[Heathkit] foldback

Mon, 13 Oct 2003 12:59:55 -0400

In over 25 years of working on these amps I have not seen more than a
handful of paper forms char beyond use from heat.

And once Heath migrated past the moisture prone "postage stamp" micas to rhe
dipped style there were no more problems there either. A 500V dipped mica is
plenty robust enough. Note that the early grid bypasses were postage stamp
also, those should be replaced.

The primary problem with the input is the ferrite cores. They are woefully
underated and even of the wrong material for the applicatiion. However Heath
wanted the parts small and the caps of cheaper low values so they went with
a high mu ferrite that is far from stable under RF temperature.
Once overheated a few times the cores lose their permeability and the VSWR
shoots up and is no longer adjustable. As mentioned above the paper forms
char as a result.in the most serious cases. From the amount of replacements
I sell it seems that 80/40 are the most troublesome.

For those who sent requests for the input value changes, I found them in an
old YCCC newsletter. I'll get the info off later today.

For the SB200 there is an early and late circuit. Initially 10 & 15 were a
Pi-Net, somewhere around manual revision #5 it went to a L network.
The input cap values are C36 = 75pf and C-38 = 68 pf. There are no C35, C37.

Contesters have been using the 220 for decades, even at 1KW + output on CW.
With a few simple mods they seem to be almost indestructible.

Carl
KM1H

----- Original Message ----- 
From: "James M. Walker" <[email protected]>
To: "Heathkit Reflector" <[email protected]>
Sent: Sunday, October 12, 2003 1:24 PM
Subject: Re: [Heathkit] foldback

> Actually,
> The problems encountered with the input circuit on "Consumer Amplifiers"
of
> the SB-220
> variety is space and heat, in a nut shell! Now before everyone goes "OFF"
I
> will try to
> explain.
>
> The alignment procedure in the manual is correct to a point, however the
> space inside
> the cabinet is NOT conducive to good air flow across the input circuit
> components, and
> they are physically small. When in operation the circuit components heat
up
> the coils
> migrate and change the tuned freq. This leads to the high reflected power
> back to the
> driver source. Paper tubes are NOT good for coil forms that handle any
> appreciable
> power, the recommended type is still ceramic forms with good point surface
> contact for
> heat-sink conduction away from the coil. The caps used in the circuit are
> mica caps or
> were, some have been replaced with higher voltage disk ceramics, in any
case
> the best
> replacement cap is the CRL 800 series of transmitting caps in the 1000
volt
> rating. This
> solution would correct the problem, however the owners of the SB-220 units
> don't like
> to modify the unit in this way.
>
> The "Bottom Line" is that the SB-220 configuration is a compromise due to
> the Table
> Top packaging of the unit. Bear in mind that the military and government
> agencies use
> the same configuration 2 X 3-500Z class "B" amplifiers, however they also
> spec that it
> WILL operate over a certain temperature range, not require depot level
> maintenance
> to maintain proper operation of the unit, and pretty much run 24-7 with no
> problems.
> When they say the source will see 50 ohms, that usually means whether it
is
> in the desert
> or the Artic, and they get this "Feature" by using large components with
> known temperature rating, adequate cooling arrangements (read air flow)
and
> over rated items.
>
> You can run the pants off an SB-220 in SSB mode but only for a little
while,
> after things
> heat up and move around, the signals get wide, and trashy, and spurious.
>
> For what it is worth, go here http://eshop1.chem.buffalo.edu/SR-120.html
> read the
> description and checkout the Eimac spec sheet for the 3-500Z then look at
> the components used in the circuitry and note that these rigs run 24/7 at
> full power output,
> most importantly note the fan and the air flow of the plenum!
>
> My two cents!
> Jim
> WB2FCN
> http://eshop1.chem.buffalo.edu
>
> ----- Original Message -----
> From: "Jim Brannigan" <[email protected]>
> To: "Heathkit Reflector" <[email protected]>; "Mike Hyder --N4NT--"
> <[email protected]>
> Sent: Friday, October 10, 2003 5:26 PM
> Subject: Re: [Heathkit] foldback
>
>
> > From "Some Notes on the Design and Construction of Grounded-Grid Linear
> > Amplifiers"
> > QST, Dec. 1970, Page 22
> >
> > For a "cold" and safe adjust:
> > The authors suggest using an Dip meter and impedance bridge (now use an
> MFJ
> > bridge) Terminate the output of the network in the characteristic
> impedance
> > of the tube. (120 ohms for a pair of 3-500's) and adjust or substitute
> > components as necessary.
> >
> > The values shown on the Rich Measures site are a good starting point.
> >
> > Jim
> >
> > > Carl, if you refer to the following response you gave:
> > >
> > > ------------------
> > > "There were several changes to the input circuit to improve the VSWR.
> > These
> > > were published in various contest and DX magazines once SS xcvrs
became
> > > popular.
> > > If no one else has something handy let me know later in the week and
> I'll
> > > try and dig them up."
> > > ------------------
> > >
> > > I did not find that your post contained any helpful information, just
> > vague
> > > reference to vague publications.  If you made any suggestions to help
> the
> > > man with his input VSWR problems, they have not shown up here.
> > >
> > > Because you obviously know more than Measures and because I am in the
> > > process of reworking an SB-220, I would like to know your ideas on
> > improving
> > > the input VSWR of my amplifier.
> > >
> > > 73, Mike N4NT
> > >
> > > ----- Original Message -----
> > > From: "Carl" <[email protected]>
> > > To: "Mike Hyder --N4NT--" <[email protected]>; "Heathkit
> > > Reflector" <[email protected]>
> > > Sent: Friday, October 10, 2003 11:49 AM
> > > Subject: Re: [Heathkit] foldback
> > >
> > >
> > > >
> > > >
> > > > > Perhaps you could shed some light on the SB-220 input impedance
> > problem
> > > >
> > > > I already did but I guess you didnt bother reading that post.
> > > >
> > > > > instead of just bashing someone else.
> > > >
> > > > Trying to present a balanced discussion is now called bashing? Gee
and
> I
> > > > thought the purpose here was to help people and possibly not make
> > > expensive
> > > > mistakes. Sorry, go on your merry way ostrich style.
> > > >
> > > > I have found Rich Measures to be of
> > > > > much more help than QST.
> > > >
> > > > He has some good advice.
> > > >
> > > > >
> > > > > And considering most of the stuff in QST, it is probably a high
> honor
> > > NOT
> > > > to
> > > > > be published there.
> > > >
> > > > Obviously you are totally clueless what transpired and are not
> > interested
> > > in
> > > > anything but QST bashing and your own beliefs.
> > > >
> > > > Enjoy yourself
> > > >
> > > > Carl
> > > >
> > > >
> > > > >
> > > > > 73, Mike N4NT
> > > > >
> > > > > ----- Original Message -----
> > > > > From: "Carl" <[email protected]>
> > > > > To: <[email protected]>
> > > > > Sent: Tuesday, October 07, 2003 10:37 PM
> > > > > Subject: Re: [Heathkit] foldback
> > > > >
> > > > >
> > > > > > You may want to read the series of rebuttals by real engineers
in
> > QST
> > > > > about
> > > > > > the Measures articles. And that he has been banned from future
> > > > publication
> > > > > > there.
> > > > > > I consider most of his parasitic theory pure garbage and have
> proven
> > > it
> > > > > time
> > > > > > and again to skeptics. This is old news as it took place on the
> AMPS
> > > > > > reflector several years ago.
> > > > > >
> > > > > >
> > > > > > Carl
> > > > > > KM1H
> > > > > >
> > > > > >
> > > > > >
> > > > > > ----- Original Message -----
> > > > > > From: "David Cook" <[email protected]>
> > > > > > To: "'Jim Brannigan'" <[email protected]>;
> > > > <[email protected]>
> > > > > > Sent: Tuesday, October 07, 2003 9:48 PM
> > > > > > Subject: RE: [Heathkit] foldback
> > > > > >
> > > > > >
> > > > > > > Wow, what a great paper! I recently acquired the first
amplifier
> > > I've
> > > > > ever
> > > > > > > owned, an SB-200. I put in all the Harbach mods, but still
> didn't
> > > > > > understand
> > > > > > > much about it. Many of Rich's explanations of the design
> decisions
> > > and
> > > > > > > performance considerations for the SB-220 apply to the SB-200.
> > > Thanks
> > > > > for
> > > > > > > posting this link. I'm going to fire off an e-mail of
> appreciation
> > > to
> > > > > Rich
> > > > > > > as well.
> > > > > > >
> > > > > > > 73, Dave, WA�TTN
> > > > > > >
> > > > > > > > -----Original Message-----
> > > > > > > > From: [email protected]
> > > > > > > > [mailto:[email protected]]On Behalf Of Jim
> > Brannigan
> > > > > > > > Sent: Monday, October 06, 2003 2:38 PM
> > > > > > > > To: Hotmail; [email protected]
> > > > > > > > Subject: Re: [Heathkit] foldback
> > > > > > > >
> > > > > > > >
> > > > > > > > This is Rich Measures' site.  There is a wealth of
information
> > on
> > > > the
> > > > > > > > SB-220.
> > > > > > > > http://www.somis.org/sb220ci.html
> > > > > > > >
> > > > > > > > The SB-220 was designed when most exciters were tube based
> > > > > > > > with a pi-network
> > > > > > > > output.  The pi net output is able to match a wide range of
> > > > > > > > loads and the
> > > > > > > > input impedance of the amplifier was not a serious design
> > > > > > > > consideration.
> > > > > > > > Many Solid state exciters are not as forgiving and some
> > > > > > > > attention to the
> > > > > > > > amplifier input impedance is necessary.
> > > > > > > >
> > > > > > > > Jim
> > > > > > > >
> > > > > > > >
> > > > > > > > ----- Original Message -----
> > > > > > > > From: "Hotmail" <[email protected]>
> > > > > > > > To: <[email protected]>
> > > > > > > > Sent: Sunday, October 05, 2003 12:10
> > > > > > > > Subject: [Heathkit] foldback
> > > > > > > >
> > > > > > > >
> > > > > > > > > I have a great SB220 with the latest Harbauch mods that I
> > > > installed.
> > > > > > > > >
> > > > > > > > > Question:
> > > > > > > > >
> > > > > > > > > My exciter (TS140S) folds back to 50 watts out on 10
> > > > > > > > meters. How do I go
> > > > > > > > about tuning the amplifier (input circuit I guess) to
> > > > > > > > eliminate the foldback
> > > > > > > > issue or is this normal?
> > > > > > > > >
> > > > > > > > > Thank you,
> > > > > > > > >
> > > > > > > > > Bruce
> > >
> > > _______________________________________________
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> > _______________________________________________
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> > ** For Assistance: [email protected] **
> > $$ See the vintage area on the HCI web site - http://www.w9wze.org $$
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> >
> >
>
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> $$ See the vintage area on the HCI web site - http://www.w9wze.org $$
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