Re: Selenium rectifiers in Tek equipment


Stephen Hanselman
 

My favorite PS used an old Burroughs transformer which I cut 4 turns off the
secondary. This meant removing 18" of 4in wide copper strap. That was fed
through a bridge to 50,000uF of filter cap. First time I turned it on the
bridge literally blew apart. Yup needed a bit of series R to "soften the
blow".

Great thread

Steve

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Tom Lee
Sent: Monday, February 8, 2021 11:50 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

Exactly. I never followed the lore because, as a teenager, I wasn't aware of
the lore. I foolishly replaced blown seleniums with whatever rectifiers I
had on hand from Poly Paks ("U test 'em and save"). Never had a problem, but
perhaps it was just dumb luck. When I later worked in TV repair, I learned
that I had violated proper procedure, but by that time, most gear with
seleniums had either been junked or upgraded. I've only replaced a few since
then, sometimes with added resistance to drop the voltage, but sometimes
not. Always in my own gear, so that I alone bear the risk.

Tom

Sent from an iThing, so please forgive the typos and brevity

On Feb 8, 2021, at 23:13, "Joe Laffey" <joe@thestable.tv> wrote:

On Mon, 8 Feb 2021, Tom Lee wrote:

The short answer is resistance. It's not only a matter of forward drop at
low currents (which the oft-quoted 1V number refers to; the drop at rated
current can easily be an order of magnitude higher). Selenium rectifiers
have considerable forward resistance (hundreds to thousands of ohms,
depending on the specific device), compared to the small number of ohms
typical of silicon rectifiers. So if you need to emulate selenium
rectifiers with silicon ones, you have to degrade the latter by adding
series resistance.

Not all circuits need such emulation, but the general practice among
old-time radio techs was to be conservative. "You'll pop the filter caps if
you don't have that resistance to soften the blow" is a common bit of lore.

I don't think you are going to harm modern caps with inrush current in
most cases (as you seem to indicate as well with your "lore" comment). The
transformer will usually help buffer this inrush. Old caps are a different
story, especially if you are one of those guys who thinks its wise to keep
20, 30, or 40+ year old electrolytic caps in your equipment (not I).

For me the easiest method is to measure the voltage with the selenium
rectifier in place (bring it up on a Variac). Then put a series resistor
after your silicon rectifier to lower the voltage to the same level (mind
the power rating for heat dissipation of course). Use Ohm's Law and then
adjust for tolerances or math errors ;-) You may even want to measure the
voltage at 117VAC input (or whatever the original expected line voltage was)
and then adjust the resistor value to match that output voltage at your
normal modern line voltage.

Most old equipment can handle fairly large swings in input voltages.
Though it may need to be re-aligned at modern voltages vs. old lower
voltages.

--
73
Joe Laffey




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