Hello guys, I got a tube preamp Tube-Tech MP-1A with some issues I really don't get it...
It's belong to a friend that, as I can see, it felt to the ground and all the switches are twisted... ok, so far so good.
The strange part of this repair, and here is where I need help, is about the Power Transformer...
It has a 2x 115V in the Primary, and it's supposed to have (written on the PCB) 270VAC (I think is too high, I'll explain it later) and 35VAC...
Well, I measured 359VAC for the 1st Secondary, and 42VAC for the 2nd Secondary...
My friend told me that he can handle burning tubes anymore, and it's clear why right?... the 12AU7 are getting 445VDC on both plates.... nothing strange if you got 359VAC before rectifying it... and the connection to it is direct through the OT.
The PSU for the 12AU7 is not regulated, and the 445V are going directly to the Output Transformer CT and appears in both plates (pin 1 and 6)... the tube with last nothing as long as the maximum rate for it is around 300V. The other voltages that is supposed to have (+240VDC, +50VDC and +33VDC) are pretty ok because there's some regulation for it, although I think that it is in effort...
Measured everything in the PSU, replace caps, everything done... but the voltage stills the same... So I thought that the problem is the PT itself... missed a few turns from the primary winding and should rise the the voltage... so I, in any case, send the trafo for re-winding... ok, the trafo's back, put it on again... same damn voltages.
The preamp was notoriously repaired before my intervention, because the switches are bended (it felt remember?) but they are not the same kind among them (Shaft different, not the same switch in both channels). There's also a black spot where a burnt resistance for the pilot lamp was set, the bulb is 24V, and should be 36V, and there's some wrong fuses (4A instead os 100mA) and so on...
Well ok, this rises me a flag for the connections for the secondary... maybe the previous tech guy did it wrong, or made some mistake... so I dig in the secondary! I attached a schematic of the trafo's secondary I measured, as long as the previous connection.
So the HV Secondary was in series, and wow, it's nice, let me put it in parallel!......... I got now 162VAC, rectified is barely 225VDC... and all the regulation circuit is now with the wrong voltages, because I have less voltage than I would have expected... in series bammmm +445VDC, in parallel +225VDC
What is happening? Am I missing something?
Hellllpppppppp please
Cheers
It's belong to a friend that, as I can see, it felt to the ground and all the switches are twisted... ok, so far so good.
The strange part of this repair, and here is where I need help, is about the Power Transformer...
It has a 2x 115V in the Primary, and it's supposed to have (written on the PCB) 270VAC (I think is too high, I'll explain it later) and 35VAC...
Well, I measured 359VAC for the 1st Secondary, and 42VAC for the 2nd Secondary...
My friend told me that he can handle burning tubes anymore, and it's clear why right?... the 12AU7 are getting 445VDC on both plates.... nothing strange if you got 359VAC before rectifying it... and the connection to it is direct through the OT.
The PSU for the 12AU7 is not regulated, and the 445V are going directly to the Output Transformer CT and appears in both plates (pin 1 and 6)... the tube with last nothing as long as the maximum rate for it is around 300V. The other voltages that is supposed to have (+240VDC, +50VDC and +33VDC) are pretty ok because there's some regulation for it, although I think that it is in effort...
Measured everything in the PSU, replace caps, everything done... but the voltage stills the same... So I thought that the problem is the PT itself... missed a few turns from the primary winding and should rise the the voltage... so I, in any case, send the trafo for re-winding... ok, the trafo's back, put it on again... same damn voltages.
The preamp was notoriously repaired before my intervention, because the switches are bended (it felt remember?) but they are not the same kind among them (Shaft different, not the same switch in both channels). There's also a black spot where a burnt resistance for the pilot lamp was set, the bulb is 24V, and should be 36V, and there's some wrong fuses (4A instead os 100mA) and so on...
Well ok, this rises me a flag for the connections for the secondary... maybe the previous tech guy did it wrong, or made some mistake... so I dig in the secondary! I attached a schematic of the trafo's secondary I measured, as long as the previous connection.
So the HV Secondary was in series, and wow, it's nice, let me put it in parallel!......... I got now 162VAC, rectified is barely 225VDC... and all the regulation circuit is now with the wrong voltages, because I have less voltage than I would have expected... in series bammmm +445VDC, in parallel +225VDC
What is happening? Am I missing something?
Hellllpppppppp please
Cheers
