[Heathkit] SB-644 problem

Sun Nov 15 11:26:05 EST 2009

Hi Bill!

First of all, the pin 2 to pin 5 jumper has nothing to do with the operation
of the SB644A. It is required for High power(100 watts) transmit. Without
it, the radio will only transmit in the low power(1 watt) mode and the HI
Power button is rendered ineffective.

Second, even though they look the same, electrically the SB644 and the
Sb644A are different ducks:
The VFO in the SB644A was updated to the electrical equivalent of the VFO in
the SB-104A; For instance, the zener-regulated supply was stabilized and the
temperature compensating capacitors were modified to reduce thermal drift.
The need for better stabilization was especially apparent when the +11-volt
supply was too low. Hence the wiring change to pin 3 on the SB644A. The
original VFO utilized a 10-volt zener diode reference and this contributed
to FM-ing of the VFO output signal. Later versions changed this zener to
7.5-volt. 
FYI, the very first VFOs used in the SB-104 used a reverse-biased #417-118
E-B junction transistors. This junction had a reverse breakdown of around 8
to 8.6 volts. This was dropped in favor of the zener diode prior to initial
release of the original SB104 probably due to cost.

Poor isolation caused problems of VFO feedthru between the two VFOs in the
SB104 when the remote VFO was in the off position. In the original SB-644,
you could hear interaction on the SB-104 while tuning the VFO on the remote
SB-644 even in the off, or bypassed, mode. The SB-644A corrected this
problem.

Looks like you're on the right track. You should be able to closely check
both the 644 and 644A schematics and easily update the 644 to the later
specs and wiring configuration.
Hope this helps,
Cheers!
Benton  wb5twc

-----Original Message-----
From: heathkit-bounces at mailman.qth.net
[mailto:heathkit-bounces at mailman.qth.net] On Behalf Of Bill Kuhn
Sent: Saturday, November 14, 2009 2:42 PM
To: ma.locksmith at juno.com; Heathkit at mailman.qth.net
Subject: Re: [Heathkit] SB-644 problem

Thanks for responding!

I think the jumper is required regardless (I may be wrong).

In my sb-644 Assembly manual, in the instructions for assembling the
plug, it has you jump between pins 2 and 5 and then connect the brown
wire to that jumper (not to pin 3 which the schematic shows). In the
assembly manual, there is no connection to pin 3 at all.

The VFO as I have been using it (I bought it used a few months ago) was
set up according to the assembly manual (no connection to pin 3, and the
brown wire connected to the jump between 2 and 5). It works as I
described.

Taking a chance, I've changed the plug wiring to what the schematic on
the website shows - the brown wire is on pin 4, and pins 2 and 5 are
jumped. No change in operation but I do now get 12v at test point C when
the remote is set for RX. I think I'm getting closer.

I'm starting to wonder if either Q6 or Q7 is bad (I haven't pulled them
to test yet).

73 DE NO0OO

-----Original Message-----
From: ma.locksmith at juno.com [mailto:ma.locksmith at juno.com] 
Sent: Saturday, November 14, 2009 2:24 PM
To: Bill Kuhn
Subject: [SPAM] Re: [Heathkit] SB-644 problem

As I look at the schematic for the accessory socket on the SB-104, it
looks like there is a jumper between pins 2 and 5....  5 being the relay
coil.  This jumper is shown with arrow points on the lines which I think
means that it's sometimes used and other times not.

My best guess is that in order to use the VFO, you have to cut the
jumper???? in the 104 and use the VFO to complete the connection.

Ed
K1ZOK

On Sat, 14 Nov 2009 12:42:51 -0600 "Bill Kuhn" <BKuhn at kuhngroup.com>
writes:
> I've got an SB-644 VFO hooked to my SB-104A.
> 
> The SB-644 works properly if I have it set for Remote T/R (transmit 
> and receive). The remote freq displays on the SB-104 and I can TX/RX 
> fine on the remote freq.
> 
> If I have it set for Remote RX (where it *should* be using the radio 
> VFO for TX and remote for RX), the radio displays the remote freq on 
> TX and RX and the TX and RX are in fact on the remote freq.
> 
> If I have it set for Remote TX (where it *should* be using the radio 
> VFO for RX and remote for TX), the radio displays the Radio VFO freq 
> and TX and RX are on that freq.
> 
> Basically, split operation is not working. I'm either all on the 
> remote or all on the radio.
> 
> Checking test points, I'm supposed to be getting 13V on test points 
> A, C, or D depending on whether I'm switched for Remote RX or Remote 
> TX. I get 0V no matter what on these test points.
> 
> Looking back a bit for a voltage source, I see I should be getting 
> 13V in on a line coming in from the radio. This (brown) wire is 
> confusing. 
> 
> >From the assembly instructions, It is supposed to connect to pins 2 
> and 5 of the accessory jack on the radio.
> 
> >From the schematic I downloaded at 
>
http://www.vintage-radio.info/download/download.php?dir=heathkit&file=sb
-
644.zip, 
> it shows a plug wiring diagram with the brown wire connecting to pin 
> 3 of the accessory jack, which is 13V switched on tx from the radio 
> (best I can tell).
> 
> The schematic I have in my sb-644 assembly manual does not show the 
> plug wiring at all.
> 
> Anyone know which is correct?
> 
> If I'm making stupid assumptions or otherwise doing something 
> stupid, please be kind :)
> 
> __________________________________________________
>  
> Bill Kuhn - MCSE , Microsoft Small Business Specialist
> President
> The Kuhn Group, Inc.
> A Microsoft Certified Partner
> 
>     E-Mail:  bkuhn at kuhngroup.com
>        Web:  http://www.kuhngroup.com
> Toll Free: 877-367-5846
>      Office: 785-437-3277
>   Callsign: NO0OO
> __________________________________________________
> 
> 
> 
> 
> 
> 
> 
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