Thanks very much for the n00b-friendly response, DrGonz. Particularly like the continuity check advice. Believe me, I do err ridiculously on the side of caution - perhaps too much so (thus, 15 years of DIY DC electronics messing about and still no meaningful live-mains-circuit experience). I hopefully will bring back some readings on this bad boy later today.

Yeah, I see what you mean now, DrGonz - there *is* a slight fracturing of the left-leg insulator on the green cap at the "back" of the red channel. I would be surprised if this would alter the performance of the cap but it sounds like you have had different experiences...

Awkward n00b question / moment of truth: I am happy to try and read voltages as rep'd on the green channel and I can figure out how they are "mapped" to the red side since the two circuits really are quite similar. However I'm not used to having to read such values on live circuits, esp. AC-based. (From the schem, I do get that the red and green channels are receiving 24VDC on the marked "E" leg of the power supply.)

To get these readings, I'm guessing I want red DMM lead on the point / leg shown, black DMM lead clipped to chassis ground somewhere. If this is not the case then please fill me in before I proceed and/or produce sparks.

...Mensur, I just saw your latest edit to your post (about routing things from the middle leg of the red channel master pot to, presumably, the power amp input?). Can you be more specific about how I would achieve that?

Quote from: mensur on December 12, 2012, 02:18:00 PM
Those two *IC* are 220nF tonestack caps, I guess that one is broken so it need to be replaced along the 2.2nF treble cap if it's leg is cut offed.If that's the case, signal only goes throughout mid pot, and energy is extremely reduced.Again from the pics, that's the only problem.Here's one trick, route the signal form master pot.middle leg to the amp input, and let's see is the tonestack the problem.

Thanks Mensur. I can't read the top of the components in question because the print is so fine / light, and I have not personally seen caps like this (knowingly), but from a few context clues elsewhere in the amp I imagine you are probably right about them being caps. The cap on the left is *not* broken, there is a light scoring line that goes across the top of the body as shown and continues on to the top of the adjacent cap. It could have been inflicted by a soldering iron chisel tip at the time of mfr and is incredibly fine - definitely does not penetrate the plastic body of the component(s).

The two green caps pictured are also solid - no leg breaks on either side. I have not yet had a chance to pull the board completely out of the chassis to look for obvious things like broken joints on that side, but I have gently poked around the components in the red channel and nothing seems obviously loose yet.

Thanks for having a look, DrGonz - I don't have the amp in front of me right now but I will look more carefully at those components a little later today. Going from visual memory of yesterday's chassis-opening, I believe those are two block resistors to the right of the clipping diode / pot, and I think the "crack" is just a surface-level scoring of the component that the flash brought out, but I'll look more carefully at it now that I see it.

And sure, I know that just looking at things is rarely likely to produce results but the last few SS amps I've had to fix did in fact have visually obvious issues - axial-lead caps with one end blown clean free of the cap body, power transistors with burn marks and/or holes, etc. Sometimes you get lucky.

Just to rule out the single-coil factor for Mensur's benefit (because I know these amps aren't normally popular with us Tele-playin' types), here's a similar demo using a bridge humbucker. Note that there's basically *no* output whatsoever on the red channel with gain on 5 / master on 2, and that the in-room output level of the red channel on gain 10 / master 10 is actually much quieter than the clean channel on gain 5 / master 2.

Posted some pics of the red-channel section of the board and the whole amp here... left them at full res for detail and zooming around.

Thanks guys. I finally had some time to pop the amp open and, as is typically the case, there is nothing visually obvious like broken solder joints / exploding or leaking caps on the red channel. (I'm super impressed by the build quality on this thing and it does *not* appear that a PO has done anything inside.) Taking a few measurements (carefully!) while in operation is next, I guess.

Single coils or no, I should be hearing a lot more gain and certainly a *lot* more volume from the red channel. Had plenty of experience with this guitar / bridge pickup through metal-esque preamps and pedals of the same ilk. I plugged my Hafler T3 into the return and used the same guitar-- the power section of my RG80 is VERY healthy. Enough to get local police attention with the Hafler's master volume set on 2.

A little late to the party, but hopefully this will still help you in your project. I think there are still some big gaps in the modeling-gear world and hopefully you'll steer your project in a direction that may help fill some of those gaps.

I'm new to the forum, but I'm going to take Enzo's cautionary approach here and suggest that you substitute "classic SS / tube amps" for "tube amps." I see you've already taken that into account. For the most part, at least when considering a good basic clean tone, I feel that good vintage solid-state amps are much closer to the performance behavior of conventional tube guitar amps than modeling amps (though, of course, you can't expect any solid-state amp to produce pleasing overdrive when the power section is hit hard), and for the most part I'm primarily using good vintage solid-state amps at this point in my playing "career." So all my answers pertaining to your "modeling vs. tube amp" questions are really responses for "modeling vs. vintage-style / purely analog SS amps."

1. The "sonic value" factor of modeling is certainly what has made it such a big deal in the last 15 years. There are certain types of tones-- particularly the super-distorted metal sort of tone in which playing dynamics are much less of a factor than they would be in lower-gain genres-- at which modeling devices excel dollar-for-dollar against the usual tube or even SS suspects that normally populate the boutique end of the equipment spectrum. The sheer number of tones to play with, along with the added possibility of DSP effects, makes them a great / somewhat pedagogically useful value proposition for less experienced players and less critical situations.

2. The biggest problems I have with most amp modeling algorithms is their lack of dynamic response and the palpable latency that occurs even on a faster, low-latency platform. I don't hear a lot of players critiquing the latter, but when I play through most modeling devices I can feel a clear "disconnect" with my guitar that doesn't occur when I'm plugged straight into a good analog pedal and amp, and I believe this disconnect to be related to inevitable DSP-related downtime, no matter how short. Most modeling algorithms do a very poor job with things like mild, dynamic overdrive that are easily attainable with even basic solid-state analog electronics, and I find that their clean tones are not much better, lacking the warmth and sparkle that any good tube / SS amp will effortlessly produce.

3. No. Further, I think it's easier for a picky modeling user to be satisfied when they are told that their model is only a "holistic" genre of tone (the Yamaha approach) vs. a specific amp (nearly everyone else's approach).

4. There are certain VST plugins (Studio Devil, some patches on Amplitube, the LePou plugins) that can generate nice-sounding results *on "tape"* with the right cab emulation selected. I also like the sound of the old Yamaha DG series amps in actual performance, and the Vox Valvetronix amps are passable if much less so, but unless I am playing death metal that day, I would generally still prefer to play through a good SS head with a couple good dirtboxes in front.

5. This is, of course, a classic "argument" not only in the guitar world but in recording and hi-fi as well. The comparatively useless adjectives you often see bandied about in regards to "digital-sounding" devices: "cold," "icepicky," "brittle," "one-dimensional," "uninspiring," "sterile." To try and describe these things more quasi-empirically, particularly in relation to various not-so-great-sounding guitar effects (and modelers) I've heard, they nearly all have to do with added (sometimes undesirable) clarity in the highs and the aforementioned missing dynamic distortion / compression characteristics. And I've heard many of these adjectives leveled somewhat unfairly at analog SS guitar amps as well. Amp modeling in general is an attempt to mimic the distortion and compression behaviors of valves, but at this stage in the game it's only gotten to a certain point, and the difference in the "grey area" of just-breaking-up is still so vast that I believe most players can still hear the difference.

6. I'm a tweaker, a guy who will occasionally get down to nuts and bolts in order to get a particular sound I can't get otherwise, so a well-designed product with a lot of depth doesn't scare me. If anything, I find that a big opportunity is being missed in terms of truly *new* sonic options. The modeling products out there, particularly effects floorboards, are more bent on mimicing "classic" devices with plug-and-play ease of use emphasized front and center over really wild new sounds and tweakability. I'm disappointed that nothing in the DSP world has come along which can baffle heads like the simple misuse of the LFO on an 80s Korg SDD-3000. This all said, when it comes to *live* use / performance and particularly my amp, I do value immediately accessible fixes and simplicity. I do not want to have to dig into a menu and hold down a value key to access the same behavior as the "bass" knob on a classic amp. An optimally designed digital device would let me dig into things to my heart's content on my own time, but keep it simple and fast to fix things that matter - volume, gain settings, basic EQ voicing, maybe the amp model itself - when my brain and hands are busy trying to make a performance happen.

7. Aside from some very expensive brand-name VST-only options (like what Waves used to offer), crazily overpriced devices like the Axe-FX, and perhaps to a much lesser extent the Yamaha THR series (a little tool I'd love to have but do not find worth the $300 asking price), modeling is about as proletariat as gear comes in the current market. This is as it should be. CPU time has never been cheaper, and when you're shoving nothing more than a basic modern solid-state power section, a processing chip and a speaker into an MDF box, there's no reason this stuff needs to cost very much money.

8. I think nearly all musicians are willing to compromise in certain situations. For instance, modeling is a compromise that actually makes more sense in many home recording situations vs. the headaches of cleanly recording a tube amp at whatever crazy required volume in a typical residential room. There are others who would feel differently and would never dream of using anything short of their nicest amp in the studio. When it comes to live situations, I want the gear to be the cheapest and simplest rig I can get away with. I have had way too much beer thrown at me on stage by overenthusiastic patrons to present them with a valuable, temperamental, fragile piece of equipment. (And that's where my love affair with old solid state amps began.) I think the flowchart of gear value / complexity is hard to sum up and will vary greatly from player to player, but I strongly suspect that most of us do have situations where equipment compromise makes a lot of sense. It's not like most guitarists can afford to be full-time divas.

9. The nod would have to go here to VST plugins; there are some really nice ones out there for guitar modeling which have zero cost to the end user, and they can definitely produce acceptable results when under the thumb of a patient and knowledgable user. Latency / interface issues aside, there's nothing that would stop you from using this stuff live, and I've seen more than a few bands that are obviously using their laptops as guitar preamps. Calculating the cost becomes trickier in this new world - do you include the cost of the laptop, especially when a) everyone has one and b) we're in an era when you can buy a VST-capable netbook for a hundred bucks? Oh yeah, and now there are also mobile iOS and, shortly, Android solutions that can achieve similar results for guitarists. In any event, your stated notion that quality goes up with cost has been totally thrown out the window in the last 15 years, and anyone designing a product to compete in this marketplace will have to deal with the reality of so many insanely cheap viable options / alternatives for those who can't justify, say, an Axe-FX.

10. I would definitely, without question, want the expensive guts of the box off the floor. I want the controls where I don't have to bend over and squint at them to adjust them in a live context. I don't want the CPU (or signal path) liable to disruption or destruction via my clumsy feet or those of my bandmates. I want to be able to replace the footswitch as easily / cheaply as possible if it gets broken and I want the show to be able to go on if it *does* get broken. 

Howdy, y'all. I brought home a Randall RG80ES head to add to the growing vintage-SS collection. Unfortunately, the big selling point of these amps - the distortion channel - is not fully functioning, and I'd like to get it going.

I am at a very basic level of electronics knowledge - happy to dig into things, avoid getting shocked by filter caps, totally willing resolder a component here and there, but I'm def not great at actual theory or troubleshooting. I do note the red channel on the 80/100ES, as shown on the schem, is a surprisingly simple circuit, which gives me some hope I can fix this without dragging it 90 miles one-way to my tech.

Clean side of my 80ES sounds great. The dirty-channel symptoms: extremely low volume (red ch gain / master on 10/10 = volume of green channel on respective 5/1.5), sounds a bit fizzy and the gain definitely sounds way too low vs. the many Youtube clips I've heard.

The amp has two other symptoms: 1) a slight power-supply hum at idle with both channels' MVs all the way down (which may be normal for this amp, I don't know); 2) the famous pull-treble diode clipping is not working at all. I realize the latter could be a bad switch in the pot but the pots are in pretty good, non-scratchy shape as is, so right now I'm guessing it's a circuit problem corresponding to the existing issues on the red channel.

Sound clip of the clean and dirty channels here for direct comparison (I'm riding the fader to add another 10dB whenever I talk, but the amp output itself is always maintained at the same in-room volume throughout the clip).

Getting ready to tear things apart - based on aural symptoms, failing obvious stuff like broken / cold joints and visibly leaky caps, what should be my next thing to examine?

Thanks y'all.