Did you fit capacitors?
The bridge rectifier feeds a capacitor input filter, i.e. the smoothing capacitor.
The datasheet will specify what capacitors should be fitted to the input and output of the regulator.
Sometimes the regulator input filter can be omitted, if the smoothing capacitor gives sufficient decoupling. The datasheet may give details.
The regulator does need a minimum output current. Does the datasheet specify?
Try 10mA as an absolute minimum, you may get away with 5mA, but you need to test and measure to confirm this.
12V and 10mA gives a necessary load resistor of <=1k2
The bridge rectifier feeds a capacitor input filter, i.e. the smoothing capacitor.
The datasheet will specify what capacitors should be fitted to the input and output of the regulator.
Sometimes the regulator input filter can be omitted, if the smoothing capacitor gives sufficient decoupling. The datasheet may give details.
The regulator does need a minimum output current. Does the datasheet specify?
Try 10mA as an absolute minimum, you may get away with 5mA, but you need to test and measure to confirm this.
12V and 10mA gives a necessary load resistor of <=1k2
It would help to see the circuit...
39 volt AC from the tranny (a 15-0-5) suggests its a small cored device with consequent "poor" (its a function of the core size) regulation, hence the almost 20-0-20 you are reading. So your 51 volts DC sounds reasonable. Assuming you have two reservoir caps in series with the centre tap going to the tranny centre tap then you should read approx. 25vdc across each cap... and I see you do.
So 25 volts DC applied to a 7812 should be fine. Are you not seeing 12 volts coming out ?
39 volt AC from the tranny (a 15-0-5) suggests its a small cored device with consequent "poor" (its a function of the core size) regulation, hence the almost 20-0-20 you are reading. So your 51 volts DC sounds reasonable. Assuming you have two reservoir caps in series with the centre tap going to the tranny centre tap then you should read approx. 25vdc across each cap... and I see you do.
So 25 volts DC applied to a 7812 should be fine. Are you not seeing 12 volts coming out ?
I am getting 12v steady from the 7812 and over 14v from the 7912. The component is a Balanced to Unbalanced converter. I can see that one channel drops LED array (L channel) and tapping the case restored the level until it dropped again, then nothing will restored the level until I unplugged the mains lead and re-inserted it. However even that is not now keeping the left channel stable. I have to say that I would not have thought that the problem was voltage related had I not got a temporary fix by tapping the case in the area of the power inlet and then unplugging the mains lead.
I thought I would check the bridge as I thought it might have been faulty. The readings I got are :-
(note : this is one of those circular small bridges and if viewed with +dc as leg 1 (top right) I have gone anti-clockwise to ac - leg 2 next to it and diagonal to leg 3 ac and then left to leg 4 - diode setting readings are in brackets.
Red probe on + 2(1) 3(1) 4(1)
Red probe on 2 3 (39) 4 (1) + (665)
Red probe on 3 2(39) 4 (1) + (665)
Red probe on 4 + (1648) 2(665) 3(662)
I thought I would check the bridge as I thought it might have been faulty. The readings I got are :-
(note : this is one of those circular small bridges and if viewed with +dc as leg 1 (top right) I have gone anti-clockwise to ac - leg 2 next to it and diagonal to leg 3 ac and then left to leg 4 - diode setting readings are in brackets.
Red probe on + 2(1) 3(1) 4(1)
Red probe on 2 3 (39) 4 (1) + (665)
Red probe on 3 2(39) 4 (1) + (665)
Red probe on 4 + (1648) 2(665) 3(662)
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If anything changes when its tapped then that points to an intermittent connection somewhere.
I doubt there is a problem with the bridge, also you can't really test it in circuit. I'm sure its fine though.
Go off what you know... the PLUS 12 volts seems to be OK from the 7812 but not the MINUS 12 volts from the 7912. Now the 79 series regs do need a minimum current draw as Andrew pointed out so at least try connecting a 1k (or bit lower) across the MINUS 12 volt rail and ground and see if the voltage is correct at approx. -12vdc
Anything tapable though and that needs looking at first.
I doubt there is a problem with the bridge, also you can't really test it in circuit. I'm sure its fine though.
Go off what you know... the PLUS 12 volts seems to be OK from the 7812 but not the MINUS 12 volts from the 7912. Now the 79 series regs do need a minimum current draw as Andrew pointed out so at least try connecting a 1k (or bit lower) across the MINUS 12 volt rail and ground and see if the voltage is correct at approx. -12vdc
Anything tapable though and that needs looking at first.
7912 vout too high, meter registring AC, tapping changes things ...
I smell RF oscillations (after all those regs contain high gain amps and FB loops)
last time I had those three things w/regs as you describe it was oscillation.
Try this: solder 100nF ceramic cap directly between output pin and reference pin as close as possible to the regs (both).
note: pinout of 79xx differs from that of 78xx !
I smell RF oscillations (after all those regs contain high gain amps and FB loops)
last time I had those three things w/regs as you describe it was oscillation.
Try this: solder 100nF ceramic cap directly between output pin and reference pin as close as possible to the regs (both).
note: pinout of 79xx differs from that of 78xx !
Update:
I suspected that the bridge rectifier was faulty and replaced it. This corrected the voltage on the 7912 to 12v. However the channel imbalance was still there. Further fiddling led me to a multi pin "wrapped" unit (wrapped in heat shrink shiny plastic type stuff) which I suspect houses maybe an SMD op-amp or discrete transistors for the audio. Touching this restored the channel balance. it may be a bad solder joint on one of the pins (which I can't detect under magnification) or something else inside the shrink wrap. At the moment a little judicious pressure on the board has done the trick.
I suspected that the bridge rectifier was faulty and replaced it. This corrected the voltage on the 7912 to 12v. However the channel imbalance was still there. Further fiddling led me to a multi pin "wrapped" unit (wrapped in heat shrink shiny plastic type stuff) which I suspect houses maybe an SMD op-amp or discrete transistors for the audio. Touching this restored the channel balance. it may be a bad solder joint on one of the pins (which I can't detect under magnification) or something else inside the shrink wrap. At the moment a little judicious pressure on the board has done the trick.
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