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

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
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

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

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

Hi again Roly,
Thanks for the extremely clear explanation. I see I had it bass-ackwards.

Quote
Quote from: Roly

    For reference, a way of testing a coupling cap for leakage is to lift the grid end and measure the voltage between the free end and ground.  A good cap will eventually charge up and read zero volts, or something very close, while a leaky cap will show a residual voltage (which in the case of some waxed paper caps may be close to HT)

Just for an exercise, I think I'll test some of the coupling caps as you describe here. I'm just waiting for my parts order to arrive, so it will give me something to do that will definitely embed the whole idea in my memory. You are gifted with being able to communicate with the uneducated, in a way that make the topic at hand seem very clear. Where do I sign up for classes? I suppose right now, my best classroom is my workbench.

I will be reporting back after I've completed at least some of the repair.
Thanks again Roly,
Pete

Thanks Roly.
Thanks for the education. So please clarify for me: In a perfect world the black banded end of the coupling cap (polarized or not) would be better facing the "driving end". By that would you mean signal end driving the grid of the power tube? Better but not necessary? The other end being driven by the signal coming from the plate of the 12AX7? Do I have that right? And by your explanation, even though the polarity of a cap under 1uF is inconsequential, the banded end COULD be considered the "-" end?

It is still my intention to replace, (and I have ordered) all the caps, including the filter caps (individual as per your suggestion). Which brings me to another question. These large, multiple capacitors in a can are on the top of the chassis for obvious reasons. But I've also noticed that a lot of the Fender amps place the individual filter caps on top of the chassis also, under a cover. Is there a reason for this?
Pete

while a leaky cap will show a residual voltage (which in the case of some waxed paper caps may be close to HT)
What is "HT"?

EYES OF EXPERIENCE PLEASE!!!!

Could someone confirm what I think I just found please!
Ok, so I think we've identified the likely suspect in this case as being one, or both of the coupling capacitors going to the grids on the power tubes. After replacing one of them with a replacement that I had in my salvage stash, most of the symptoms diminished markedly. So the plan was/is to replace all the caps, just because they're all so old. I do not disagree with this. Seem like cheap insurance to me. So while I'm waiting for the parts order to show up, I'm sitting here gazing alternately at the schematic and at the amp itself in an attempt to get better at reading one against the other. Most of the time I feel like I'm doing pretty good. Suddenly I notice something that seems totally wrong to me concerning the two coupling capacitors in question. It looks like their polarity is reversed!! If you look at photos 2 or 4, you can see them, one just above the other, just to the right of the input jacks. The lighter colored one (C11) on the top, and the black colored one (known replacement C12) just under it. I also notice that the lighter colored one MAY have been replaced also. I do notice that the legs on it are a lot longer than all the other legs in the amp. Most of which have been trimmed to a more compact length.

So please correct me if I'm wrong, but as I understand it, the end of the cap that has the solid line going around it is the negative end, and on the symbol for the caps in the schematic, the strait line represents the positive end, and the curved line the negative. How am I doing so far?

Ok, so what I think I see is this; (please reference schematic in first post)
C11 should have the + lead going to pin #1 on V3 (phase inverter), and - lead to pin #5 of V5 (6V6G).
C12 should have the + lead going to pin #6 on V3    (same tube),   and - lead to pin #5 of V4 (6V6G)

If this seems right to you, please look at the pictures again. I don't think you can make out the pin #s but you can see the negative end of both those caps is facing left, toward V3 (12AX7 inverter), and the positive end is facing right, toward the power tubes. What i see in the chassis is exactly opposite of what I've described above!
Negatives of cap are going to pin 1 and 6 (of 12AX7) and positives are going to pins 5 on each power tube.
Just for confirmation I traced the location of R24, 25, 26, and 28, just to make sure everything seemed to jibe. All these did agree with the schematic. For further confirmation I checked the polarity of several other caps as I interpret, and found them to be in agreement with the schematic.

Did I find the problem?, or am I all wet?
Dying to have someone look and render an opinion and or conclusion.
Thanks a million in advance.
Pete

Hi again Roly, and G1,
OK, let's clear this issue up first. "Guilty" on the charge of viewing the YouTube video mentioned. This IS exactly where I got my information. G1, thank you for the link. I DID read Roly's article. Makes perfect sense to me. I have been converted, and educated. Thank you so much. (I'm learning as fast as I can!!)

Roly,
Now to the amp. I was hoping to retain as much of the original circuitry as possible. In this case, what you say does make perfect sense. Doesn't mean much to me to be original if it won't work. I WILL be replacing all the caps. I found these at "Just Radios". According to their product information these look (to the untrained eye) like they will fill the bill. They are available in the values that I need, and are very reasonably priced. Would you please render your opinion, and/or make me aware of other options or even a recommendation.

http://www.justradios.com/DMEcapacitors.html

By "electrolytics" are we talking about the large filter capacitors, four in one can? If so, I have found a new replacement with identical specs for somewhere just under $40. I also have an old, but very unused one that is 20, 20, 20, 20, 450 volt. Do you think it would be false economy to try using that. It's not an easy one to replace. (Very tight and lots of stuff soldered to it.) And I know caps go bad just from age. They don't really need to be used.

I will be following your specific advice on the resistors. I'm hoping at least SOME of them read ok, just so I don't have to replace SO much stuff!!

On the issue of being an "over the top" repair. I don't view it that way at all. If in the end, the result is a very nice sounding, somewhat rare, nice vintage tube amp, it's worth every minute of work, including all the research I've had to do to accomplish this. The learning part that I get out of it is the most important part of my reward, in addition to having the amp. It will take me some time to get all the parts in, and get all this done. I assure you I will post again on this topic when the repair is completed. In the meantime, I've included some pictures of the amp taken before disassembly. Hope you enjoy.

I can't wait to get this completed!!!!! Thank you Roly again. I think it probably would have taken me quite some time to stumble onto the solution, had it not been for you bringing those coupling caps to my attention. This is where experience comes in. What's totally baffling to a noob is so obvious to someone who's been around the block. Your knowledge and experience have been invaluable to me, and I don't take it for granted. God bless you for your generosity in time and advice.

Waiting to hear your comments on my post. Oh, and I got the 300W bulb at Home Depot. You wouldn't happen to know anyone that could use a 300W bulb would you?

Pete

I meant 6L6s.

Another update Roly.
I got to thinking about those coupling capacitors. I can see how if one was even partly shorted, and letting some of that 125vdc into pin 5 (grid ?) on the power tubes, that seems like it would spell big trouble. I don't know the proper way to test them but . . . My multimeter will put out and audible test signal. One of those caps seemed to be shorted, as the test signal would go right through it. It would not beep if I bridged any of the other caps I tried it on. I checked it several times and got a beep on that one cap (C11 or C12, not sure which one). Funny thing is, after I went to my salvage stash to look for a matching value cap, I tried it again and got no beep. I decided to try replacing that one cap anyway. (good luck in the salvage stash) After doing so, . . with both 6V6s plugged in, my test light would glow a little, but the amp was working!! I didn't want to leave it on long enough to test any voltage readings as I don't know what really should be checked. What do you think I should do?

Where should I go from here??? Do you think it would be wise to just wholesale, replace all those paper caps?, or just all the coupling caps? Would that be C6, C7, C8, C9, C11, and C12? What do you think would be the safest bet? Or am I not necessarily on the right track at all?

Thank you again for checking this out for me. I really need some guidance, and I can't express how much I appreciate your time.

Pete

Hi Roly,
Back again. I tried my best to follow your instructions. I hope I got it all right. Here's what I did, hoping I didn't misinterpret.

Disconnected C5 (?) at cathode end. (Pin 8 ?) With the 6L6's out, zero voltage before AND after disconnecting C5.

Coupling caps (C11 and C12 ?) are different from each other in appearance, but of same value, as specified in schematic. One does look like you describe as waxed paper. The other is black plastic looking. I have to say though, that they both look original. You can see them right next to each other in one of the photos.

With C5 still disconnected, and getting a good reading on the cathode resistor (R27 ?) of 197ohms;
I plugged in the new 6L6's and clipped the meter on each end of R27. Turned on power. Voltage quickly climbed past 25 volts, to 50 volts and still climbing quickly, at which point I powered off.

Please let me know if the pictures are adequate, or if you're trying to see something specific. I'll get a better shot for you.

My safety light IS 300W. I included a picture for your amusement. Is there a reason a 300W bulb is not a good idea? I was told the heavier filament in a high wattage bulb would act more like a wire, and not light up so easily. Please advise if this is not recommended.

Hi Roly,
Well, my anxiety/frustration has already diminished some. Thank you so much for taking a look. I'll follow your advice/instructions and be back to you sometime later today or tonight.
Pete

OK I know this is a SS amp forum, but I'm hoping someone will have mercy on me.
Knowing almost nothing about electronics, I'm faced with the following problem. Working on a 1954 Masco model MU-17. (I've attached a schematic, but it is quite small)

The schematic says i should have 330v at the plates of the two 6L6GT power tubes. I have 468v.
I'm supposed to have 340v off the 5Y3GT rectifier. I have the same 468v.
Checked that with a new tube also. Same reading.

If I plug in either of the two old power tubes (one of which I KNOW is bad), into either of the two sockets, I get some kind of short as soon as the tube warms up, and my in-series 300w safety light starts to glow. Same thing with two new replacement tubes, again in either socket. I see no conspicuous faults in the circuitry or components. Everything is clean and appears to be in outstanding condition. No mess in the tube sockets or anything like that. If someone could have a look at the schematic and maybe tell me what to check. I've checked all the voltages from the power and the output transformers. Can't find anything that isn't just a few volts higher than what they should be. I sure would appreciate any suggestions. Note; All I have for test equipment is a multimeter.
Thank you in advance,
Pete

Hi Roly,

Status report on the SoundTech 4150;

First of all though, thank you for your time, expertise, generosity, and willingness to take a look at the schematic of my problem amp, and render some possible solutions, or at least a couple things to check out. This whole thing of amp repair is my latest hobby endeavor. It's something I've wanted to make time for, for decades. I guess better late than never, as it is said! I find it amazing and remarkable that there are people out there willing to help for nothing more than a thank you.

So far I've been able to salvage a Peavey XR700 powered mixing console, a Peavey XR500C mixer amp, and a Peavey KB100 keyboard amp. (that one still needs a little tweaking). And now, with your help, it looks like I've got this SoundTech amp back in business.

I followed you advice on checking out the grounding first. One thing I thought was kind of cool about this amp is the way the power amp was on its' own chassis that mounts to the rear of the cabinet, while the inputs and equalizer (and preamp, I believe) mount separately on its' own chassis (face-plate really) on the front of the cabinet, connected by a ribbon cable. One thing that occurred to me though, about this arrangement, that didn't dawn on me until after I read your suggestions. There's no real good ground connection between the two chassis.  I'm sure there's one in the ribbon, but how good? So I added a nice substantial ground wire from chassis to chassis. At the same time I got some real cables to run from guitar-to-amp, amp-to-speaker. All my regular cables were left at a gig on Friday night because we were coming back on Saturday. I was forced to kind of mouse-up some adapter cables in the meantime. I'm sure this wasn't helping anything. So anyway, I put it all back together and voila! Everything works! No buzzing! All seems well so far. Thank you so much Roly. I will definitely be addicted to this forum for some time to come. I truly hope that at some point in the future, I can offer somebody useful advice as well. Thank you for the warm reception.

Pete