Peavey KB2

[barri]

transformer voltage

hi sir thanks for the answer but i measure it the positive side withe the ground and its really 45v dc i measure also the secodary of the former and its 30volts ac maybe there' something wrong with the transformer what can you say

[52 Bill]

Please verify that you are saying that the power supply is producing both + and - 45vdc as measured to ground.

Is your amp the same model that the earlier schematic shows? What country are you located in? What ac voltage reading do you get on the primary winding of the transformer?

[barri]

former volt

sir im from the philippines i use stepdown trans what is the max volt for lm3886 for woofer andlm1876 for the tweeter thanks again for your help

[52 Bill]

Look up the datasheets for each of these chips. They are current National Semiconductor products.

LM3886 maximum voltage is listed at 94 volts, LM1876 64 volts.

[J M Fahey]

On Post #14 Enzo kindly posted the power amp schematic.
On page 4 you see the PCB.
To its right you find main filter capacitors, C25 and C26.
The C25 + leg is joining C26 - leg, they are connected to chassis ground.
With the multimeter back probe touching the chassis , touching C25 - leg with the red one should read about -20V ; with black probe *still* touching chassis, touching C25 + leg with the red probe should read around +20V.
Multimeter range scale set in 200V DC.
Please confirm.

[barri]

burned lm3886

mr 52 bill pls. me again i changed the burned ic thats lm3886 i got a new one when i replace the dead one and turn on the amp it burned again but the tweeter ic amp is ok im wondering about that thanks again for the help

[52 Bill]

You never answered our questions regarding the actual voltages from your power supply. If there is too much voltage there, any new chip you put in will probably burn up as well.

Did you check the other components in the power amp section as well. There may be other things that have shorted and are causing the damage to the new chip.

Did you check the speaker to see if it has a shorted out voice coil?

Please start by fixing the power supply problem that you have before trying to make the rest of the amp work.

Do you have access to an 60 watt incandescent light bulb? Use it to make a current limiter that will help you save some of those new parts.

[guitarman1970]

ANdy, you are beiing misled. I have to point out to the well-meaning folks posting above that this amp uses a LM1875 to drive its tweeter, and an LM3886 to drive its woofer. So those won;t be in three-leg TO218 cases, or for that matter, three leg anything. There is a small push pull discrete amp for the headphones, runs off +/-15.

If there is DC on the woofer, your LM3886 is almost certainly shot. The two ICs I mention are indeed on the heatsink, but you won;t be making meaningful tests on them with a meter unless you find the output pin shorted to a supply pin. We still don;t call them op amps, they are full fledged power amp ICs.

The LM3886 is that 11-leg thing. I'd remove it, and check that there are both +20 and -20 volt power supplies getting to it.

I attached the powr amp diagrams for those who need them.

Hi, Enzo I'm fixing a Peavey KB2 power, the LM3886 was burnt, but I found a faulty transistor between a pin of IC and the power supply, and I can't find any reference of it, the printed number is F0330AB...I've tried to open the attached file you post 3 years ago but it doesn't open ,,,,is still available? if not the case, Would you mind to posting it again?

Thanks a lot for your support,

[Enzo]

What pin? What shape transistor? What was wrong with it?

I see from pin 8 of the 3886 a small JFET to ground. That would be a type J174 in a TO92. They have a low resistance from source to drain when no gate voltage is applied. A blown 3886 might have damaged it.

[guitarman1970]

You're right Enzo, I did as you told me, I've tried to buy a new one J174, but only I found a equivalent 2N5462, i've just checked the 3 pin configuration, because isn't the same as the original, and worked fine! The Peavey KB2 sounds fair....The problem was to find what kind of Tr is Q3 without schematic or other reference...

Thanks a lot for your support my friend, was so valued! I really appreciate it indeed....

[Enzo]

I'll try to find the scan and post it when I get a chance, but you can get ANY Peavey schematic for the asking from customer service at the Peavey factory.