faulty peavy 5150 mk 1 combo

[blobby]

good day people
i hope you maybe able to help me repair this big beast,

the power amp is working ok and i have cleaned the send and return sockets, i had to replace R 111-110-116-117 as i was not getting my + - 23 volts now the preamp heaters work and the chanel switching lights work,

i have replaced all the valves (tubes) for known working ones and have replace the high voltage caps, and have checked resistors on the preamp board for open or short circuits.

after scratching my head for a while i have noticed that q6 seem to be shorted with 14 volts on each leg and and i have no volts on q1-q4-q12 and cr9. i think i maybe in the right ball park now , but not sure weather to replace these now or if some other componant may be causing these to mall function. sorry q1 also seems to be shorted. thank you in advance ... blobby

[blobby]

any ideas ?

[Enzo]

So then what is the symptom you are trying to fix?

Q1 is a mute transistor. It is a JFET, so that means it is ON untill turned OFF with a voltage at the gate. It is normal to measure 100-200 ohms or so across it when no voltage is present. AMp powered there should be about +20VDC on the gate leg ONLY.

If Q1 is a dead short, remove it, the amp will function without it. If that restores sound, then replace Q1.

Q4 works the same way, but mainly affects the reverb.

If there is no +20 at the gate of eiother one, see if triac CR9 or transistor Q12 is shorted. The amp will function without it if removed.

[blobby]

thank you for the reply enzo,

the amp has very low volume on both the crunch and the lead chanel. when i plug in through the return loop the volume is quite loud, but going though the front end volume is very low... thanks

[blobby]

hi enzo
i have removed q1 as you said and the amp has bust in to life, i dont have + 20 volts on either of the gates of q1 or q4, but this might be because i have q6 removed from the circuit at the moment as this was shorting.

cr9 and q12 dont apear to be shorting but still i have no voltage across them........ and thank you for you help and time.

[Enzo]

Middle left on your schematic, the CLAMP circuit. Fix it.

If no voltage as across CR9 or Q12, then see if R55 is open or c36 is shorted.

And either Q1 or Q4 can short the CLAMP signal to ground, and in so doing kill it for the other one. So with Q1 gone, the amp works. Remove Q4 and see if the voltage returns to CLAMP. Check R55 first.

[blobby]

hi enzo.

it turns out that q4 was faulty, i checked r55 and that seemed ok and when i removed q4 from the board i got 20 volts on q1-q4 cr9 and q12 just need to get the transistors now and will let you know it turns out.

thanks again for your help and time on this , as ihave been trying to fix it for a while now and was begining to grind me down...thank you

[oegoeh]

Hi,


I'm new here. I'm also have a problem with my 5150 combo. All eq pots, post volume, and reverb pots is not function. The amp sound is very loud, I can only use pre pots(both channel), resonance and presence pots. I Have check all the pots and they good.
ANy sugeestion for this problem?

thx.

[Enzo]

Hi, welcome to the forum.

It would be a good idea to start a new thread for your amp instead of adding on to the end of this old thread. You will get better response, especially since your problem is different from the original post problem.


All those controls in your amp share the same ground return. I think your ground connection to them has been lost. First, make sure the jack nuts are tight on the FX loop jacks, they are involved in grounding that section. But what I fear is the problem is a broken jumper wire.

Please be careful inside this amp, the voltages it uses can KILL YOU.

With power off, the amp unplugged from the wall, and power supplies discharged, look at the circuit boards. There are three sub-assembly circuit boards that are connected together with a row of short bare wires - "jumper wires." One row of jumpers connects the control panel board to the small board with a couple relays on it. Then another row of jumpers connects that relay board to the preamp tube board. Use a small tool of some sort and get under each and every little bare wire and gently tug on it. If any come away, it was a broken wire. And that would need repair.

[oegoeh]

Hi, welcome to the forum.

It would be a good idea to start a new thread for your amp instead of adding on to the end of this old thread. You will get better response, especially since your problem is different from the original post problem.


All those controls in your amp share the same ground return. I think your ground connection to them has been lost. First, make sure the jack nuts are tight on the FX loop jacks, they are involved in grounding that section. But what I fear is the problem is a broken jumper wire.

Please be careful inside this amp, the voltages it uses can KILL YOU.

With power off, the amp unplugged from the wall, and power supplies discharged, look at the circuit boards. There are three sub-assembly circuit boards that are connected together with a row of short bare wires - "jumper wires." One row of jumpers connects the control panel board to the small board with a couple relays on it. Then another row of jumpers connects that relay board to the preamp tube board. Use a small tool of some sort and get under each and every little bare wire and gently tug on it. If any come away, it was a broken wire. And that would need repair.

Thx Enzo, I'll try to check the ground and the jumper.
Sorry I'm wake up the old thread. I'll make a new thread after this.