How do you "break" a TVC?

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shib

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How do you "break" a TVC?
« on: 20 Jun 2007, 11:26 pm »
Sam Tellig stated in a recent Stereophile that he connected the Prometheus TVC backwards, inputs to the outputs, outputs to the inputs.  What happened?  Did he oversaturate the core and ruin the laminates?  Is that possible with the TX102 units? 

On another note, it is good to hear that John is making progress moving forward. 

John Chapman

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Re: How do you "break" a TVC?
« Reply #1 on: 21 Jun 2007, 08:49 pm »
Hello!

The short answer is that it is very hard to actually break a TVC - TX102 or I would expect most other units as well. I would think the Promethius would be fine and I can't imagine it would break just from that.  I would not think that the CD player (or whatever source) would put out enough current to actually cause a wire to pop like a fuse.  Connected backwards the unit could act like a large ratio step-up transformer and if there were no load on the other side it could develop a high voltage on the primary - just guessing at that one but still I would not think it high enough to pop the primary to secondary wire insulatio - although in this particular scenario it could I supose be the culprit.

I only once had a break-in issue and it was from a VERY aggressive cable cooker - It put out a HUGE signal and it could source gobs of current. Maybe at least 100x the current a typical source would be able to pump out..... Great for cable burn-in I am sure! In that case it popped a couple switching relays (actually turned them into little black bits of ex-electronics) but even in that case the TVC was fine. 

The only other thing that could affect a TVC is prolonged exposure to DC voltages - which can magnetize the core. Running through a de-magnization cycle can correct it. I have never out of many 100's of TVC's had to do this but it is there in theory so I figured it worth a mention.

Never had one die like that and since they are all quite similar in physical respects (ie approx wire sizes, etc) I would be surprised to hear of another brand popping.

Thanks!

John




1000a

Re: How do you "break" a TVC?
« Reply #2 on: 21 Jun 2007, 11:04 pm »
how many turns can one expect to recieve trouble free on a TVC volume controll itself, the designer of my passive preamp tells me my Alps Blue is usually trouble free to 100,000+ turns easily.

I would imagine after studying the images of some of these controlls with all the wires they might be considerably less Robust?  I really do not know but I have already read of a couple of messed up TVC volume controlls.

If they break how do you fix one can a regular audio shop do it?  Or once a warrenty is gone your out of luck?

any ideas knowledge or feedback would be very helpful, thanks in advance.

samplesj

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Re: How do you "break" a TVC?
« Reply #3 on: 22 Jun 2007, 12:12 am »
how many turns can one expect to recieve trouble free on a TVC volume controll itself, the designer of my passive preamp tells me my Alps Blue is usually trouble free to 100,000+ turns easily.
...
If they break how do you fix one can a regular audio shop do it?  Or once a warrenty is gone your out of luck?

A tvc is just a pair of transformers with LOTS of taps (at different attenuation).

Your volume switching is done by selecting one of those taps with just a normal multi-position switch.

Any wear from the turns is just to the switch itself and not the txs.  You could also replace the switch if needed.  However realize that a 23 position switch is 46 time consuming wire-switch solder jobs.

Audiovista

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Re: How do you "break" a TVC?
« Reply #4 on: 22 Jun 2007, 12:17 am »
how many turns can one expect to recieve trouble free on a TVC volume controll itself, the designer of my passive preamp tells me my Alps Blue is usually trouble free to 100,000+ turns easily.


ALPS lists 15,000 cycles in their datasheets. Bourns has some pots with 25,000 cycles and some MIL grade audio pots list 30k rotations. Not sure how they define useful life, but I'll suspect that there will be some scratching well before end of life.

Mechanical switches probably last less.

John Chapman

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Re: How do you "break" a TVC?
« Reply #5 on: 22 Jun 2007, 01:42 am »
Hello!

2 issues mixed in - Aging and death by execution. The original question asked about how much pain a TVC can take before it gives up and dies. As far as aging goes samplesj summed it up nicely - the TVC itself does not wear at all with time - no moving parts at all. The switch used to select output taps will have a life of some kind. The shallco I used much of the time for that has a life of a minimum of 50,000 full ccw to cw rotation cycles. The high quality sealed relays used in the TAP boards  have a min life of 1 billion cycles - not million but billion...... That's a lot of volume adjustments...... I use that same part on all TAP boards - TVC boards, Resistor Boards, Source select boards, etc....

scottw

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Re: How do you "break" a TVC?
« Reply #6 on: 5 Jul 2007, 09:44 pm »
Quote
The only other thing that could affect a TVC is prolonged exposure to DC voltages - which can magnetize the core.

Hi John,

I recall reading this years ago and always wondered at what level does it become a concern. Does a little DC offset (mV's from opamp output) cause a problem, or how about turn on/turn off thumps than can put out a momentary couple volts?



Scott

John Chapman

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Re: How do you "break" a TVC?
« Reply #7 on: 5 Jul 2007, 10:35 pm »
Hello!

I have never got a firm "this is o.k. and this is not o.k." kinda  answer from transformer guys. Certainly we have a lot of history of TX102's running with a random selection of sources and there are many with op amp output stages running a mv or two offset I am sure. The inrush issue might be there but would only be on source power up and is very short - the AC music signal I would think would tend to cancel that out after a short time of listenning.

In theory if you have phase invert you'd run inverted every so often (to run the dc offset the other way though the transformer primary) and this would tend to cancel it as well but I have never done this - too many other things to think about and I am sure most users would be the same!

Sorry to only have my guess based on experience and not a solid answer for you. Will post one if I get a nice clear description from someone.

Thanks!

John

scottw

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Re: How do you "break" a TVC?
« Reply #8 on: 6 Jul 2007, 12:22 am »
Thanks for the reply John.

Your "guess" is good enough for me. This was just one of those curiosities that float around in the back of your mind.


Scott