Phihong PSM-5341 bad PG

[markTT]

Hi all

I'm trying to resurrect a TT with the Phihong PSM-5341 PSU. I've found the voltage on PG (orange output) is around 33v. All the other voltages are spot on.

There are no shorts on Q3 or Q4 or R30 & R32 Any ideas?

-Mark
One time 520ST, 1040STFM, Mega STE2 owner and now TT.

[exxos]

I don't see any way you could end up with 33V with this circuit...

PG is switched to 5V.. You say 5V is good ?

I would re-check your measurements..

Also check D10 isn't shorted.. Then start checking voltages on both sides of R33 and see what's there for starters.

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[markTT]

ah 31.2 mV - need to put my helmet magnifier on!

under load.

Red 5.11V
White -5.00V
Blue -12.5V
Yellow 12.1V
Orange 33.2mV

Looks like R33 is bad. 5.2V in and 31.2mV out. For some reason I was focussing on R32 back to C22 and missed it. Time for a rummage in the resistor box.

Thank you for putting me on the right path.

[exxos]

Orange 33.2mV

Looks like R33 is bad. 5.2V in and 31.2mV out. For some reason I was focussing on R32 back to C22 and missed it. Time for a rummage in the resistor box.

Does it not measure 330R ?

Don't forget if PG isn't "good" then Q4 will be "on" and shorting the bottom side of R33 to 0V anyway.

[markTT]

ah, R33 does measure 323 ohms, so it is good. Of course 5V was on one side as it is on the 5V rail which is good.
Back to square 1. It does look like 71V is coming out of D10 which seems high. I expected something more in the region of 40V

[exxos]

ah, R33 does measure 323 ohms, so it is good. Of course 5V was on one side as it is on the 5V rail which is good.
Back to square 1. It does look like 71V is coming out of D10 which seems high. I expected something more in the region of 40V

Are you sure its not 7.1V ? I would check across C22 and see what you get there. Also check voltages both sides of R31.

Check there are no bad solder joints on the transformer as well.

[markTT]

Thanks exxos. Yes, definately 71V.
After much prodding and poking, it looks like R26 is bad, and C20 physically next to it seems to have leaked a little. It may also have taken out SHR1.

[exxos]

Thanks exxos. Yes, definately 71V.
After much prodding and poking, it looks like R26 is bad, and C20 physically next to it seems to have leaked a little. It may also have taken out SHR1.

I guess if you removed Q2.. Then the base of Q3 will get turned on via R29, D11, which will then ground the base of Q4 turning it off, so PG should then go to 5V. If that happens then you know the circuit before Q2 and related parts is having issues.

But even so, 71V on C22 doesn't sound right. The coil is grounded with RTN so on a -12V rail it shouldn't be that high. Its going via a diode so would be half the voltage of the coil, so should be more like 7V. Its why I suggested check the grounds on the transformer are good, RTN top end of R35.

Might be worth checking the voltage across C18 and make sure it's like below 20V.

71V could be peek voltage, but again still seems to high. if R26 is blown then there no load and likely would kill C22. Might be worth just removing C22 and R26, and put something like 1K in place of C22 just to load D10 voltage, then measure across the resistor and see if you get something more sensible like 7V there.

[markTT]

Spare parts arrived today. Quick swap of R26 and C20 and the PG line is back to +5v. The power supply is fixed

Thank you for guiding me through and helping me to understand the circuit. My first PSU repair!

[exxos]

Still would be interesting to know what the voltage is across that cap now it's working again.