The AM Forum

Hi all,

Replacing the selenium in my 5100B. Thinking 1n4007 diode. Opinions?

And what is that large mica covered disc thinga-ma-bob. A resistor of some kind? It looks good and clean, no signs of cracking or burning, even on the wires. I should know, but alas .... brain fart.

Also, I would like some thoughts on upping the uf's on the supply caps. I have a source for a new (non NOS) 50/50/50 twisty can (original was 40x3). I would like to stay with a can, and not just stick a bunch-o-caps in the chassis. Is adding 10uf on each enough to show a difference in performance. Or is 40x3 fine.

Also, does anybody have a source for the AC feed-thru input caps, as well as the .25uf/2000V oil can (maybe this might be increased also in uf's?). I think the oil can is the choke bypass for the HV.

Opinions welcomed please. Thanks in advance.

John
W2WDX

The thinga-ma-bob is the dropping resistor for the 120 volt line.  It is in series with the AC when you have the tune/operate switch in tune.  That is nichrome wire wrapped around the support.

Dropping resistor .... of course.  ::)

Now where could I get one of those I wonder, if I ever needed it. It measures fine. Nichrome wire eh? Can that be had anymore?

As far as the can cap, I can get the original values for the supply multi cap. I was just wondering if anyone had ever noticed an increase in performance by upping the values a little. I know these rigs work great as designed. I guess I'm trying to reinvent the wheel a little and over thinking. Ha!

I wasn't sure about oil caps and their longevity. It doesn't appear leaky, electrically or literally, so I guess your saying it's probably cool. Ok ... cool.

Saw your website on your restores on both B&W's. I'm hoping to get the SSB unit someday, hopefully in similar condition to my rig. Maybe I'll take some shots and post them while I'm doing this basic restore.

One last question, what happens if the bead chains on the multiplier mechanics break (knocking on wood while saying this)? Not functionally of course, I mean about replacing them.

Other than making your own dropping resistor, I don't know where you would ever find one.  You might find a junker with one in it but that would be luck.  Check with Gary/WZ1M, he has some parts and may have that.

I have never deviated much from factory installed caps except where you have 30 mfd and the replacement available is 33.  Today most 40s are 47 and so forth.  I don't like to increase values very much because it does put a bit of extra strain on the transformers that does not add much to the signal.

The B model does have a couplate between the first and second stage audio which can be removed for a bit better sound.  There are some other things in it that can be done easily but it won't make a big difference.

That type of chain used on the multiplier stage is available, but there are different sizes.  I never messed with mine because of that fact and finding the right one  would be nearly impossible.  That is a continuous chain without a coupling joint which makes it kind of rare.  If it isn't broke, don't fool with it.

Check BAMA for some factory errata sheets on that transmitter.  Mark/K3MSB put them out there and there is good information.  Your transmitter may have them all but check to be sure.  Mark can be a great help if you have questions.

You may have to adjust the bias on the transmitter. I don't know much about the B&W 5100 bias supply, is it fixed or adjustable? Well anyhow selenium rectifiers have a much greater drop in voltage than silicon types.

From: "Barry L. Ornitz" <ornitz@dpnet.net>
Subject: Re: Selenium Rectifier
Date: 02 Dec 1998
Keywords: selenium, rectifier, toxicity, safety
Newsgroups: rec.antiques.radio+phono

Mark Densel wrote in message <36659183.9596D249@agouron.com>...
>Where can I find a selenium rectifier for a 1950' kiddie record player?


I wrote a piece for the Boatanchor radio mailing list a while back on
selenium rectifiers.  When these short, they produce rather toxic products.
Thus they should always be replaced when restoring older electronics.  They
also tend to age and increase their internal resistance over time, so
replacing them will likely help the operation of the equipment too.  Just
remember that a modern silicon diode will produce a greater voltage output
than the selenium rectifier, so you may have to add some series resistance
accordingly.

                  Barry L. Ornitz     ornitz@dpnet.net


                   Selenium Rectifier Replacement
                          (c) 1997 B. L. Ornitz

Several people have recently asked about the toxicity of selenium and
its compounds since selenium stacks were common as rectifiers in many
Boatanchors.  Like others here, I too have learned the hard way just
how bad a selenium rectifier can smell when it is overheated or
shorted.  One of my first electronic projects has a small selenium
stack that I wired in backwards.  Naturally I didn't plug it into the
wall socket at my workbench - I carried it into my bedroom to test it!
I couldn't sleep in there for three days!    ;-)   Murphy Rules!

Selenium, a p-type semiconductor, is coated on steel plates in a thin
layer to produce a selenium diode.  Normally seen as a black or gray
shiny coating, the metal is a close relative to sulfur (in fact, I
have made homebrew copper sulfide rectifiers).  It was named after the
Greek word for the moon, Selene, by Berzelius in 1817 because he found
it associated with tellurium which is named for the Latin word for
earth.  If I remember my chemical history, his housekeeper accused him
of eating loads of garlic, when he had not.  This garlic odor is
characteristic of many selenium compounds.  Selenium metal is, in
itself, not terribly toxic.  Its compounds are, however, even to the
point of making some plants toxic to animals when grown in soil rich
in selenium.  [Hopefully we don't have any mega-supplement health food
nuts taking massive doses of selenium here.  Some effects of chronic
exposure include depression, lassitude, fatigue, liver and spleen
damage, yellow skin, garlic breath, giddiness and emotional
instability - and reproductive effects which nature gladly provides to
prevent the stupidity gene from being passed on.]       ;-)

Selenium dioxide is the major compound produced when a selenium
rectifier is overheated.  It can cause severe burns to the mucous
membranes and severe respiratory tract, skin, and eye irritation.  It
is also a dermal sensitizer in that it can promote allergic reactions.
Fortunately it is not consider a carcinogen.  Another fortunate thing
is the BAD smell.  When I say BAD, I mean really, really, _really_
_BAD_.  Our odor threshold for selenium dioxide is 0.0002 mg/m3.

The allowed exposure for selenium and it compounds (expressed as
selenium) is:
   0.2 mg/m3 OSHA TWA
   0.2 mg/m3 ACGIH TWA
   0.2 mg/m3 NIOSH recommended 10 hour TWA
   0.1  mg/m3 DFG MAK TWA (total dust);
   1 mg/m3 DFG MAK 30 minute peak, average value, once per shift

Note that the odor threshold is far below these.  If you smell
something really rotten, like decaying onions and garlic, coming from
your equipment, it is best to leave the area immediately, opening some
windows on the way out.  Allow the selenium dioxide vapors to
dissipate for several days before you go back.  If you have ever
smelled this odor - believe me - you will not want to go back very
soon anyway!  The odor is very distinctive to say the least.
I almost always replace selenium rectifiers as standard practice with
BA gear. If you have the schematic and know the voltages, choosing the
proper silicon diode is easy.  With no other information, a reasonable
rule of thumb is that a single stack handles about 25 volts PIV.  Thus
the typical stack for rectifying something off the power line would be
five plates.  Another "eyeball" approximation is that the area of each
plate in the stack is about a square inch per 300 milliamps.  For most
boatanchor applications where small stacks are used, common 1 amp
diodes are fine.  Remember that selenium rectifiers have a much greater
forward voltage drop with current than do silicon diodes.  Thus when you
replace a selenium diode with a silicon diode, expect a higher voltage
out of your circuit. For most selenium stacks, the DC output is about 2
volts less than the RMS input voltage multiplied by the number of plates.
This implies a considerable internal resistance.  Silicon diodes generally
have about a volt of forward drop associated with each diode, but the
internal resistance is very low.  A series resistor may be added if
necessary to drop this voltage.  For applications like the T/R relay
rectifier in a military R-390A, this series resistor is hardly necessary.
However, for large low-voltage DC supplies using selenium stacks (often
only one or two plates per leg, but all four legs of a bridge are usually
mounted in one assembly), a series resistor may be necessary when a
modern silicon bridge is substituted.  Many such supplies included
multiple taps on the power transformer, fortunately, so investigate this
before wiring in a power-wasting resistor.

From the interest of safety, I believe it is always wise to replace
selenium stacks with modern diodes. Some restorers leave the original
selenium stacks in older gear to make it look authentic.  Since modern
silicon diodes generate so little heat in comparison, they may often be
hidden in the circuitry replacing the selenium units while not being
noticed.

        73,  Barry L. Ornitz  WA4VZQ  ornitz@dpnet.net
 

Bias adjust on the 5100 is via a large resistor with multiple taps.  It is akin to the Valiant 1.  The big resistor has one end to the negative voltage and the other to positive  voltage (as I recall) with the center of it tied to ground.  The tap for the bias is then taken from the the resistor on the negative side.

If you have the S model, then the mod for variable pots probably are there.  If yours has the big resistor, then consider making the modification to variable pots.  All it takes is a couple of terminal strips, 4 two watt fixed resistors and two pots.  The pots are Allen Bradley Type J and can be found at Mouser.  Pull the errata sheet from BAMA and look it over.

Holy crap. Someone should tell "Net Control" right away!

I used to,as a kid scratch those plates with a 6 volts ac source just to smell them:)

hey Barry ... I tried to email you with your provided link ....was rejected after trying for 96 hrs ...what gives?

Well, that posting is 11 years old!    :P