm, or even a slight drizzle, please do so.

Pete

[phatt]

Hang on,, just had a thought.
As the tone section on the drive Ch has little or no effect then the drive Ch may not even be switching.

Q Peter B,
Does the Gain pot (R29) have any effect on the volume of Drive Ch?

If so then I guess it's switching ok,, if not there maybe issues with Q12 and Q14 or U6A. I think these switch the channels, Switching circuit is hard to read

Better minds here may will know more.
Phil.

[Peter Blair]

phatt

Thanks for reading the post, and thank you for all the time it takes to look everything over. Everyone on this forum continues to amaze me with their generosity.

When I first got the amp (see first post) I did check the function of everything. EVERYTHING was working EXCEPT for the tone and bass controls for the "overdrive" channel. This is what prompted me to investigate in the first place.

* The channel switching DOES work.
* Reverb does work.

  "Normal" channel -
* Everything works A-OK,

   "Overdrive" channel -
* The Gain, Volume, and the Contour work, though the Contour maybe seems not quite as effective as I 
   would expect, but I could be wrong about that. It DOES impact the tone to some degree.
* Tone and Bass controls - very little, or no response.


Going in, I thought my ONLY problem was somewhere in that tone stack. I have come to understand that the two burnt resistors (R75/ R76) are totally unrelated, and are a separate issue. Looking at the schematic, I do see they couldn't possibly have any affect on the tone stack. How could it impact one channel and not the other? It's all so downstream from the pre-amp and amp. I have to admit, I am somewhat puzzled that those two resistor, completely shorted out, did NOT seem to interfere with the functioning of the amp.
And as if I haven't caused enough trouble . . . 
I do have another question about this, and I don't want anyone to get into a real lengthy explanation, because I probably won't understand it anyway, but . . . isn't the output signal to the speakers AC? If so, what's that capacitor (C49) doing there, directly linked to the + speaker output? Is it actually allowing some -DC from ground to pass to the +AC output? Is it sending stray +DC in the output signal to ground? I don't get it.
(Please excuse my ignorance. I'll mention again, I'm trying to learn.)

So back to the tone stack. I thought likely, just a bad pot or something. All the resistors that I've checked so far are all very close to, or right on spec. I don't know how to test the caps, but they all LOOK OK. And I know they can look OK and still be bad. To tell the truth, where they are so inexpensive, I wish I had ordered replacements just in case I did somehow find one to be bad. I know that the preferred approach is NOT to just start replacing parts willy-nilly, without knowing that they're bad, so I resisted the temptation to do so. I don't even know what the symptoms would be, of a bad cap in the tone stack. Maybe someone could answer that question for me. And/or instruct me on how to test said caps. I'm assuming there would be some really obvious symptom that I haven't mentioned, as so far, nobody has brought it up. I understand that the general consensus seems to be that caps in the tone stack rarely fail.

So thanks again for reading. If anyone has any more ideas, please chime in. Please correct me on any erroneous statement I may have made. I AM very much a novice (as if you couldn't tell), and trying my best to learn. Thank you all for your help. In the meantime, I'm waiting for replacements for those two burnt resistors so I can reassemble and re-check functions.

Pete

[Enzo]

I think if you measuer them, R75,76 are not shorted, there is just a low resistance.  Considering they  are 4.7 ohm resistors in parallel, they will measure less than 3 ohms normally.

Yes, the output of the amp is going to be AC, but at audio frequencies, the resistors and C49 do not offer a low impedance.  But at higher frequencies, like above audio, they do conduct well and thus off stability.  When you want to study this aspect, read up on "zobel network."  Caps do not allow DC to pass from anywhere to anywhere, if they are working.


yes, caps in tone stacks rarely fail, which makes them unlikely suspects.  Add to that that you do not have just one tone control not working or compromised, you have two.  That would require at least two caps to have failed, meaning even way more unlikely.  The explore what we do it try to systematically find where the system breaks down.  This could be at a part, a connection, a copper trace, something else.  Once we know WHERE the problem is, we can determine WHAt it is.

Some general advice for you.  Don't over-think this.  Guitar amps are not rocket science, they are not precision circuits with critical adjustments all over like some laboratory equipment.  They are just guitar amps.  That means it won't matter if some part value is off a little.  Old Fender schematics had a note on them that read all measurements were approximate and should be within 20%.  Close is good enough.  You will learn that measuring parts while they are still connected often leads to low readings because of parallel circuit paths.  By the nature of resistors, the actual parts almost never go down in value.  So when a 100k resistor reads say 60k, it won't be the resistor's fault.  Resistors do sometimes go high in value, especially old ones.  So a 250k plate resistor in a 1950 amp might actually read 400k.   But mainly when I test them, I am mostly just looking to see if they are open or not.  Off values rarely cause an amp not to function.  They might make it function differently, but not usually fail.  In other words a tone stack might sound funny, but would still work.  A tremolo circuit is probably the one place value makes the most difference.  leaky - moreso than off value - caps can cause it not to oscillate.