Tandberg 74b noise

[LeeYoo]

Losts of other noise, because of an open input, so hard to tell if the hum is gone.
One way out is to use a resistor (1Kohm or so) in series with a cap (C106) from base to ground. Positive of the cap to base. Ground has to be CLOSE to Q1, like ground of R107. Wire has to be short..
Leo..

[midnightmarauder]

Originally Posted by LeeYoo

Losts of other noise, because of an open input, so hard to tell if the hum is gone.
One way out is to use a resistor (1Kohm or so) in series with a cap (C106) from base to ground. Positive of the cap to base. Ground has to be CLOSE to Q1, like ground of R107. Wire has to be short..
Leo..

And the result is: no hum. ch1 and ch2 identical (same amount of noise) on the output.


Can the hum come from the bias circuit? I'm just guessing..

[midnightmarauder]

I tried swaping the out/in's on the pb/rec head. ch1 with ch2. hum still present on the same channel...

[LeeYoo]

Wow, this is getting complicated. If the hum is fairly low, it may have been in there from new.
If there was no hum with the resistor/cap, that means that the pre-amp is fine.
Must be the wiring, or switch A, pin 10/11, or the magnetic field from the mains transformer.
The bias circuit has nothing to do with playback. As you can see, in playback, the negative (lower) wire of the PB head is grounded via switch A, contact 10/11. Pin 29/30 are open.
Just some correction C's are added for different speeds with the EQ switch. They can't introduce hum.
Leo..

[midnightmarauder]

Originally Posted by LeeYoo

Wow, this is getting complicated. If the hum is fairly low, it may have been in there from new.
If there was no hum with the resistor/cap, that means that the pre-amp is fine.
Must be the wiring, or switch A, pin 10/11, or the magnetic field from the mains transformer.
The bias circuit has nothing to do with playback. As you can see, in playback, the negative (lower) wire of the PB head is grounded via switch A, contact 10/11. Pin 29/30 are open.
Just some correction C's are added for different speeds with the EQ switch. They can't introduce hum.
Leo..

Tried some more.. it's almost insane to spend so much time on this thing but when I first start its hard to settle with no improvement.

- Cleaned the head-mute switch and ohmed the leads from the head. No improvement.
- Replaced the coax going from the head to C106. I even tried running the ground path(this was the shield of the coax) with a seperate lead, leaving grounding the shield on one end. No improvement.

So now I'm looking at the board where the "lower" signal from the head enters the bias caps (c120B and c120). And what I find is that c120B is not there, not c170b either(channel 2). On the board scematic it's next to c120, but it's not on my board.. Anyway, is there no chance the hum can enter here somewhere around the bias curciut? I don't think I left anything else out now for testing. How can it be transformer hum(unproper shielding), when it's not picked up in the head, wiring or switch, and the pb amp is working fine....?

[LeeYoo]

I admire your perseverance.
Don't worry about the 8pf C's. They might have used a trimmer (C120) with a bigger range, so those C's were not needed anymore.
If you think, the bias circuit could be the problem, just pull V3 out.
I don't think it will be any different.
I am running out of ideas soon.
Can you solder the head coax to R107 ground, and to and to C106, floating in mid air.
The shielded wire should not be grouded anywhere else.
Pertinax switches do go bad with age and moisture.
Leo..

[midnightmarauder]

Originally Posted by LeeYoo

I admire your perseverance.
Don't worry about the 8pf C's. They might have used a trimmer (C120) with a bigger range, so those C's were not needed anymore.
If you think, the bias circuit could be the problem, just pull V3 out.
I don't think it will be any different.
I am running out of ideas soon.
Can you solder the head coax to R107 ground, and to and to C106, floating in mid air.
The shielded wire should not be grouded anywhere else.
Pertinax switches do go bad with age and moisture.
Leo..

Thanks for explaining about the 8pF caps, I should have seen that I did try pulling out V3 earlier, and it didn't help.

What happened when I lifted one side of the shield earlier, was that the head pcb lost connection to ground. So I tried connecting a separate lead for ground instead. Which didn't help.
I did check the switches with dmm for shorts and bad connections, but they seemed just fine.

Time to give this up and live with the hum maybe.. Maybe I'll run into a better working unit one day Anyway thanks for all help, I really appreciate it. At least I learned some more about electronics along the way

[ruffrecords]

Did you try swapping the tubes?

Cheers

Ian

[LeeYoo]

Hi Ian. Yes, he did that in post #12.

I did some more browsing, and found this:
Tandberg Hum - UK Vintage Radio Repair and Restoration Discussion Forum

Problems with the shielding of the PB head.
It must be something like that, because the pre-amp is quiet with the head replaced by a resistor.
Leo..

[midnightmarauder]

Thanks for that link Leo. It certainly seems the same thing is happening with that 3000X. I'll look more into this when I have the time. I'll post back if I find out anything. Thanks for all the help!