[Heathkit] SB-104A SSB drive problems

Wed Nov 3 12:43:32 EDT 2010

Bob, Heinz,

First, thanks for your comments.

I am using a Heathkit monitor scope (an I/O-10 that I also use with my Drake 
TR-4C and other rigs). The RF output of the rig goes to the scope, the other 
scope connector to the dummy load (internally the scope connectors are just feed 
thru's).

Let's repeat the info. The Heathkit spec is for a mic with an output level of 
-45dB to -55dB. This info is from the SB-104A specs in the Operations Manual. In 
those days it was dBV (i.e- referenced to 1V). This translates to a signal level 
of between 1.6mV to about 5.6mV. From two sources on the web, I have seen notes 
that are said to be from Heathkit service techs. In these notes, they checkout 
the TX audio stages by applying a 1KHz, 150mVp-p signal into the mic input and 
then measure the outputs and inputs at the various stages. Compared to the mic 
requirements, this is about 30 to 100 times larger than what should be 
needed.The TX audio stage of my rig seems to match closely the Heathkit service 
tech info. For example, on the emitter of the emitter follower buffer 
transistor, the techs says you should see 0.63Vp-p. I see slighty over 600mVp-p. 
Under these conditions my rig will produce full output for the power mode in 
use. This tends to make me think that the TX audio stage is okay. Do you agree 
with this? Or is there a significant difference in circuit performace with 5mV 
signals compared to 150mV signals? I would think that both would be considered 
"small signal gain". 

The next stage this feeds is the balanced modulator. From the schematic it seems 
that a signal from the mode switch (about +11V) is used to 'unbalance' the 
modulator allowing unmodulated RF to go through the modulator's transformer. In 
CW mode, the rig produces full output.

Any input is appreciated.

thanks,

Bill

________________________________
From: Bob Macklin <macklinbob at msn.com>
To: Heinz Breuer <hbreuer at debitel.net>
Cc: William Houghton <rdrbill at pacbell.net>; heathkit at mailman.qth.net
Sent: Wed, November 3, 2010 7:48:54 AM
Subject: Re: [Heathkit] SB-104A SSB drive problems

I'm sorry but if it works properly with a signal generator the fault is not in 
the transmitter.

We don't know how he is making his judgment that he is not getting full output. 
The meter will not tell him!

He needs to look at the RF output with a scope!

Bob Macklin
K5MYJ
Seattle, Wa.
"Real Radios Glow In The Dark"
----- Original Message ----- From: "Heinz Breuer" <hbreuer at debitel.net>
To: "Bob Macklin" <macklinbob at msn.com>
Cc: "William Houghton" <rdrbill at pacbell.net>; <heathkit at mailman.qth.net>
Sent: Wednesday, November 03, 2010 7:41 AM
Subject: Re: [Heathkit] SB-104A SSB drive problems

> No it is a fault in the TX section, probably the mic amp. He needs a signal 
>about 100 times stronger than specified. A LM3900 should still  be available.
> Vy 73 Heinz DH2FA, KM5VT
> 
> Von meinem iPhone gesendet
> 
> Am 03.11.2010 um 14:53 schrieb Bob Macklin <macklinbob at msn.com>:
> 
>> If you get full output with a signal generator the problem in not in  the
>> transmitter. It is either the mike or the way you are using it.
>> 
>> Are you using a monitor scope on the RF output or just reading the meter?
>> 
>> Bob Macklin
>> K5MYJ
>> Seattle, Wa.
>> "Real Radios Glow In The Dark"
>> 
>> ----- Original Message -----
>> From: "William Houghton" <rdrbill at pacbell.net>
>> To: <heathkit at mailman.qth.net>
>> Sent: Wednesday, November 03, 2010 12:16 AM
>> Subject: [Heathkit] SB-104A SSB drive problems
>> 
>> 
>>> Hi fellow Heathkit fans,
>>> 
>>> I'm having a problem with an SB-104A that I'm trying to get up an running.
>>> It
>>> seems that I can't drive to full output with any of my mics. Heathkit
>>> specs the
>>> mic requirements at -45dB to -55dB (aprox 1.6mV to 5.6mV of drive).  My
>>> D-104
>>> provides about 20mV at normal voice volume. It should be able to  drive
>>> this rig
>>> easily.
>>> 
>>> In CW mode, I have full output.
>>> 
>>> Per the Heathkit technician notes on the web, I did drive the mic  input
>>> with a
>>> 1KHz signal at 150mV and did get the proper amplitude at the input  and
>>> output
>>> stages of the TX audio section. With a 150mV input signal the rig  does
>>> drive to
>>> full output. This seems to vindicate the TX audio stage (LM3900 and  the
>>> emitter
>>> follower transistor).
>>> 
>>> 
>>> I'm wondering if the balanced modulator has an issue. I have to  admit,
>>> that I
>>> don't fully understand how the modulator circuit works - I'll have  to do
>>> some
>>> studying :).
>>> 
>>> Anyone ever had this type of problem before? I'm open to all suggestions.
>>> 
>>> Thanks & 73,
>>> 
>>> WA6NEQ (Bill)
>>> ______________________________________________________________
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>> 
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> 