SB-104 (A?) help

[Keith Rowland]

Hi Bob:

The SB-104, as Chuck Penson correctly points out in his book, was on the
cutting edge of the technology of the day, and for this reason and others, it's
not surprising to find that it had many problems, which very soon caused Heath
to do what amounted to a re-design, resulting in the SB-104A.  They made a
modification kit to upgrade one's SB-104 to the "A" model.

The mod kit instruction manual runs to 41 pages, and calls for many parts that
I suspect strongly can no longer be found, and some that are probably beyond
the capability of the average ham to manufacture.

For example, hard to find items might include the 6110-ohm, 1/8-watt, 1%
resistor, as well as these other odd values:

1210-ohms, same power rating and tolerance, a 15-ohm, 1-watt unit, a .15-ohm,
2-watt resistor, an .82-ohm, 2-watts, and a 175-ohm jobbie at 5-watts.


There are also 3 trimmer caps that might or might not be hard to find:

3.2-18 Pf
5-25 Pf
15-60 Pf

I think the clincher are the six inductors called for in the upgrade:

Two .82 microhenry peaking coils
One .44 microhenry toroid
Two 1.31 microhenry toroids
One 2.3 microhenry toroid
One 13.25 microhenry toroid
One 350 microhenry choke (the type that looks like the old standard 1.5
millihenry RF chokes that abounded in older gear)

The mod kit included new circuit boards for the VFO/Filter, a pre-assembled
receiver front end board, an ALC/filter board shield, and several other parts
no longer available.

Mods were made to the VFO Oscillator, VFO Buffer, Audio/Regulator, Transmitter
I.F., HFO/premixer, Carrier Generator/Xtal Filter,Receiver I.F./Audio, Power
Amplifier, Driver, ALC/Output Circuit, and the VFO/Filter circuit boards.

There are also 16 pages of changes on the main chassis.

Possibly one way to tell if yours has been upgraded is to take a look at the
driver circuit board.  The changes were:

Remove the heat sink from the board.

The following parts were added:

R904, 470-ohms, mounted just below the RF choke near the upper left hand corner
of the board.

C909, .001 ceramic, added just below this RF choke..

R903, either 220-ohms, 2-watts or 175-ohms, 5-watts, depending on whether the
rig used TRW6619 or CTC driver transistors (respectively).

C906, in the lower left hand corner, .1 mfd mylar cap.

Reinstall the heat sink.  This was one of the easier boards to do!  But if
those changes have been made, you can be pretty sure you have an SB-104A.
You'd need the modification manual, and the huge 104A schematic, plus the new
assembly and operation manuals to check all the changes to make absolutely
sure.

Hope this helps,  and was not too much bandwidth, but I figured there are a lot
of 104s and 104As out there (I have two A models, and until recently had an
unmodified 104), so this info may be of wider use.

73, Keith


Robert Nickels wrote:

> I've been working on an SB-104 which according to Penson's book should come
> as no surprise.  There are no markings that this unit was upgraded to
> SB-104A but I've found several areas where wiring differs from the 104
> schematic.  Does anyone know what the changes were to upgrade a 104 to a
> 104A?
>
> There is a sticker indicating this rig went back to Heath for servicing in
> 1987.  Would the factory have done the upgrade?  And if so, wouldn't they
> have identified it as having been done?
>
> If anyone has experience with the 104 and wouldn't mind dropping me an
> e-mail I'd appreciate a second opinion.  Thanks for any help!
>
> 73, Bob W9RAN
>
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