Fender Frontman 15 G problem

[moosemouse]

Great a newcomers forum!  I'm in the right place!

I'm working on a Fender frontman that will not power up.  I've included an enlarged portion of the schematic that shows my test point readings.  You will notice that there are readings in both volts ac and dc for tp1 &2, I misread and started in dc volts, then got confused when the dc reading was closer to the schematic values.??  I hope someone can correct my mistake and thinking, also if someone could trace the flows or flow patterns associated wih the power supply I would appreciate it greatly. 

thanks!

[moosemouse]

  I think you neen to be logged in to see the drawing

[J M Fahey]

Hi Moose.
I assume you are using a plain vanilla digital multimeter.
Check that there's 0 ohms between P6 and actual chassis (if you read around 0.5 ohms it's normal, it's the internal resistance of the wires and contacts)
Black probe to chassis or P6 , red probe measures, 200VAC scale, you should read the AC voltages indicated on schematic, +/-2V. (every value has some tolerance).
Now 200VDC scale.
TP3 and TP5 look normal.
TP4 and TP6 do not, at all. Besides, they should show a negative sign.
Recheck that black probe sits on chassis, then measure the actual pad where C30 is soldered to the board, and the anode (negative) leg of CR5 and CR7. They are *supposed* to show the same -24V, but their solder might be cracked, or the track that joins them, or the diodes might be open.
With power off, check using the diode test function that the diodes shoe a reading of about 0.65 (or 065 or 65) ; check all four to compare, because CR8 and CR10 look good.
Good luck.

[moosemouse]

JM Fahey'

Thank you for all of your suggestions, while following your instructions I found a solder point that had "come up" under one of the big caps C 30 I think.  After that every test point tested in range with the exception of tp 1 & 2 I guess my meter doesn't like them for some reason.  The little amp is up and running.  I hope to do this again, I have several questions including the path that is taken through the power supply in my attached photo.  And how to test a TDA 2050V see attached drawing.

[J M Fahey]

IC chips, being *very* complex inside, usually can only be checked "in circuit", where they either work and stay or blow/short and go to the junk bag.