Magnecord Bias Oscillator voltage

[Levente]

Originally Posted by ms660

I don't know what the resistance across the head windings should be in your machine with the head disconnected, but usually if they are not open circuit or gone high resistance then the head will be ok from an electrical point of view.

Lawrence.

Thank you... I did order another set of 12BH7 as I dont have these. I did try and swapped with a 12au7 with no better results, but maybe the 12au7 is not suitable here.. the preamp section at the "recorder" socket is giving a superb signal quality. The problem seems to be somewhere at the transport and the bias circuit. I will update here when the new 12bh7 is here. I really hope thats the problem and not something more serious.

[Levente]

..hmm one more thing made me thinking. The wiring coming from the playback head. I have an XLR however, I only see two connections inside that male XLR coming from the playback head. Pin2 is the signal ( a copper wire) and pin1 is the shield. pin3 has nothing.

The XLR socket p106 on the wiring diagram attached seems to be different. I dont know to be honest how to read those circles at the pin 2...i think something might be wrong here...

[ms660]

The dotted circles represent the cables screen (the outer shield braid)

Lawrence.

[Levente]

Originally Posted by ms660

The dotted circles represent the cables screen (the outer shield braid)

Lawrence.

Thanks Lawrence... then the P106 should have 3 wires in total including the screen but I only have two coming out of the transport the screen, and likely the signal..thats odd....

[ms660]

Originally Posted by Levente

Thanks Lawrence... then the P106 should have 3 wires in total including the screen but I only have two coming out of the transport the screen, and likely the signal..thats odd....

If the connections to the plug are original it means that the circuit in the P63A schematic is different to your PT75A.

I can't find any reference to a PT75A on the internet except for what's in this thread.

Lawrence.

[Levente]

Thanks Lawrence, appreciate that you did try and looked this up. Yes, it seems to be a rare ish model, or not so common. So basically from the playback head there is an xlr coming out with two pins. What I had is pin 1 and pin 2 connected to the playback head. (no shield). This XLR cable is not original, someone changed this. Maybe I am completely wrong but the pin1 is wrong..this should may be pin3 instead...as pin 1 "normally" the shield...maybe someone did this wrong, I don't know... i am just trying possibilities here.