What does it take to 'blow a head' ??

[pizzahotline]

Hi there,

I've heard horror stories of cutting heads being 'blown' by people sending too loud a signal or signals with an abundance of high frequencies to them. Does anyone have any personal stories or close calls? perhaps someone can tell me how many Amps to takes to 'blow a head' ? I take lots of precautions and am always very careful but I'd love to know more about this so I can avoid it at all costs. I'm using a Neumann VMS80

[flozki]

On a vms80 I guess you have an sal74b rack and sx74 head.

Make sure your rack is in top condition.tested and recapped.the amps like to oscillate at 1-2Mhz and your drive coils don’t like..
Also cables connectors like some attention and tweaks...
If not send the two amp channels to someone who knows and who can test it seriously..
Also power supply likes a refresh
That helps a lot.

The sx74 can blow anytime.even if not used for long time or even with reasonable levels it can happen.if head was serviced recently chance us low. The reason is glue aging (the last heads left 94/95 the neumann company and epoxy has a certain shrink per year..so just a matter of time. Additionally neumann used some corrosive paint to mark polarity on fab heads..that is also a great and sure source of planned obsolescence..
So it will happen one time.i Hope you have s 2nd Head or the patience of a few weeks downtime...

So mostly not horrible stories just some almost silent failure.

Happy cutting

[jjgolden]

Thanks for the info flo.

I also want to add:
Never be afraid to say no to a client if the program material is risky.
It's just not worth it, especially when you don't have a backup head.
Having said that, I would love to be able to find where that threshold of destruction is... but can't afford to find it!

[pizzahotline]

Thanks for the replies and the help with this question. It's incredibly interesting to hear your words on the glue and paint of the head flozki. Sad to hear that it's a potential matter of time until it blows.

Do you have a personal limit to how many amps you see on the Amp meter while cutting? I keep away from 1amp, I use lots of de-essing to combat the highs. It's nice to hear jjgolden's advice on never being worried about saying no to a client.

[dmills]

You kind of have to separate the "Head blows because of corroded coil connection/Glue/Mechanical failure/Electronics issue" from "Head blows because of overload", the first being something which apart from a serious maintenance regime you can do little about, the second being something that you have rather more control over.

Setting the circuit breaker to trip at say 180c instead of 200c feels like the sort of thing that would have value, as does testing that the breaker actually works correctly (Over 300 - 500 kelvin, copper is more or less linear in resistance, so calculating a resistor load to test the circuit breaker is not hard). Note that in the SAL74b, the detector bridge bias is produced by the power amps, so that needs to be tested and calibrated as a system.