Whose the Resident V76/80 expert?
- Thread starter mkiijam
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Ericbazaar
Well-known member
the first tube stage and the input transformer are assembled on a plate, this plate is decoupled with three rubber feets, they're almost dried out and broken. they need to change, but you need to disassemble the hole V76. its not easy...
best
Andreas
best
Andreas
It came in "working", but for a high pitched whine when using phantom power. The unit is about 16dB lower than another one in the same rack.
I did see that the gain switch for sure looks like it was touching some of its terminals against the input transformer. I can see what looks like it had scratched through the paint/clear coat. But I'm not sure if all the way through to metal. I have cleared that possible short and it is still low.
I did see that the gain switch for sure looks like it was touching some of its terminals against the input transformer. I can see what looks like it had scratched through the paint/clear coat. But I'm not sure if all the way through to metal. I have cleared that possible short and it is still low.
Michael Tibes
Well-known member
Where was the picture with a V72 or V76 input transformer arcing with 48 V applied? The isolation on the tiny wires in the transformer had already been very thin and didn't get any thicker over the decades, so 48V applied could actually kill the transformer on a bad day. And before you dig too deep, the silver foil in the input transformer supposedly contains dangerous materials, maybe better don't touch it. Is the frequency range of the input ok?
Fixing the rubbers is definitely not a fun job. about 25 years ago I was still able to buy these standoffs in a local shop in Hamburg. Maybe they are still available somewhere? Later V76 had one of these rubbers replaced with a solid standoff, probably braking the isolation between the mounting plate plate and the chassis. I guess they didn't like having to fix these even back then. The rubber gets hard, and finally breaks, it probably resembles bakelite then.
Fixing the rubbers is definitely not a fun job. about 25 years ago I was still able to buy these standoffs in a local shop in Hamburg. Maybe they are still available somewhere? Later V76 had one of these rubbers replaced with a solid standoff, probably braking the isolation between the mounting plate plate and the chassis. I guess they didn't like having to fix these even back then. The rubber gets hard, and finally breaks, it probably resembles bakelite then.
Michael Tibes
Well-known member
Can anyone rewind the input transformers?
Ericbazaar
Well-known member
try to ask [email protected]
Michael Tibes
Well-known member
Thank you, who is that person?
This is the 'M3 rubber standoff' used in the V76, the thread needs to be cut to fit.
Michael Tibes
Well-known member
The picture you took is the gain switch, what did you want to show?
Ericbazaar
Well-known member
Its a Guy with lot of experience in repairing Transformers and Chokes, he repaired V41 Chokes and V241 Power Transformers for me.
He told me he can repair the V72 or V76 Input Transformers in "Disc Wounding" Technique.
If its possible please ask him in german
best
Andreas
Ericbazaar
Well-known member
You can isolate the side of the transformer shielding with a thin part of pertinax.
Best to use a ramping circuit for 48v on old telefunkens, even then the coating on the wires they used in the v series do get brittle and break apart causing issues from time to time. The grommets break down and even with all this they hold up surprisingly well for their age. Have seen two bad input transformers due to phantom shocks, and various wire coatings broken in repairs.
Michael Tibes
Well-known member
I believe it shouldn't have scratched anything relevant. The transformer is shielded in 3-layers of mu-metal, unless you open that you might need a gun to scratch the actual transformer ;-)
I'm not completely sure what you're worrying about? The components shown in the picture are the input transformer housing and the gain switch. There is no power switch in the whole unit, you just plug it in or have an external switch to turn it on. If I recall correctly they used two of the rubber standoffs with one of the threads cut of, screwed to the gain switch to prevent the input transformer from crashing into the gain switch in case the unit gets bumped around. When the rubber standoffs which hold the input transformer mounting plate break, the input transformer is just loose and easily bumps into the gain switch. I didn't experience any damage from that yet, so I keep my fingers crossed. In my opinion the two first steps could be to determine whether the transformer is fine. If / when it is, the mounting plate should be secured. You might also visually inspect the gain switch. If there was any damage, it should be visible as cracks or broken off bits. Does this help?
Michael Tibes
Well-known member
Regarding 48V: After understanding the problem I've only used separate 48V supplies with coupling capacitors in order to keep the transformers safe.
