Heath SB 104A problems

[John Farrington]

At 09:55 AM 02/05/2004 Vin wrote on the Heath list:

 >How would one determine if a 104A is really a 104A and not a 104?
 >I suspect that the one I have has had its front panel changed.
 >Also,  what's a good replacement for the PA transistors?
 >Regards from a vy windy and cold Maine.
 > Vin


The 4 PA transistors were #417-831:  either CTC CD-2664A or 2N6456,
which, according to the manual, had the same Heath number but slightly
different mounting steps. NTE cross-references the 2N6456 to a NTE325
with the note that "Minor mechanical differences exist, but device
will fit most applications".   Hmmm?
Rfparts.com used to have the 2N6456;  some hams might know of another
substitute that will work - someone once suggested the MRF 455A, and
another the MRF450A-MP, but I don't know.
NTE does not list a substitute for the CTC CD-2664A.

There were quite a few mods involved in upgrading the 104 to a 104A:
Heath put out a field update kit with a 40-page manual (manual number
595-2016-03), which included, among other things, 5 new transistors,
7 diodes, 9 coils, 60 caps, 45 resistors, a new front panel trim strip
(with 104A printed on it), a new VFO/filter circuit board, a complete
new rcvr front end board (board G), and a new 104A assembly/operation
manual set. Numerous chassis wiring changes were also made - it took
some time to make all the mods, definitely not a casual one-evening
job, and with no room for sloppy work. To complicate matters, there
may have been a series of mod kits, so they may have varied somewhat
from year-to-year, plus there was an errata letter making mods to
the mods.

There was also a mod kit for the HP-1144 power supply (manual #595-2219)
which added a crowbar overvoltage protect circuit, but I don't recall
now if it came with the 104-to-104A kit or was separate.

Probably the easiest way to tell what you have is to look at the G-board
(the rearmost - of course there were unique part numbers printed on them,
but I don't have them handy):

the old one had 5 transistors on it and the IF out (pin 1) comes from the
drain terminal of Q704 through an 820 pf cap (C779);

the new one has only 2 transistors and the IF out (pin 1) comes from the
center tap of transformer T704 through a .01 cap (C754). This new board
uses 2 balanced mixers each consisting of 4 diodes in bridge circuits
(D719-721-722-723 and D724-725-726-727); the old 104 board G did not use
diode balanced mixers.  If you have the new board and it has encapsulated
balanced mixers instead of individual diodes, it's probably a later mod
made by a ham who followed an article in QST magazine for upgrading the
balanced mixers.

There were some under-chassis wiring changes to accomodate the new
board G - you could not simply drop the new board in the old 104.


Another thing to look at is the VFO/Filter circuit board, as it was
replaced:

the old one may have a part # like 85-1516 on it: it has 5 diodes among
other parts, and 1 MPSA20 transistor (417-801);

the new one's part # may be like 85-1930-1: it does not have any diodes,
and the 1 transistor is a 2N5770 (417-293).

It's a small square pcb mounted under the chassis at the edge amongst all
the plug-in board slots, between board slots C & E. If you turn the chassis
upside down with the back side nearest to you, the VFO/Filter board is on
your left right next to the side of the chassis

I seem to recall that there was another effect of making the mods:
on the 104 there was a front panel button to turn on/off the last digit
of the frequency display, but maybe that button was either disabled or
used for another purpose on the 104A so the last digit was always on.
On the other hand, that might have been a side-effect of adding the
noise blanker kit. Well, memory fades, like other things I won't mention.

73

John    KE5ZB

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