C'mon guys, this is only an SB-220. It's a barebones amplifier
with no complicated protective circuits. It's easy to work on,
since it was built as a kit.
To troubleshoot this amp, you should need only one spare fuse.
Somewhere in the procedure below, the fuse will blow, and you
will know where the problem is.
First, remove the surge limiter circuit. It is a troublesome,
unnecessary item. Restore to factory configuration.
Disconnect the primaries of the plate and fil/bias transformer
and see if the fuse blows.
Connect the primary of the fil/bias transformer, with secondaries
disconnected, see if the fuse blows.
Connect the bias secondary of the fil/bias transformer and see if
the fuse blows.
Connect the filament secondary of the fil/bias transformer, with
the tubes removed. See if the fuse blows.
Plug in the tubes. See if the fuse blows.
Connect the primary of the plate transformer with secondary
disconnected, see if the fuse blows.
Connect the secondary of the plate transformer, with the B+ line
to the tubes disconnected. See if the tube blows.
Connect the b+ line to the tubes and see if the fuse blows.
It's a lot more fun to work on Tetrode amps with all kinds of
digital and analog protective circuitry!
Dave Hachadorian, K6LL
Big Bear Lake, CA
-----Original Message-----
From: donroden@hiwaay.net
Sent: Monday, July 10, 2017 5:50 AM
To: amps@contesting.com
Subject: Re: [Amps] SB220 step start blows fuses
Agree with Gary !!
Make a tester out of a garage light with a 300 or 500 watt
halogen element.
( or pair ) relatively inexpensive and I always need an extra
light
when looking for things in my garage.
Don W4DNR
Quoting Gary Schafer <garyschafer@largeriver.net>:
You don't have a big enough bulb.
I use two 300 watt bulbs in parallel. 300 watt bulbs are about
the largest
you can find easily.
I made up the bulb unit using a square electrical box and
mounted two
outdoor light sockets on it, an outlet and a regular house
switch to short
out the bulbs when I want to bypass the series bulbs.
It works great for checking amps. It is really nice if you have
an
intermittent arc or want to let suspect capacitors cook for
awhile with the
amp is in standby as no harm is done if something arcs, the
bulbs just light
up bright.
I sometimes plug the light bulb setup into a variac too.
73
Gary K4FMX
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of
Warren Volz
Sent: Sunday, July 09, 2017 9:53 PM
To: amps; gudguyham@aol.com
Subject: Re: [Amps] SB220 step start blows fuses
I may shortly be looking to purchase a variac.
After building a dim bulb tester this weekend I have a minor
update.
With 120V connected, the HV primary disconnected and one tube
in a
socket (doesn't matter which I use) the 100W bulb in my tester
lights up
bright. No fan and the meter lights aren't on. If I take out
the tube
the fan and meter lights work. So I guess I have a shorted
tube? Would
it be good to double check the DC/AC voltage on the filament
supply with
the tubes plugged in? That would show any sags that might
exist.
-Warren
> On Jul 7, 2017, at 12:58 PM, gudguyham--- via Amps
<amps@contesting.com> wrote:
>
> I know I sound like a broken record but yes, a Variac is the
> way to
go. Especially with amps that can have power supply problems
and gassy
tubes. You will hear the plate transformer groan and won't
see HV
climbing on the meter if there is a problem in the PS long
before the
smoke Genie appears. My Variac gets used daily several times
after
doing ANY work on an amp. It's saved me loads of time fixing
blown
parts that would normally occur if I had just turned an amp on
with
problems. Best investment I ever made. 0-250 volts at 20
amps.
>
> Sent from AOL Mobile Mail
>
>
> -----Original Message-----
> From: Bill Turner <dezrat@outlook.com>
> To: Amps group <amps@contesting.com>
> Sent: Fri, Jul 7, 2017 12:34 PM
> Subject: Re: [Amps] SB220 step start blows fuses
>
>
> ------------ ORIGINAL MESSAGE ------------(may be snipped)
>
> On Fri, 7 Jul 2017 10:21:36 -0400, Mark B. wrote:
>
>> A variac varies voltage. It is not designed to limit
>> current.
>
> REPLY:
>
> At the very low end of its range, a Variac limits current
> just fine. I
> worked as a calibration technician at Tektronix for years,
> taking
> brand new scopes off the assembly line and powering them up
> for the
> first time. Believe me, a Variac is the only way to go with
> a unit
> which could have any number of problems at first. Over the
> years I
> powered up literally thousands of scopes this way and never
> a problem
> with current limiting.
>
> 73, Bill W6WRT
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