[sdiy] P.S. troubleshooting help

[Roy J. Tellason]

On Thursday 02 April 2009 03:42:31 pm Scott wrote:
> I have tried w/ a dummy load and get the same results.  It's not the synth.
>  I must have done something to the ps last night while I was testing it.
>
> I need to desolder the main transistors before I can look at the traces on
> the PCB.  I'll do that in a few minutes.  (The big transistors mounted to
> the frame are basically what holds the PCB in place!)
>
> I'm starting to think it's the transistor...  On the positive side, the
> case reads 23v and the pins read 15..  on the negative side the case reads
> .66v(what should be -15v if it were working) and the pins read -23.  (or is
> it supposed to be backwards, since it's the negative side?)

Are they the same transistor?

The "pins" you refer to are base and emitter,  probably,  and should show a 
difference in voltage of 0.6V or so between them.  If they're at zero volts 
then you should find out why.

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Scott Gravenhorst
> Sent: Thursday, April 02, 2009 2:09 PM
> To: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] P.S. troubleshooting help
>
> I would start with a dummy load on both + and - sides of the supply and
> see if you can replicate the failure outside of the synth cabinet.
> Power resistors calculated to draw a certain amount of current, perhaps
> something in the range of 15 ohms for 1 ampere 60 ohms for 0.25 A.
> Remember to consider power dissipation and provide for that.  Really
> sucks not to have the schematic.  Do you know how much current the
> synth draws?  Are you possibly causing it to shut down due to
> overcurrent?  The part number - does that suggest 3 amperes per rail?
> Is it possible to trace out the schematic by looking at the PCB?
>
> "Scott" <Scott at scottwick.com> wrote:
> >Can someone help me troubleshoot my power one hcc-15-3-A? I took
> >it out of my synth cabinet last night to test it. It tested fine.
> >(checked for ac ripple w/ the scope, checked to see if it was
> >loosing power when my AC power drops, etc.. was all fine) I put
> >it back in my synth today.. doesn't work. Take it out and test
> >it. I get 15 volts on the + and .66 volts on the - side.
> >
> >The pins of the main power transistor on the + side read 15
> >volts. On the negative side the pins read 23! (I'm guessing it
> >should read -15) Then Im not sure what happens to the signal,
> >because almost everywhere else I test on the - side of the pcb, I
> >read .66 volts.
> >
> >I did try replacing the regulator IC (lm723n) (swapped sides, put
> >negative on pos, pos on neg) and the positive side still works
> >fine, so that isn't it. (If you aren't familiar w/ them, they
> >aren't positive and negative regulators.. they are the exact same
> >chip)
> >
> >So, having never tried to fix a power supply, and not having the
> >schematics for this thing... im not sure where to go from here.
> >
> >let me know if anyone has any ideas. Hopefully that .66 v on the
> >negative side will give someone more informed than me, a clue as
> >to what is going on. I also tested the transformer, and it is
> >outputting exactly what it should on every 2ndary.
> >
> >thanks a lot!
> >
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>
> -- ScottG
> ________________________________________________________________________
> -- Scott Gravenhorst
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