D&R4000 PSU Issues

[Siegfried Meier]

Hey guys,

I have a PSU that is not giving me anything on the +18v side.  A resistor burnt up, and I cannot seem to find the proper replacement.  Added a few and they all burnt up.

I've replaced C5, which is a 100uF 50v part since it was bubbling...?  I stuck a random value resistor in, and at least it doesn't burn up.  However, the board isn't marked in any way at all, so I have no way of knowing what part is what really.

Any advice?

Thanks!
Sig

 

[mjrippe]

Hi Sig,

You have the schematic and the parts list, and you know that the cap you replaced is C5.  It seems like you have all the pieces to the puzzle here, is there something I am missing?  If you have narrowed it down to the ones shown in the schematic, a little logic and probing with your meter set to continuity will tell you the answer.  R2, 3, 4, and 5 all connect to C5 at one end.  Measuring where the other end is connected will tell you which is which.

Yours,
Mike

 

[Siegfried Meier]

I shouldn't say it was C5...I don't know that it was C5.  I do know that it was a 100uF cap, that's all...

I have a pic of what the PSU looked like before I bought it, and according to the colours I see on the old resistor, it makes no sense as it's saying it's a 470 ohm...which according to the parts list appears to be R11?  I don't see an R11 in the +18 side though...

I've circled the resistor that's going bad.

Is it possible the voltage regulator is killing the resistor?  I've been fighting on and off with this PSU for nearly 2 years now.  I give up, and always come back to it...

Thanks,
Sig

 

[Siegfried Meier]

Hmmmm odd...it appears that this section is not the +18V after all...it says that IC3, which is that IC you can see with the heatsink, is actually in the 48v side?  Then that makes sense - R11 is 470 ohm, and it is in fact the resistor that is burning up.  This board is so confusing...would it have killed them to silkscreen the damn thing?

There are only 2 trimpots, so I assumed I was seeing one for the +18v side and one for the -18V side.  Seems that this entire section is actually phantom.  I guess that's why I measure 40v on all 3 legs of that 7824 regulator.  Is this normal?

 

[Siegfried Meier]

Ok, so those 220 uF caps in that section...the pads were all broken off the bottom.  The guy before me "recapped" some stuff, and clearly did more damage than good.  I fixed that, but the resistor still burnt up.

I thought maybe I had the wrong value...perhaps it's a 3k9 as stated, since I still don't know which part is which.  Now, it's not burning up but I still don't have a +18 rail...I'm measuring -10.5v on that pin...

Any other ideas?

Thanks,
Sig

 

[mjrippe]

Hi Sig,

Can you post the entire schematic?  Hard to tell what's going on with half the picture ;-)  Maybe go over the entire board and check for other "repair" work? 

Mike

 

[Siegfried Meier]

I've pretty much gone over the entire board now and repaired everything that I could physically see.  Still no proper +18 voltage unfortunately...

 

[walter]

If you are unsure about what is what you could draw that section of the circuit by looking at the board, and compare it to the schematic. Or you could locate an obvious part on the schematic, and then trace from that point on the pcb to the problem area, identifying each component and branch as you go. Like reading a road map. Take notes on the schematic and the pcb.

 

[Siegfried Meier]

One question - I did notice that all 3 legs of that IC3 in the 48v side conduct to each other, and I have 40V on all 3 legs.  Is that odd?  It seems odd to me...could this be an indication that the IC is dead?

 

[radardoug]

You have two separate problems. R11 is 470 ohms, in the 48 volt regulator circuit.
Yes, the 48 volt regulator is fried.
On the +18, change the 723 i.c., and check the output transistors.
723's often die for no apparent reason.

 

[Siegfried Meier]

That's what I thought!!

Do you know what voltage I should be seeing on those transistors?  It appears normal.

I'll order up a new reg and get that proper resistor in there.  Thanks!

 

[mjrippe]

Ok, first off: "Supply Ooops"!?!  That says a lot!

Next, IC3 is a +24v regulator and definitely needs replacing.  But why a +24 in the +48 supply?  Maybe the +48 supply is not referenced to ground, but rather to the +18v rail (note the "+V" connection) which gives a total of +42v.  Not sure where the other 6v comes from...someone smarter want to jump in?

 

[Siegfried Meier]

Haha ya - they also left the first number off the front panel on the PSU as well - I guess they were for different units?  Add a "4" in front, and you 4000PS ;-)

It's a weird PSU, there is no question.  I thought I was mucking about in the +18v section for the longest time before I realized that was the phantom section...

I'm gonna order that reg, see what I can do.  I have a line on another PSU from the Netherlands, along with a master section (which this dufus that owned it before me ALSO "modded", and said he couldn't get it working anymore - I trust nothing this guy has done), so I think we'll snag those as extras anyway just in case.  Finding parts for these is getting more and more rare, but I have a feeling this could be a really cool little console for a smaller studio.

 

[Siegfried Meier]

Ok, we're getting closer...but still not there.

I replaced both the 723 regulators in the + and - side...I don't think those are the problem, since I tested them and they both work perfect in the - rail.

Replaced the 7824 - the legs do not conduct, looks good here - you were correct, the other one was def fried.  Replaced the 470ohm resistor, no more burning up - this is good!

I also replaced D3 and R12 (3k9) - still nothing.

However, here are my voltages:

19.4v - on the +48v
-9.6 - on the +18v
-18 - on the -18v rail - perfect

I also checked the 2N3055 transistors - looking at the back of the PSU, the one on the far right reads 37.5v and the one to the left of that reads +19.2v.  I don't know what to make of that, but that appears to be ok?  I did buy 2 extra transistors as well, just in case I had to replace them.

Any other advice on what to do next?  Thanks!

 

[mjrippe]

Well, since your +18 is still no good, I'd say swapping the two 3055 for that section (T1 and T2) would be a good place to start.  But first double check the caps and other components in that area for previous "repairs"!

 

[Siegfried Meier]

OK, will do.  Everything "seems" to be ok now.  Man, so many pads are lifting that the PSU is starting to look like a point-to-point design!! 

I'll swap out the transistors and see where we're at!

 

[Siegfried Meier]

Might I ask, what is the job of the output transistors in a power supply?

In a power amp,  they add power to the signal that drives it.  If it's a power amplifier, you drive it with a little power, and it passes greater power out in step with the power level in.  It draws the extra power from the power supply directly, it does not actually "amplify" the input power.  But, what do they perform in a PSU?

 

[Siegfried Meier]

OK!  Somewhat success...

I replaced the far right (higher voltage) output transistor, and now we have +18V LED!

However, I cannot turn down the +18v at all...no amount of turns will do it.  I'm getting roughly 34.4v on the +18V side, and over 60v on the +48v side...

At least it's progress?

My 7824 IC is a 7824ABV, and the replaced transistor is a 2N3055G instead of just a 2N2055 - would this really matter?

What else can I look at now?

 

[mjrippe]

Replace the other 3055?  Often if one of a pair goes, it take the other with it.

 

[Siegfried Meier]

Ok, replaced the other transistor, no change.  Appears that one was ok.

For curiosity sake, I put back the old 723 regulators that I know were working fine.  It's perfect, still, in the negative rail, but I notice something different in my positive.  I see 22.4v now on the +18v rail, and 51.6v on the +48v rail.  Could it be that those replacement values I got won't really work?  There were extra numbers on the part number, something could be different about that.

I also pulled the trimmer out to measure it out of circuit, and it appears to be working fine on its own, but I still cannot trim the level down at all on the + side...what would cause this?  Is there another part near the trimmer that's faulty?  Could the trimmer be bad after all, even though when I'm measuring it outside of circuit, it appears to be ok?