5 v rail pulled down hard, blowing fuse

[fredcapo]

Working on a Peavey XR800D mixer/power amp. Rebuilt the output stage and was burning it in when the 5v fuse blew, taking out the effects processor board and effects display on the main board.

The 5 v rail seems to run off of a 3 pin regulator on the power supply board, to the main board, where it takes off to the effects processor board, then back via flat ribbon to the effects led display on the main board. With 5 v disconnected the mixer and amp runs fine but no effects function.

Measuring ohms to ground from the 5 v regulator pin I see 1.7 k pretty hard. If I unplug the ribbon to the display I see about 2.6 k with some slow capacitor charging effect.

When powered up the OC 5.6 volt drops to 4 volts with the ribbon in or out. The fuse is rated 2A. A 1.5 blows in about 1 second. A 3 did not blow for a number of seconds though I powerd it down did not push it.

Where would I start lifting components/chips given the numbers?

Thanks

[Enzo]

So if you apply a similar AC voltage to the bridge, you get a good +5 unloaded?

You can remove the fuse and connect a DC current meter in its place, just be ready to kill the power.

So if +5 goes to the digital board, then on to the main board, pull the ribbon between the last two. Now does +5 hold if it is allowed to the digital board but not on to the main?

Do any of the ICs get hot on the FX board? Or the ICs by the FX selector rotary encoders?

[fredcapo]

Enzo. Thanks for jumping in. If I unplug the ribbon cable,which appears to power the led displays and some misc, the voltage rises slightly , say 4.6 vdc,while with it connected the voltage is 4.2 vdc. Open circuit I am seeing 5.6.

I didn't mention that with the 5vdc fully connected it is adding significant hum to the output. Could ths be some corrupted signal fron the effects circuit?

No signs of burning or heat,mthough the fuse blows or I power it down quickly.

Considering jumping the fuse and waiting for something to burn...somebody please slap me out of it...

[g1]

Considering jumping the fuse and waiting for something to burn...somebody please slap me out of it...

If you're ready to do that kind of damage, try this first: cut the trace at the output of the 5V reg. (after C101) and put your current meter in series. Which board is pulling the bulk of the current?
And if there is not enough current being pulled to be stressing the fuse, then the problem is probably the reg. or before it.

[The Dude]

It's not horribly uncommon for a regulator IC itself to give up when loaded. I might first try a new regulator.

[fredcapo]

Thanks for the suggestion. I will try a new 3 terminal regulator, though the fact that the specified fuse is blowing i tend to think it is a load fault somewhere...

[The Dude]

Is the fuse on the supply side of the regulator or the output?

[fredcapo]

I think the regulated 5vdc is passing thru a 2 amp fuse, but need to check the schematic at the shop. I also will measure the current when I have the oversized fuse temporarily installed.

I don't know enough to recognize what a normal ohms measurement should be between the 5 volt and ground. Only that I see a steady ohms reading, instead of a charging curve is suggesting there is a hard short somewhere.

Thanks.

[g1]

The fuse is on the AC feeding the bridge that supplies the regulator.
Page 3 of attached schematic:

[The Dude]

Thanks g-one! Since that is the case, I would definitely have a look at the regulator itself. The posted resistance readings don't seem all that low to me. I would suspect the regulator is "giving up the ghost" when it gets a little warm.

[Enzo]

It is hard to detect many overloads on a 5v rails simply becuase the stuff is full of transistors. If a shorted transistor is on the other side of a semiconductor junction inside an IC, your meter will never see it.

[fredcapo]

Guys. Thanks for your comments and insight. I will work with that... Was getting worried that the forum was lost yesterday and would have to rely on help from Danica Patrick...guess could be worse though...

[fredcapo]

Replaced the regulator but still have the 5 v pulling down whenever the load is connected. I just will have to start pulling chips or otherwise trying to isolate segments. Thanks.

[g1]

Disconnect the output leg of the 5V reg. and put your current meter in series between the leg and it's trace. Now you should at least be able to tell which board is drawing too much current.
But before even doing that, make sure the voltage going into the reg. is stable in terms of DC and ripple when the load is applied.

[km6xz]

It would be pretty rare for a junction to short that is isolated from the rail enough not to show lower than your measured 1.7k ohms, but it could be voltage dependent breakdown of all sorts of parts such as a filter or bypass capacitor. You are going to have to isolate the region that is pulling current. Based on the 1.7k figure, it should only pull about 3ma. Do you have a bench power supply? Try powering the module or +5volt line with a 5volt current regulated power supply set for foldback at less than an amp and see if the effects module works.. That would isolate the 5 volt supply or the effects module as the problem. You can also unplug the ribbon cable and connect a 15-20 ohm resistor to the +5v power supply output to see if it behaves well with a known load.