I was following along and meant to send this out last night, sorry if it's useless now.  xP
I've been drugged up lately from some recent medical work so I'm not all here.

Just wanted to repost the schematic with a bit of touch up for readability, hope this helps.

Quote from: BungleFever on August 26, 2010, 02:57:10 AM
would that cause the main amp to buzz? or just the extension speaker. i've ran stuff through the extension alone and it's fine.

If this is what I'm thinking it would (usually) cause everything except the extension speaker to buzz, because plugging something in solves the problem temporarily. I've put a picture in below to help show what I mean. There are several types of switching jack, but this sort let's me demonstrate most easily.

Switching jacks like this are usually used in such a way that when nothing's plugged in the jack is effectively bypassed. When something's plugged in the contacts are under good tension, so everything's ok (right side of picture). With nothing plugged in the contacts have a smaller spot to contact and are under much less force from the natural spring of the metal. If the ground contact was dirty or weak it could cause a buzzing, much like if you hold just the tip of a guitar cord in your fingers while it's plugged into an amp.

When I've faced this problem it was with a headphone jack in a practice amp. Cleaning the contacts didn't help, but if I held the connection closed from above with a toothpick it cleared right up. I wound up replacing the jack figuring it was only going to get weaker over time.

I'm a bit new at all this myself, so this is just a first guess at what might be an easy fix. Mr. Fahey is one of the many professionals here, so working with him should be much more promising.

Hope that helps.

The first thing I'd look at is the jack for the extension speaker. The contacts in the switching portion might be dirty or may have lost their tension. It might be as simple as that, but if not it's the sort of thing you'd definitely want to rule out.

Quote from: J M Fahey on August 16, 2010, 10:39:39 AMHi DJPhil
1) Thanks for pointing to the spammer. (Sigh!)

No worries. It's like whack-a-mole, I just report them as I see em.

Quote from: J M Fahey on August 16, 2010, 10:39:39 AM2) You are not crazy, you are absolutely right.
That amp IS a piece of cr*p.

Woohoo! I'm not nuts!

Quote from: J M Fahey on August 16, 2010, 10:39:39 AMPlease post some interior pictures, boards will tell us a lot.

Board Bottom 1
Scratches are from me cleaning out flux residue from resoldering after component checks.

Board Bottom 2
Sharpie marks are mine, the brass standoff is my addition as the board was unsupported except by the pots and jacks. The nasty looking solder gob near the standoff is the chassis ground.

Board Bottom 3
The tacky looking stuff is electrical tape residue. The isolated solder pads in the upper left are for the fuse clip and blade terminals. I was sad to see this arrangement used and I don't really consider it safe.

Board Top 1
The electro that looks like it's in the way of the switch is on long enough leads to bend out of the way safely. This is the input jack and preamp section.

Board Top 2
There's the poor RC4558P. The lighting makes the chip look blank but it's clearly marked with a TI logo (though who knows these days). The three pin sockets go to the power transistors, which are TO-220 Isolated types with small breakout boards. They're screwed into the chassis with no thermal paste.

Board Top 3
Power and output section. There's the fuse holder I hate so much. The black and white wires are speaker output and the three pin connector top center is for the transformer.

Enclosure Back 1
They were very careful to make sure you know that this is patented pretty much everywhere.

Enclosure Back 2
I wish they'd have used an IEC connector, but that's asking a lot of a budget amp.

Enclosure Front 1
For the sake of completeness.

Enclosure Top 1
This is what makes me crazy about the fuse, there's a D punchout right there in the back! They seem to have cheaped out on a proper fuse and covered it up with the giant label. I've rigged up a temporary fuse holder for testing, and I've got some proper fuse holders on order. I'll feel a whole lot better when I can't see metal that's at mains potential.
The grommeted hole is for the speaker wires, and the washer scarred hole just down from the ground is for the board ground. Any two of the four holes in the center can be used for the power transistors, they're all threaded. There's a bit of spare room in this chassis, if you can imagine the board only extending to the fuse holder all the way across.

Transformers
This shows the label on top of the transformer. For a size comparison I put the 36VCT 180VA transformer I picked up a while back next to this one.
I mistakenly said in my first post that it was rated for 36VCT instead of it's marked 32VCT. I hooked up everything again (no speaker connected) and read voltages just to be sure. With the transformer attached to the board and no substantial load other than the rectifier and filter caps I was reading slightly over 18VAC RMS on each side of the secondaries. I tested the wall outlet (I've got a good, safe meter for this) at 121.8VAC RMS. The voltage across the power pins of the RC4558 was 48.02VDC with a little ripple.
I'd expect such a small transformer to regulate poorly at low load but I'd think that they'd factor that into the design. The AC output would have to droop all the way to about 26VCT before it was nearly safe for the opamp.

Quote from: J M Fahey on August 16, 2010, 10:39:39 AMI also suspect Taiwan speakers and not Eminences. Frames tell a story too.

The rest of the box is with my friend at the moment so I can't include pictures. I built him a LM386 baby amp to keep him going while I was working on this. The case is a mix of plywood walls and particle board front/back strip. The speaker is an unmarked 6.5" that actually sounds pretty good on the pocket amp.

Quote from: J M Fahey on August 16, 2010, 10:39:39 AMI suggest you drop that power amp and install an LM3886 board , powered by the existing power supply.
You will have around 35W available 

My plan was to slap some zener's in for the opamp just to get things under control, add a proper fuse holder, and begin work on an entirely new amp. I've got a few TDA2030s on order to play with and I might just wire one up to take care of the power section on this one.

The new amp will likely use that monstrous 180VCT transformer from the picture and a LM3886. It's really tempting to go with a discrete build, as I've already got a bunch of TIP31/32Cs handy. Either way he needs a real speaker and cab as well, so I figure we'll take it slow. If we do it right, we'll have a good speaker cab (lots of good woodworking tools handy, my father's hobby) and an amp with enough power. Then the fun begins for me, and I'll spend all winter tinkering with preamps. 

Quote from: J M Fahey on August 16, 2010, 10:39:39 AM
3) What are those Leprechaun jokes anyway ?  :trouble :trouble
My Father's family surnames were Fahey Ronnan Clavin Clarke, from Galway , Eire.  :grr

Beannachtai! My family's from Cork originally (about four generations back). I'm half Irish and half French Canadian myself, which can be a bit confusing at family gatherings. When we moved out here to the middle of the US, by some quirk of fate we wound up living on Leprechaun Lane. It's the first thing I reach for when looking for a mythical creature.  :tu:

I think I've finally figured out the problem with my friend's amp after five months of on again, off again head scratching.  xP

I'm going to try to resist the urge to turn this into a fifty page writeup on how slow I've been to see the obvious. I'm posting this to get a sanity check on my conclusions and also to leave a record of this issue somewhere for others on the internet to find if they have the same problem.

The original problem was excessive hum. I had no trouble getting the schematic from Peavey, they were quick to help. I did have to specify that I was after the Backstage II schematic, also known as the Backstage '04. I've included the schematic in attachment for review. The difference between the two is striking. The Backstage II is in my opinion a cheap knockoff, cheaply and possibly dangerously constructed. More on that by request. The important detail is that the Backstage II is rated at 10 watts and has an 8Ω speaker. Though I can't find any info on whether that's peak, rms, or leprechaun watts I'm pretty sure it's RMS.

Over the course of testing the amp I verified almost every component, many out of board, that was not specifically contained in the overdrive circuitry. All of the obvious things were slowly ruled out. The first problem that really tripped me up was finding ~50VDC across the poor RC4558 opamp's power pins. That's way over the max rating of 36VDC, and the chip would heat up to about 70C at idle. I'd guessed that such an overvoltage would very likely cause poor supply rejection, and that I was lucky that hum was the only problem!

Going back to the schematics showed that there was nothing in the design stopping the Vcc and Vee rails from delivering their full voltage to the RC4558. The part number on the schematic matched the part number printed on the (clearly labeled, yay!) power transformer, which is rated at 36VCT. This The opamp is essentially configured as a summing and voltage gain stage for the output transistors.

My first thought was that they left out (among other things) some way to drop the rail voltage down for the opamp. I cursed them for being cheap enough to skip a pair of zeners and a pair of resistors that would have made this amp less of a complete lemon, especially after seeing odd value E24 1% resistors all over the board. As my understanding of amplifiers grew I realized that the output transistors aren't providing any voltage gain, so it occurred to me that there was no reason whatsoever to use the transformer they did. They could have used a 24VCT transformer from the start and wound up with the same output power without baking the opamp!

:duh

My conclusion now is that either by design error or supplier miscommunication they wound up with the wrong transformers for the job. However it happened, they let at least some of the amps out the door like this, and the results are strewn all over the review sites. Many complain of loud hum, noise, and early failure. One poor fellow was told he blew out his guitar cable. People can be jerks sometimes.

So that's the short version. I've got a half finished spice circuit, pictures galore, datasheets, and measurements if anyone needs them. If anyone has questions ask away.

My question to the more experienced is: are my conclusions sound? I'd really love to stamp 'Solved' on this mystery.

I'd guess that the primary complication would be the power supply, followed by inter-stage coupling. I know Vox does something like this, but I haven't dug up schematics yet to see how they did it. I believe they run a dual triode like the 12ax7 at low B+ voltage in an amp with high supply rails to begin with. Come to think of it, I haven't managed to find VT50 schematics anywhere yet as Vox is tightfisted with their in-production schematics. I'm pretty sure Teemuk's book has a bit on this in the preamp section.

I'd imagine that you'd have to do a lot of work to make it worth the effort too. From what I've read it's not as simple as just wiring in a tube, it takes some careful design to make good use of the non-linearity inherent in a tube without undesirable side effects. There are a lot of 'tube preamp' amplifiers out there where the only thing the tube is really doing is increasing the manufacturing cost by 20% and the retail price by 300%.

Just a noob here, but hope that helps some.

Quote from: jtayres on August 09, 2010, 08:37:43 PM
thanks a lot for the hints. i pulled the trim pot but haven't got it on the meter yet. I'm adding a couple more pics. one of the bottom an the second is a close up of the area you isolated in your picture. I thought that looked like it might be grounding out but it seems like there is a plastic insulator inside the hole. but maybe its not enough.

Ah, it's tough to see in the pictures but it's good to know there's an insulator in there. I'd recommend seeing if any of the pins are loose enough to wiggle (think vibration from sitting on a speaker stack) and testing to be sure that there's no conduction between the wires and heatsink. I left out the transistor testing link in my last post, guess I was more tired than I thought! That'd be what I suggest next. Also worth checking would be the rectifier diodes, the ones hiding next to the huge filter cap on the transformer side. Be sure to at least desolder one side for testing, if you leave it in circuit the transformer windings will (hopefully!) give you low resistance in both directions.

Last thing, it's probably just a trick of angle, but it looks like there's not much of a connection on the pilot light lamp's lower leg. This shouldn't really mess with anything unless it's failed as a short circuit somewhere inside (unlikely, but easy to test quickly). It looks like it's on the primary side of the transformer, and I get a bit nervous seeing exposed metal with the possibility of mains voltage. I'd recommend testing it with a meter quick to be sure it's not a dead short (should be at least a few hundred ohms) and taping it up. If you have heat shrink tubing that'd be ideal, it's worth desoldering wires to do each side and then a big one over the whole thing. If I'm wrong and it's a low voltage connection then don't sweat it too much. I'm just thinking ahead to when you fix whatever's blowing fuses and you go to test the amp under power, I'd feel bad if I didn't say something when I noticed.

My (underdeveloped) intuition for consistent fuse blows is to look to the power transistors and rectifier first and then check the transformer. Power semiconductors like those sometimes act like ancient royalty, they'll gather together their favorite servants and bring them with when they go to the afterlife. When they fail they unleash so much energy into the circuit that it sometimes pays to go about checking the local relatives (transformer, speaker, output connector) to make sure they escaped before adding power again.

I hope that helps some. I'm still somewhat new at amps, but I'm sure the pros will speak up if I'm too far off the mark. 

That is a grungy looking trim pot! It's likely there to set the bias, and replacing it should be fairly cheap (shipping aside). You could desolder it and test it with a meter if you have one handy. If you do, you should probably reset the bias when it's all back together (ask if you need help, I'd have to look it up to be sure). Lots of the older cheap amps use a system for setting bias that's very unhappy if the pot wiper lifts, and it could be related to your problem.

I attached a highlight below of something that gave me pause. If that wire touches the heatsink, well, I'm not absolutely sure what would happen but it's not good. That's probably the Base connection (assuming it's a standard TO3), and it's usually grounded or attached to ground through a low value (<5Ω) resistor. You might take the opportunity to test the output transistors while you have it open, here's a guide on doing so with a multimeter. It looks like the mounting bolts are insulated, and if so be sure that there's no conductivity (infinite resistance) between all three pins and the heatsink. Assuming it's a standard complimentary pair it's likely to have a pinout like these transistors from ST, but don't take it for granted, try to get a number off of them and look them up.

I know this is barely a start, the pro's will likely be able to give you better advice. I just mop the floors on the late shift sometimes.  

Final note: Pictures are a great idea, and always a help. You may want to unscrew the circuit board and get a couple of the bottom side in case there's something obvious there.

Hope that helps.

Edit: Would help if I attached the photo, I suppose.  xP
Edit 2: Forgot the link for transistor testing too, sheesh.

Quote from: shinychrome0 on August 08, 2010, 08:47:12 PM
ok i put  68 ohm resistors in the feedback loop to kill the feedback(and replaced the chip, which was apparently fried) and now it works great!  I'll just have to play with some capacitor values to find the right cut off point, and then i'll be ready to solder!

Excellent! Here's a calculator that might help. Sallen-Key Low-pass Filter Design Tool

There's a bunch of good filter calculators on that site.

Hope that helps.

Given that your problem is so frustratingly intermittent, my first instinct is mechanical. It's probably because I just had a horrible run with a headphone jack myself.

In my case the jack's innards were exposed, so I could cram a wooden toothpick between the switching contacts. This verified in my case that one of the contact arms had lost it's spring, so it was intermittently open circuiting the output. A quick cleaning and a pinch with the pliers solved it for me.

Hope that helps.

Just wanted to throw in my 2pf.

I've been neck deep in filter literature lately for one reason or another. It seems that as a general rule analog filters represent a trade-off of adjustability, performance, and component sensitivity. As I learned more I remember thinking that the engineers were crazy, after all, a simple tone stack is a bare handful of (usually cheap, low performance) parts. They were lamenting the need for several precisely matched capacitors and resistors per circuit. Ok, we don't need that level of precision, but something's off.

In studying some of the passive tone stacks I found that often the settings were interdependent, i.e. one filter's settings could change the response and basic function of another. The engineers would find this messy heaving of load around a circuit to be insanity, but the early amp designers didn't care about precision. They just wanted a low parts count circuit that'd be as flexible as possible. I suspect a lot of their work was experimental 'see what it sounds like' sort of exploration, and when you could get six components and two pots together to make an interesting range of tone it was time to celebrate.

It was a bit of a revelation to me, but in hindsight it should have been obvious that these two camps had different goals. I'm sure I've read exactly this sort of breakdown before, but it didn't sink in for me until a few days ago as I was trying to make different values of pot work in a tone stack.

Hope that helps.

Greetings,

Sounds like a good find! There's a bit better schematic here, the one you posted is a bit blurry. The schematic lists the tank as a model 4FB2A1A. You should be able to use that part number to find an exact replacement ([urlhttp://cgi.ebay.com/PAIR-ACCUTRONICS-REVERBERATION-REVERB-TANK-FENDER-/350375068243]ebay is one option[/url]). To install it all you should need to do is bolt it on, plug it in, and try it out. The tank itself is basically a microphone and a speaker glued to each end of a spring, so all the electronics are in the chassis. There's no other adjustments necessary that I'm aware of, though you might wait to get a second opinion from one of the pros just to be sure.

Hope that helps.

Just looking at it I'd say that S1B is simply unused. It may be that there's more to the system on a different model that uses the same schematic, and in this version it's just grounded to keep it out of trouble. The loop is barely there in practice, as pins 3, 4, 5, and 6 will be shorted to ground on the circuit board.

Quote from: jordileft on July 16, 2010, 10:40:34 AMWhy should I change C33? this filter cap value has nothing to do with the amplifier used, just with the fc, isn't it?

I said that to be conservative, but yes, it's essentially part of a 50Hz high pass filter. If I were to build a separate board for the power amp section I'd have put everything from C33 down on it, it's just a conceptual delimiter.

Quote from: jordileft on July 16, 2010, 10:40:34 AMI've been looking the datasheets and schematic but I don't know how to calculate the gain of the TDA1514 on the Frontman schematic. Otherwise, I don't know what would be the difference from the LM3886 datasheet figure 2 feedback which has Rf1, Rf2 and Cf than the feedback on Frontman schematic, with Rf1, Rf2 and a resistor replacing Cf and a capacitor tied to ground between that resistor and Rf2. If I knew the gain on the frontman TDA1514 gain and how the difference on the feedback between LM3886 datasheet and Frontman schematic affects on the formula of gain which appears on LM3886 datasheet, I'm sure that the most difficult part would be solved. Could someone help me with those two things?

I can't say I blame you there. The datasheet has a note on the closed loop gain spec on page 7 for calculating gain. The feedback path is a bit convoluted, but the info in the LM3886 sheet regarding the feedback loop applies to the TDA1514. The gain works much like an opamp, with the extra bits rolling off gain at DC and high frequency for stability. The gain calculation for the TDA1514 is roughly equivalent to R3/R2 on the test circuit. Looking at the Frontman schematic and taking R58 in parallel with R59 + R60 I get a gain of about 13, compared to the datasheet circuit of about 30. That sounds . . . close to what makes sense.There may be something going on I don't understand, so I'll pass on certainty and refer it to the pros.

In the end, you can build the preamp section up to around C33, test the output, and configure the LM3886 for whatever gain suits you. There's a guide at the end of the datasheet to help. This is probably what I should have advised in the first place, but I didn't know the Frontman circuit would be such a beast!

Quote from: jordileftI've been thinking and I hope someone could tell me if I'm right, no matter what's the gain on the frontman schematic with TDA1514, as gain is the relation between Vi and Vo given by the components on the feedback then any components on the feedback on a TDA1514 would be the same gain than in LM3886 if the values are the same, is that right?

Sort of, but frequency matters too. There are factors which might cause the specific values of the resistors to change, but the ratio of resistances between Vo and Vi, and Vi and ground will mostly determine closed loop gain. I wouldn't copy/paste the parts values, but it should be close.

Hope that helps.

Quote from: jordileft on July 16, 2010, 03:11:47 AM
Despite being my first SS amp I actually have build a tube amp (a Fender Blues Junior similar amp) and some effects pedals, so I'm not a beginner on building amps, but my electronics knowledge on amps is just a little more than the basics. The problem for me is not building or the complexity of the preamp, the problem is having a part on the schematic that I can't find nowhere, so I think that if with your help we can find the changes needed the rest won't be difficult.

No worries. That's a sweet sounding tube amp!

Looking at the schematic, everything from R55 (which I think is limiting the current draw from the opamp buffer upstream) down is your power stage and should look somewhat familiar. Essentially you'd be replacing C33 (DC blocking cap) down with the LM3886 reference schematic and adjusting the feedback and gain to suit. The TDA datasheet is short and sweet, but the LM3886 has a heap of appnotes explaining the reference circuit and changes you can make to it. You may have to tinker a bit with the finished product to be completely satisfied, but that's half the fun!

I think you'll be surprised how easy it is. These sorts of chips are designed for low external parts count, so they tend to be fairly simple to use. You can also copy off of a schematic for a 3886 powered amp and see what they did. I've got no knowledge of amp models, so I can suggest any, but I'm sure there's folks around who'd probably be able to give you a half dozen examples.

I know that wasn't to specific, but I hope it helps some.