Heathkit SB-200 audio distortion

[Jeff Goldman]

Tom,

After reading a few of the related posts, I want to clarify some  
wording.  In your last paragraph below, you refer mention installing  
the "soft key to connect it to the FT-840".   The Harbach soft start  
option is used to slowly start up the AC power to the power supply  
when the main power switch is first turned on.  It has nothing to do  
with keying the amp from the exciter.  I am not sure exactly how you  
have things hooked up, or if you may need some sort of keying  
interface between the amp and your radio, but whatever it requires,  
this interface is definitely not the "soft start" option.

I agree with one of the other commenters that it sounds like you amp  
is locked in the keyed position as soon as you turn it on.  Disconnect  
the cable going to the amps relay control connector on the back panel  
and measure the voltage at the connector (ground to the center  
conductor) - - it should be about negative 130-175 volts DC, which is  
the standby cutoff bias being applied to the tubes, which in turn  
causes the plate current to go to zero ma.  If the bias is not there,  
its time to open up the amp and find out why there is no cutoff bias.

Good luck with the fix.

73,
Jeff, K3DUA



On Feb 28, 2009, at 11:23 PM, 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 and the audio  
> sounds
> raspy. 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.
>
> 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.
>
> 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.
>
> Tom
> KG4KGY
>
>
>
> -----Original Message-----
> From: Heathkit Owners and Collectors List [mailto:HEATH at LISTS.TEMPE.GOV 
> ] On
> Behalf Of Tom Buchanan
> Sent: Saturday, February 28, 2009 5:24 PM
> To: HEATH at LISTS.TEMPE.GOV
> Subject: Heathkit SB-200 audio distortion
>
> Hi,
>
> I am working on a SB-200 for a winter rebuild project.  I have the amp
> working with full output, but the problem is the receiver audio.   
> The amp
> has a new power supply board, soft start and soft key interfaces  
> plus 2 new
> electrolytic capacitors under the chassis.   When I turn the amp on,  
> the
> radio audio is somewhat distorted and fuzzy. I turn the amp off and  
> the
> receiver audio is clear and clean.  Could it be picking up noise  
> from the
> fan?  Any ideas?  Thanks.
>
> 73,
>
> Tom
> KG4KGY
>
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