The thread concerning the Crate amp that won't stop buzzing motivated me to post my own sad story. But first a question. Are the suggestions for troubleshooting that were given for the Crate pretty much standard, in the sense that they can be used for an SS amp that's acting up? I was especially interested in the tests of the transformer, and the little doodad involving a resistor and cap placed between the amp (I'm guessing output jack) and the speaker.
Now for my tale. I've had this old amp sitting around for some time, but never use it because it is just a noise generator, and a loud one. With nothing plugged into the input jacks, you can't turn the volume up past 1 without getting a very loud hissing sound. Turn past 5 and the hissing is accompanied by an even more ear-shattering buzz. Plug in a guitar and forget it. You can't even turn past .5 on the volume or the buzz just takes over. I'm not doing any of the obvious mistakes either. I don't play next to a flourescent light, not using high-gain overdrive pedal, and am not living next to a Tesla Coil.
I've taken the amp apart, but couldn't find anything loose or, bad solder, etc. The pots make no real noise, and everything SEEMS grounded ok. From the photo you can see that this really is a simple design; it's just what it looks like, a 4 channel amp with 4 identical amp circuits leading into one "master." The hum is the same no matter which amp I plug into. There's no headphone jack, just two 1/4 output jacks, wired together (for stereo?) in the back. Hum is the same no matter which I use to connect the speaker.
I'd really like to be able to use this amp. I've never seen another one like it, and judging from the way it puts out compared to my 30watt, it's much more powerful than that. Plus it's got the "I could plug in FOUR guitars at once if I wanted to" vibe, and you know how crucial that is....
Any thoughts? I truly need this thing to justify my love. :trouble
See if you can bypass the preamp and locate the noise source to either pre or power amp. After this you could go on locating it to certain stages. Is what you hearing hum or hiss? ...There's a difference between them. Hum has a frequency of either 50/60 Hz or 100/120 Hz. Same you'd hear from fluorescent lights. If it's hum then most likely the preamp is inducing an awful a lot of it (because it seems like you can control it's amplitude by volume control). If it's not hum it could be almost anything - for example oscillation triggered by any input signal. Locate the source - then it becomes easier to troubleshoot.
Edit: That "little doodad" is known as Zobel network and it is used to attenuate high frequencies (i.e. oscillation). It's actually not a cure for anything - (more like a solution of sweeping dirt under carpet) - and best would be to leave it totally away. Sometimes it's however the only thing keeping the amp stabile. If your amp doesn't have one then I'm pretty sure it shouldn't need it in the first place.
Thanks for the suggestions.
Maybe this will help answer your questions.
1. It's hiss if I unplug the guitar and just crank the "master volume" (on the right in the photo). Loud hiss, no hum. Once I plug a guitar into one of the 4 amps (same with all of them, I tried), and I try to turn the master volume pot even a smidgen (i.e., less than "1"), I get a hum (it's a buzz to me, but if hum is the technical term, let's go with that) that's just so loud that you have to shut it back down. It's note the gentle hum that you get from, say, a tube amp. This is a "something is WRONG" hum.
2. Now, here's the weird part. Whenever I tinker around with amps, I always use my Pandora as a "signal injector." Plugging it in is a lot more convenient than plugging in a guitar when you want to test something. I just set it to play a drum fill, and connect it with the cord into the input jack. So just for the heck of it, I was plugging it into each of the four amps, into the Aux in jack, etc. Whatever orifice I found, I plugged Pandora into it and fired up the drum. No hum whatsoever. Still some heavy hissing if I crank it, but to be honest, at that level I'll get evicted from my apt before I care about the hiss. So i'm thinking, ok, that's cool if I only want to listen to a drum or metronome running through my amp, what about the guitar? I plug the guitar into the pandora (as it's meant to be, nothing fancy), then plug the pandora into one of the 4 amps. Hum is gone. It's like it was never there. Clearly the pandora is getting rid of the hum, but I'm not sure how. And I don't mean it's just adding noise or something to cover it up. Even cranked, the guitar makes no hum, and it does this regardless of which setting the Pandora is on.
So on one hand, I guess i can live life knowing that I can never play through this amp without Pandora acting as a mediator. On the other hand though, I'd like to know what's going on.
To the folks at Korg: great product! :tu:
Some ideas:
-Make sure no ground screws are missing, that go through tabs on the PCB. If they are all there, make sure they are tight.
-This is a pain, but remove the PCB and check the solder connections, especially on the jacks/pots. You'll have to remove the knobs and washers on all the pots first.
-Sometimes due to age etc corrosion forms around part legs, and sometimes you can even pull them straight out if it's real bad. Resolder bad joints if you find them.
Replace your power caps, if they are old, they will dry out and cause noises like this.
Hmmm... This seems like a grounding problem: The Pandora is between the amp and guitar so it possibly breaks a ground loop or something. How are the inputs grounded?
Thanks for all the advice guys. Ok,
I will tear into it again and look for bad joints.
Don't mind checking the pots, probably could use a cleaning.
The capacitors should be easy to replace. They are all out in the open (the amp is mostly empty space). All of them, or just the big egg-roll size guys?
Re: the pandora grounding theory, I think I understand, but the pandora's just a handheld box, it's not grounded to anything and runs on batteries. Wouldn't any grounding issues just transfer through it? ???
Here's some jpgs of the innards. The one photo shows the front panel pulled out, with the 4 amp modules and the 4 sets of input jacks. The other jpg is the back panel, not much there except the transformer and the power section, and two jacks for output to the speakers....
http://pg.photos.yahoo.com/ph/idle_chatterbox/my_photos
Thanks for all the suggestions. I'll get to work on it. 0:)
Just replace the two big guys for starters, those are the power caps.
Ok, will do. Thanks for the tip. For now I guess running through the pandora is an acceptable workaround. Still wish I could figure out what's going on though. Checked all the grounding points that I could think of checking inside, looked at the solder joints, and cleaned out all 22 (!) pots. Still hums, loudly, unless pandora is plugged into 'er.
Thanks for the advice guys. My ears thank you 8|
My theory is based on an assumption that Pandora is a digital effects unit. Somewhere inside it the signal is converted from analog to digital and then back. At this point the ground path may also be "cut" since analog and digital never actually should share an equal ground plane (only one separate point truly at 0V potential). This might be enough to cut a ground loop.
You should focus your troubleshooting on the input and see if the input / preamp grounds share any other (high current grounds) - for example, by making a connection to chassis via non-insulated jacks (your amp seems to use those). Ideally they should not do this: The input grounds should be connected either at the ground of their preceeding gain stage or or straight to main ground point (usually located very near rectifying / filtering ssection). Consider all other grounding points than the latter as conductors with resistance. Any current that does not belong there will be heard some way. i.e. Speaker return currents in the input ground are definitely not wanted. You'd also may want to hunt down all points where a ground conductor is divided (i.e. ground via chassis and ground via wire).
The idea to tighten all screws and check for oxidization / corrosion etc. was good: for example, a screw tieing the pcb ground to ground plane formed by chassis may have become loose. Also, have you tried with any other signal source i.e. radio/mp3 player - do they suffer from the same problem? You should override the possibility that your guitar might be causing this. You have already narrowed the source to input, try to narrow it more.
Those capacitor changes, cleaning and tightening all pots, screws etc. are always a good way to start fixing any old equipment.
Ok, thanks. I'll tear into it again (I'm thinking I need to attach the front panel with velcro!).
Once I order the big caps, I'll replace those too. Here's some additional thoughts. I've tried more than one guitar, and I've tried those guitars on other amps (solid state and tube). The humming only happens with this amp.
I tend to think, but could be wrong, that it's something to do with the "Master" section of the circuit, since the hum is present in all 4 amps, equally.
The comment about input and output ground made me wonder. Originally, the amp had the power switch (it is a 3 position switch, on-off-on) located on the back panel, right next to the transformer, fuse, etc. I thought this was a bit impractical, so I left that switch where it was, and spliced in another two throw switch and stuck that on the front panel. I was thinking that I had just done something pretty cool, since I now have a "standby" switch and a "power" switch. But did I do something really stupid? The new switch is now grounded on the front panel, but I guess I figured that the back and front panels were already grounded (they certainly were according to my multimeter) together anyway. Did I make a really amatuer mistake? I have to say, it's a lot more convenient to not have to reach to the back to turn the thing on.
Secondly, the problem is sometimes intermittant in the sense that it can be loud or louder (it never goes away completely). This really doesn't seem to reflect anything that I'm doing, i.e., one morning I'll fire it up, leaving the guitar and pandora just like it was last time, and it will be humming louder. Come back in a few hours and its just moderately loud (as opposed to "this things gonna blow" loud). If it was a ground problem, woudlnt' that stay the same?
Lastly, when it gets really loud, if I listen on the outside of the cabinent, there's a serious vibration inside. Tell me that doesn't mean the transformer is shot. I'm guessing a tran will cost more than I paid for the amp.
RE: the connections, corrosion, etc. I'll double check everything again. I didnt' think anything was loose or poorly connected, but who knows I might have missed something. Thanks for all the advice and the moral support 8)
Remeber to replace the preamplifier filter capacitors too. If they are bad they can cause serious hum. Still I think this is not the case in your amplifier. If the problem would be the bad capacitors the amp would hum all the time - even with the Pandora plugged in. Actually, the amp should hum with nothing plugged in as well. Same thing with the "mixer" section: if the hum would be induced there you should hear it even without having to plug someting into the amp. If you hear hum only when you insert a guitar then there's a 99% chance that you have trouble with the ground connection.
If a bad grounding is not the problem then the easiest way to find the hum source would be to probe the signal output of the various preamp stages with a capacitor coupled wire feeding a high impedance amplifier plugged to small speaker.
However, this made me worried: You grounded a power switch to chassis... How can you ground a power switch? Did you connect "N" of the mains to chassis? You should never ever do that!!! The neutral wire is sure "0V" but when you reverse the power coord (at least where I live you can easily do that) you will have the chassis in mains potential. The only grounds you ever wish to appear on chassis are the safety ground (the "E" of three-prong coord systems) or the secondary ground (optional). Also: If you brought the mains wiring near inputs (sounds like you did that) it might induce hum. Remove your switching arrangement and see whether it makes any difference.
If you want to switch the amp on and off from the front panel then use a DPST switch (rated for proper voltage) that switches on/off both L and N. If you for some reason could switch only one of them then switch the L - (since it's obviously safer). Neither one of these wires should never make a contact to chassis! They should also be insulated so you never can touch them by accident. Keep this switch a safe distance away from the inputs of the amplifier. Actually keep it safely away from any part you could actually touch. Read: Make sure the mains wiring can never touch any part of the amp if it would ever come loose. ...In other words, wrap the wires to heatshrink tubing.
I couldn't see the inside pics in full res but it looked like all input jacks are non insulated from the front panel. If your inputs now share the same ground with the neutral wire (return currents of the whole amplifier) I wouldn't be surprised you have a hum problem. Also, you do not need a separate mixing stage to induce hum problems if the jacks share the same ground path (chassis) at the input.
Vibration you hear could be a bad tranny but I doubt it. In my opinion it isn't very easy to blow up a power transformer - most of them tend to tolerate even long term short circuits. Anyway, it's normal for transformers to vibrate slightly. If your amp now greatly amplifies the hum from the input stage it will be under relatively small load, during that time it's quite natural for the transformer windings to vibrate (more) and make noises. Even when amps runs normally I can hear noises from all transformers except toroids. Transformers make noise even when the amp is idling. Only, if you hear the transformer constantly emit a "loud" "howling"-like noise then suspect a short circuit somewhere.
Switching should be done like this. If your amplifier switches the secondary side (some amps do this) then tear off the damn thing and rebuild like this. (Switching only secondary is both inefficient and dangerous). If your amp has a two prong coord (only L and N conductors) then do yourself a favour and replace it with a three-prong one.
Note: I haven't included the color codes for mains wiring since due to different variations (Europe/US & old/new) they may become quite confusing to some people who blindly follow instructions without thinking first. If one can not figure out which is which by him/herself then I suggest to stay away from mains wiring...
Sorry, I didnt' mean to say that I grounded the main supply line to the chassis (I'm clueless, not crazy). I meant to say that I ran the line that had previously been going to the fuse through a DPDT switch and then back out (and back to the fuse). By making that wire longer, it can now reach to the front panel (where it's installed in a hole). So the only real modification, from the little electrons' perspective, is that they will now get to the fuse a few milliseconds later, because they have to go through a second switch that either makes or breaks the circuit going to the fuse.
When I said that the switch is grounded I meant that the metal part of the switch (the barrel, washer, nut) is of course touching the front panel where it's attached, and THAT panel is connected to ground (the green wire on the 3 prong plug). Sorry about the bad description earlier. In any event, just to satisfy my curiousity, I wired everything like it had been, with just the original switch. It didnt' make any difference.
Thanks for the diagram. I will re-mod my front-panel switch that way.
:)
QUOTE:
" If a bad grounding is not the problem then the easiest way to find the hum source would be to probe the signal output of the various preamp stages with a capacitor coupled wire feeding a high impedance amplifier plugged to small speaker."
hmmm.... sounds like a good idea. I'll try it. Wish I had a schematic, but from what I can gather the company that made the amp was in business about 6 months, so they're not online anywhere.
Thanks for the suggestion and the help.
OK. I guess you can pretty much ignore the theory of sharing the mains ground return I presented in the previous post then. What you describe sounds like an adequate switching configuration - something found from many amps. IMO it's just safer to break the connection of both L and N. This way you can be sure that only the switch (and probably the fuse are in mains potential). I assume the sleeve of the switch has no electrical connection to any part of the switch either...
I edited my post while you wrote your's but my main points were to say:
"If the problem would be the bad capacitors the amp would hum all the time - even with the Pandora plugged in. ...Same thing with the "mixer" section: if the hum would be induced there you should hear it even without having to plug someting into the amp"
and...
"you do not need a separate mixing stage to induce hum problems if the jacks share the same ground path (chassis) at the input."
I hope this helped to see thing in a better light. Anyway, just to make sure I have not completely misunderstood your description let me ask this once again: Does the amplifier hum with nothing plugged in?
If hum is present only when something is plugged in it means that disconnecting the input breaks the signal path of the hum. To visualize the thing better examine the attached picture and consider how it affects your amps input signal in different situations. This schematic most probably shows what you're dealing with. Now, you should find out what causes the signal loop of the hum and how you could get rid of it.
BTW, What's your amp model?
Ok, thanks again. If that's the safest way to run the switch (with both of the mains instead of just the one going to fuse) I'll do that. The switch can definitely handle it since it's got the lugs on both sides.
As to your question: no, the amp does not hum when nothing is plugged into an input jack. Even if I crank all the volume knobs, I only get a hissing (maybe better to describe it as "seashell against your ear" sound?). It's a loud hissing, but it's pretty much the same sound that you get if you crank up an ordinary stereo that's switched to "Aux" or something like that.
The humming only arises if I plug in a guitar. At that point, I can't even break past the "1" on the volume of the master. The hum is just too loud and overpowering. Increasing the bass/treble, etc. has the expected effect of making it worse.
Like I said earlier, running the same guitar through the pandora doesn't just quiet things down a bit, it completely eliminates the hum. I've tried to make it hum, and even on some of the pandora settings that are meant to model a high-gain amp with distortion, it just won't. Interestingly, if I just run the Pandora's drum fill (with no guitar attached), there's no hum either. Since I started trouble-shooting this, I've switched the guitars to amps that I know are good, because I really want to believe that this is just a guitar issue. But on other amps they are silent, especially the guitar with the EMGs.
RE: the model of the amp, it's called "Electric Spirit 4" by Holmes. Fairly simple, in fact most of the amp head is empty space inside. There's a small reverb pan, but no other on-board effects.
With the hum gone, the amp sounds wonderful. I'll never be able to use anything but a few watts, but I don't care, it still sounds great. Just for the heck of it, this morning I ran the guitar through my Art SGX (which is usually noisy) and into the amp. No hum. But then again, that is grounded through its own 3-prong plug. Either way, it sounds good. I'll keep trying to figure out the mystery, but it's nice to at least have a way to avoid the hum.
Thanks. I'll look at the schematic you posted. 8)
Ok. I could not locate a schematic either.
Here's a simple and quick test you can try: If the inputs have non-insulated jacks and if they are connected to ground trace on the PCB then lift one of the jacks off from the chassis, insulate the jacks's ground pin so it won't touch anything by accident (i.e. wrap it it in plastic film) and then plug in a guitar (without Pandora) to the lifted jack, See whether you still have the hum problem. If you can do this you have either located the source of hum or eliminated one possible source.
ok, I'll try it and let you know. I take it that the point is to "unground" the jack from the chassis? Or disconnect the ground wire coming off the back of the jack as well?
You can try both. Start with "ungrounding" the jack from the chassis.
Another point: Do the input jacks have a switching mechanism and if they do how is it connected?
If you can post a detailed picture showing one input section and it's connection to PCB that would be helpful. I see the amp photos in a size of a postcard. I guess haven't figured out how to view your photos in higher resolution.
This is what I highly suspect: The chassis acts as a high current ground return path which causes it to appear as an alternate voltage source between input stage's input and reference ("ground") nodes. The signal amplitude of this voltage source doesn't have to be greater than few millivolts in order to cause a notable hum. Note my deliberate use of quotation marks: There can exist only one true ground point in your system.
Now consider what happens if you would unplug the guitar cable, thus removing the "voltage source" from the place of the jack symbol: The same hum voltage would now affect at both input and reference nodes. Therefore the voltage difference (V input) reading would be zero. If there is no voltage difference then there will not be amplification either.
Note: Your guitar pickups do not have a ground but if you were to really ground the input "ground" - and at the same time the chassis - to another system, i.e. through Pandora or any other device then the current return path on the chassis would become grounded as well. If both ends of the hum voltage source are connected to ground it means the hum can not be in series with the input jack voltage source. The result: No hum.
If you lift the input jack "ground" from the chassis you can test whether my theory is right or not. In worst case, you may have to unmount all input jacks from the chassis to check this out. The PCB looked like it contains a ground trace where it hooks all the ground wires from the input jacks, therefore the input can make a connection to chassis through any jack still left mounted.
I need some sleep now. Good luck in troubleshooting.
ok... I've now disassembled and cleaned all 22 pots again, re-checked all solder connections (most of them are large, so that's easy), and tried plugging the guitar into a jack that's not secured to the front panel. No luck... I have to admit I'm starting to get disillusioned. I need to order some parts for other projects, so will order some caps and replace those.
Still the same issues. With nothing plugged in, fairly loud hiss. Guitar plugged in, loud hum as as soon as the volume is increased. Run guitar through pandora, hum is pretty much gone. Gone enough to not worry about. Just to see what happened, I ran it through a Yamaha digi-stomp. That didnt' take away as much hum, but it did make a difference. Even with the gain turned up all the way, the hum didn't really get noticably worse.
Now that I've been messing with the thing so much, I do notice that the hum doesn't really increase proportionally with adjustments to the master volume. I.e., sometimes it will become 100% louder with a tweak from 1-2, and other times only 50%. Could all of my problems be just a bad pot, or is the pot just "reporting" a problem elsewhere in the circuit? I guess it wouldn't be that hard to swap in a new one and see what effect that has, given that I'm now intimately familiar with all 22 of those stupid thing.
Thanks for all the suggestions. I tried to upload a bigger photo of the front panel, but there's a size limit for this site.
Ok, you should try my suggestion and lift all jack grounds from the chassis. The trace circling around the PCB is the "ground" and it seems to make chassis connection through several screw mounting points. Something like this can be an open invitation for ground loops. Then again, I repaired an amp which used a similar grounding method few months ago and it didn't have any problems with hum whatsoever. Sometimes grounding issues can be very tricky...
Another easy method to test my theory of ground return currents in the chassis is the following: Plug the guitar in normally but make an electrical connection from the amp's chassis to a known good ground point i.e. a chassis of another amplifier. Before doing this check if a notable voltage difference exists between these points.
Here's a result of intuitive thinking:
1) I doubt that you have no mechanical component fault in the amplifier (i.e. faulty / dirty potentiometer etc.) If your amp works fine with Pandora or any external effect this option should be overruled. If your controls would have a fault it would appear in every situation regardless of the signal source.
2) The problem is not induced ripple in the supply line from aged capacitors: You would hear this as well. You could still control the volume of preamp induced hum but it would appear regardless whether the guitar is plugged in or not.
3) If you can control the amplitude of the hum with master volume potentiometer then the source of hum must be located before this potentiometer. You can apply the same rule for every control that adjusts the sound and amplitude of the hum. This should help you narrow down the problem.
3) But I consider this point as the all narrowing you need: If your amp works alright but hums when you plug in a guitar then you have found the source. The way I see it there's mainly two options:
a) the guitar is faulty or picking up external interference: You have overruled this option by testing the amp with other guitars and signal sources.
b) the ground path induces hum. Have you tested the amplifier with another mains outlet? The one you use may not neccessarily have a safety ground return which means your chassis is most likely floating in other potential than zero. Have you checked the continuity of the safety ground wire in the power coord?
Even though all points would prove the chassis is connected to ground adequately it doesn't imply the potential of all points in the chassis is zero. Remeber, there can exist only one good ground point in your system. Ideally all grounds should connect it directly (star ground topology). The only problem causing the symptoms you describe I can think of is overlapping ground return paths. The current in this return path must likely be fairly high since it can induce a notable voltage over a (assumably) small resistance.
Anyway, I assume your amp did not induce this hum from the beginning: The injection of hum is not a normal condition and definitely something has gone faulty in your amp. I still suspect a bad solder joint or mechanical connection somewhere. Now, you should check what high current stages overlap their ground return with the inputs. In order to do this you must locate the secondary ground (hint: three most common places to look for are secondary center tap, the main filter caps or rectifier). Follow the ground returns of inputs and high current stages (mainly PA and speaker jack) to this point and see whether they overlap. The chassis is tricky but most likely the electrons find the shortest/least resistive path). Check the path for for loose connections, cracked traces, corrosion, broken parts and anything abnormal.
Quote from: idle_chatterbox on August 15, 2006, 09:44:26 AM
Thanks for all the suggestions. I tried to upload a bigger photo of the front panel, but there's a size limit for this site.
If you need something really big you can always use photobucket or another image hosting site.
Exactly. I've had good experience with http://imageshack.us/ so far...
Thanks for the suggestions on the photo sites. I tried both. ImageShack seems to require more intelligence than I have to upload photos, and with photobucket, I can upload, but if there's a way to keep the photos their original size, it's apparently beyond me. In any case, I tried.
http://s96.photobucket.com/albums/l164/idle_chatterbox
I know when I'm beaten....
Anyway, back to the amp. Took it apart again, checked with eyes and meter for bad connections. Recleaned the 22 pots. No change.
grounded the amp to another grounded unit. No change in noise. Tried other guitars, cables, speakers. Unconnected one of the input jacks, plugged in, no change.
I've got no ideas.... Replacing the caps at this stage seems like a shot in the dark, and I'm wondering if it is cost-effective to solder good caps into a noisemaker.
:grr
If you disconnect only one input jack from the chassis it will still remain connected through the wiring of other jacks. Grounding the chassis to another unit should however indicate that the chassis is not the hum source... at least I think so. You could try probing various ground points with a wire that has the other end connected to central ground point. (Where is it located by the way?) This should show you if another ground route would stop the humming - what it most likely should do if the hum is indeed ground induced. Start with the input jack's ground lug. Remember to keep the amplifier's master volume low as possible - you only need to hear if the hum goes away or decreases. Be careful not to short anything by an accident.
And another thing to check out: Do the input jacks have switches. (Check if the inputs are grounded when no plug is connected.) It looks like they don't have a switching mechanism but then again your amp pics are postcard size and only show an overview. At this point it has become impossible for me to help you anymore unless you provide more details (i.e. schematic, sketch of the grounding paths or more detailed and larger photos - preferebaly with the important sections labeled (central ground point, power amplifier, power supply, etc... you got the idea.)
Replacing the capacitors as a cure for the problem seems like a far fetched idea indeed. I sincerely believe that an increased amount of ripple voltage (= hum) due to bad capacitors would be heard in all situations. It might work in case the capacitors have began to leak current to ground path. Curing the hum problem with an external effect unit is a very clear sign of some problems existing in the grounding path and topology.
....However, I advice you to change the filter capacitors anyway (both main and preamp filters of course). Actually, I advice you to change all electrolytic capacitors while you're at it. Your amp doesn't seem to have a massive amount of them - actually it has amazingly few - so doing that is not a big deal. Capacitors cost what, few bucks - not a big deal either. Decoupling capacitors can start leaking DC and a bad bootstrap capacitor in the power amp doesn't sound good either. Sometimes aging capacitors do make strange problems: My 20 years old Yamaha multitracker makes sudden loud pops, hums now and then and the "lowest" VU meter leds sometimes burn constantly (DC in signal path). I don't want to touch it: this thing houses at least 100 electrolytic caps. Compared to work needed for monster circuits of 80's (where virtually every capacitor was an electrolytic) your "cap job" seems like a child play. If you replece those capacitors you have both increased the life of your amplifier by, say, 20 years and eliminated the capacitor's as being any (even strange) source of problems.
Yeah, I wish I had a schematic. But at least last time I searched online for one, I came up with nothing.
I guess you're probably right that it won't make it sound any WORSE if I replace the caps, and like you say, there's not that many. The only good part about it is they all seem to be out in the open and not under or tangled up with other components.
As far as where the ultimate ground is, I haven't been able to tell (yet). The main cord ground wire (green, the "3rd prong") of course grounds to the chassis right next to the transformer, but there is as far as I can see only a few wires that connect the back panel (where the trans is) to the front panel (where the inputs and pots are). There's no obvious wire leading from either PCB to the chassis, for example, or metal screws/mounting points that would do that. Or at least I haven't seen them yet, and I've been staring at this thing a lot lately. Two of the wires connecting the back and front panels probably don't count, since I think they are only the "On" light circuit.
Thanks again and I'll keep tinkering with it and let ya know. :-X
For starters, it looks like the PCB on the front panel makes ground connection to the chassis through the mounting screws and input jacks. ...Should I even use a word "chassis", your amp seems to be built on two separate metal sheets? Does the casing have a metal sheet or anything tying these two panels together?
The interesting point is indeed how these two panels connect each other, It's quite fair to assume that the only ground - not just a ground return is located somewhere in the rear panel and near the power supply. Therefore you should have at least three wires running from rear panel to front: Signal, ground return and Vcc. If the supply is dual (which one is it?) then you should also have Vee. (Unless the preamp uses single supply). If the whole amp is single supply then the ground counts as Vee.
The above configuration seems reasonable if the front panel houses only small current stages - ideally, if you have to bundle small current stage ground returns together the signal ground returns should use a return path separated from the supply ground returns (ie. filter caps). Shared supply and signal ground returns can cause hum so it seems like you should be concerned about this at some level. I think that grounding the front panel to a separate system could actually cure hum caused by this. However, a quiet amp without a guitar plugged in doesn't fit this theory. There's a heatsink looking thing attached to front panel also but I assume that it is not a section of power amplifier but the case for spring reverb? Note: The input impedance of most spring reverbs is quite small so this should be - at some level - considered as a stage using high currents. After all, it is a small power amplifier driving the unit.
Front panel:
The section on the left (by left I mean the side housing that "spring reverb case" ) interests me and a better picture of it (or a set of them) would definitely help. I assume that this is the section which ties the circuits together. Looks like three wires run out from it - can you identify them? The section at the far left looks like the switch you installed but there's something under it as well. Now, I saw some pictures of this amp while looking for the schematic and recall that this place holded direct inputs to power amp - is this true? I can't seem to locate the (ebay) site with those pics anymore. Looks like you lifted one jack off to house the switch.... make sure the jack can not short anything or that removing it has not ruined any important continuity. If this indeed is a direct input to power amp the next thing is important: Do these jacks hum as well. If they do is the hum amplitude equal, smaller or higher. This might tell you a lot about where the hum source is located.
A mains wiring near small signal stages looks like trouble but if it induces hum it should - again - do it all the time. You have also removed your switching arrangement once and it did not make any difference. I guess you can pretty much ignore any effect of this wiring.
The rear panel:
Three wires run from the transformer so I strongly suspect you have a center tapped configuration. Now, I see only two diodes - might be a single supply configuration or dual with half wave rectification. If it's single I see no output capacitor - unless it's the other one of those huge caps. Enough assumptions, the picture does not show all the details and forces to guess. You can easily measure with a DMM against ground to find out. If the configuration is dual then the number of wires (three?) does not sum to equation of 2 supply wires, and at least 1 ground and 1 signal wire. (Note: Unless the preamp uses single supply). Identify these wires, it will help you - if not now, then at least in the future if you ever have to work on this amplifier again.
The two jacks between PCB and transformer - are they speaker jacks? If so, the point where they connect the PCB is assumably the main ground point - at least reasonably near it anyway. The PCB spot would fit perfectly.
Those two green wires, are they for the lamp? Interesting sections here (I need more pics) are the bundle of wires near the place where the power switch is located and the three wires leaving the PCB and connecting the other one at front panel. I assume one of these wires must be the ground return of the preamp, trace out where it connects. (Ideally it should run straight to ground without overlapping any high current return path.
The safety ground: Only in rear panel? Use a very thick (grn or yel/grn depending of the country code) wire to hook it up to the front panel as well. If you use "fork end" in the wire you can mount the other end, for example, under any of those screws holding the PCB on place. If there indeed is not any other ground return between those two panels except a thin wire - which also runs through PCBs - I do not consider it as adequate "safety grounding". All conductive places that the user can touch should be safety grounded.
Now, adding this wire could cause a ground loop - as well as prevent one - so finding an optimum hookup place is essential. For safety point of view, optimum is near that switch you put to the circuit. I guess this is also the shortest place to connect the wire. Even better if that lamp is near this place as well. See, I guess the lamp is hooked to mains potential in parallel with the transformer (a usual configuration when leds were expensive). It might have a resistor for voltage drop and current limiting but "mains potential" is the key word here. Should any of those wires ever come loose and touch the chassis the current would find the least resistive route to ground instead of running through the fragile PCB traces and thin wire connecting the boards together.
I could email you a large scale photo if that would help.
Let me try to answer your questions.
1. I agree there is no "chassis" in the traditional sense. Like I said before, the amp head is mostly empty space, an empty box. On the front is a cheap loking piece of galvanized tin (?) that the long PCB with the input jacks and pots is screwed to. From the location of the screws, this seems to be where the panel is grounded, through the screws. Same with the back panel, which is also fairly thin sheet metal containing the PCB with the large egg roll caps. The transistor bolts to the sheet metal, just where the main cord comes in from the back. What ground there is between the front sheet and the back i'm not sure. I can try to figure that out. There are I believe a total of 5 wires coming from the back sheet to the front, but as I said in the last post, 2 of those (the green ones) are going to the indicator light. The other 3 I will have to check on.
2. yes, the little box is a spring reverb unit (which sounds terrible by the way). It has four very fine wires connected to it and going to the front PCB.
3. near the main power in and the transformer is a wire that is grounded to the back panel with a spade connection and a screw (that screw also holds the trans in place). I assume that's the main ground? Nothing else is connected to that screw.
4. QUOTE: " I think that grounding the front panel to a separate system could actually cure hum caused by this."
So connect a jumper wire from a front panel ground to another unit, like an effects board, that is grounded would work?
5. QUOTE:
"The section on the left (by left I mean the side housing that "spring reverb case" ) interests me and a better picture of it (or a set of them) would definitely help. I assume that this is the section which ties the circuits together."
Yes, that's the "master" section, with master volume, bass, treble, etc"
" Looks like three wires run out from it - can you identify them? "
I will try. No schematic, but I can try.
"The section at the far left looks like the switch you installed but there's something under it as well. Now, I saw some pictures of this amp while looking for the schematic and recall that this place holded direct inputs to power amp - is this true?"
Yep. Good memory! There were originally 3 jacks there. Monitor, aux in, and aux out. I took out the monitor jack when I installed the switch in its place. The aux in and out jacks are still there as they were.
"Looks like you lifted one jack off to house the switch.... make sure the jack can not short anything or that removing it has not ruined any important continuity."
The old jack is tied off and insulated so it can't short anything. And when I reinstalled the jack, it made no diference to the hum.
">If this indeed is a direct input to power amp the next thing is important: Do these jacks hum as well. If they do is the hum amplitude equal, smaller or higher. This might tell you a lot about where the hum source is located."
Let me try. I seem to rember that plugging my guitar into them did nothing. But let me try again tonight when I get home and I will let you know.
I'll have to wait to work on it when I get home... But I'll be back with the results.
Thanks again :)
1) Yes, the panel is definitely "grounded" through the screws. You want to check them out for corrosion which could make this connection more resistive. What would that do - hard to say, probably nothing: The jacks are already grounded. A failed connection would ruin a large groundplane though. If the PCB design was bad (it seems to me that at least the ground routing is very bad) it might matter.
...The transistor? I assume you meant transformer.
3) "a wire that is grounded to the back panel with a spade connector" ...Where is the other end of this wire connected? Don't make quick assumptions of what this is. What you want to locate is the ground that the circuit uses as the reference. That must exists somewhere in the secondary circuit. The wiring of the transformer is pretty hard to visualize from the picture but those three wires connecting the PCB must be the secondary. That most likely means these should be two AC nodes and one center tap node. Besides these you would have two transformer nodes that connect to mains (there might be unconnected spare ones for other mains voltages). If there is a wire coming from transformer which connects straight to chassis it would most likely be a ground connection for the transformers casing.
4) "a jumper wire from a front panel ground to another unit, like an effects board, that is grounded would work?" ...Yes, this works. Your coord's "ground" wire has the same function. However, this wire is not a ground, it is a ground return with resistance and current through resistance means voltage. This again means that the ground voltage on the other end of the coord can not be the ideal 0 V. If the wire is very long you will surely have problems and the reason why you should route the ground returns inside the corresponding equipment is to avoid the problems caused by the resistivity of the ground return. Note: Every time you hookup the guitar wire to another system you connect the jack's ground return and the (another) system's ground return together.
5) I see, probably no direct in to power amplifier then but something which comes very close. The aux in is what you should test, it is probably designed for "line level" signals, which usually contain higher signal voltages than guitars output. Also the input impedance is most likely different. You should, hear something when you plug a guitar into it though. If not, try to amplify the signal input or try an external line level signal source. Remeber that by doing so you increase the signal to noise ratio - listen the same source through guitar inputs to have a reference of the amount of gain. The jack might be preceded by a gain stage before the signal is fed to power amp, however I suspect that the total gain of this auxliary channel is lower than of the guitar channels. If the hum source is the input you should hear this as a reduced amount of hum.
I don't know the difference between monitor and aux out. Monitor sounds like it could be a high current insert for monitor speaker (very unlikely) - aux out would be the same as preamp out. Plug this jack to a suitable system and test whether you have the same amount of hum that you hear through the power amp present there. Monitor could also be the direct connection to power amplifier - you need to trace this out to be sure. Better avoid plugging anything into it in case it truly is a jack for monitor speaker.
Higher resolution pictures will help me. My email address is teemukyttala[removespam]@gmail.com ....you know what to remove to make that a valid one.
Quote from: teemuk on August 17, 2006, 03:02:31 PM
3) "a wire that is grounded to the back panel with a spade connector" ...Where is the other end of this wire connected?"
I was referring to the main ground wire coming into the back from the wall. It's the 3rd prong wire (green in the US).
"The aux in is what you should test, it is probably designed for "line level" signals, which usually contain higher signal voltages than guitars output. Also the input impedance is most likely different. You should, hear something when you plug a guitar into it though. If not, try to amplify the signal input or try an external line level signal source. Remeber that by doing so you increase the signal to noise ratio - listen the same source through guitar inputs to have a reference of the amount of gain."
Ok, I tried that. I first ran the guitar through the Aux in, and ran a cord from aux out to a small test amp (probably no more than 2.5 amps). The sound was reasonably quieter in terms of hum, but I'm not really sure if the Aux was really doing anything. It wasn't noticably louder than when I just plug into the test amp.
I then ran the pandora (no guitar) into the Aux in, and used the test amp as the aux out again. No hum whatsoever. But again, I don't know if it was any different in tone or volume than what I would have heard if I'd just have gone straight to the test amp.
I then ran the guitar into my Behringer pre-amp, ran that into Aux in, and ran a cord from aux out into the test amp. I'm not sure what that showed, to be honest. It produced a high pitch feedback type howl, so loud you couldn't hear the guitar at all.
In each case, no matter how I had things situated, I could turn the "Auxiliary" pot all the way up without really hearing a difference (is it possible I wasn't hooking up the right auxiliary?), but as before, as soon as I touch the "Master" volume, the hum is there again.
Overall, I'm not sure what I learned. In some ways, I'm not sure I heard anything different than I would have if the Aux in and Aux out was just a patchbay. I could make the signal louder if I cranked the Master volume, but that came at the cost of the hum.
"Higher resolution pictures will help me. My email address is teemukyttala[removespam]@gmail.com ....you know what to remove to make that a valid one."
they're on the way. Thanks
Your tests did show very important points:
1) The master volume control introduces the hum regardless where you plug in the guitar.
2) The hum is not amplified by the early input stages of the preamplifier.
3) Only the signal heard through the power amplifier has a substantial amount of hum.
The only ground return path from the preamplifier to power amplifier seems to be that green wire. It is as well the only thing grounding together the front and rear panels. Your amp relies on use of large ground planes as conductors for return currents. Sometimes this is as effective technique as careful routing of the returns but what your amp fails to do is to make an adequate connection between the panels.
This causes some issues:
1) The ground return currents of both signal and supply are still forced to share the same path (wire). This ruins the idea of using large ground planes.
2) If front panel for some reason goes to mains potential the mains current has to flow through the preamp PCB traces, then through the green wire between the boards, then through the power amp PCB traces, not to mention all solder joints between. What you want is that fault currents flow straight to safety ground lug on the rear panel through a thickest wire possible. What you also want is that the voltage potential on the front panel (ideally zero) will not float according to amount of return currents and the resistance of that green wire connecting two boards together etc. - thingie. You want a solid ground connection which increases the size of the ground plane between these two panels.
Here's what I suggest - again: Hookup a very thick green wire under the screw holding up the upper left corner (seen from back) of the preamp PCB. This place seems to be reasonably close to both wiring in mains potential and what seems to me to be the point where the green wire leaves the board. Connect the other end of this wire to the safety ground lug, see if this removes the hum or weakens it. This should at least enlargen the ground plane not to mention improve the safety of the amplifier.
If this does not help try connecting to the upper right screw point holding together the power amp/supply PCB instead. This would be the (quite) ideal path to the secondary's main ground point which is somewhere in the area containing the speaker return, main filter capacitor return and the connection of the center tap wire (yel). However, I have a doubt this makes any difference if the hum was still present after the first procedure.
Even a better option would be to use a metal bottom plate in the case. However, you'd have to make sure it retains the electrical connection between both panels and mounting such a plate might prove much more complex than using a thick wire.
Thanks for the pics, they did help.
Something like this...
Thanks Teemuk.
I connected a large gauge wire from the ground on the back panel to the front panel. No change. Then, I tried connecting a number of jumper wires from ground points on the back panel to ground points on the front. No change. Wish it had worked, because now I'm wondering what it can be, if not a ground problem. :-[
It could still be a ground problem. My LM3886 amp had a bunch of noise no matter where I added ground wires, I then found it was due to one poorly placed wire near the power supply.
Well I guess that's a little bit of encouragement JoeCool. Just out of curiousity, though, was the problem that you found one of a ground where it wasn't supposed to be, or the lack of a ground where one was supposed to be. The reason I'm wondering is i'm curious about what tipped you off. Was it something as simple as finally seeing a disconnected solder junction, or did you have to do a lot more analysis than that? ???
Well, I guess the only thing left to do is to start probing the thing to find out where the hum is actually induced to the signal. You already eliminated the input stages as the source so plug straight into aux input (it's probably closest to the power amp in signal path) and work your way up there.
Before that, I suggest you try a complete disconnection of the preamp from the signal path to see if it cures the hum thus totally eliminating the possibility this is caused by sections in power amplifier.
You can do this fairly easily:
1. Connect the green wire from the power amp to the ground leg of a jack not connected to preamp in any way (i.e. hanging in the air). You either need a new guitar jack or have to remove one completely from the amp.
2. Connect the white wire from the power amp (I assume this is the signal) to "tip" lug of the jack.
3. Don't plug in yet. Power on and check if there exists any DC between the tip and the ground in the jack.
4. If you had DC you need to decouple the signal with a capacitor. At this point it is also worthwhile to check if the amplifier does the decoupling in either pre or power amp. If there's a decoupling cap in the power amp but you still have DC you have located a leaky capacitor.
5. Plug in a guitar (with it's volume control turned down). Raise the volume gradually and see if the amp still hums. It should not - you plug staight into the source which is always on and perfectly runs without hum. If you still have hum then for now on we can completely ignore the effect of preamplifier. Either the guitar is faulty or the bad return path exists in the power amplifier.
Next, (unless you already located the problem) put everything as it was and then connect the jack (preferably signal bypassed with a cap) in parallel with the power amp connecting wires. Once again you plug straight into power amp, this time with the addition of preamp grounds. See if you have hum. If the hum is gone it's most likely due to insulation of the jack from any other ground retuen paths than the one going straight to power amp. See if connecting the jack ground to chassis brings back the hum.
Quote from: idle_chatterbox on August 19, 2006, 12:53:18 AM
Well I guess that's a little bit of encouragement JoeCool. Just out of curiousity, though, was the problem that you found one of a ground where it wasn't supposed to be, or the lack of a ground where one was supposed to be. The reason I'm wondering is i'm curious about what tipped you off. Was it something as simple as finally seeing a disconnected solder junction, or did you have to do a lot more analysis than that? ???
The ground was grounding what it was suppose to, but because the wire ran near the power supply, it was picking up noise. I figured it out by doing a TON of research on making a quiet clone amp.
I didn't get a chance to re-read the entire thread, but has anyone mentioned rectifying diodes? They can go bad eventually, and it could cause some severe noises.
Quote from: joecool85 on August 19, 2006, 08:37:22 AM
I didn't get a chance to re-read the entire thread, but has anyone mentioned rectifying diodes? They can go bad eventually, and it could cause some severe noises.
Yes, but in this case wouldn't the noise be heard constantly? I mean, he can plug in a guitar with Pandora and the amp works fine.
With my logic this indicates a grounding problem - something tricky is happening there - possibly in the way the return currents run to ground point. What causes this is another question: There might be a faulty component somewhere - something difficult to detect because the amp seems to work normally, except that it now generates a current flow to the signal ground returns. (With the term component - besides electronic parts - I mean as well solder joints, screws etc..) Somehow this effect is also canceled with some external effect units.
Edit: JoeCool85, As soon as I have more time I can start a thread about proper grounding methods, I think this is pretty important topic after all. ...Unless someone else is faster than I am of course. Then I will happily contribute the results of my research as well.
You have a good point, it probably is a grounding issue. I think a good grounding thread would be a great thing to have, maybe we can add it into the wiki as well.
Just have a minute, but wanted to say thanks to you guys for all the time you're putting into curing my patient.
I'll try the stuff you suggested and get back with ya. By the way, last night saw Eric Johnson play. After the first song, when he was introducing the band members, he complained about "the hum" (while pointing to his marshall full stack). I was thinking, "dude, that's NOTHING...." :lmao:
Quote from: idle_chatterbox on August 19, 2006, 04:57:41 PM
Just have a minute, but wanted to say thanks to you guys for all the time you're putting into curing my patient.
I'll try the stuff you suggested and get back with ya. By the way, last night saw Eric Johnson play. After the first song, when he was introducing the band members, he complained about "the hum" (while pointing to his marshall full stack). I was thinking, "dude, that's NOTHING...." :lmao:
You should have been thinking, "Dude, you can totally fix that!" haha
Hi Teemuk:
I had a few questions before I tear into the amp again.
QUOTE:
"1. Connect the green wire from the power amp to the ground leg of a jack not connected to preamp in any way (i.e. hanging in the air). You either need a new guitar jack or have to remove one completely from the amp."
I'm guessing that the power amp is the PCB in the back. Is that correct?
"4. If you had DC you need to decouple the signal with a capacitor. At this point it is also worthwhile to check if the amplifier does the decoupling in either pre or power amp. If there's a decoupling cap in the power amp but you still have DC you have located a leaky capacitor."
Could you maybe elaborate on what it means to decouple the signal?
Thanks for the help. Just want to double-check that I know what I'm doing here....
:)
Sorry for the delay... Yes, the power amplifier circuit is located in the PCB mounted to rear panel.
"A Decoupling capacitor decouples one part of a circuit from another. That means it makes one part of a circuit unaffected by things going on in another part of the circuit." -Wikipedia
In this case you want that DC voltage potentials in different parts of the circuit connected together will not affect each other. Since capacitors block DC and pass only AC signals they will "decouple", for example, two gain stages from each other. i.e. The DC potential in the output of the first gain stage will not pass to the input of another preceding gain stage - only the audio signal (which is AC) will pass.
Thanks Teemuk, I think I follow. I'll give it a shot. :tu:
whew.... ok, finally found the time to tear into the Noisemaker 2000 amp again.
Here's what I found, using the diagnostic of swapping in a guitar jack to the two wires coming off the power amp.
1. hooked up the jack, using alligator clips. As soon as they were hooked up, checked for DCV. Didn't have any. There was a low hum. Nothing like the hum I had been dealing with (which is anything but low).
2. of course, turning the volume knobs on the front panel has no effect, but I admit I tried it anyway ;D
3. just for the heck of it, plugged in trusty Pandora. low hum went away. As in completely silent like the amp was turned off (seriously). Now, if I switch pandora to a high-gain setting (e.g., "distortion") the hum gets reasonably loud. But I thought it was interesting that on the low-gain, clean settings, the amp is dead quiet.
4. grabbed a guitar and plugged it into the test jack. No pandora, just guitar. Reasonably loud hum. Not like the original bad hum, and has this significant difference: it varies with the signal from the guitar, which the original bad hum never did. In other words, if I play a string, the hum gets louder. If I mute the string, the hum settles down (but still hums).
5. plugged in guitar through pandora. Sort of like before; on high gain simulations, hum got louder, on clean settings, the amp was almost dead quiet, and didn't do the thing of humming louder as the strings were played.
6. grabbed another guitar. A little quieter. Admittedly, the first one does have some fairly hot pickups, and is noiser no matter which amp, than the other guitar. But as long as the guitar signal goes through Pandora, it is playable, and there's none of the increasing/decreasing hum.
any thoughts?
this amp..... :trouble
Sounds like you may have it as good as you can get it. I've seen plenty of amps from the factory that have hum like you speak of. On that same note, if you work at it hard enough, its not impossible to totally get rid of hum from the amp. However, hum from guitar you may not be able to get rid of without working on the guitar. Single coils are known for hum, so that could be part of your issue with the guitar if its a SC setup.
naw, not a p'up issue. Wish it was, but it aint. It's pretty much the same noise whether I plug in a single- or humbucker guitar. Oh well. I'm tempted to gut it for parts. xP
Got plenty of working amps, thank god, so this one was just a project.
Joecool85 is right: The amp can be repaired to be better but it will most likely mean a significant amount of work. Judging by your description the hum is introduced by a bad grounding path in the preamplifier - probably located in one of the last stages in the preamplifier. However, sorting that stuff out in the particular amp will not be a very easy job; the grounding between the two panels is done very poorly. Actually, IMO the whole grounding is done poorly. If the hum is caused by a part gone bad you at least have some hope of easier repair job but it means pretty much "shotgunning" the design (swapping all the parts that seem likely to have failed).
"First aid" hum curing could be:
1) Replacing all jacks with insulated ones, insulating the front panel from the PCB or reducing the amount of connections to it to 1. (Could even make things worse - experiment).
2) Shorting some ground paths with jumper wires to main ground point. Again, experiment.
3) Connecting the front and rear panels together with a metal plate. Recommended.
4) Replacing all electrolytic capacitors or at least filter ones. Very likely will not solve the problem but might hide it since this reduces the amount of ripple.
Some extra work would include complete rerouting of the ground return paths. I started a thread about grounding methods and I guess you saw it as well. Following the instructions presented there you could make the amplifier almost hum free, problem is: the stuff is very difficult to implement into finished designs. Very likely you would have to do stuff like cutting PCB traces etc. so I would not bother unless the amplifier's tone is otherwise worth to die for. Personally I'd just replace the jacks with insulated ones, make a solid chassis with external metal plate and experiment with shorting the ground return paths to main ground point from few logical places. This is quite a lot of work if you get into it. (You have to trace the amp schematic to see what parts of the circuit are worth looking into etc.) ...and it may still prove fruitless.
BTW, the hum getting louder during louder signals is normal: The amplifier is drawing more current to produce its power output, while it does this the supply voltage will sag, the filter capacitors can not charge fully since the current is required elsewhere and as a result of all of that the amplitude of ripple voltage grows higher. Because you do not have a clean DC supply the output signal will be modulated by the ripple voltage. Since your amp is most likely a single supply design the effects of ripple in the supply will be even more notable. The guitar plugged straight to power amp is also most likely a serious impedance mismatch as well, which might exaggerate the effect of hum. Since this was intented to be merely an experiment to see if the problem was in the line feeding the power amplifier I guess you should not care too much about the effect. If it is a great concern two things might help:
1) Replacing the filter capacitors. Aged filter capacitors can grow a high internal resistance which will weaken their ability to charge and discharge properly.
2) Adding more filter capacitance. I do not recommend this: Large supply capacitances stress the amplifier during power on (while the capacitors charge your amp is basically shorted) and can stress the rectifier diodes even during normal operating conditions (larger capacitance requires larger supply current thus the rectifier has to dissipate more waste power).
Thanks Teemuk. I think you're probably right. I will probably just reincarnate the amp into something else. The case is very large and sturdy, so I was thinking of turning it into a nice cabinet/speaker enclosure. I'm sure I can use the pots for my other DIY stuff too.
Thanks for all the advice in trying to save the patient! 0:)