Peavey session 500 (400 B/Gmodule)

[Sandwich]

Hi im new here.

Im working on a session 500 that is exact by month and year as old as i am.
Problem is that it powers up but it doesnt give any output. Then i started doing some research on this thang. And
i changed all capacitors on the main and the driveboard. I also changed the transistors on the output board. And... also some transistors and the voltageregulatorson the driveboard (400 B/G module). And tried power it up but no luck this time either. So have any of you guys any ideas what this could be?

[52 Bill]

Problem is that it powers up but it doesnt give any output. Then i started doing some research on this thang. And
i changed all capacitors on the main and the driveboard. I also changed the transistors on the output board. And... also some transistors and the voltageregulatorson the driveboard (400 B/G module). And tried power it up but no luck this time either. So have any of you guys any ideas what this could be?

Welcome to the board.

By "powers up", do you mean that all of the power supply voltages are up where they should be or do you mean the pilot light goes on?

Try and isolate the problem. Do you have a schematic? A voltmeter? Swapping out transistors is fun for the experience, but is not a very smart approach to troubleshooting.

Check the power supply first. Then check the power amp and then the pre-amp sections.

[Sandwich]

well im pretty new in this electrics, but after some studying ive got the point.
amp goes on and the green led is on. and you hear the bruzze in the amp as when electric is on.
I have schematics for it and went trough every component and found some wrong components in wrong places so i replaced them with those that had to be there following schematics.
I have a voltmeter wich im not so familiar with cause there wasnt any good manuals with it, and im not sure where to start with the meter when for example measuring ohms if it has to be 200k or 2m or 20k etc etc. dont know where to start from. to isolate a problem is a thing also im not familiar with so information is welcome

[52 Bill]

amp goes on and the green led is on. and you hear the bruzze in the amp as when electric is on.

OK, so maybe the power amp is working. Plug your guitar into the power amp in or the FX return jack. Do you hear any sound from the speaker?

Does the amp have reverb? If it does turn the reverb control up a little and shake the reverb tank. Do you hear the springs through the speaker?

If neither of these tests work, start by taking voltage readings on the dc power supply. Set your meter to the 200 vDC range, hook the black lead to the chassis of the amp and use the red lead to carefully read the voltages. Look for the two large filter caps in the power supply. Each one should have a fairly high voltage on it. One will be for the positive supply and one will be for the negative supply, maybe around 50 vDC.

The next section of the power supply will be for the ic's, so the + & - 50 volts will be reduced to + & - 16 volts. Follow the schematic and check for these voltages.

Let us know what you find.

[Enzo]

WARNING WARNING WARNING

I was immediately struck when I saw

I have schematics for it and went trough every component and found some wrong components in wrong places so i replaced them with those that had to be there following schematics.

That right there looks to me like the kiss of death. We have to assume this thing left the factory in working condition and that someone has been using it. Now it has failed. So either those wrong parts were always there and it worked anyway, or they have been added since the failure. I would be very suspicious you might have misinterpreted something here.

In any case, if no sound comes out:

Check the speakers and their wiring. Touch a 9v battery's terminals to the speaker terminals briefly. Disconnect them from the amp first. The speakers should pop with each touch.

COnnect different speakers to the amp for a listen.

Then follow Bill's advice too. Are power supplies present? All internal fuses intact?

[Sandwich]

WARNING WARNING WARNING

I was immediately struck when I saw


That right there looks to me like the kiss of death. We have to assume this thing left the factory in working condition and that someone has been using it. Now it has failed. So either those wrong parts were always there and it worked anyway, or they have been added since the failure. I would be very suspicious you might have misinterpreted something here.

In any case, if no sound comes out:

Check the speakers and their wiring. Touch a 9v battery's terminals to the speaker terminals briefly. Disconnect them from the amp first. The speakers should pop with each touch.

COnnect different speakers to the amp for a listen.

Then follow Bill's advice too. Are power supplies present? All internal fuses intact?

i hope death uses lipgloss .
The wrong components have been added there since the failure and those components have been added there cause the repairman didnt even try to get schematics, so he tried his own luck.

power supply is present and internal fuses are ok. The research continues...

[Sandwich]

ok little more research....
i measured the powersupply transformer that is connected to the driveboard wich gives 52 volts and 15 volts. The readings of the measurements werent stable numbers bumped up and down. So i started wondering if the transformer that is the issue?

[Enzo]

Oh well.

If your voltages were jumping around and that was what was wrong, the amp would still make sound. If both polarities of 15v and both of 50v are present, then the transformer is fine.

Isolate the problem. Plug the guitar or some signal into the POWER AMP IN jack on the amp. ANy sound out the speakers? If so the power amp works, if not, it doesn't. Then plug a guitar into the front inout jack, and connect a cord from PREAMP OUT to some other amp with speaker for a listen. Does sound come out the other amp? if so the preamp works, and if not the preamp is bad.

HAve you verified the power amp is indeed a 400BG? I added the schematic below for anyone following along.

If the power amp is bad, pull the Molex connector off the end of its circuit board farthest from the large capacitors.. That connector handles the cable from the preamp. Fire up the amp with speakers connected. See the four pins sticking up where that connector went? The one closest the corner is the inout. Touch it with your finger. There should be hum out the speaker when you do. If hum results, the amp works. If no hum, then look on the schematic. There are numerous DC voltages specified on there. Get out your meter and start checking the amp versus the drawing in that regard.

Also, if the ICs are in sockets (I think they were on that model) try this. On the power amp board, the small 8-leg IC closest to the input connector we removed should have a part number on it ending in 478. Like most ICs it will have either a notch at one end or a round dot in one corner. Note which way that feature is pointing, then gently remove the IC from its socket. Now fire up the amp- and see if it now makes sound.


If the preamp was bad, we must reconnect the input cable to the power amp. The preamp gets its power down that cable, the power supplies are on the power amp. We need to verify that both +/-15VDC supplies are making it to the preamp board. That four pin Molex connector uses two of its pins for those voltages, so you can probe down into that connector, then go to the other end of the cable and check there. Right in the center of the preamp board is one chip twice as long as the others. It is a type TL074. The center leg on each side is one of the 15v supplies. SO carefully check the voltage there on those two pins. if it gets that far, it should be throughout the preamp. Beyond that we need more specific information.

[Sandwich]

Oh well.

If your voltages were jumping around and that was what was wrong, the amp would still make sound. If both polarities of 15v and both of 50v are present, then the transformer is fine.

Isolate the problem. Plug the guitar or some signal into the POWER AMP IN jack on the amp. ANy sound out the speakers? If so the power amp works, if not, it doesn't. Then plug a guitar into the front inout jack, and connect a cord from PREAMP OUT to some other amp with speaker for a listen. Does sound come out the other amp? if so the preamp works, and if not the preamp is bad.

HAve you verified the power amp is indeed a 400BG? I added the schematic below for anyone following along.

If the power amp is bad, pull the Molex connector off the end of its circuit board farthest from the large capacitors.. That connector handles the cable from the preamp. Fire up the amp with speakers connected. See the four pins sticking up where that connector went? The one closest the corner is the inout. Touch it with your finger. There should be hum out the speaker when you do. If hum results, the amp works. If no hum, then look on the schematic. There are numerous DC voltages specified on there. Get out your meter and start checking the amp versus the drawing in that regard.

Also, if the ICs are in sockets (I think they were on that model) try this. On the power amp board, the small 8-leg IC closest to the input connector we removed should have a part number on it ending in 478. Like most ICs it will have either a notch at one end or a round dot in one corner. Note which way that feature is pointing, then gently remove the IC from its socket. Now fire up the amp- and see if it now makes sound.


If the preamp was bad, we must reconnect the input cable to the power amp. The preamp gets its power down that cable, the power supplies are on the power amp. We need to verify that both +/-15VDC supplies are making it to the preamp board. That four pin Molex connector uses two of its pins for those voltages, so you can probe down into that connector, then go to the other end of the cable and check there. Right in the center of the preamp board is one chip twice as long as the others. It is a type TL074. The center leg on each side is one of the 15v supplies. SO carefully check the voltage there on those two pins. if it gets that far, it should be throughout the preamp. Beyond that we need more specific information.

ok. i take it in steps. i tried out the 9v battery on the speaker and it worked so the speaker is ok

And im 100% sure that it is the 400 B/G module.

15 volts come out fine in the middle foot TL074

i genttly removed the ic 70487478 and powered it up
but no sound

And every little hint i keep learning from
so keep em coming

[Enzo]

How about disconnecting the Molex connector and touching the pin with your finger?

The point of doing this is the same as if you hear nothing from an amplifier and pull ther cord from the guitar and touch it just to see if the amp is on.

If the power amp stays silent, and it has reasonable DC voltages throughout, then try a new TL074 on the power amp. You can exchange the power amp TL074 for the one in the preamp.

[Sandwich]

you mean that put finger on these two in the picture

http://s714.photobucket.com/albums/w...rent=400bg.jpg

[52 Bill]

you mean that put finger on these two in the picture

No, I think the one you circled is the reverb tank connector.

Edit:Sorry, it goes to the compressor led and switch not the reverb.

Look all the way to the left edge of the board. As Enzo noted, two of those pins will have the pre-amp power supply of + and - 15 volts. The other two pins are the input to the power amp and a ground.

Touch the pin that goes to the input to the power amp.

[Enzo]

Bill is right. All the way left upper left corner. The top pin is the input. That is the pin that has C16 going to it.

[Sandwich]

Bill is right. All the way left upper left corner. The top pin is the input. That is the pin that has C16 going to it.

ok now i get it . so i touch that pin and no sound

[Enzo]

If the amp is powered up and a speaker connected, then touching that post should make sound. If you get none, then the power amp is faulty. Now start through the circuit and check the various DC voltages that are on the schematic.