[Hallicrafters] HT-37 Rebuild

[Al Parker]

Hi all,
    I think we need to qualify the statement, "replace ALL the
capacitors."  If you mean "ALL," then I think most of us will disagree
with you.
    Filter caps (electrolytic filters, for those that need more
definition), and other electrolytics, which are often used as audio
bypass/shaping in the cathodes of audio amps, plus elsewhere, like
sometimes in AGC lines in rcvrs - these, YES, probably.  If you have an
ESR (Equivalent Series Resistance) meter, take a look at them and maybe
they're OK, but still suspect they'll fail in the not too distant future.
    Micas, used as coupling between stages and in frequency determining
ckts, PROBABLY NOT, but do check them.  Don't replace them shotgun style,
especially in freq. determining ckts.   They do fail occasinally.
    BBOD's  YES.  (Black Beautys Of Death- these are the black ones WITH
colored stripes to show value.)  Most of the Hallicrafters units I've
worked on have not had them, but did have black plastic encased caps that
are shaped the same.  They apparently are not exactly the same, but better
quality, in my experience and measurements.  They still should be checked.
The bad ones, with stripes, are common in SP-600's, and do go bad with age
only, even split open.  If they're not bad now, they will be very soon
with use.  There are also similar GLOD's, (Grey Ladies Of Deception) that
are the same, just gray, not black.  I've also seen, in Collins, similar
BBWS's (Brown Blobs With Stripes) that look the same, but may be better
quality, and in my limited experience have checked OK.  These all are
usually used as bypass caps, sometimes coupling between stages, and
decoupling caps in DC supply lines.  In each application they can cause
excessive current to be drawn, overloading/overheating dropping resisitors
and power transformers.  As coupling caps between stages, they can forward
bias the 2nd stage and cause excessive plate current, with possible
disastrous effects.
    Do consider, as Chuck points out, putting SS replacements in for the
tube rectifiers.  There's been a recent thread on thes topics right here
on this reflector (I think).  Take a look at the archives for this month.
To see some details on the SS'ing of an HT-37/32, take a look at one of my
webpages:
<http://www.thecompendium.net/radio/HT37.htm>
    The SS stuff is down toward the bottom.  I've got some pix, a sketch,
and some notes/comments there and I hope it'll be of help.
    I'll expand a bit on "SS diodes"  it's just replacing the tube type
rectifiers (both of them in this instance) with solid state diodes.  This
reduces the heat loading on the pwr xfmr, and by also removing the
filament leads you remove the HV on them inside the xfmr.  Hallicrafters
power transformers haven't stood up well with their advancing age, they
often develop HV to filament winding shorts, and sometimes also to ground.
These failures can be attributed to deterioration of the insulation by
heat and or moisture.  I've had it happen to me in an HT-37, an SX-101,
and an SX-42.  Also I have bought both an HT-32 (which is used regularly)
and an SX-62, which had already lost their pwr xfmr.  That's why we so
often hear the caution to warm up a new acquisition with unknown history,
or any one that's been stored a long time, by putting heat under it with a
light bulb, etc, for a day or 2.  Then slowly increase AC line voltage
just to be sure, and to allow filter caps to reform if they haven't been
replaced.  (even if you've just been in there and replaced "everything".)
    I hope this hasn't been too long-winded, but maybe you get the idea
that I don't like sweeping, all inclusive, non-qualified statements about
our boatanchors.  No offense intended.  Whatever, please think about
things when you do it, look at the schematic, learn as you go.
73,
Al, W8UT
New Bern, NC
BoatAnchors appreciated here
http://www.thecompendium.net/radio/
http://www.hammarlund.info




----- Original Message ----- 
From: "JM/CO" <jmerritt2 at capecod.net>
To: "Bill Gerhold" <wpgerhold at elongo.com>; "Hallicrafters"
<hallicrafters at mailman.qth.net>
Sent: Wednesday, October 15, 2003 5:53 PM
Subject: Re: [Hallicrafters] HT-37 Rebuild


> It does sound as though you have  found a low mileage HT-37. Be warned
that
> the power transformers in these are fragile. They were designed a bit
> "under-size" for the application.
> It is generally a good idea to replace ALL the capacitors, except the
> ceramic types before you fire the set up. Do not think that, just
because
> there is little use evident, that your capacitors are not leaky /
shorted.
> Time, alone, is all that is required for these to go leaky or shorted.
Many
> who restore HT-37s, and certain "other" Halli sets, convert the low(er)
> voltage power supplies to solid state, mainly in order to reduce the
total
> overall current pulled by the primary of the power transformer. These
> transformers are always hard to find and expensive to have rebuilt.
> Prevention is the thing here, so change out those nasty ol' caps.
> And--No, I don't think Halli ever branded their 6146s, but you had
better
> solicit other opinions. The finals are often the first tubes to get
replaced
> as a rig piles up the hours, so they very well may have been changed.
> Chuck N1LNH
> ----- Original Message ----- 
> From: "Bill Gerhold" <wpgerhold at elongo.com>
> To: "Hallicrafters" <hallicrafters at mailman.qth.net>
> Sent: Wednesday, October 15, 2003 10:49 AM
> Subject: [Hallicrafters] HT-37 Rebuild
> > I have thoroughly cleaned the chassis of my HT-37.  Did it by using a
> > small solder paste brush
> > And a small vacuum attachment.  The chassis cleaned up nice and shiny.
> > No rust spots, no
> > Grease.  All the tubes (except finals) are stamped Hallicrafters, the
> > panel lights are 2 # 44's and
> > 1 # 47.  All 3 bulbs appear to be new, no internal blackening.  The
> > bulbs had the same amount
> > of dirt on them that the chassis did.  The rubber feet are original
and
> > show no signs of wear at all!
> >
> > I am in the process of changing all electrolytic caps.  Once completed
I
> > will bring it up slowly,
> > Monitorinig key voltage points.  This should happen in about 2 weeks.
> >
> > All the above indications lead me to believe that I have acquired a
rig
> > with very little or no use at all.  Is
> > This possible?  Did Hallicrafters ever put their name on 6146's.
> >
> > K2WH