[Heathkit] SB-104A SSB drive problems

[William Houghton]

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)