BE-5 - summary of recommended changes

[Robert Groh]

Bob,

Here are my recommended changes/additions for your BE-5. I am attaching, for you, a good pdf of the schematic (obtained from Jim Bromley). For the rest of my recommendations, I am adding a list of URLs to this email for you to get details on possible/suggested parts.  By the way, I am assuming you are not trying to restore a 'collectable' item but just want something to use which looks like the good 'ol BE-5.

#1:  recommended change to silicon diodes to replace selenium rectifiers
There are a lot of possibilities here. I've chosen some Fairchild silicon diodes (PN FFPF3060S) rated at 30A, 600V, 180A surge current, TO-220 style mounting package.  Of course, gross overkill but why not?
Links to the Mouser page and the data sheet page:
<http://www.mouser.com/Search/Refine.aspx?N=254459&Keyword=FFPF30U60S >
<http://www.fairchildsemi.com/ds/FF/FFPF30U60S.pdf >

Price: about $ 0.91 each.  Get 4 of them.

#2:   mounting
I am not sure but it looks like the back side of the package (where you would butt it up to a heatsink) is NOT electrically connected to the diode. I hoped for this but can't really tell from the data sheet (?) - still looking. 

IF the physical/thermal mounting is electrically isolated from the diode parts, mounting is simple - just bolt it to a suitable heatsink (e.g. the enclosure itself or just a hunk of aluminum bolted to the case) and you are ready to go. 

IF not isolated, you will need to buy a suitable mounting kit with a mica washer or the like.

#3:   Thermal
Based on 1 V drop in the diode and a continuous current of 15A, we will have 15W dissipation in the diode. Divide by 2 due to duty cycle - so say 10W.  Temperature rise over the heat sink temperature will be about 10 degrees C.  Just keep a good heat sink and you it should run cool and comfortable.

#4:   Wiring up
Somehow figure a way to connect wires that went to the selenium rectifiers to the new diodes.

#5:   Existing filter caps.
Probably should plan on replacing these.  Kind of old.  You can try them but don't be surprised if they are a bit (lot??) leaky and run hot.

#6:   Input ac wiring
I definitely recommend replacing 120 VAC line cord with new 3-wire cable.  Also you need to rewire the input switch and fuse so they are both in the AC 'hot' (black) line.  The AC neutral (white) should NOT be broken but should go directly to the transformer. Physically connect the line ground conductor to a separate (i.e. not shared with any other connection) grounding point on the metal chassis.

#7:   Add a NTC resistor for surge current limiting
When you turn a low voltage power supply (with large caps), you get a heck of an input current spike.  That is the 'grunt' you hear sometimes on some power supplies. The NTC resistor (looks like a large ceramic capacitor):
<http://www.gesensing.com/downloads/datasheets/920_325a.pdf >
<http://www.mouser.com/catalog/636/584.pdf >
is connected in series with the 'hot' side of the AC line input (i.e. in series with the fuse and on/off switch).  It is a large resistance when cold and a much lower resistance when it gets hot (i.e. after the current has been applied for a time). As an guess, I would suggest a CL-210 device (30 cold, 1.5A maximum, 30 second time constant).  This will greatly decrease the input current surge on turnon but will have a very small effect on operation.  

These are cheap - about $0.90 each.

#8:   Notes
The silicon diodes will give you a couple of volts higher output (both minimum and maximum) - this is my guess based on a swag at the nominal drop across the selenium rectifiers.  I would not be surprised if the output regulation is a bit better.

That's it.  Any other questions, please feel free to drop me an email at <rgroh at swbell.net>

73
Bob Groh, WA2CKY

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