Ok this guy had this sitting around and needed it gone. I purchased it from him. It is a combo amp the speaker is good. When powered on I hear one bump from the speaker, plucking strings it doesn't say a word. I plugged in headphones plucking the E or A string I can hear very faintly that there is something there. I can't find a schematic on the net. I really need help. I am doing this as a confidence builder. What does the faint output from the headphones imply, if anything? I have the head out of the cab and a visual turned up nothing. Where should I start with this?
Start by the classic: plug a known good cable from loop send to return or pre out to power amp in , get sound now?
If the power amp clicks or thumps on power on but does not hum very loud and send the speaker full forward or backwars then it probably works.
Put your ear by the speaker, does it softly hiss or hum?
In this case these are signals of life.
Thanks , but just so I know I am understanding correctly. Take a good instrument cable place one end in pre out. The other end right around to amp in. Or are you saying to run the pre out to a different amp?
No sound from the speaker at all. Not even a hiss. I did look good and it doesn't appear that the speaker is getting a straight dc signal if that is what you mean full forward or backward. It is 1am here so i had better check in the morning.
He was suggesting plug a cord from send to return. As if you had an effect unit connected there set to bypass.
A simple test is also just to plug the guitar or other signal source into the power amp in jack, or FX return jack, whichever you have.
And you can additionally plug into the regular input with your signal, and connect the preamp out or FX send to some other amp to listen to what is there.
DOing these things will; tell us if the power amp is the problem or the preamp is the problem. (Or possibly both. In that case, probably a missing power supply voltage)
If you hear it faintly, that tells me at least part of the preamp is probably working, and it is not able to get to the power amp. The thump at power up tells me the power amp is probably working, or at least mostly OK.
Between the power amap in jack and the actual power amap circuit, there are a couple stages for the compression/limiting. If there are power supply problems affecting the preamp, it will affect that stuff too.
COntact Samson and ask for the schematic.
Oh, your schematic is at the bottom of my previous post. I found it in my files already.
Thanks. So with with preamp looped back into amp in I get the same intermittent bumps I hear in the headphones. Of coarse they are much louder. Bass plugged into amp in the samething.
So look into the other stuff. Are all power supply rails present?
Enzo I am a absolute beginner when it comes to amps. You will have to give me detailed instructions
I really don't want to mess this up. If that is asking to much if you can point me to something that has a detailed photo and explanation as to what I am doing and why that would help also. Thank you for taking the time to even answer at all. Maybe if i ask enough questions i can one day help others.
I guess I am trying to say I don't understand how to check the power supply rails.
Ok from the power supply to the power switch two black wires and, two white wires plug into the switch. One green wire is screwed to chassis ground. If I understand correctly you are saying check each of the four wires that plug into the switch ( 2 black, 2 white ) black probe on volt meter to chassis ground red probe to each of the four wires????
NO please!!
Those wires are *hot* meaning they can kill you !!
Do not get even near to the power switch, fuse, and whatever gets plugged into the wall.
Yes. I understand very well what those wires are. That is why I asked this question. I don't know what would be wrong about checking them the way i explained though? Perhaps you would care to elaborate. I think rail voltage means voltage that is present ...well it could be anywhere in a circuit. It was stated check the power supply rails. I took that to mean voltage in voltage out. Could someone please explain what was meant by that statement? If I am asking for too much I apologize. That is how I thought this fourm worked. I am here to learn.
I am also taking photographs if this works out I am hoping that it will be a help to someone else who may want to learn.
Just to reinforce what JM said; all the wiring between the mains power inlet/lead, fuse, main power switch, to the power transformer, is deadly.
Normally these days this area is pretty well covered up, but you still come across amps where some of these points, such as the rear of the fuse for example, is exposed metal. When I encounter this I put something over that area, such as heavy cardboard or a plastic folder to prevent accidental contact.
Switch off, unplug from the power socket, disconnect speaker etc, remove amp chassis from its case.
When we say "pull the power plug out" we don't mean just turn off the switch - we really mean PULL THE PLUG - okay? This is a habit you must get in to - except when you are doing power on tests, the mains plug must be out of the socket and somewhere on the bench in front of you where you can easily see it.
Take the best camera you can find and two or three desk lamps ONO; pour light into the chassis from two or three different directions close up and take the sharpest pix you can, one overall, and say three more end/middle/end up as close as you can get while still staying sharply in focus (use "tulip" macro if available, and manually turn the flash *off*).
Post pix. Then we will all have some common ground.
If the printed circuit board has component numbers marked on it, then try and find D9 which will be close to where the one black and two red wires from the transformer go to the PCB. This is the main rectifier and appears to be a four-diodes-in-one-glob in this amp. Somewhere nearby you should find electrolytic caps C32 and C33 (marked 470uF/16V).
You should also try and locate diodes D12 and D13. These may be near where the speaker wires connect to the PCB (but they could be just about anywhere).
You will also need to knock up a limiting lamp, as per;
http://www.ssguitar.com/index.php?topic=2093.0 (http://www.ssguitar.com/index.php?topic=2093.0)
You will need to get the chassis up on a table or bench where it is securely propped up and won't fall over (or into your lap!)
Having got your amp secure on the bench, well lit, and the internal mains connections covered, and having located diodes D12 and D13, (and since you have already been running it without a limiting lamp), plug in, switch on, and measure and record the DC voltage across D12 and D13, measure and record the voltages across C32 and C33 - switch off and unplug. Post voltages.
The first two are the main amp supply voltages aka "rails", the second two are the preamp supply voltages aka "rails". "Rail" normally implies DC, so the mains feed side would not normally be called a "rail".
HTH
Dear oleskool.
Fully agree with Roly's suggestions and add: this Forum is a mutual help place, and no question is too dumb, so there's no problem on that side, but I saw you walking towards a *real* minefield and promptly wanted you out of there.
Imagine you walking on a street and apparently about to fall into a hole , I'm sure a stranger would first push you aside or at least *shout* a warning, not much time for introductions and social small talk. ;)
And please be welcome to the Forum, it's just that we love to have alive members. :tu:
As a side note: just a week ago I had to stop a guy who was testing his amp by putting one probe of his multimeter (on a current setting, meaning it behaves as a short) on one side of his mains fuse, and touching various circuit points at random with the other.
He was happy because he got the power light to light on , "although some points in the board started to smoke" :loco
And there's this other guy who was about to clip the ground lead of his oscilloscope to what he thought was the SPMS "ground" : -170V *live* voltage.
I'm not making this out, answered him 10 minutes ago.
By the way, our friend Enzo here , also warned him about the same.
NOTE: in this context, "LIVE" voltage means, paradoxically, voltage which will kill you.
Repeating what was told above: "rail" voltages are DC voltages used by many points in the circuit , so there is a continuous line or wire which carries there.
This Hartke has two power amp rails , around + and - 45V DC , and 2 preamp rails, +/- 15Volts.
You can see them in the power supply section.
The suspicion (to be confirmed by you) is that maybe one (or both) of the 15V rails is missing, which would kill your preamp and also the compressor-limiter which apparently lives in the power amp board.
Check all Op Amps (JRC072), they should all have -15V on pin 4 and +15V on pin 8 and basically 0 volts (or at least less than 100 mV) on pins 1 and 7.
Except on U5 which might have a few volts on pins 1 or 7.
U6 (CA3080) should have +/-15V on pins 7/4.
Briefly touching a screwdriver to the net (union of) C22/R41 on the power amp should produce a click on the speaker.
Plugging a known good guitar cable into J4 , power amp in, should also produce a click, but I guess there's the problem we are searching.
Now, go and do it, on the double !!! :trouble
Just kidding.
Test and post. :tu:
In general, when we, or at least I, say "power supply" we mean the internal DC ( usually) voltages the circuit runs on. We generally do not use the term power supply to refer to the power coming from your wall sockets.
Specifically here, I was interested in the two power supplies of something like 30-40 volts, one positive and one negative. Are both present?
MY use of the term "rail" just means the distribution of those power supplies through the circuit. If an amp has a 15v power supply, and many different parts of the circuit connect to it, we often refer to the supply as a "rail." SO "15v power supply" and "15v rail" mean the same thing.
Hello again guys. thank you all for assisting me.
I did make all of the measurements you all instructed me to. They were all as you all expected. +/- 51v, -54v, +15v,-15v and all of the other measurements followed suit. pin 1 on U5 did however measure 13v.
To update I decided to look at the board again. One of the first things I learned was a good visual is the first thing you do. Well I found a spot on the board that is not etched. it appears to be some kind of foil. It may even have been a thin line of solder. Anyhow it had a small amount of corrosion on it. I brushed at it with a small brush, and it was not even connected to the board anymore. I took a bad pic.
(http://i50.tinypic.com/9ierg2.jpg)
It is a straight line from the collector's of Q8 & Q9. I am about to just jump a piece of wire from the solder connections, and see if that could be my problem. I will post the results. That line of foil is only 1/4" from the heat sink.
:-[ :-[ Oh well the line fuse went as soon as I hit the power. will get another tomorrow to see if it happens again.
Quote from: oleskool on July 21, 2012, 12:04:21 AM
:-[ :-[ Oh well the line fuse went as soon as I hit the power. will get another tomorrow to see if it happens again.
Don't do that again - it will, and you will only cause
more damage.
Do this;
Quote from: Roly
You will also need to knock up a limiting lamp, as per;
http://www.ssguitar.com/index.php?topic=2093.0 (http://www.ssguitar.com/index.php?topic=2093.0)
Thanks Roly!!! Will do.
Okay, so if it's blowing fuses, once you get your limiting lamp in series you will find that it lights brightly because the amp is shorting.
With limited power on the amp the thing then is to try and isolate what, exactly, is shorting.
You also need to disconnect the loudspeaker, and a simple way to do this in this particular amp is to plug a blank plug, or lead with nothing connected to the other end, into the headphone socket. However you also will need to get inside the chassis to take voltage measurements so it may be easier to disconnect it altogether, however.
With the limiting lamp all the voltages will be very low compared to their normal values, but what we are looking for is some indication of what is shorting to what.
You will need to locate the main electrolytic caps C34 and C35, 4700uF/63V. Using the chassis as the reference we hope to see a few volts positive on the +ve end of C34, and a few volts negative on the -ve end of C35.
We also need to find the voltage on the output line, the ungrounded speaker connection. What is of particular interest here is if the output line is the same as the +ve supply, the -ve supply, or somewhere in between. (D12 and D13 are between these three points, +ve, -ve, and output).
If the output is +ve this suggest you have one or more shorted transistors on the +ve side, Q11/12/13, if on the -ve side Q5/8/9.
If there isn't much voltage at all it is possible there are shorted transistors both top and bottom, and the bridge rectifier D9 may also be shorted as a result.
The next step is to remove any suspect transistors from the circuit, carefully noting/marking where each came from, and testing out of circuit. While Q13/14 and Q8/9 are out of circuit you can also power up for a few seconds and see if the short has cleared (the lamp settles to a dim glow or less). This will clear the bridge rectifier D9 (and you should get tens of volts on both +ve and -ve supply rails), but if the short remains then D9 has most likely failed as well and will require replacement. You can do this and retest before replacing any of the transistors.
Anyhow, using the limiting lamp, take some measurements and report back.
Hello again.
Well the first test C34=1V, C35=-1V, speaker=1V.
Pulled Q8, Q9, Q12, Q13 They all test good with a multi meter.
Plugged back in switched on, very dim bulb for about two seconds then it fades out.
D12= -.384V, D13= 104.7V ????
I guess I should test Q5 & Q11?
>Well the first test C34=1V, C35=-1V, speaker=1V.
+/- 1V supplies tell us we have a short right across.
Speaker at +1V says we have an output transistor shorted between the +ve rail and the output, i.e. Q12 and/or Q13.
>Pulled Q8, Q9, Q12, Q13 They all test good with a multi meter.
Given the measurements above it seems to me unlikely that Q12/13 are actually okay.
Firstly you need to measure the Base-Emitter and Base-Collector junctions. These should both look like diodes, between 500mV "500" and 700mV "700" forward, and open "1___" in reverse (on your multimeter diode test range). Then you need to measure the Collector-Emitter path which should be open both polarities.
>Plugged back in switched on, very dim bulb for about two seconds then it fades out.
>D12= -.384V, D13= 104.7V ????
Now something significant has changed because you no longer have the short between supplies, but the output rail (join of D12 and D13) is still stuck to the +ve rail, again strongly suggesting that Q12 and/or Q13 is short C-to-E.
>I guess I should test Q5 & Q11?
Yes, this is a good idea, but in any event I think you still have a shorted transistor at Q12 and/or Q13 (and it's possible that you also have Q8 and Q9 blown open).
Because this amp uses pairs of output transistors it is possible to remove only Q13 and Q9, then Q12 and Q8 to help isolate which are faulty, just as long as you don't try to drive the amp hard into a load (but you should have the speaker disconnected anyway).
I have to say that this power amp circuit is not well drawn; it's actually quite conventional but drawn in a confusingly offset manner.
This is not identical - it only has one set of output transistors instead of two, and the initial input stages are "upside down" to yours, but this is the same general arrangement, and seeing this may help you find your way better around the Hartke circuit.
(http://www.ozvalveamps.org/repairs/solidstateamprepair/circuit-power-amp-ocl-100w-by-transister-mj15003mj15004.jpg)
OK I hope i didn't mess up, but earlier in this post there was a spot I needed to jump from collector to collector at Q8 and Q9. Well when I pulled Q8 and Q9 I also pulled that jumper. I replaced the jumper Q8,Q9,Q12, and Q13 are still out. Across D12= -51.3v. -51.3v. Black lead connects at end of diode with the band for both. At first these were jumping up and down. Now they are fairly stable with these two measurements. I was talking to a guy he said the zener D5 is most likely bad. He also advised to change all the transistors. He also noted which does sound correct, that if J5 was bad it would cut the speaker out. But he is getting said parts.
I'm getting 20mv at the speaker. C34=51v and C35= -51v.
> I'm getting 20mv at the speaker. C34=51v and C35= -51v.
I'm having a bit of trouble following what you are doing, but again the conditions have changed (for the better) and you are now getting very reasonable readings - plus and minus supplies, and very little DC at the speaker; this is all good. As you may have worked out, D12 is connected between the +ve supply and the output rail, and D13 is connected between the output rail and the -ve supply, making these two a handy point to find the three most important voltages.
> I was talking to a guy he said the zener D5 is most likely bad. He also advised to change all the transistors. He also noted which does sound correct, that if J5 was bad it would cut the speaker out.
Measure the voltage across the zener - measurement trumps a wild guess any day. If you have 15 volts it's okay.
Given that you now have good supplies and only 50mV across the speaker, advice such as "replace all the transistors" is, at best, let's say, not very well informed. ( :duh) This is what I call "blunderbuss" servicing and often leads to more trouble and expense than doing it properly. "Properly" means locating which component(s) are faulty, and replacing them. The blunderbuss mode is often used by people who can't actually locate faulty components, it's hit and miss, and not at all professional or reliable.
While he is right that J5 could interrupt the output, it in no way explains a) why both supplies were being loaded down to only one volt, or b) why the output rail was previously stuck at the +ve rail. This guy isn't even considering the available evidence, and when you're done you will be able to give him (better) advice.
Thanks. I was hopeful that these measurements were more reasonable. There is a spot on the circuit board that is fried. It is a very short run. The connection between the collectors of Q8 and Q9. I used a piece of wire to bridge that gap. The first measurements when the supplies were 1v, and more important stuck + were taken after I removed Q8 and Q9. When I removed them I also removed the wire that bridged the gap. Another thing I learned along the way, if something changes and doesn't make sense, look at the work you did. Look at the schematic and imagine the entire line that connects the collectors of Q8 and Q9 erased. Well that is what my board looks like. I have placed a piece of wire there to bridge the gap, and then took new measurements. I will try to put up a few pictures. My camera is eating batteries.
Ahh so - track blown off. Happens.
May I then suggest that you fit a couple of fuse holders somewhere between C34, +ve rail, and C35, -ve rail, and everything else?
P=150 Watts
R=8 Ohms
P=I^2*R
I^2 = P/R
I = root(P/R) = (P/R)^0.5
(150/8)^0.5 = 4.33012702 amps
So initially try say 4 amps in each rail and see how you go.
These won't prevent transistor failure, but they will help to minimise damage if that should happen again.