Heathkit SB-200 audio distortion

[Kenneth G. Gordon]

On 28 Feb 2009 at 22:23, Tom Buchanan wrote:

> Here is a bit more detail for everyone.  The exciter/transceiver I am
> using is a Yaesu FT-840.  The received audio in the FT-840 sounds
> clear until I turn the amp "ON".  It attenuates the signal somewhat

Ah! That is important!!! Do you hear a relay close when you turn the amp 
on? It may be hard to hear it, since the power switch makes so much 
noise itself. 

The reason I ask is that it sounds to me as though when you turn the 
amp on, something is keying the amp, opening the antenna relay (which 
would attenuate the signal), and un-biasing the amp (which would cause 
noise in the receiver).

The amp SHOULD NOT key when you turn the power on. It should only 
key when you operate the soft-start circuit, which then should cause the 
antenna relay to switch, and unbias the amp from idle.

> and the audio sounds raspy.

This sounds EXACTLY as though your amp is being keyed on when you 
turn the power on to it.

> I have moved the radio away from the amp
> and in close proximity with no change in poor audio.  Turn the amp
> "OFF" and the signal in the FT-840 comes back up and is clear.  

Antenna relay being keyed open.

> 
> I bought the amp in a non-working condition and it included an
> unassembled power supply filter board with 6 new HV caps along with a
> soft start module which may be from Harbach Electronics. Initial
> testing, the amp has a little over 2400 HV and idling current 50 mA. 
> Idling current is a bit low.  

Is the idling current showing AS SOON AS you turn the amp on?

If so, that is another indication that it is being keyed when it shouldn't be. 
There should be NO IDLE CURRENT showing when it is turned on, but 
unkeyed. The amp should be biased off when it is unkeyed. You should 
only have idle current when the amp is keyed, but with no input such as if 
you had not yet pressed the key in CW, or started to talk in SSB.

> 
> Two paper caps C3 & C19 on the bottom of the chassis were replaced
> with modern electrolytics.  I installed a Harbach soft key to connect
> it to the FT-840.  Disconnecting the keying interface has no effect on
> the audio in the FT-840.

I think your soft-key module is defective, shorted, or incorrectly hooked 
up, and is keying the amp on constantly as soon as you turn it on.

Ken Gordon W7EKB

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