Need Help with this Circuit Please

[stj]

Re: Need Help with this Circuit Please

well, nothing a couple of jump-wires wont fix.

[tfs2015]

Re: Need Help with this Circuit Please

You just feed the DCV directly to the two main filter caps. Your amp has SMPS power supply or linear power supply?

The whole PA system uses a SMPS Power supply to supply +/- 15vdc to the mixer and +/- 52vdc to the amplifier section. Going into the amplifier section is a Red Wire that is +52vdc and blue wire for -52vdc and two black wires that provide a return(ground) back to the SMPS. The odd thing is once the two black wires are connected to the amplifier section they are electrically connected together to from one 0V ground wire. You can see this on page 25 of the service manual.

If there was a way I could get this voltage dropped to +/- 40vdc then I could just use a Amp boards from Sure electronics, or one of the other many designs that are available but with +/- 52 vdc I am kind of stuck on what to do.

[budm]

Re: Need Help with this Circuit Please

Which post has the link to your unit service manual? Post 17 diagram matches your board?
Modifying the SMPS to drop 52V to around 40V may not be practical (feedback circuit will have to be modified) but can be done, if you drop the 52V to 40 the +/- 15V will also drop by the about the same percentage, 11.5V which mat be still enough to run the OPAMPS..
Notes from FENDER about the power supply: page 7, it is not tightly regulated for powering the power amp section, so it does not look like it monitors the output to maintain 51V, only monitors it for protection if it gets too low. 52V to 36V is about 30% drops.
The SMPSU incorporates several protection circuits other than the fuses, including over current protection in which
the unit will shut down and the protect LED will light and must be reset with the main power switch. Secondly, the
unit will protect itself if the voltage on the rails sags to below ±33V dc after which it will go into protect mode. The
reason for this is that during normal operation with 8 ohm loading, the rails will remain at about ±48V dc and with 4
ohm loading are allowed by design to sag to around 36V at full output. If loads lower than this are applied, either
the rails will sag lower or the current will be exceeded and the unit will go into the protection mode. The protection
PCB connected to the amplifier board serves this purpose.

[tfs2015]

Re: Need Help with this Circuit Please

So After reading all of this information that you posted, am I stuck with trying to repair the board? Btw there is some sort of glue all over the bottom of it. I have up loaded some more pictures of the bottom and top sides of the board.

Thanks
Tom

[stj]

Re: Need Help with this Circuit Please

glue?

[tfs2015]

Re: Need Help with this Circuit Please

ok, maybe not glue, some sort of protective coating I guess.

[stj]

Re: Need Help with this Circuit Please

you mean the blue coating?

[budm]

Re: Need Help with this Circuit Please

Blue solder mask?

[keeney123]

Re: Need Help with this Circuit Please

So the blue solder mask will not prevent you from desoldering the solder. It looks like you have plated throw holes for the components to go in. That is the metal of the hole is all the way through the board and there is a solder pad on each side. This is a good thing. If somehow when you pulled up the components you ripped the trace to the solder pad you can gently scrap off the blue solder mask to expose the trace underneath. You would use an X-acto knife to scrap the blue solder mask, but gently. Then if the trace is close to the solder pad you can use a bare wire about the same diameter as the trace/run. Bend the wire to go throw the hole and enough on top to go to the trace/run. If you are using flux core solder you would clean your iron then tin it place it beside the wire and heat everything together and apply solder. Watch carefully as you do this making sure the wire says in place. Sometimes the wire has a tenancy to stick to the iron. You can kind of roll the iron off to one end if that happens or get some hemostats and clamp it in the middle and solder each end separately. I use to use lacquer red nail polish to then cover the exposed run and jumper back up, by not on the solder pad. I do not know what they use now one of these guys will tell you.

[tfs2015]

Re: Need Help with this Circuit Please

If I remove all of the power transistors from the board I should still be able to test it to see if the rest of it is working OK, right?

[stj]

Re: Need Help with this Circuit Please

depends what you have to test it with.

[tfs2015]

Re: Need Help with this Circuit Please

I would test it by hooking it up to the rest of the PA system and seeing if i get output from the board to the speakers using a input signal from the mixer section. The only thing that I am worried about is burning out the emitter resistors or one of the other components. I can only test the one side because the Left channel bias VR needs to be replaced.

[budm]

Re: Need Help with this Circuit Please

I would ALWAYS check to see if you have DCV >1V at the speaker terminals fist before hooking up any speakers or dummy loads to the speaker terminals.

[stj]

Re: Need Help with this Circuit Please

no output transistors = no output.

[keeney123]

Re: Need Help with this Circuit Please

So do we have a schematic of this board? Could always test it with a bed of nails if you could afford to by one and then program it to test that board.

[fzabkar]

Re: Need Help with this Circuit Please

The service manual was uploaded earlier, but here it is again for those who don't wish to become forum members just to see it:

http://music-electronics-forum.com/a..._pd_250_sm.zip

[rievax_60]

Re: Need Help with this Circuit Please

If I remove all of the power transistors from the board I should still be able to test it to see if the rest of it is working OK, right?

With the power transistors removed, the rest of the circuit should still actively balance the output to zero volts if all is well.

[stj]

Re: Need Help with this Circuit Please

the output to the power-transistor base pins.

[keeney123]

Re: Need Help with this Circuit Please

So let me understand this a little more. What have we establish with the voltages on the power supply? What do we have and what don't we have? Have you read the Theory of Operation and did you do the preliminary test before you opened up the unit? Also, did you verify the updates to make sure the amplifier is running as smooth as can be?

[tfs2015]

Re: Need Help with this Circuit Please

I updated the components that needed to be updated and reinstalled the power transistors on the right side of the amp and went to power it up and when I did power it up the BD140 transistor Q229 went up in smoke but i don't know why, so I need some help to figure this out.

Thanks
Tom S.