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[Paolo]

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.

[phatt]

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.

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.

[Roly]

"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.   

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.

[Paolo]

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

[teemuk]

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.

[Roly]

Nope. Obscure-tronics; all I get from Google is Kermit and Miss Piggy.

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.

[Paolo]

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

[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.

[J M Fahey]

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.

[teemuk]

I 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.

I 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.

I 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.

[phatt]

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.

[Paolo]

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

[teemuk]

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.

[phatt]

OK,
    You obviously have some interest in the inner working of circuits,,,,,
Well Now is your chance to actually learn something really useful.

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.
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.

[Roly]

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).