Hi, i've just discovered this forum, so hello to everyone.
I hope someone can help me with an issue i have with my Prince (Piggy) PS-200R. It was made in Japan 25w SS blonde tolex combo with a 10" speaker modelled somewhat on a Mesa Boogie Mk2B. It was made around 1979.
It has a gain control, but all it seems to do is work just the same as the regular volume control. It was like this when i bought it off ebay a few years ago. If i turn the gain all the way up, and just have the volume on a tiny bit, you can hear an overdrive sound but it is very quiet and in the distance/hollow sounding. If i turn the volume up more than that everything goes clean again.
As i've modded amps and pedals, i 've had a look inside and can't see anyhting loose or burn't out. Because it's a 70's amp the circuit board is a bit different than a modern pcb. It is also pre- op amp chips. If i knew what was wrong i could fix it myself. These amps are very rare now and there is no real info about them on the internet.
I love this amp. It has the best clean sound; It's warm, woody and chimey and sounds like a valve amp. I use pedals for overdrive, but i'd really like to hear what the amp's original gain sounds like! Any help would be gratefully appreciated.
Hi Paolo,
It is possible that with the aid of some close up pics of circuit board (both sides)
someone here might be able to help. You could get lucky and someone might know that amp well.
You have the advantage that it looks like a small uncomplicated circuit. :tu:
Meantime resizing pictures
Try this;
http://www.irfanview.com/
Find *Re-size, Re-sample* under *Image* drop down menu. The rest should be self explanatory.
This is so handy because you preview printout so schematics fit to the page.
I'd be lost without it. oh and it's free.
Phil.
"No real info" is right - I can't see any trace of the maker never mind the model, but as Phil says it won't be rocket surgery, and failing an actual circuit some good clear pix inside around the gain control area and where that connects to the PCB might help.
IrfanView - used it for years now prepping pix for my website and I couldn't live without it. :dbtu:
When it comes to internal pix for diagnosis, if they are well lit and in sharp focus it is generally better not to reduce them as detail is lost.
Thanks Phil and Roly for the advice and heads up for Irfanview.
Just to clear it up. The manufacturer is Prince and the model name is Piggy-2, and the technical model reference name is PS-200R. They also came in Black tolex and a wooden case like the original Boogies. There is also a a Prince Piggy 3 which looks very similar , but also has a mid control is slightly bigger like a Fender Deluxe size. I have pictures somewhere.
Anyway, i've re-sized the original gigantic photo (above) of the amp, and here are some photo's of the PCB and insides. The gain control area is over in the right component (top) side, and on the left from the underside view. Again, any help in solving the gain problem would be much appreciated. If anyone wants even more closeup photo's i'll try and do that.
Paolo
It's not about getting more closeup photos, it's about how well you can interprete the photos. To visualise the circuit from the photos one should be able to see EVERYTHING in a manner that practically leaves no room for second guessing.
From the posted stuff...
- I couldn't interprete resistor colour codes even if my life depended on it
- I couldn't see all the components since they are hidden by shadows or by things such as wires running over them or parts of other components blocking them
- I couldn't see where the offboard wires go
- I couldn't accurately follow the solder side traces because of low resolution of the photo and the skewed angle of the shot.
- I couldn't accurately tell which component pin corresponds which solder pad at the bottom, mostly because of the aforementioned reasons.
I don't even bother to list everything I couldn't do working with such weak photos. Even few points are enough to warrant a fail. Don't get me wrong, as amp porn those photos are great, but as something that should provide enough information to trace the circuit in proper detail they are lacking.
Also, the reason i'm ranting about this is that photos taken closer to the board can still fail in all of these accounts. I've seen it happen dozens of times. It's about the quality of what's in the photo. If all things were allright the size would actually be just about perfect. The key is, the circuit must be interpretable from the photo.
You have an advance since you have the thing right there in front of you. The faulty area is probably one or two gain stages at max, three to four transistors at max, and a handful of generic passive components. Sketching a schematic should be a breeze, even for the entire amp.
You want some help: Based on your description the problem is most likely found from the stage(s) incorporating the gain and the volume controls. The basic functionality of the gain potentiometer as a variable resistor seems to be allright, the effect of this functionality in the circuit is not. Why? I would need more data to be able to answer that. The functionality of the volume potentiometer leaves room for doubt but then again the two could be interacting in the circuit in a way that...
Well, let's just say guessing isn't really all too helpful and I'd be more inclined to discuss functionality of a circuit when I would know something concrete about it. We can spend a year discussing what a circuit could theoretically do and what a circuit could theoretically be alike but that doesn't really help to fix an amp that's broken in real life.
It's a basic circuit, attack it with those basic troubleshooting techniques that would follow visual inspection and you probably should be able to find what's wrong in a very short time.
You have worked with related electronics before, treat is as a generic stompbox if it helps at all. It's transistor or FET-based stages introducing gain and a bunch of passive components doing what passive components usually do in such circuits. Not rocket science.
But for us to be able to help you in a greater detail you also need to provide us information in much greater detail.
Nope. Obscure-tronics; all I get from Google is Kermit and Miss Piggy.
Quote from: Paolo
It was like this when i bought it off ebay a few years ago.
It is quite possible that it isn't actually
faulty, but was built like that, which is why finding an actual circuit would be very helpful. It is pretty unusual for an amp to simply lose gain and still sound clean, without distortion or crackles &c.
If you could trace the circuit from the input, past the gain control, and up to the main volume control, and post that here it would be a great help.
You can also try two other approaches, signal tracing, and signal injecting.
In the first you apply a signal to the input, then use a lead connected to the input of another amp (via a dc blocking capacitance) to follow the signal and see how the gain ramps up (or doesn't) as you move from input to master volume.
Signal injection is similar except that you inject a signal starting at the input and moving to the volume control.
But in both methods the idea is to get an idea of how much gain each stage is providing, and if one stage isn't providing any.
A possible single fault is that an emitter bypass capacitor has gone open for some reason, which would give a clean reduction in gain.
Teemuk -
Firstly, i posted this question in the 'Newcomers' section beacuse it suggested BEGINERS post their amp questions here. I am a BEGINER in electronics and have only modded a few things, where i have been told, or given diagrams of what to replace on a PCB. I can barely read a schematic never mind trace one out. I can see a burnt out component or loose wire and have a multimeter and soldering iron.
I took the photos to show a general overview picture of the front and back of the circuit board, so people could see it was an old fashioned pcb and what type of circuit. From there they could make suggestions as to what components create gain in these type of amps, and hopefully suggest testing/replacing certain types of components, that i could do myself to see if it could be fixed. I wasn't even really sure WHAT to photograph at this stage. People might recognise the type of circuit and have similar experience of what went wrong even if it's a different make of amp. You suggested this type of circuit has transistors for the gain stage - that's the sort of thing we need to know.
The pots are wired direct to the PCB and i could barely get the thing out of the faceplate as the connecting wires to the jacks, etc have obviously been wired after the PCB has been screwed down and positioned (you can see the strain on the wires in at each end on the underside photo), therefore i did the best i could photographing the PCB. Like i said, if anyone wants closer, more detailed pictures they only have to ask, though i can only zoom in on what's already there.
As for seeing the whole thing, that is the whole thing. There is just a transformer and reverb pan that you can't see which is in the photo below.
Again, i'd be really grateful to anyone who wants to help with this problem and offer advice. Please remember though that i'm just a beginer with this stuff.
Thanks, Paolo
Roly -
Thanks again for replying!
Unfortunately though, i'm just really a beginer like i said above and don't understand alot of this advice, even though i'm sure it's excellent! I could just probably test with a multimeter and replace components as directed.
The gain is there in a weird distant manner like i described before, so i assume it must have been there originally (otherwise why have a gain knob if it didn't do anything?).
I suppose i'm not technical enough to be on this forum. I thought it might be quite easy to diagnose and fix.
Dear Paolo, big problem is that the amp is unknown, there's no schematic available and it's too complex for you to trace it.
Which is never easy, even for experienced people.
Even if you see components on top, and tracks and pads below, sometimes is easy to lose count, and assign one leg above to the wrong pad below.
Oh well.
On other sites, I've followed guys degooping and tracing boutique or "mystery" pedals, and even then, when Boards are smaller and simpler, and where small parts allowed to superimpose (using photoshop) both top and bottom views, simulating a translucid PCB, things have not been straightforward
They had many guys working on the same, yet schematics were corrected many times, and so on.
Look in this post how many guys tried to lift the design of a pedal, with less parts than your amp does, and how even then there are many obscure points.
http://www.freestompboxes.org/viewtopic.php?f=7&t=17919&sid=909648517da3c0c70afd4a5a7a3253ed
Back to your amp: if you can't solve the dirt problem, but works great as a clean amp, use it as is and maybe in the future you can do more for it.
QuoteI could just probably test with a multimeter and replace components as directed.
Problem is, we can't really direct you since we know practically nothing about the device.
You could test basic functionality of parts but most of these would be "in-circuit" tests and this practically means you would need to trace out the circuit anyway in order to get any sense to things you are testing.
QuoteI thought it might be quite easy to diagnose and fix.
Probably would be if we had access to the amp and could know more details about it. Based on mere problem description, a handful of more or less hazy photos, and your inability to help further.... not so.
QuoteI suppose i'm not technical enough to be on this forum.
Well, this forum is really about discussing solid-state amps and as far as I see the topic doesn't really segregate anyone based on technical skills. Even without knowing anything about how they work people can still talk about amps.
But this thread in specific is about trying to to fix and troubleshoot an amp and that thing indeed requires technical skills and knowhow of electronics.
--- Quite frankly, I'm not too big of a fan of these kinds of threads. In the words of a car analogy they way too often go like this:
- My rare and obscure vintage car that you probably never even heard about has this weird side noise in the motor. Can you guys help me fix it?
- Well we can try. Since it's a rare car we never heard of could you provide us some service manuals or other helpful info so we have a clue of what we're actually talking about? Besides knowing that it has a combustion engine, that is.
- No.
- Well, tear down the motor and tell us what unordinary and suspect things you find and we'll see if we can help.
- Thanks, I would but I have never fixed engines before and I know practically nothing about how they work.
So, you see how it can get quite abit of frustrating. On my behalf I must admit that I usually stop at the point where I realize the thing is going way over head of the thread starter. In this case the easy fixing and troubleshooting tip is: A visit to professional electronics repair shop.
Hi Paolo,
a possible suspect for you.
I notice that the Amp has two separate inputs, one says *Boost*.
OK a fair chance it works independently from the clean.
First I'd check the input setup,, my guess is the gain knob is only going to have effect IF the guitar is plugged into the *Boost socket.
So try that> let us know if the gain now works.
Phil.
I've tried that Phil, along with every other variation on the dials and inputs. The only thing that really makes any difference is if i turn the reverb all the way up, the background faint overdrive is a bit more prominent.
The boost input is really just like the high and low gain inputs that you would get on a standard Fender amp, although there is more of a difference between the two. The gain control seems to act more like a mid and treble boost on either input.
BUT,Since you mentioned the boost channel i think i may have now found out something that may be helpful regarding this:
I have a EHX LPB1 (clean) booster pedal, and the thing i've found is that if i plug into the Boost input of the amp and turn the level knob on this pedal up past 9 o'clock (normal unity gain) the overdrive starts to work! Similar to the way a valve amp overdrives with more of a hotter input signal. Turned up fully there is a massive Marshallesque crunchy overdrive. (This only seems to work using the Boost input at low house volume. I assume it would do the same in the normal input with the amp volume cranked up more.) So there must supposed to be something that boosts the signal once it enters the amp and before it hits the gain knob or stage that is at fault. I hope we are getting somewhere with this idea!! If anyone would like a more detailed close up of that input/volume/ section of the PCB, let me know.
J M Fahey - I was thinking that i would have just had to use it as a clean amp with pedals as i have been doing, but the above discovery has proved that it did indeed once work as a gain knob. By the way, thanks for the link to the Freestompbox place, but i couldn't register on that site to see it for some reason (i'll try again later). I would have thought that the old skool PCB on this amp with big patches of solder and no component i.d marks and numbering would make it even harder to trace!
Here is a site where there are pictures of a few Prince amps: http://fr.audiofanzine.com/prince/
Included in this is the Prince Piggy-3, the deluxe version of my amp, which has additional Mid and Distortion controls. When i saw this i thought the gain on my amp must just be for mid/treble boost like JM Fahey suggested, since this had an actual knob for distortion, but now i'm pretty sure the gain is there waiting to be reborn!
There are also a few owner reviews of the Prince Piggy-3 on Harmony Central here: http://www.harmonycentral.com/products/123165
PS - I've just re-read those Harmony Central reviews for the Piggy-3 and realised that the last review by Egbert Griffioen was his review for the Piggy-2 which was on the old Harmony Central site (before they junked all the thousands of user reviews and started again from scratch - what a waste of all that wealth of information!) There also seems to be a review of a Piggy-1 in there also. They must have grouped all the old Prince Piggy reviews under Piggy-3, assuming that was the last model.
Anyway, i'm eager to hear your comments on this new development!
Paolo
As far as (I think) I can follow the circuit based on the photos the "gain" control looks like following:
Potentiometer wired as two-terminal variable resistor. One end is grounded the other connected to series resistor. The series resistor connects to a capacitor and the capacitor in turn connects to one of the terminals of the nearby transistor. I don't know if it's anything more fancy than a generic variable emitter/source resistance thingie but it might as well be something like an adjustable negative feedback loop.
There could easily be a handful of different kinds of circuits with that kind of basic part, all achieving the function of controlling stage's gain. This is why we actually need to know details about the circuit, it's all guessing game without that information.
Anyway, that kind of circuit would practically operate pretty much like a volume control - after all, it's literally just introducing user-adjustable gain control for the stage. NOTE: Gain does not mean distortion. How much distortion you get actually depends on overall signal levels such as - as you figured out - the overall amplitude of the input signal. Probably some part is faulty causing way too much attenuation so overdrive never really takes place without help of external devices introducing even more gain ...but this again just goes down to theoretizing and in order to know something concrete you need to test what actually happens in the circuit.
OK,
You obviously have some interest in the inner working of circuits,,,,,
Well Now is your chance to actually learn something really useful. :dbtu:
You have an amp that at least works, is not extremely complex but has a small fault.
You could start teaching yourself electronics by pen and paper. 8|
Just start drawing the input circuit.
Hold the board up to strong light and you can follow the connections to those first two transistors.
Google the numbers on the transistors for pinouts and before long you will have some semblance of the input circuit.
BTW, this is how I learned to understand circuits and NO help from the internet.
It takes a long time and you waste a lot of paper LOL but just by looking at other similar schematics gives you the idea of how they connect and how to draw decent schematics.
My guess is one of the can caps have died and that pot likely connects a bypass cap to the ground/common trace in local feedback.
Thousands of similar BJT or fet circuits have worked that way,, Draw a few circuits, cruse around old transistor Amp circuits and it won't take long to get an idea of how your input circuit works.
It might seem like a really hard thing to do but then you don't learn guitar in a week do you. winky.
Phil.
Paolo, is there anyone who knows something about electronics you could get to give you a hand tracing the input circuit, science teacher, local radio ham, hackerspace etc?
All we really need is an idea of the circuit from the input sockets past the gain control to the main volume control.
Failing that we need two photographs of the input end of the board which need to be;
- taken with a camera that has macro "tulip" mode, important to get them crisp and in focus
- square on, directly above the middle of the board between the "gain" and "volume" controls, not at an angle
- fill the frame with the end of the board (see example)
- no flash, and lit from two or three different directions so that shadows are minimised (e.g. three disk lamps close left, right and in front, positioned so there is no reflected shine)
- both sides of the board like this
- do not reduce these, post full size
Also need two shots at different angles of the backs of the input sockets that show how they are wired (as above, but these can be reduced).
Meanwhile; making heavy use of my crystal ball, here is a bit of what I think the input and part of the first preamp looks like.
If there isn't a dry joint in the gain setting path, then the most obvious suspect has to be the can electro (but I still remain to be convinced that it isn't working as designed - unsatisfactory maybe, but as originally built).
Hi Guys
I really appreciate all the time your spending on this and advice you're giving me!
I think i really have to step up to the plate now and get stuck in, now that that you guys have identified the potential faulty area and slung some theories out there. I don't understand most of the technical language but i'm sure it's informative for the other parties to speculate on.
Unfortunately, i don't have any one at hand with electronics knowledge that can assist me, but I definitely need to try and trace that part of the circuit myself and start learning about this stuff, as Phil suggested. I think i will probably have to draw it out in my own way though, as i can't relate schematics to actual pcb's. In other words, it will be a drawing of the PCB
with linking lines denoting the connections.
I also think Teemuk and others are right about the quality of the photos, so i will do some new ones in the next day or two, focusing on the problem area. (Thanks for those photography tips Roly!) As i said, initially i just wanted to get the whole pcb in the photos and i'm a beginer with this stuff, so i didn't know what people would want to see the most.
Until i get the new photo's done and try and check the circuit path, i have another (definetly simple!) question about the reverb, which i will make a new post for.
{Still do as Phil suggested as best you can, your own way. One tip I can suggest is that you make a drawing/tracing of the copper side area of interest, then try to locate the components on the upper side onto that drawings. Done as carefully and accurately as possible, that alone would be very helpful. When trying to locate components a powerful desk lamp shining from the copper side can give you an "X-ray" view.
Resistor colour code charts;
http://www.google.com.au/search?q=resistor+colour+code (http://www.google.com.au/search?q=resistor+colour+code)
On-line calcs;
http://www.csgnetwork.com/resistcolcalc.html (http://www.csgnetwork.com/resistcolcalc.html)
http://www.hobby-hour.com/electronics/resistorcalculator.php (http://www.hobby-hour.com/electronics/resistorcalculator.php)
}
Okay, a few things you can check/try before posting photos.
See bottom view, attached. Measure the resistance between ground "Gnd" and point "A" while varying the gain control. The resistance to ground at this point should vary between nothing and whatever the value of the gain pot is (a few k ohms?). If this appears to be open circuit try remaking the solder joint to the pot directly above "A".
Resistance swings up and down? Good.
Next do the same thing from ground to point "B". In this case the resistance should go from the upper value found above, plus about 1k, to around 820 ohms.
Don't get this result? Then the 820 ohms (grey/red/brown) resistor is faulty, (but this is quite unlikely).
Do get this result?
Try replacing the electrolytic cap (shown in green circle in the top view) with the same value (this doesn't have to be exact, 10-20% more capacitance and/or voltage is fine). Note value and post here as soon as you know. This cap is polarised and must be replaced in the same polarity (use marking pen to mark on the board where black negative mark is on the cap before you remove it).
Cured fault? Post and we can all enjoy a beer.
Same as before?
Now we get out of repair territory into mod territory.
When you go to buy the new cap, also buy the following resistors (if you don't have to hand or can't scrounge from some dead radio or such); 680 ohms, 560, 470, 390, 330.
Replace the 820 ohm resistor (blue circle in top view) firstly with the 470 ohm (don't cut the leads short, mount it up in the air a little bit, say half and inch).
The gain control should now produce more gain at the top end.
You may get the following results; just right, not enough, too much, wild oscillations near the top.
If not enough, try a lower value, if too much try a higher value, if wild oscillations try a much higher value.
Once (if) you find a value that gives you satisfactory results you can remount in down close to the board like the original (and make a note on the board of your mod, original value, the date, and your initials).
Post results. {Drink well earned beer. :dbtu: }
Hi
I have been trying to draw/trace the circuit around the gain area, which has took around 6 hours!, but i've just looked at the photos i took, and i think the x-ray pictures are actaully better. I think this will leave no doubt where everything is, and someone will be able to see it as a schematic.
Roly - I will try and work through your suggestions shortly, but i might not have the components at hand to try some of the suggestions and will have to order them.
Anyway, i hope the new photos are helpful! If anyone wants any components or connections identified more specifically i will try to see what is written on them or where they join. The ony thing i can't tell from when i was drawing it out was, where the silver metal towers are connected underneath.
I think this site is terrible for being able to view pictures! Each picture should have a scroll bar to move the view along, and/or show them full size when you click on the thumbnail. The best thing to do seems to be to click on the blue file name under each photo and open them in your prefered image viewer and make your own copies.
I have also updated and put photos on the reverb problem post.
Paolo
Hi Roly
1. Measure the resistance between ground "Gnd" and point "A" while varying the gain control. The resistance to ground at this point should vary between nothing and whatever the value of the gain pot is (a few k ohms?).
A: This reads from zero to 2.7 ohms while turning the dial. The gain pot itself says B3K ohms.
2. Next do the same thing from ground to point "B". In this case the resistance should go from the upper value found above, plus about 1k, to around 820 ohms.
A: At full clockwise it reads 820 ohms, then gradually gets into the 1000's as you turn it back to Zero. (I have to point out here that i always have a real problem reading the meter, as i'm never sure which setting to put it on in the ohms catagory. i don't know the difference between ohms and k ohms and stuff like that! The meter i have is also very basic).
3. Try replacing the electrolytic cap (shown in green circle in the top view) with the same value (this doesn't have to be exact, 10-20% more capacitance and/or voltage is fine). Note value and post here as soon as you know. This cap is polarised and must be replaced in the same polarity (use marking pen to mark on the board where black negative mark is on the cap before you remove it).
A: The only writing on the elec cap is Tracon and 10/16.
These are the spare elec caps i have: 4.7 uf/25v
10 uf/35v
22 uf/10v
22 uf/50v
Should i try any of those?
As for the mods, my playing style doesn't need loads of overdrive, as i like a gritty, open medium break-up overdrive. Sort of keith richards to george thorogood tone. So i probably don't need even more overdrive than the amp does already! The overdive i was getting with the EHX signal booster pedal sounded pretty good, a bit like my Fender tweed Bronco, but with it's own 'unique' 70's Japan Solid State touch!
Photos much better, thanks. You really only need to work out the first bit, first two controls or three transistors from the input ('tho having an entire tracing to work from and offer others is helpful with any problems down the track, and to others struggling).
"10/16" will be 10uF/16V, but the value nor the voltage rating are critical here, anything within 2:1 will be fine.
Reading 1. is inconsistent with reading 2. but your comment explains that; "k" = "kilo" = x1000.
Reading 2. is as it should be.
Any of those caps will do, it's mainly a matter of what will physically fit. I'd select the 22uF/10V on the grounds of voltage first, then the 10uF/35V on grounds of value, then 4.7/25 on voltage. Just watch you get the polarity right - the black stripe means the negative lead and will almost certainly go to the 820 ohm resistor+pot.
What this bit of the circuit does is change the voltage gain of the first preamp stage, and the 820 ohm resistor sets a minimum resistance which sets a maximum gain. By reducing this resistor you allow the stage to go to a higher gain, more crunch.
If you carry this to extremes (such as putting in a wire link instead of any resistor) the gain will go to a very high value. This may work but the amp may also become unstable with so much gain, and oscillate at high gain settings.
Something for you to experiment with.
Try and find some sort of old electronic junk, radios, Hi-Fi amps, whatever, you can recover components from. Anyone who is into electronics has a well stocked "junk" box (although the class of the junk varies quite a bit). It is just shocking how many perfectly good components go to landfill every day. I haunt my local rubbish transfer station and tip shop, garage sales, op-shops (thrift stores), roadside hard rubbish collections, &c&c.
Keep us posted on how you get on. :tu:
Hi Roly
I've tried a few things from your suggestions with some positive results!
1. I changed the electrolytic cap with a couple of the ones i had spare (22uf/10v & 10uf/35v, but this didn't seem to make any difference at all. Eventually, after the whole testing session, i put the original back in as i don't think it was the problem? Is this what you thought was the main problem?
2. I took out the 820 ohm resistor and I tried a 467 ohm resistor. This didn't seem to make much difference.
3. I didn't have any resistors of a lower value, so i just went straight for a jumper wire. This gave the massive overdrive that i got when i used the EHX clean booster pedal before it. This was in the 'Boost' input, but i also plugged into the 'Normal' input and got similar results at a lower volume and reduced gain. So now we know the amp works on the same basic principle as most amps with a high and low input.
4. From 1 - 8 on the gain knob, nothing really happens different from before (just gets louder), then the overdrive kicks in dramatically between 8 - 10. , with little scope for much fine adjustment. I assume this is because there is no resistor, just the jump wire?
The overdrive itself is pretty raw in a crunchy marshall distortion way, with an undercurrent of fuzz. There is artifacts in the sound at higher volumes, but this is probably due to vibration from the amp being open and not all screwed down. There i also a high pitched whine when using the headphone socket with the gain on full using the Boost input.
Some people would love this tone as is, but i need less (or the more scope to go from less to more) and a more gradual sweep on the Gain dial. It's amazing how one tiny part can make so much difference!
I've sent off for some of the other resistor values you suggested, so i'll try those when i get them.
Only recently, i chucked out alot of old hi-fi equipment/ ghetto blasters that were partly broken. Some good made in japan stuff from the 70's. I took out some electrolytic caps and normal caps and some other parts, but didn't take out any resistors because i didn't think i would be able to identify them, or even need them for that matter.
I really appreciate all the help and advice so far. Getting some results now!
Paolo
1. okay, this suggests that there isn't actually anything
wrong and it is working
as designed, just not to your satisfaction.
2. no it wouldn't be dramatic, roughly a doubling of gain. (820 / 470 = 1.7 times)
3.
Quote from:
so i just went straight for a jumper wire.
Yikes! Oh well, okay, so now you know what it will do with the preamp first stage pedal-to-the-metal. 8|
Just guessing on the numbers you gave I'd say that distortion due to overload will start with the "gain" control flat out with a gain-limiting resistor of about 330 ohms (orange/orange/brown), so if you want a bit of crunch right up the top end of the gain control I'd go for something like a 220 ohm (red/red/brown).
I assume the "467" was from your ohmmeter (maybe with your fingers in parallel?) and it will be a nominal 470 ohms (yellow/violet/brown). Holding the meter probes onto the resistor lead with your fingers works up to a few k ohms, but above that you start to get errors due to you body resistance being about 20k, and not constant at that either, so you can only hold one end.
The high pitched whine may be the oscillation I was worried may happen with very high gain, but it could be that it is external electrical crud (light dimmer, electric drill or blender, &c&c) and you can now hear it because the amp has a whole heap more front end gain to amplify it (and it may be reduced or eliminated when the amp is reassembled).
Yes, well, the pot sets the first stage gain and the resistor limits how high that gain can go, so replacing the limiting resistance with a bit of wire seriously supercharges the first stage. Now it's a matter of tweeking the value of that resistor until you get the result you want, or at least can live with.
I have a large box full of printed circuit boards ripped out of sundry stuff. The cases are generally useless and just take up space, but you can get 10 or 20 PCB in a small cardboard box, and whenever you are short of a cap, LED, resistor, or even transistor you can hunt one down and recover it.
Learning how to read the resistor colour code on sight is very helpful, but you are normally only dealing with about a dozen values and their multipliers. Get a colour copy of a resistor cheat-sheet and pin it up above your soldering iron, and pretty soon you'll be picking up resistors and seeing their values without looking them up.
It's one of Murphy's very many Laws, you'll need tomorrow what you threw out yesterday.
Have we now moved closer to what you originally wanted?
Hi Roly
Yeah i think we/you are really starting to solve this problem! I just want to get the amp to sound how it should have done originally or better. I disagree though that the gain knob was designed to not produce any gain! I think the fact that we can get a big overdrive sound now proves that it's always been there?
I'm also learning alot from doing this. While i'm on the subject, could you explain what the electrolytic cap does on my amp gain and why you thought of changing it? I'd like to learm as much as possible. I still don't really know what causes the overdrive other than the resistor controls the amount. I only know about diode and LED clipping from pedal mods.
Yes i probably had my fingers contributing to the ohm reading! I've included a picture of the meter i use. As you can see on the left, there are several settings for testing ohms and as i said sometimes i'm not sure what to set it on, but i think i'm getting the hang of it more now. Prior to problems with my amp i'd only used it to test the resistance of pickups and to test battery power.
The 467ohm resistor was blue with dark blue stripes. I got sent some with some LEDs i ordered for a pedal mod.
Perhaps the real problem with the gain on this amp (as i've mentioned over on the reverb problem post), lies with the ghost overdrive, which is still there under the regular crunchy overdrive. It's like two guitarists playing the same thing through the same amp, one using a distortion pedal and the other on overdrive pedal where the batteries in it are dying and it's like a splatty fuzz tone that has a longer sustain than the first guy's guitar.
This is also present in the reverb and increases when the reverb knob (and/or the gain) is turned. It's always been like that. It's not present if the gain knob is completel off. I'd rather have the gain working than reverb, so it's not a big deal but i'm hoping we can fix this issue also, once i get the right ressistor to set the gain at the right balance. Do you have any ideas what causes the ghost fuzz? 'Phatt' seems to think it can't be fixed (at least in the reverb).
I'll definitly have to start saving old cicuit boards. It's a good idea.
I really appreciate all the time you've spent on this and the step by step advice. You have a lot patience!
Paolo
Paolo, I've been working on guitar amps for fifty years odd and I can sadly assure you that amps that are poorly designed are far too commonplace. Even very well known names like Marshall have amp models that are dogs to live down.
The fact that we have found no actual fault tells us that this amp is working as it was out of the factory.
"Gain" is just another word for "volume" and as I understand your posts this control has been changing the volume, just not as much as you would like. "Overdrive" means enough gain to drive the amplifier into clipping or limiting with the attendant crunchy or grinding tone of distortion due to the amp being driven past its clean range.
This amp has two inputs, "normal" and "boost". Well actually what it has is one input that cuts the input signal back a lot, "normal" and another that cuts it back a bit less, "boost". These two signals are then taken to the same place, the input of the variable gain stage where the gain is set by the gain control.
Why did I suspect the electrolytic?
A couple of reasons, firstly that these types of caps "dry out" over time and lose capacitance which would, secondly, be consistent with a remark you made that suggested that the gain control was acting like a middle/treble control.
This could be simply subjective, or it could be caused by a cap in that position that wasn't fully open circuit but had only lost a lot of it capacitance, so I got you to test it by substitution.
Since this produced no change we could then assume it was okay, and move on to modding the circuit to get more gain from the first stage, i.e. by reducing the resistor that sets the upper gain limit on the gain pot. When you put in the wire link you experienced how high the gain could go.
As I said above, once the gain got high enough some stage down the chain ran out of headroom and started clipping, giving you "overdrive" gunge. As it happens (and this is very common with solid-state amps) this clipping didn't sound too nice, and may give us a clue as to why the manufacturer selected a resistor value in series with the gain control that prevented you getting into this gain territory.
The resistance ranges, like the other ranges on your meter, define the highest reading on that range, so "200" is for measuring resistance up to 200 ohms, "2000" to 2k, and 20k, 200k and 2meg, ditto. It is particularly important with these last three higher ranges where you have to be careful about your fingers - just try measuring your own resistance between your hands and you should see why - you are in parallel with what you are measuring, and you ain't no insulator. ;)
Just another point about resistance measurement is that they must be made on circuits that are dead - voltage free - or you will a) get incorrect readings, or b) blow up your meter. All of these style of meters are also quite prone to dirty connection on the range switch, so if you get readings that don't make sense, just try wiggling the range switch a little bit.
A resistor with all blue stripes would be 660megohms. Google "resistor colour code" and learn how to work out the value of resistors from their colour bands.
(http://members.optusnet.com.au/nswmn2/ResColour_files/image003.jpg)
...and...
http://www.michaels-electronics-lessons.com/resistor-color-code.html (http://www.michaels-electronics-lessons.com/resistor-color-code.html)
Phil has been paying much more attention to your reverb problem than I have, so I would trust his advice/opinion.
HTH
You know alot more about this stuff than me, so i'll just have to accept it was manufactured like that. As far as i'm concerned though, it's been fixed, BY YOU, ha ha!
Well the main part anyway. I still don't know what is causing the ghost overdrive/fuzz, and you didn't mention it in the last post. Unless you are calling this "Overdrive Grunge", in which case if it happens "further down the line", i'll just leave it, as any more messing around by me might damage the amp.
When you suggested changing the elecrolytic cap, i thought this would make the overdive happen, so that is why i said it didn't do anything. I wasn't listening for changes in the mid/treble. Should i try replacing it again, once i've got the right value resistor in place.
In a way, the gain knob working in this mid/treble manner was a good thing as it contributed to the woodyness of the middle and the chime of the top end at low volume. Some of that will now be lost in favour of a fixed overdive and an improved reverb (with new tank). The gain knob will have to be on zero, to avoid the ghost overdrive coming in for clean tone with reverb.
Ideally, i'm hoping for the great clean tone to remain, with good reverb available to that clean tone. I don't use reverb with overdive so that shouldn't be an issue really. Then also i'm hoping for a nice range and sweep of overdrive from the gain knob (after i've found the right resistor value). Obviously, given the manufacturing limitations, these two things will have to remain strictly seperate to both work effectively. If i get all that i'll be really pleased, and we can say we have made a great improvement to this amp!
Yeah, Phil has been the main guy focusing on the reverb issue, i might attempt his stand alone reverb tank one day.
Thanks for the explanation of the gain, the components and the resistor chart. Very useful info. I've learned alot from this forum. I even have some idea now of what is probably wrong with the excessive bass on my Fender Princeton Chorus. So keep an eye out for that post! I even have a schematic!
What does HTH mean at the bottom of the post Roly?
Well, apart from playing around with the resistor value in the preamp you can also play around with the value of the capacitor. For large values the gain control will effect all frequencies equally, but as you reduce its value the gain will tend to be tilted towards higher frequencies.
As the original cap was 10uF you could try some larger values and this might make the bass end more responsive to the gain control (and maybe not, maybe it is already large enough) but you can certainly try some lower values such as 1uF, 0.1uF and 0.01uF.
The smaller values will be non-polarised types, such as "greencaps", that can be put in either way. These will confine the effect of the gain control to higher and higher frequencies.
My failure to mention your "ghost overdrive" was deliberate. Where ever possible in electronics you try to deal with only one thing at a time, or things can very rapidly get confusing. Now that you seem to have the preamp under control to your satisfaction we can move on to the next problem.
I suspect that what you are calling your "ghost overdrive" is actually some distortion arising later in the amplifier, and my guess would be in the output stage.
You will notice that there are two output transistors. What happens is that the signal is split into the top half and the bottom half, and each transistor amplifies its half, either the positive half or the negative half. Since the signal consists of waves that swing back and forth between positive and negative so each of the transistors take turns to amplify the output.
Now ideally when one stops, the other should take over seamlessly, however that isn't always the case and you can have a tiny pause in between. Since the signal is crossing over from one to the other this sort of distortion is called "crossover distortion" and happens to be a rather nasty sounding form of distortion even at very low levels.
To overcome this, or at least minimise it, the two output transistors are biased so that they are both slightly on or conducting when at rest, so that when driven one starts to conduct a bit before the other stops (somewhat like the next runner in a relay race starts running a bit before the hand-over).
In inexpensive amps (and even some expensive ones) it is easier to leave a bit of crossover distortion rather than put in the components to set the bias correctly, and to hold it correct for changing temperature.
Lacking a circuit it is hard to say, but I suspect that this is the cause of your "ghost overdrive".
If you follow the wires from the transistors on the heatsink back to the board, it could be helpful to have a photograph of this area of the board where they go.
Also, the transistors on the heatsink will have type numbers such as 2Nnnnn or 2SAnnnn, 2SBnnnn, 2SCnnnn, or perhaps just A, B, or C followed by some numbers - these would be handy to know since they will give a hint to the circuit being used.
HTH = Hope This Helps.
Well, i might have the preamp under control to my satisfaction, if the electrical store could get my order right. They've sent me the wrong thing twice now, so i still haven't been able to settle the resistor value equation. But that should be solved in a day or two.
What i can do though at the moment is tell you the codes on the heatsink transistors, which are TD526, then underneath that code is the code 01 E. I'm assuming the input to the heatsink is on the left (viewing from amp front) and the output on the right. I will take some photos of the area later today.
The white gunk underneath those transistors that you mentioned previously is bone dry. I don't know if that has any bearing on anything?
Thanks for the explanations and further advice. You have a real knack for explaining things in a way that i can understand!
Get back to you soon
Paolo
Quote from: Paolo on June 19, 2012, 08:04:20 AM
The white gunk underneath those transistors that you mentioned previously is bone dry. I don't know if that has any bearing on anything?
Paolo
If by "bone dry" you mean crumbly, then that's not good. I would unmount the transistors, wipe off the crud and reapply some new thermal paste.
Hi, thanks for your interest.
Well from Rolly's initial description i was assuming it would be a thick sticky substance, but it's a thin coat/film. It's not exactly crumbling, as it's not thick enough to really crumble! It IS bone dry though. Probably to be expected after 30 years or more?
Paolo
Quote from: Paolo on June 19, 2012, 02:20:17 PM
Hi, thanks for your interest.
Well from Rolly's initial description i was assuming it would be a thick sticky substance, but it's a thin coat/film. It's not exactly crumbling, as it's not thick enough to really crumble! It IS bone dry though. Probably to be expected after 30 years or more?
Paolo
It wouldn't be sticky, at least not in my experience. Silicone based thermal paste is slippery and should stay gel-like (similar to toothpaste, but slippery like I said) to work properly.
So is this thermal paste issue contributing to any of the problems with the amp (gain and/or) reverb)?
Anyway, i'll have to get some of that paste to refresh the connections.
Thanks for the advice on that.
Paolo
Quote from: Paolo on June 19, 2012, 03:44:20 PM
So is this thermal paste issue contributing to any of the problems with the amp (gain and/or) reverb)?
Anyway, i'll have to get some of that paste to refresh the connections.
Thanks for the advice on that.
Paolo
I doubt it. If it was a thermal paste issue it would only start acting up after being on for a while and fully warmed up.
Nah, something isn't right there; the TD526 comes up as a half watt transistor in a small D-package, and that certainly isn't what you've got there.
Not sure what you mean by...
Quote from: Paolo
I'm assuming the input to the heatsink is on the left (viewing from amp front) and the output on the right.
These two transistors act as a "push-pull" pair, one driving the speaker during the positive half of each signal wave, and the other during the negative.
The output stage of your amp will look something like this (with thanks to Rod Elliot)...
(http://sound.westhost.com/p12_fig1.gif)
At rest the join of R10, 11, and C5 is at about half the overall supply voltage (35/70V), therefore this is called the "half rail", and it's also the output node.
The output pair are Q4 and Q6 (the ones bolted to the heatsink). When the input swings positive transistors Q1, 2, 3 and 4 all turn on, causing the output via C5 to swing positive and drive the speaker cone in one direction.
Similarly when the input swings negative the transistors above turn off and Q5 and 6 turn on pulling the output negative and driving the speaker cone in the other direction from its rest position.
There are several different ways output stages can be arranged, but this would be one of the most common.
The white gunk is thermally conductive gunk intended to fill the small voids between the nominally flat surfaces and improve heat transfer. Once it's in place drying out doesn't matter much since the solids are what do the thermal conducting, it's only a paste form so it's nicely conformal when it goes on.
When you remove a device however if the paste has dried out you need to clean of the old residue and apply a dob of fresh. Any place that sells power transistors should also sell mounting kits, variously shaped insulators, and stepped insulating washers for the mounting bolts.
This stuff was particularly necessary with mica washer insulators, but there are now grey floppy rubbery insulators that are conformal and thermally conductive and don't actually require the white goo.
(PS) Joe mentions silicon grease thermal coupling compounds, and there are several different types. The older silicon grease was a milky paste and not sticky at all, but the while gunk that I get, "Unick", is like artist's white paint and sticks to everything, and it sounds like Joe uses something else again that is rather more friendly.
Remounting the output transistors with fresh goo certainly won't hurt, but be careful of the insulating wafers and washers, the mica wafers are
fragile and it all must go back so the tab of the transistor is isolated from the heatsink (test with ohmmeter after remounting).
Quote from: Paolo
Thanks for the explanations and further advice. You have a real knack for explaining things in a way that i can understand!
My pleasure. I have been doing this stuff since the days of valves (tubes) and for much of that time I have had trainees. When it comes to electronics it's us imperfect humans against Murphy and the very picky universe, so I do like a win, and the moreso when I've helped an "improver" to a win and a better understanding. And you can always award chip points, top left. ;)
Far too many musicians are far too often victims of their gear. Rock 'n Roll has always been somewhat technical with amps 'n all, so really the modern guitarist needs to also know enough about electronics to survive.
Here are some photos of the area where the heatsink wires connect to the board.
Assuming that the input wires to the heatsink are front left, those wires are on the right on the PCB in the pictures.
INPUT to heatsink: Right hand yellow and the top blue and the (partially hidden) orange wire underneath that.
OUTPUT from heatsink: The next blue wire under those and the lefthand yellow and left hand orange beneath the two big red resistors.
For orientation, the controls pots from left to right in the picture are Tuner/Reverb/Bass/Treble. From the underside of the pcb they are featured at the top of the picture so you can see where the front corresponds to the back. There is also an x-ray view of the area.
I hope these photos are ok. If not i'll take some more.
Not sure what you mean by...
Quote from: Paolo
I'm assuming the input to the heatsink is on the left (viewing from amp front) and the output on the right.
[/color]
******************************************************************
Ahh, i see what you mean. I thought you mean't transistors on the pcb in your previous explanation/post. I suppose i've overexpained then in my last post!
Here is a picture of the holy grail for those non-believers!
(I put the 'L' pen mark on by the way, because i thought i might have to unscrew those to get the PCB out. Glad i didn't now i know about the thermal paste! (which i think i'll leave until everything else is sorted)).
Thats a 2SD526 transistor many mfg leave out the 2S part. Data sheet here http://www.datasheetcatalog.org/datasheet/toshiba/3449.pdf
The T is the manufacturer's logo - Toshiba.
That's great Tony.
Thanks for the ID on the transistor. Roly should be happy with that info.
Paolo
Thanks Tony, that's much more like it and helps a lot.
Paolo, take a look at your (excellent, crisp, and well lit) pic 874 - a bit below the middle there are a pair of red-ish resistors connected to the yellow wires to the output pair, and marked "0.5 ohm".
These are a dead giveaway as R10 and R11 (Rod's cct above) and when you are dealing with an unknown amp you look for something like these to help orient yourself.
Another thing you look for is if the type numbers on the output transistors are the same (quasi-comp, generally both NPN as here), or different but perhaps by only one number (fully comp NPN/PNP pair, e.g. BD139/BD140, 2N2955/2N3055).
That would make the two blue wires just to the right the output transistor base connections (between the two 150 ohms brown/green/brown/gold, like R8 and R9), and the two transistors to the right will be the drivers (like Q3 and Q5 above).
A couple of variations; your amp appears to use a split rail supply and direct speaker connection rather than a single rail and output coupling cap (C5), so the supply will have a grounded mid-point between the two caps at left, just above the four diodes for the power supply bridge rectifier.
(http://www.ozvalveamps.org/repairs/solidstateamprepair/dualrailpsuhp.jpg)
(http://www.ozvalveamps.org/repairs/solidstateamprepair/directcoupling.jpg)
The transistor just above the centre of the pic will be doing the same anti-crossover bias and thermal compensation job as diodes D1, 2 and 3, and the transistor just to the right of that will be the VAS - Voltage Amplification Stage, Q2 above.
I'm guessing, but I'd say that the two transistors on the right-hand edge are a differential "long-tailed pair" doing a somewhat better job of the task of Q1.
(http://www.ozvalveamps.org/repairs/solidstateamprepair/circuit-power-amp-ocl-100w-by-transister-mj15003mj15004.jpg)
Here Q1 & 2 are the long-tailed differential pair, Q4 is doing the bias/thermal comp, and in this circuit Q6 & 7 are providing overload current limiting protection (generally not used in combos where the speaker is permanently connected and not externally accessible).
It's a bit of guesswork, but it looks like your amp uses a transistor bias rather than the string of diodes, but yours doesn't appear to be adjustable, which means that it will be fairly conservatively set, and could well be the source of your "ghost fuzz" - crossover distortion. Tell me, is it worse at quieter levels than louder?
(http://www.ozvalveamps.org/repairs/solidstateamprepair.htm (http://www.ozvalveamps.org/repairs/solidstateamprepair.htm))
{Here endeth the guided tour of your output stage - postcards and tourist geegaws are on sale on the other side of this post.}
Awww Roly, you've gone all JM Fahey on me ;) with those technical terms and complex schematics!
I didn't understand hardly any of that and i can't read a schematic. I know a few of the symbols but that's it. Mainly beacuse it's non-linear, non-heirachical, or any other historically acceptable form of information transferrence! I don't think i will ever understand them. For a start there are no direction arrows from the input through to the output, or between components.
I have included a diagram for everyone's amusement, but if it was like that i could understand it!
It's going to take weeks to figure anything out, if at all. At least they've sent me the correct resistors this time, so i will be able to get that original gain problem sorted.
{You're kidding, right?}
Signal flow goes;
INPUT -----> OUTPUT
By convention forward signal flow is from left to right.
In the "elecfree 100W" (cough cough) amp above the signal flow goes - Input, C1, Q1, Q5; it then splits, the +ve half wave going via driver Q9 and output transistor Q11 and R17 where it recombines with the -ve half of the waveform which has gone ia driver Q8 and output transistor Q10 and R18. Then the recombined signal goes via L1 to the speaker. See attach; forward signal flow in red, negative feedback in blue.
Q4 provides the bias offset between Q9 and Q8 to reduce crossover distortion.
Q6 and Q7 are overcurrent protection transistors (which you can ignore for the moment), and Q3 is a load for Q5 which you can also ignore.
One thing you can do is to fire up your amp, and after it has been running for about ten minutes with the volume and gain at zero, measure the voltage across each of the 0.5r resistors. It will be quite small, in fact it's possible it won't be anything much at all. What we are hoping for is about 25mV (0.025 on the 2VDC range) across each. I suspect it will be lower.
The input stage.
(http://s17.postimage.org/y9k2hj8bx/princeinput.jpg)
Here are the answers to a couple of tests Roly suggested:
1. One thing you can do is to fire up your amp, and after it has been running for about ten minutes with the volume and gain at zero, measure the voltage across each of the 0.5r resistors.
A. (Looking at the picture)
The left red resistor reads 0.015
The right red resistor reads 0.013
2. "ghost fuzz" - crossover distortion. Tell me, is it worse at quieter levels than louder?
A. Increases with the gain knob. It's about the same on the volume knob. The more the reverb knob is turned up the worse it gets, i suppose because the sound is being echoed.
I'm pretty sure that it's not so prominent (in general) when the reverb unit is disconnected. (I'm actually suprised the amp still works with the reverb wires disconected!)
Bearing in mind, i still only have a jump lead in place of the first resistor, there is a high pitched whine when a cable is plugged into the boost channel (but not the guitar) and the Gain is between say 8-10 (even with the Volume knob low). It's not feedback from the guitar obviously.
Also we have to remember that the ghost fuzz was there at the begining with the original 1st resistor in place, so it's not because of the jump wire (although it ain't gonna help).
I really should get that resistor set!
Paolo
Just a quicky 'coz I'm away from home at the moment.
Applying Ohms Law to your 0.5 ohm resistor readings;
I = E / R, = 0.015/0.5, = 0.03 amp, or 30mA, which is a very respectable idle current for this output stage and tends to make my guess about crossover distortion due to insufficient bias look wrong. This should be about right.
Your observation that "ghosting" is less with the reverb spingline disconnected suggest that at least some of this effect is due to vibrations from the speaker being directly picked up by the reverb springline, which I'm sure you have discovered is mechanically sensitive.
If that is the/a source then you need to improve the physical isolation of the springline by resilient mounting and by padded enclosure - there really is no other answer since this is much the same as the feedback you get by bringing a mike too close to a PA speaker.
The reason the amp continues to work with the reverb disconnected is because the reverb is in what we call a "side chain". The amp "dry" or un-effected signal goes straight through, but some of this signal is sampled off and fed to whatever effects ("Fx") are fitted, or via external Fx loop connectors. The "wet" or effected signal is then brought back and mixed with the dry signal down the signal path a bit, giving a mixture of dry and wet which goes on to the main amp and speaker.
Like i said, the ghost fuzz is more like a cheap distortion pedal with the batteries dying mixed in with the amps overdrive. You can hear it as a seperate thing as it has a longer sustain. It's metallic like some one playing down a ventilation shaft. It's not like feedback from standing too close with a guitar or microphone.
I can't say for sure about more or less with the reverb tank attached. The full on gain with the jump wire was clouding the issue. I'll have to try it again.
Without a schematic this will go on forever.
If this Amp means that much to you then,,Time to put in the hard yards and back engineer the circuit and draw up a schematic.
If you Think I'm cruel? :loco
Yes it's bloody hard to do but I probably knew less than you when I started and
Oh Yes,,,did I mention,,, There was NO internet for Me. :'(
So if I can do it? :tu:
Phil.
No Phil, i don't think you're being cruel, but you said a similar thing back on page 1.
Since then we have got the gain working and sounding great; we have discovered that putting in a different reverb tank vastly improves the reverb; we have found Roly! There is a minor gliche in the circuit that still needs to be ironed out and we are working our way through some ideas.
One day i hope to understand schematics, but this day is not it and this amp is not the place for me to start! I need to start small and learn by actually doing stuff and finding how the little bits work, and hearing for myself what happens when a part is changed.
Back when you started, all the components where all probably handwired along a turret board of a valve amp, where the reality gap between a schematic and the board was much less than it is now.
The issue is the ghost fuzz. It is something that must happen in a lot of amps. Someone will know what causes this. They might look at this thread or join the forum at any time and know what causes it.
Let's be positive out there!
Hi Paolo,
OK but Amps don't come much easier to draw up a schematic and learn something very useful.
My best guess is You have DC getting onto the signal.
1/ decoupling interstage cap or 2/ a DC filter cap dying.
Where exactly is any ones guess.
now if you had a schematic ,, :-X arh well I tried. 8|
If your Amp oscillates then that wire link has made that stage unstable,, wise to back it off.
Save the pain mate,, use pedals for gain. xP
Something that might help get your head around these suckers;
Understand that everytime you look at a schematic you are looking at 2 separate but interlinked circuits.
Amplification works by floating the AC Signal (What you hear) on a DC potential.
All the DC parameters are set so as to pass the best possible AC signal. Guitar Amps are often DC offset or deliberately limited to create distortion of the AC signal for obvious reasons.
With most schematics the Positive DC is at the top with ground under the active element. So looking at a simple transistor stage most of the DC is North South whereas the AC Signal usually goes left to right.
AC signal path is not that hard to work out with common sense but the DC stuff is a little more tricky and requires more knowledge.
Needless to say most problems are related to DC as once setup correctly the AC happens by design.
Phil.
The thing is Phil, even if i could trace out the circuit to a schematic, i wouldn't be any the wiser.
I have schematics for a few projects that i can't do. One of which is to un-mod an Electar Tube 10 amp back to the Fender Champ it was based on and bypass the underdeveloped, unfinished extras that colour the tone. This only involves adding a few connecting wires to bypass parts of the circuit and changing a couple of values. This amp is much simpler than the Prince amp circuit and i haven't been ale to fully grasp it. But after this, with the things i'm learning, i think i might be able to do it. (I'll be running that schematic past you guys in the future for your opinion on it's competence, before i electrocute myself!).
I also have a schematic from Carl Martin to make his trem pedal true by-pass but can't do that either (at the moment).
At the end of the road there will always be gain pedals, but NO PEDAL iv'e heard can do the gain tone of this amp. It's unique and sounds amazing, except for that damn ghos ....
Paolo
Hi
The amp was stressing me out too much so i had to take a break.... then it broke! :'(
Firstly an update:
GAIN - What i settled on for the resistor in the boost channel was 120 ohm, which gives me a great crunchy overdrive at the top of the dial. I could have done with 150 or 170 for a just breaking up tone but with 120 the extra is there and i can back off the guitar volume a bit. I'll see how it goes in 'the field' anyway. The normal channel is sounding great also (it always did).
REVERB - I soldered in some new wires with RCA connections for the accutronics tank i borrowed from another amp, which is in the base of the cabinet. The reverb sounds 100 times better than the fool that was in there originally. It doesn't really go very deep but it's just right for my needs.
GHOST FUZZ - Seems to have lessened somehow. It only occurs on the gain input at on 7-10 on the dial but is very faint. Doesn't get any louder when the volume is turned up either. If the reverb is on though wile using the gain input, it's a different story. It's very much there fighting with the main signal. Luckily, it's not really there using the normal channel.
HELP!! PROBLEM!! HELP!! PROBLEM!! HELP!! PROBLEM!! HELP!! PROBLEM!! HELP!! PROBLEM!!
So ... everything was going great until i put the circuit board back in screwed everything back in place, and was getting ready to fix the reverb tank in the base of the cabinet. I turned it on and something is wrong. The whole amp sounds like it's powered by 9v battery on it's last legs. A splattery distorted quieter tone.
Curiously though, everything seems ok through the headphone socket (does the headphone socket come straight from the pre-amp?). I have opened up the amp and checked all the wires and connection and can't see anything. Before i joined the new RCA phono cables to old ones it was working fine, then i put it all back together and bad sound. The only thing i could think of is that i had dropped some solder or stray wire fragments on the circuit, but i can't see anything. Maybe it's linked to the ghost fuzz - something that was on it's last legs has now given up?
It's really frustrating because i'd got it running really nice and thought we'd solved all the problems.
Please does anyone know what happened?
Undo what you just did - go back one whole step to the situation where it was disassembled and working; can you revert to that previous situation? With what result?
Hi Roly
Everything was working great and the chassis was back in and still working great. Then i took the chassis back out to solder some RCA connecting wires to the existing reverb wires, then when i put it together again the problem started.
I've disassembled the connecting wires i'd done but it still sounds bad. That's the only thing i'd changed since it worked. I've checked for loose wires, bad connections, etc. Sounds OK like it used to through the headphones though.
Paolo
On looking at the amp again, i've noticed a small indentation to the resistor which is connected to the treble control, which i assume is from a droplet of solder, but the treble control is still functioning. In any case i've ordered a replacement 10 ohm resistor.
Another thing which could have some bearing, is that one of the speaker wires had become disconnected at some point during the connection of the extension reverb wiring. I've heard that switching on an amp with no speaker connected can blow the output transformer? I hope this is not the case!!
I can't understand how everything sounds fine through the headphone socket tho?
C'mon people - any ideas. I'm heartbroken here!
It is common for resistors to have bumps and paint voids, little bubbles that have burst and set into a crater-like hollow. Nothing to worry about I suspect - post close-up.
Ten ohms, in the wiper of the treble pot - you sure it's only ten?
That it sounds fine through the 'phones suggests you may have a speaker problem.
Have you tried running the amp into a different speaker cab?
Similarly, try running another (hopefully known good) guitar amp into your speaker (gently) and see how they compare.
And no, a valve amp must never be driven into an open circuit and must always have a suitable load; transistor amps must never be shorted but are fine open circuit.
Here is a close up of the resistor. I think it's definitely a solder melt. I was also wrong about the value, it's 100 ohm! I've tried a jump wire in it's place and it made no difference, but there was no sound without anything there.
It's in between the volume and the treble controls. The burnt end in the photo goes to the centre lug of the treble (ground?) The bottom goes way off the right of the photo to another resistor approximately (for cross reference) in the middle of the board and between the volume and boost pots (if looking at previous photos or the 'x-ray' photo you will see it).
Originally i also thought it was the speaker, but I've tried connecting different speakers and it's the same. Also following your suggestion, i've just tested the speaker with another amp hooked up and the speaker is fine.
I can't say for certain that the speaker wire didn't touch the speaker, the chassis bottom, etc when it was loose. So it might have shorted something.
This probably has nothing to do with it, but my soldering iron stopped working at the same time as my amp broke!
I'll try and describe the sound in more detail:
The normal input has virtually no clean tone anymore. It's sounds more like the boost input was on full but a really bad version. The decay is splattery and has a bit of tremolo/ocillation breaking up the signal. The boost input sounds like the ghost fuzz has completely taken over and killed the main signal.
Is there anything i can test with my meter?
Depending on the output setup,,I'd hasza guess and say you have blown the output devices.
Small low end SS Amps with internal speaker are often hard wired direct to speaker and have *no Short Circuit Protection* as do other SSAmps.
It only takes a micro second short to blow power devices in Amps like that. :'(
Well now that it is likely stuffed you might like to take the slightly faster long cut to fix it and learn something useful in the process. 8|
Paper , pen and start drawing up a schematic. ;)
Otherwise don't mess with stuff. :trouble
Phil.
QuoteThe bottom goes way off the right of the photo to another resistor approximately (for cross reference) in the middle of the board and between the volume and boost pots
This kind of "explanation" really explains
nothing.
Please draw a proper schematic.
Well, because of the colour rendering I can't make sense of that resistor value, looks like violet, black, violet which would be a very non-standard and unexpected 70Megs. Did you measure it while out of circuit? Whatever, that's a paint void, albeit a bad one, but not likely to be the cause of any of your troubles.
Accidentally flashing the output to ground is why we generally disconnect things at the source end rather than the load end.
Given that you think you may have shorted the output, and given things seem to have changed dramatically for the worse, it's a fair bet you've blown one of the output transistors.
A simple test, with the speaker disconnected, is to measure the half-rail voltage in see if it is indeed close to half rail, or close to the supply or ground. However, lacking any sort of circuit...we don't know what sort of output stage we are dealing with, and can direct you to the output half-rail.
Compare this;
http://www.OzValveAmps.org/repairs/solidstateamprepair.htm
... with what you've got, and see if you can find any relationships that make sense.
Plan-B; find somebody locally who knows a bit about electronics to give you a hand, maybe sketch the output stage circuit or at least identify what type of arrangement it uses.