Maestro Echoplex EP-3 help

[eurekaiv]

Hi all, I have a friends Echoplex and I'm trying to figure out what's up with it. The unit powers up and the motor runs but it's passing no signal at all. I've taken a ton of voltage measurements and I think I've narrowed it down to a problem with the power supply but I can't seem to find a bad part or anything actually wrong so I'm hoping someone here can help. As you can see below, I marked up voltages on a copy of the schematic.



I lifted and tested all the filter caps except the one on the motor circuit, it's too hard to get to and there doesn't appear to be trouble there. I also replaced the bridge rectifier with a known good and the 100, 1k, and 680Ω resistors all check out also. I appear to be losing all my voltage between the first and second filter. Also, the 680Ω resistor drops the voltage down to 11.5v, runs until it hits the pilot light (circle with the M in it), and then goes dead at .003v on the other side.

I'm totally a novice (though I have built and repaired a number of amps/effects over the years) and this one has me stumped so any help at all would be greatly appreciated!

[Enzo]

OK, so your power supply starts out with 30v, but is zero on the other end of the 100 ohm resistor - well. .05v is close enough to zero in my book.

If the 100 ohm resistor is hot, then that cap after it may be shorted.

I suspect though that it is cold, so either it is open, or the circuit path for it is broken. SO measure continuity from the cap with the 30v to the cap on the other side of the 100 ohm. SHould be 100 ohms or less. Keep aware of resirual voltage hiding in the circuit to confuse your meter. Alternatively, put your meter on the 30v cap, and probe voltage to the near end of the 100 ohm then the far end of it, then to the next cap. SOmewhere in there will be a loss of voltage as you cross the break in the circuit.

[eurekaiv]

I actually checked that resistor previously and it measured in the 90Ω range so I figured that's within tolerance and a-ok. I actually pulled this particular PCB and lifted a leg up on all the parts in this area—tested values and everything came up fine. Like you say it almost has to be at that spot but I'll be damned if I can find anything actually wrong there. Obviously I'll need to go back and double check all those values again here after I eat some food. Thanks for the reply!!

[eurekaiv]

So the 100Ω resistor reads 92 which I think is fine and I double checked all the caps, the diode, and the two 680 resistors (there's a 680 in place of the 1k in this guy) in the PS and all seem good to me. The only thing I can come up with is that the bridge rectifier (it's a enclosed unit with 4 legs) seems to read open from the side facing the filter caps (positive leg) and the grounded leg. This to me says a short in the power supply. I had the same problem when I first opened this unit up and that's what had me replace that part in the first place. Could there be something else wrong causing the rectifier to go open to ground immediately after powering up?

[Enzo]

Um... You are getting +30v at the first filter cap, right? If the bridge were open you would get nothing. I don;t expect to measure resistance between + and ground there. test the bridge as if it were four diodes.

If the powr supply were shorted, it wouldn;t read open.

You mention the resistor measures 92 ohms instead of 100. That would not leave you with zero volts at the downstream end of it. You mentioned lifting it from the board to measure it. That is fine, and verifies that teh part is OK. But I don;t see you trying what I suggested - measure from the one cap terminal to the other. In other words, don;t just read the 100 ohm resistor, measure from where it came from to where it is going. The resistor could be just fine and a crack in the circuit board can be opening your circuit. That is why I suggest not measuring the part itself, but measure from things the part ultimately is wired to. measure from the + corner of the bridge to the + terminal of the cap with the 13 above it. Still get 92 ohms?

[eurekaiv]

Yes, +30v and I definitely get 92 ohms measuring across the filter caps and not the resistor leads. I even went so far as to check traces on this particular PCB for continuity as the three caps, three resistors, zener diode and the bridge rectifier are the only parts that live on the one PCB.

I'm curious why you would expect the bridge rectifier to show continuity from the DC side to ground. I always thought that diodes shouldn't go "backwards" so to speak and then only forward once a certain voltage threshold is met.

[Enzo]

One or more of us is not communicating with the other effectively. From my end: I don;t expect continuity to ground from the + corner. In post #4 you mentioned finding open from + to ground at the bridge. I assumed you meant you expected otherwise. I don;t expect continuity to ground, an open reading would be expected.

YOu have +30 at the first filter. You still have essentially zero at the second cap? At point 13? if there is no voltage there, then either the path from the +30 is open or the top of the second cap is shorted to ground.

SO we are back to this: does the 100 ohm resistor get hot? is point 13 grounded? or any of terminals 1,2,3 on T2?

Does your meter have a diode test function? If so, use it to check all four sides of the bridge.

[eurekaiv]

Sorry, I have a bad habit of confusing "open" for having continuity.

I tested the bridge rectifier and all diodes in it are a-ok.

The 100Ω resistor gets VERY hot very quickly.

Point 13 reads 0Ω to ground.

I get 93Ω from the DC side of the bridge rectifier to ground (now we're getting somewhere I think).

I lifted the board with T2 and wouldn't you know every leg reads 0 to ground. So I think that's why I get 93Ω when measuring from the rectifier to ground—it's taking the path through T2 to get there. So I think this would mean a bad transformer right?

Thanks SO MUCH for your help!!

[Enzo]

yes, powerful clues.

So T2 seems shorted to ground. Is T2 pc mounted or are there wires coming out of it to the board? In other words, can we disconnect wire from pin 2 of T2? or do we have to remove T2 from the board entirely to disconnect it? Either way, that is what we have to do.

Disconnect pin 2 of T2 or remove T2. Now is ther still grounding at 13? If not, then it sure looks like T2 is shorted. Look closely for any solder bridges associated with T2 and its wires. of course if 13 is still grounded without T2, then we look further for a short. But if the voltage is now restored to 13, then we have to fix or replace T2.

T2 has two windings. Check for continuity through each winding. Then check for shorts from one winding to the other. And check for shorts from either winding to the fframe of it. In this case the primary winding would be the one grounding 13. Of course these tests are done with the transformer removed.

And make sure the collectors of Q6,Q7 are not touching ground.

[eurekaiv]

T2 is a plastic box mounted to the PCB. Thankfully these things are fairly well engineered in terms of how everything fits together so pulling it isn't all that hard really. I'll do what you've suggested tomorrow after work and report back.

[Enzo]

You understand what I am doing, I think. If you have to remove the whole thing, OK. we just need the transformer disconnected from the power supply to see if it is the problem.

[eurekaiv]

Yeah, this transformer doesn't have any leads, it's mounted directly to the PCB. Here's a pic...

[eurekaiv]

So I removed the transformer from the board. I'm not reading a short from one winding to the other on any of the legs but without the transformer pulled I'm getting exactly what I should be voltage wise (according to the schem) at all marked points in the power supply. Unfortunately, there's no way for me to read from any of the pins on T-2 to it's case since its plastic and the actual transformer inside appears to be epoxied in. I also can't see any other path to ground from point 13 that would incur additional resistance somewhere along the way so it almost has to be T2 at this point. I fear the hardest part is going to be finding a replacement as I have no idea what the specs for it are.

[Enzo]

We're not done. Your voltage recovered, but we have not proven the transformer to be at fault yet. Go on the board where the transformer was. Pin 2 must not be shorted to ground or the voltage problem would still be there. But check pins 1 and 3. Are either of those shorted to ground? Grounding one of those would kill your power supply just as easily as a pin 2 to ground short.

See the thing is, somehow the transformer completed a circuit to ground. If the winding shorted to the frame, but the frame was not grounded because it is buried in epoxy, we are still left wondering how the grounding occured.

So don;t toss the part, check pins 1 and 3 on the board. if one of those transistors short, it places a 22 ohm resistor betwen ground and the transfoermer. And a shorted transistor is a lot more likely than a bad transformer. I think your transformer winding is merely serving as a wire to the real grounded point.

[eurekaiv]

I checked for continuity (with the transformer removed) and it's open from pads 1, 2, 3, 5 and 6. Pad 4 should be grounded and is indeed. The emitters of the two transistors read 22Ω as it seems they should without a plug in the footswitch jack, and the collectors (pins 1 and 3) are open.