Hi all! My first post!
I have a Princeton 65' guitar amp which I have always treated with care. The other day however when I turned it on the signal (playing through the clean channel) was distorted and lacking of any treble response. The distortion sounds a bit like the effect from a dist. pedal however when the notes decays they all end in a fart-like crackling sound.
I have no idea how I came to harm the amp. Does any of these symptoms sounds familiar to anyone?/Any suggestions? I'm planning to change the fuse and unscrew the speaker to check if it is torn in any way. I have found the schematic, but cant make a lot of sense out of it.
any response is highly appreciated! <3)
Who says you "harmed" it? That light bulb in the bedroom ran for 15 years, then one day, POOF, it blew out. Is that anyone's fault? No, it just happened. Like that light bulb, paqrts in an amp can fail, so can connections between perfectly good parts.
First thing to do is connect a different speaker tot he amp to see if the problem is the speaker.
Hm yes Ill try that and get back to you. Thanks for the input ;)
Ok so I've checked the speaker and the PCB and both look alright apart from the thermistor which is covered in a white substance. This might just be isolation but I thought it might be worth a mention. Any suggestions on what to do next?
A thermistor covered in a white substance??? I want to see this... Take pics for me and all to see.
Or it's just the glue they put in there to hold in place just in case? This sounds strange.
Five will get you ten it's mounted against a heatsink (or should be) and the white substance is thermal compound for thermal coupling.
10 points to you Roly!!
The amp was put together in 1999. Would breaking down in 2012 be considered long durability? I really don't know as this is the first amp I have ever owned apart from a Roland MicroCube :P
The thermistors are there at the entrance of power stage and they are resistors that change their resistance in response to the amp heating up. This is different than the output transistors tip142 & tip147 that are mounted to a heat sink w/ the thermal compound coupling the connection between the two. Now those transistors I can imagine a white substance, but have only seen glue holding thermistors tightly to the PCB board.
So I have worked on a few of these 1999 boards and that would indicate to me that it is made in Mexico rather than China. My best recommendation is to fully solder up that board and make sure every connection is connected to the board nice. As you take off the board you will need to make sure that you document where each connection belongs. Take a sharpie and write J9 or J11 on the plastic part. When you get to putting it all back together you will need thermal grease on the heat sink and make sure the heat sink bar is on the right way. Take a sharpie and write an arrow pointing up to ensure this is correctly put back together. This amp probably just needing some solder, but then after you do all this play the amp and see if there is any noticeable changes. At that point this only can be good for the amp if you are handy w/ a solder iron...
If the amp is still acting up then we can start looking into troubleshooting.
Great!
Ill do this and upload pictures as soon as I can find the time. Thanks a lot for your input! I'm hopeful for this project!
13 years is not bad, but it's not great either. It depends a lot on what sort of life its had, just in a bedroom or gigged hard, lived by the seaside in salt air and had drinks poured in it, or kept in an air conditioned studio...
Ok so here is a picture of the thermistor labled "RT1" (in the center, covered with white stuff). Its not mounted against the heat sink, but placed right next to it. That was just my bad English running loose ;)
I dont know how to crop it for this topic, but Ill supply a link to it instead: http://i.imgur.com/GYK4g.jpg
I have removed the board from the housing and looked over the soldering. The soldering looks very good, though I will go over it with my soldering iron to make sure. Also I will check for contact between the various connected nodes.
Okay I see... Yeah I am learning as well as we all go here too!! lol :duh But I was imagining other type of thermistors than this one. Larger one's located near the fuse and transformer input stages on the board. Maybe I am even thinking of a different Fender FM65, princeton212 or something like the RocPro??? Props to Roly for his greater experience than mine. :dbtu: Sorry I even had the wrong idea going on the output transistors for this amp...
Yeah if all the soldering looks nice and shiny that is good to know about the condition of the amp. If you flow solder on at least the components like wire wound resistors and filter caps that might be just enough. I like to practice soldering a whole board at a time as I am new at this and it helps to practice.
This might not fix anything yet, but now you will know the condition of the board as a whole. Also, this will get you really familiar with the entire amp. Be careful and take some care putting it back together. Remember to clean off the heat sink and apply new thermal grease when you put the board back together.
Update:
I have, at long last, gotten around to re-solder the whole board. Next up is checking all the cables for damage. Is changing the thermal putty-thingy for the transistors mounted on the heat sink important?
The thermal compound is old and used by now for over a decade. I would take some isopropyl alcohol and some Qtips to really get that stuff out of the way. It is important that this compound fill in the cracks that may exist between the heat sink and the chassis. Fresh grease is way better and more reliable. The last thing we want here is for the output transistors to overheat.
Keep up the good work and let us know if the soldering makes any difference on the amp. Good luck.
Hi knutolai,
I just had a quick looky at schematic and assuming we are both looking at same then
Check voltages at TP3 and TP4 about +/- 40ish VDC either side of common. (GNDA on drawing)
If they are OK then check TP5 and TP6 you want to read close to +/- 15VDC.
My guess is these are marked on the PCB so should not be hard to find em all :tu:
Also check those efx loop sockets they are prone to fail one day. :-X
Test by inserting a short cord into loop which bypasses the internal switch which is the bit that fails and can cause all sorts of problems. :grr
There is also a mute switching setup on the input socket and that maybe faulty causing issues.
hopefully better minds here will have more info.
Phil.
Hi Phatt
Thanks for the tips!
I found the points you listed on the PCB however I have never done this type of troubleshooting before so I have a few basic/stupid questions:
-What do you mean by "either side of common"? I can't find a GNDA label on the PCB (I might just not have looked hard enough xP ) Im guessing GNDA is a ground and that I should measure the voltage between the TPx points and ground?
-When I do these measures should the amp be powered or shut off?
again thanks everyone for all the great input!
"Either side of common" is common ground. Read on and reference the schematic as you read too. Look at the P4 connection on the schematic and observe what is on each side.
To find the GNDA reference it is found all over the place on the schematic. In this case look at the power supply part of the schematic where it shows TP3 TP4 etc... On the PCB board you will find a center tap part of the transformer that is connected by way of the P4 faston. That is essentially ground or GNDA, and also you can pull out the DMM to test continuity to ground. Do the test for continuity to ground with the amp off to become familiar with all these point that reference back to ground. The chassis is a reference to ground as you will soon discover. I would get some alligator clips to clip the black probe of the DVM to chassis ground. Look on the schematic and find common grounds to double check it is connected securely. Look for the AC safety ground wire that is bolted to the chassis... That is ground reference point. With amp off clip one probe on P4 and touch the other probe to safety ground clip on the chassis. Do this with the continuity setting of the DMM and you will see .002 or something very close to zero ohms. Most DMM's have a beeping noise that lets you know there is continuity too. So, what are you noticing?
You will need to have the amp on to test for voltage. So secure that black probe to chassis ground and then use red probe to touch the test points to read voltage. Read as much as you can about safety tips before you proceed testing a live circuit, especially if this is your first time.
Well DocGonz beat me to it and thanks chum great advice. :tu:
Anyway I'm not about to rewrite my whole post again so some stuff will be repeated here. :cheesy:
Ok I'll try my best to explain it.
First comes safety!!!
All watches rings and sacred crosses or any kind of metal pendants around the neck are OFF. :grr
Ware shoes and maybe even put a thick mat under your feet.
Ideally work on a wooden table/bench.
Now find the MAINS wiring and understand exactly where those wires run.
Most modern stuff seems to cover really dangerous terminals but don't bank on it.
Yes, Common is ground or GNDA
First maybe learn to understand the basic principal of operation.
Amplification is achieved by floating the AC voltage on a DC voltage potential.
The noise you hear is the AC wiggle made larger while ridding on the DC . (not to be confused with Mains ACV at the wall socket)
To do this you first have to convert/transform the AC mains voltage down to the needed voltage, Secondary winding of power transformer does this.
Then the secondary AC is Rectified to DC and then filtered. Rectification is via the 4 diode bridge.
In this case you have both a *Positive rail and a Negative rail* and of course Common is zero volts and often ground in the audio circuit.
The common rail will be where those 2 big can capacitors are joined on the PCB.
Nearly always the biggest things on the board so it's hard to miss them.
The other 2 cap terminals will be one positive and one negative. You are looking
for C65 and C66 both 2,200uF.
Than is the main supply for the circuit. So you have +40/0/-40 VDC supply rails.
The other lower voltages for opamps are derived by a drop resistor from the above rails (R118 and R119 330 R wirewound ceramic) and the voltage is held constant/regulated via a Zener diode (D46, D47 16 volt Zeners) across the rail to common. These are often prone to failure.
also if C69, C70 have failed that can give problems.
So now (if you have grasped the idea) Turn the power on and With DMM set to the highest setting, usually 1,000VDC range,, black lead on common rail and red lead on the two test points you should read one at 40 VDC positive and the other obviously will read the same but with a - sign in front.
(I say highest setting because you never know for sure what you are about to read.
Once you see the 40 volts then switch down to the 200VDC range for better accuracy.)
Now again for the lower voltage test points, It does not have to be exact so within a volt or so.
If one side reads 39 and the other reads 41 it's ok.
Same for low rails. one side might be 15 the other 16,, close enough. :tu:
If these voltages are way off then we know there is a problem with DC rails and you need to fix those first.
Thing to look for;
Burn marks under those zener diodes and also those 5 Watt drop resistors, R118 and R119.
Bulging or leaking can caps,, not to be confused with the brown glue that might look like leaking goo. It's usually hard and brown while electrolyte is clear wet and sticky muck.
Having said all that I'd still say start at the easy things first,, like the input socket.
I believe it has two inputs so try both and wiggle the plug gently side to side to see if that mute switch is stuffed,,Same for efx loops.
Also if it has 2 speaker sockets,, then try the other one.
Meantime google some tecko words like rectification, diode operation, transformer concept,, It is amazing how much stuff you can find once you catch onto a few teckky words the Technicians use.
If you don't understand anything then ask first ,,, Oh yeah and my favorite advice for those trying to get there head around electrical stuff;
""Don't touch anything you can't outrun"" 8|
Phil.