Need help safely powering old signal generator

[antonis]

Hiring a tech to do it right may be the best money you ever spent, and cheaper than a funeral.

Not in case of public expenditure funeral..

Seriously Patric, mains wiring looks pretty simple job - with many (potentially lethal) hidden details..
(especially in case of old/rusty/(possibly) internaly violated/modified instrument..)

[patrick398]

Was away from my computer for a while so only just seen all these replies, thanks so much!
Part of the reason i went and picked this up was because i think it's beautiful, i really love the look of old gear so for me it was a bonus if i could get it working or if it was actually useful for my needs. I did think i could potentially use it for pedal building, as a signal generator but from what some of you have said it's not really much use to me.
But that's fine because i did also have thoughts of gutting it and turn it into something else. My first thought was to house a few effects in there but i am starting to really like the idea of making a preamp out of it. Once i've got a few other projects cleared out the way i'll start looking into that properly.
It ways a ton as it is so hopefully i can shed a few pounds from it and potentially take it out on the road.
It is a 78 pro reverb you see in the background, i have beautiful fender clean tones, and a completely broken spine.
Any preamp schems specifically you recommend i take a look? Or other ideas for what to do with it?
Thanks, you guys are d'best!

[thermionix]

It is a 78 pro reverb you see in the background, i have beautiful fender clean tones, and a completely broken spine.

Not completely broken, you need a Twin for that.  My amp is basically a Pro Reverb, but only the "party" channel, and a mid control.

[patrick398]

  Which is the party channel?

[thermionix]

Reverb/trem.

[patrick398]

Ah makes sense, thought maybe there was some secret party channel going out behind my back...story of my life

[Rixen]

Mains input filter capacitors C20 & C21 should be replaced with modern Y class capacitors. Alternatively, if you decide to fit an IEC inlet, you can buy ones with inbuilt filters, fuse and switch (maybe not all three) but they do take up a bit of space.

https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/

I have an Advance AF signal generator, which I still use sometimes in my day job. It originally had an inlet connector like yours, but I replaced it with a cable gland and flexible cord.

[EBK]

Just want to throw this question in here: 
How do you know what type (rating, fast/slow) of fuse you need?

[Rob Strand]

This is probably where it was built.

The closer I look at it the more I'd have to agree.

[Rob Strand]

ust want to throw this question in here:
How do you know what type (rating, fast/slow) of fuse you need?

Working out fuses properly is a real pain in the butt. 
There was a thread on it some time back and RG and I too'd and fro'ed a few things.

The manual states 25W.     That usually amounts to a 250mA to 500mA.

[Edit:
Actually this is fairly good account of the issues fusing small transformers.   
Small being less than say 20VA to 30VA, where the winding resistance becomes significant and limits the fault current:
http://sound.whsites.net/articles/fusing.htm
]

[EBK]

Very nice article at that link.  Frightening though....  The short version (no pun intended) appears to be that you specify a fuse by guessing (but not haphazardly) and hope for the best.   

[DIY Bass]

Very nice article at that link.  Frightening though....  The short version (no pun intended) appears to be that you specify a fuse by guessing (but not haphazardly) and hope for the best.   

... and if it keeps blowing try a slo-blo

[EBK]

[Rob Strand]

Frightening though....  The short version (no pun intended) appears to be that you specify a fuse by guessing (but not haphazardly) and hope for the best

The article does say that but it's not as bad as he phrases it.   You need to choose a fuse large enough so it won't blow under normal use.    The first guess is the operating current but it is likely that will blow at some point.   So then he suggests a safety factor of 2 to stop that happening.  So you have narrowed down the choice of the fuse current to within a factor of two.

There are well known ways to come-up with a better estimate but they are quite complicated and require a lot of measurements and also require data about the specific fuse you intend to use.   Sometimes this info isn't available.  Going down this path can be a bit painful and a job for the "professional".

The second important point is the relative size of the fault current compared to the operating current.    If the fault current is much larger than the operating current then even of your estimate of the minimum fuse current was off the fuse is likely blow under most faults.   However, if the transformer is small the fault current isn't much different to the running current.    This is where is becomes a problem.   It also means you need to have good estimate of the minimum fuse size and a good estimate of the fault current.  If those are too close together the choice of a suitable is very narrow and if you use that fuse it might blow in use and/or not blow when you have a fault.    You hit this problem when a transformer is below about 20VA.      The modern solution for small stuff is to give up using a fuse.   You use thermal thermal fuses instead which measure the transformer temperature, this is actually a better indication of a fault and of a dangerous situation.  These type of fuses trip and reset or fail permanently.

[Rob Strand]

@ EBK
Some of my friends were quite fond of the 250A model and they did manage to cook some wiring (only 12V though).

[thermionix]

Good luck finding a 7.62x51mm fuse holder these days.

(The multi-dispensers are still quite common though)

[patrick398]

If i can get another project finished up i might well end up gutting this thing tonight. But first, what's going to fry me? Big capacitors i'm guessing. Discharge them first with a screwdriver or something right? What about the transformer? I don't want to go digging around in there with wire cutters unless i know it's not going to roast my chestnuts 

[Rob Strand]

. But first, what's going to fry me? Big capacitors i'm guessing. Discharge them first with a screwdriver or something right? What about the transformer? I don't want to go digging around in there with wire cutters unless i know it's not going to roast my chestnuts

No offense meant but you shouldn't take on such a job without at least being able to identify those issues yourself.  A few posts will put you on the right track but if you can't work out one of the 20 items not covered you can get into serious trouble.   

Here's some areas:

- Mains wiring
http://sound.whsites.net/earthing.htm

Note the cable must be correctly rated.
There are correctly rated fuse holders.   Modern ones are designed to reduce risk of shock.

- High voltage on the secondary side.  AC, DC.
Yes discharge the caps with a 100R to 1k 1W resistor.   But you have to wire it up so you can't slip and touch the wires.  Don't forget one.

- Check your work before you turn it on.

- Be aware of things that will catch fire if you have a fault.  No paper.

- Work practices:
  - Remove the plug completely from the wall when working on it. 
     Don't just turn it off at the switch. And definitely don't rely on the equipment switch.
  - No children or animals in the vicinity.   Close the doors.
  - Tell someone what you are doing
  - No drinks or liquids on the same table.
  - Don't leave it plugged in if you aren't there.
  - If you have unfinished work don't leave it in a state where it is dangerous for someone else to plug in.

[DIY Bass]

If you are just talking about gutting it, then you should be right as long as it hasn't been on recently.  Capacitors do hold a charge, and power supply caps can have charge for a while.  They are probably OK but it is always good to discharge then as a matter of course just in case. If all you are doing is disassembly then you will be fine with that.

[patrick398]

Yeah just gutting it, i won't be building anything with high voltages until i feel comfortable and safe about it. More reading to be done for sure. I just wanted to make sure there wasn't anything specific i should bare in mind when taking it apart, Thanks