Losing my mind soundcraft cps-150 help

[Ptownkid]

So i have a client who wanted to replace the caps and regulators in a soundcraft cps-150 psu...ok, easy peasy

Switched them all out and now it no longer works properly. R1 and R21 both smoke...

I am getting 32V at the input of the regs
The pots are good
The electros are good

I've been beating my head against the wall on this, i just can't figure it out. I didn't want to post this, because I should be able to figure this out...

Any suggestions?

http://www.soundcraft.com/downloads/fetchfile.aspx?cat_id=user_guides&id=550

 

[sahib]

Why the regulators? Were they kaput? They are TO3 package. What did you replace them with? Have you connected them correctly?

 

[zebra50]

And have you increased the cap values at all?

 

[jwhmca]

I don't know how much repair work you've done so sorry if this al sounds basic...

1. Check the polarity of the capacitors you have replaced against the schematic... I have seen the silkscreen be in error or the schematic.

2. Check for solder blobs/ introduced solder shorts.

3. Broken wires. You lifted PCB's, parts knobs... you took the thing apart wires will break or come off.

4. You broke a pcb trace... similar to #3

5. You put the thing together wrong something is touching something it shouldn't be... usually grounding out...

6. Of course wrong parts.
Okay... now we can start the LOGICAL procedure.... With the unit off and unplugged, what is the resistance across R1 & 21's pad's with them out of circuit?

 

[Ptownkid]

He just wanted the regs replaced...they worked.

I've done a decent amount, but don't worry about being too basic, I don't know it all...

The caps are all in correctly, but I will check again.
Solder shorts are non-existent, that's where i started.
No broken wires, if there were, I would not get the 32V at both inputs.
Pcb trace...doubtful, but I will check again for posterity.
Incorrect assembly, no, i hardly had to disassemble anything.
Wrong parts, no, everything is a letter for letter match except some of that caps are rated for a higher voltage.

R1 pad R = 1.7M
R21 pad R = 0.8M

 

[moamps]

Hi,
check D7, Tr1 and Tr2.
Regards,
Milan

 

[Ptownkid]

D7 is good, I replaced all the diodes.

I know that TR1 and TR2 are used as shutdowns in case of a fault...can someone shed some light onto what happens if those fail? Also, there's nothing special about those is there...possible subs?

 

[Ptownkid]

So there definitely appears to be something up with the two transistors... the 2sc2240 reads an hfe of 260, which is low as it's the BL part (350-700) and the 2sa970 reads infinite...

 

[riggler]

Did you have to use a lot of heat for an extended period soldering/desoldering around those transistors? I've toasted a couple transistors this way not even realizing.

 

[Ptownkid]

NO, i'm thinking there might have been a temporary short way back at the beginning of this saga as the connectors from the regs to the pcb were garbage...

So I no longer have any smoke in the quick test I did with both transistors out...I think the problem is solved...

How critical in this application are the specific transistors used? Is it as simple as replacing with a suitable npn/pnp pair?

 

[sahib]

Not critical. Mr. Self has got BC184 and BC212 in his book and the rest of the passive components are pretty much the same.

That explains why your client wanted the regulators replaced.

 

[Ptownkid]

His thinking was simply that brand new ones will last longer before failing. This supply is notoriously under powered for a 24 channel console, but I couldn't talk him into doing an upgraded supply.

 

[Ptownkid]

Still can't sort this out...R1 still smokes...

Replaced all the diodes, cheacked all the caps, replaced/checked the resistors and trimmer, tested the regs out of circuit and they function as they should.

I'm getting +/- 30V and i'm seriously ready to light this thing on fire. I simply just don't get it...

 

[3nity]

Its not your fault.
he should have brought it to a qualified tech!  ;D ;D
Just kidding...
why dont you take pix? maybe someone here can spot something..

 

[abbey road d enfer]

Still can't sort this out...R1 still smokes...

Replaced all the diodes, cheacked all the caps, replaced/checked the resistors and trimmer, tested the regs out of circuit and they function as they should.

I'm getting +/- 30V and i'm seriously ready to light this thing on fire. I simply just don't get it...

R1 smokes means the regulator is either fried or not connected properly. In normal situation, the reg mainyains 1.2V across R1.
To answer an earlier question, if the shutdown transistors fail, if they go open they just don't do their job, if they go short, they shutdown the voltage.
Have you replaced the regs with the exact same type (LM338K)?
IIRC the regs are PCB mounted, right?

 

[Ptownkid]

It is the regs...they are toast. I'm using bc550 and bc560 for the shutdown pair...this should be ok?