Peavey 2.6C Question

[Enzo]

Those little transistors are what we call a TO92 type. That refers to the shape: transistor outline #92. The little flat faced half-round black things. Looking at the flat face, legs down, left to right, the legs are emitter, base, and collector: EBC helps to refer to emitter or base or collector.

The part is an MPSA13, common enough. Peavey would sell you one, or any general electronics part supplier like Mouser or Digikey. or you might find one locally. Where are you located?

I am not a fan of extra part moving, but you could swap Q108 and 208 and see if the problem follows the part.

[BowerR64]

Those little transistors are what we call a TO92 type. That refers to the shape: transistor outline #92. The little flat faced half-round black things. Looking at the flat face, legs down, left to right, the legs are emitter, base, and collector: EBC helps to refer to emitter or base or collector.

The part is an MPSA13, common enough. Peavey would sell you one, or any general electronics part supplier like Mouser or Digikey. or you might find one locally. Where are you located?

I am not a fan of extra part moving, but you could swap Q108 and 208 and see if the problem follows the part.

I thought about trying that, i have some random audio boards i might check and see if there are any transistors on it that might crossover. I thought about swapping the part i have a stack of solder wick that works really good at removing solder and cleaning up the foil i may try it later. and see what it does.

So in your opinion the amp doesnt have a problem? its just the DDT monitoring section has an issue? i could use it with the DDT button on the back turned off and if i use the amp at low levels it wont blow up on me?

The DDT is like a dummy light that tells you when your pushing the amp to hard and its near blowing right? or what is it exactly?

When i flip the DDT switch on the back does it turn the DDT off or take it out of the cirquit? or what is it doing? i never thought to take voltages with it off. I thought maybe i could use the amp while i wait for the part.

[Jazz P Bass]

I would go back in & measure the base of Q211 & Q212.

O/k, you saw the base of Q208 getting -1.5 Vdc.
Find out where that voltage is coming from.

You are assuming that the transistor is 'breaking down'.
That may or may not be true.

[Enzo]

The DDT is not a dummy light, it is more like a governor on a motor. The DDT stays out of the way until the amp is ready to clip. Then instead of clipping, the DDT circuit limits the drive so it can't. Nothing to do with almost blowing up, the DDT light simply indicates that your peaks would have clipped.

If your voltages were all the same through those four transistors, and only Q208 was different, I am led to suspect it. sure, but swapping them is just a quick and dirty way to find out if they are the problem without ordering something first. The Q208 doesn't monitor anything, it sits there waiting for the circuits that DO monitor things to tell it to light the LED or not.

MPSA13 is a darlington transistor, and is seldom used in the audio path. You will note it has teh job of turning an LED on and off here. If you know anyone with an older Bally or Stern pinball machine, the lamp drive board has a sea of MPSA13s on it.

[BowerR64]

I would go back in & measure the base of Q211 & Q212.

O/k, you saw the base of Q208 getting -1.5 Vdc.
Find out where that voltage is coming from.

You are assuming that the transistor is 'breaking down'.
That may or may not be true.

Can i take it out of the cirquit and test it, or test it in the cirquit? Yeah i figured it was breaking down i shut it off once the LED came on which was -1.5 but im not sure how high it will go. I didnt let it keep going. I thought a transistor is like a little switch or something. One leg is the voltage in, and one controls the switch which then allows the last leg to energize or something like that? It didnt seem to do that it seem to just slowly creep up till it reached a high enough voltage that was enough to light the LED. I kept my eye on the LED this time and i could see it just start to slowly fade on and im sure it woudl of eventually got brighter the longer it was on.

The DDT is not a dummy light, it is more like a governor on a motor. The DDT stays out of the way until the amp is ready to clip. Then instead of clipping, the DDT circuit limits the drive so it can't. Nothing to do with almost blowing up, the DDT light simply indicates that your peaks would have clipped.

If your voltages were all the same through those four transistors, and only Q208 was different, I am led to suspect it. sure, but swapping them is just a quick and dirty way to find out if they are the problem without ordering something first. The Q208 doesn't monitor anything, it sits there waiting for the circuits that DO monitor things to tell it to light the LED or not.

MPSA13 is a darlington transistor, and is seldom used in the audio path. You will note it has teh job of turning an LED on and off here. If you know anyone with an older Bally or Stern pinball machine, the lamp drive board has a sea of MPSA13s on it.

ok i havnt read the manual i was just kinda going by what the guy told me i got it from. So it comes on when the output gets to a point of clipping? or the input?

[BowerR64]

The DDT is not a dummy light, it is more like a governor on a motor. The DDT stays out of the way until the amp is ready to clip. Then instead of clipping, the DDT circuit limits the drive so it can't. Nothing to do with almost blowing up, the DDT light simply indicates that your peaks would have clipped.

If your voltages were all the same through those four transistors, and only Q208 was different, I am led to suspect it. sure, but swapping them is just a quick and dirty way to find out if they are the problem without ordering something first. The Q208 doesn't monitor anything, it sits there waiting for the circuits that DO monitor things to tell it to light the LED or not.

MPSA13 is a darlington transistor, and is seldom used in the audio path. You will note it has teh job of turning an LED on and off here. If you know anyone with an older Bally or Stern pinball machine, the lamp drive board has a sea of MPSA13s on it.

I swapped the 2 transistors around 208 and 108 and its still tripping the DDT after a few minutes the bottom leg of 208 starts to climb till it gets to a voltage that the LED turns on. So its not the transistor breaking down.

Should i get -1.5V on both sides of R240?

[Enzo]

You have a good channel, right? Use it as a model. What are the voltages doing there?

[BowerR64]

You have a good channel, right? Use it as a model. What are the voltages doing there?

What about this, if i just turn the DDT off using the switch will that turn off the DDT cirquit so it doesnt burn up what ever is causing the DDT to trip?

The DDT has a problem, not the amp right?. Im afraid if i keep poking around in it i might jack up what i have now. I think im giving up on trying to fix it.

Im going to use it as a guitar stereo amp with a Line 6 modeling unit. Its just amplifying the modeling preamp to my guitar speakers i doub ill get it much past halfway on the volume nothing bass heavy like PA or DJ stuff.

[Enzo]

Turning off the DDT switch only disables the DDT's ability to do anything about clipping, should clipping occur. However, all those transistors and things will still be connected to the circuits, so turning it off will have no effect on whether some problem grows.

Other than the lights, have you HEARD and difference due to the DDT?

[BowerR64]

Turning off the DDT switch only disables the DDT's ability to do anything about clipping, should clipping occur. However, all those transistors and things will still be connected to the circuits, so turning it off will have no effect on whether some problem grows.

Other than the lights, have you HEARD and difference due to the DDT?

Well i havnt used it but just for about a half hour the first time we fired it up, the light came on and we shut it down and havnt used it since. It still worked when the light was fully lit up he strummed the guitar just before we shut it down and it still sounded normal we didnt even notice till we just happen to see the LED lit up.

This is just a guess but say there are like 6 points that run the LED what would the 6 points be that i would check backwards from the LED? like the LED gets voltage from point 6 then from point 5 working backwards to the problem? or does it work that way? even though i have a good working side i cant seem to fallow the schematic back correctly

[52 Bill]

This is just a guess but say there are like 6 points that run the LED what would the 6 points be that i would check backwards from the LED? like the LED gets voltage from point 6 then from point 5 working backwards to the problem? or does it work that way? even though i have a good working side i cant seem to fallow the schematic back correctly

The two leds are controlled by Q108 on channel A and Q208 on channel B. The only job of Q108 and Q208 is to turn on and off the DDT leds.

Each of the two transistors gets the voltage signal to turn on or off the led from the resistor connected to the base R135 or R235.

That voltage signal comes from the circuit consisting of Q106, Q111, Q112 and Q115 or Q206, Q211, Q212 and Q215.

The 4 transistor circuit monitors the input and output signals at the inputs of the IC U100 or U200.

So compare the voltages at the bases of Q108 and Q208.

Compare the voltages at the resistors R135 and R235.

Compare the voltages at the bases of Q111, Q112 and Q211, Q212.

Hopefully, you will find out where the voltages are different, and then why they are different.

[Enzo]

Great Bill, I didn't have the energy to do that, glad you got there first.