5150 Combo problem

[oegoeh]

Hi,

I'm new here. I have problem with my 5150 Combo. The amp is so loud, cause the post pots is can't uses. Also the eq and the reverb. I'm only can use pre pots(both channel), resonance and presence.

Herewith the quote answer from enzo from faulty peavy 5150 mk 1 combo

Originally Posted by 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.
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.

I have check all control ground, and the share ground is OK.
1 think confuse me, the post pots if measure on the board, is only read 23 Ohm (the pots is 1 M), but if I'm pull out the pots from the board, the pots read 1 M by DMM.
Any another suggest with my problem?? THX

[Amp Kat]

Do the pots vary from the wiper to the 3rd leg ? Pot can measure full resistance from pin 1 to 3 and still be bad in between. I would replace them if there is any doubt at all and it seems that is the problem unless of course they aren't grounded.

[oegoeh]

Do the pots vary from the wiper to the 3rd leg ? Pot can measure full resistance from pin 1 to 3 and still be bad in between. I would replace them if there is any doubt at all and it seems that is the problem unless of course they aren't grounded.

sorry for my late respond.
thx for your advice. Yes I'm measure full resistance from pin 1 and 3, for post pots (both channel) if the pots still on PCB, the read is only 23 Ohm, and if pull from the board DMM read correct 1 MOhm.

Any suggest?

[Enzo]

SOmething isn't adding up for me here. You report that the amp is very loud and the tone and post volume controls have no effect. Now we find the 1 meg pots measure 23 ohms across in the amp.

Oh wait, you took the readings with power off. Turn both controls to zero, and turn the three tone controls to the max and then measure again in circuit. Now I bet you get more like 200k. The two post controls are in parallel, so right there they would measure only 500k instead of 1 meg. the tone stack is also in parallel with them. SO maxing them means another meg and a quarter across the pots. AND... with power off, JFET Q1 will be "on" and measure very low. So if either post control is turned up, then that ground path through Q1 will shunt the controls.

Measure resistance from the grounded end of the post pots to the metal threaded bushing of the main input jack on the panel. And I mean take the reading directly from the leg of the pot, not from some place it is supposed to connect to. We know whay it SHOULD be, but we want to know what it ACTUALLY is.

Did you powr off and test EVERY one of those short bare wires between boards for being broken off?

[oegoeh]

SOmething isn't adding up for me here. You report that the amp is very loud and the tone and post volume controls have no effect. Now we find the 1 meg pots measure 23 ohms across in the amp.

Oh wait, you took the readings with power off. Turn both controls to zero, and turn the three tone controls to the max and then measure again in circuit. Now I bet you get more like 200k. The two post controls are in parallel, so right there they would measure only 500k instead of 1 meg. the tone stack is also in parallel with them. SO maxing them means another meg and a quarter across the pots. AND... with power off, JFET Q1 will be "on" and measure very low. So if either post control is turned up, then that ground path through Q1 will shunt the controls.


Measure resistance from the grounded end of the post pots to the metal threaded bushing of the main input jack on the panel. And I mean take the reading directly from the leg of the pot, not from some place it is supposed to connect to. We know whay it SHOULD be, but we want to know what it ACTUALLY is.

Thx Enzo for your answer. I'll measure it again tomorrow (I'm at GMT +7)

Did you powr off and test EVERY one of those short bare wires between boards for being broken off?

Yes the power is OFF.

[oegoeh]

Thx Enzo for your answer. I'll measure it again tomorrow (I'm at GMT +7)


Yes the power is OFF.

HI Enzo,

I have do all the advice from you. That's correct as what you said that after turn the tone control to max, the post pots measure around 300 kOhm.
So, I think all the pots is fine. What should I'm check to solver my problem? replace Q1 and Q4?

by the way, what is clamp circuit for on this amp? If I remove Q1 and Q4 what will be happen?

thx

[Enzo]

Clamp is a brief momentary mute that is triggered whenever you switch channels. If you remove them, the amp will work as normal, but there will be greater possibility of pops and noises as you change channels. When they go bad you generally lose signal, the exact opposite of the problem you are having.

[oegoeh]

Clamp is a brief momentary mute that is triggered whenever you switch channels. If you remove them, the amp will work as normal, but there will be greater possibility of pops and noises as you change channels. When they go bad you generally lose signal, the exact opposite of the problem you are having.

Hi Enzo,

OIC, for now if I'm switching the channel there was huge pops. Now, I check the pot with the Amp is ON. THe result is for post pots measure around 300k if turn min and O ohm if turn max (measure from 2 and 3 leg). What suppose to check at another location on th e PCB?
thx

[Enzo]

I still want to know what resistance is between the grounded end of either post control and the sleeve contact of the input jack.

[oegoeh]

I still want to know what resistance is between the grounded end of either post control and the sleeve contact of the input jack.

Enzo,

Herewith my measure result for either post pots :
All tone pots to max ..!
With Power OFF :
* post rhythm turn to min :
leg 1 - 2 = 0 ohm
leg 1 - 3 = 347 kOhm
leg 2 - 3 = 347 kOhm

- post lead to min :
leg 1 - 2 = 0 Ohm
leg 1 - 3 = 347 kOhm
leg 2 - 3 = 347 kOhm
- post lead to max :
leg 1 - 2 = 347 kOhm
leg 1 - 3 = 347 kOhm
leg 2 - 3 = 0 Ohm
* post rhythm to max :
leg 1 - 2 = 0 ohm
leg 1 - 3 = 0 kOhm
leg 2 - 3 = 0 kOhm
- post lead to max:
leg 1 - 2 = 0 ohm
leg 1 - 3 = 0 kOhm
leg 2 - 3 = 0 kOhm

*With power ON give the same result..!!

* end ground either post pots resistance to input jack sleeve:
Power OFF :
- with input jack bushing connect to chassis = 0 Ohm
- with input jack bushing not connect to chassis = 46 Ohm

Power ON, standby OFF:
- with input jack bushing connect to chassis = 0 Ohm
- with input jack bushing not connect to chassis = 46 Ohm

Power ON, Standby ON:
- with input jack bushing connect to chassis = 0 Ohm
- with input jack bushing not connect to chassis = 110 Ohm

[Wailin]

Interesting ... I have a 5150 head on my bench with the exact same problem. Pots measure ok, and have good grounds. I wonder if there is a specific commonality between the combo and head that may point to a cure to the problem.

[Enzo]

Look at the schematics and layouts. The combo and heads have utterly different layout arrangements, and circuits are circuits, these are not the same. The tone stack is passive and immediately followed by the post controls - a circuit from time immemorial.

The volume controls are simple voltage dividers, if they don;t work - that is, if the volume remains maxed no matter what the pots do - then voltage division is not taking place. SO either the signal path no longer even flows through the pots, or the bottom end of the pots is not being terminated.

Same thing with the tone stack. Easiest way to disable a tone stack is to open its bottom.

And I have to think if your 5150 head has a wide open reverb, then you have a very unusual 5150 head.

[oegoeh]

Look at the schematics and layouts. The combo and heads have utterly different layout arrangements, and circuits are circuits, these are not the same. The tone stack is passive and immediately followed by the post controls - a circuit from time immemorial.

The volume controls are simple voltage dividers, if they don;t work - that is, if the volume remains maxed no matter what the pots do - then voltage division is not taking place. SO either the signal path no longer even flows through the pots, or the bottom end of the pots is not being terminated.

Same thing with the tone stack. Easiest way to disable a tone stack is to open its bottom.

And I have to think if your 5150 head has a wide open reverb, then you have a very unusual 5150 head.

Enzo, this answer for 5150 Combo?
So, the next step I should do is to measure the voltage from post pots?
If no voltage, what should be replace?

thx

[Amp Kat]

There isn't any voltage (DC) on the pot. What Enzo means is it's wired as a divider for AC voltage which is very small because it's in the preamp. The best way to test it is to send a signal to the input with some type of tone generator and use a scope to see if the sine wave is varying in signal amplitude when you turn the pot. If there is no signal at the wiper of the pot you can almost guarantee the ground is open. Either way this will give you a good idea exactly what is going on with the signal and the pot.

[Enzo]

Exactly, the signal voltage is what I refer to there.


Set up a test signal, doesn;t much matter what, just something steady, into the amp. Now scope it or otherwise measure the signal at the top leg of the post control. I don;t know what you will have, but we can turn it up and down in the preamp. Looking at the circuit, the signal from those post controls goes through a cathode follower to the FX send jack. That follwer has essentially unity gain, so that means the signal at the post control is more or less line level. So. Let's call it 1 volt of signal and set things accordingly.

SO we should have 1v of signal at the top end of the post controls, which also happens to be the wiper of the treble control, right? Now scope or measure signal at the other end leg of the post controls. You better get zero signal, if you don't, they are not grounded. Either way, scope the wiper of the post controls, the center leg, turn them up and down. Does the signal level go up and down with that? Or does it stay the same?

We are still trying to solve the post controls having no effect and the sound stays all the way up, are we not?