Hartke B90 no sound

[oleskool]

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?

[J M Fahey]

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.

[oleskool]

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.

[Enzo]

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.

[Enzo]

Oh, your schematic is at the bottom of my previous post.  I found it in my files already.

[oleskool]

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.

[Enzo]

So look into the other stuff.  Are all power supply rails present?

[oleskool]

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.

[oleskool]

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?

[J M Fahey]

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.

[oleskool]

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.

[Roly]

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

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

[J M Fahey]

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:

[Enzo]

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.

[oleskool]

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.

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.