[Heathkit] DX-40 Keying trouble

[Curt]

Always use a VTVM on this old stuff for serious measurements, but the DVM 
was in the drawer and just wanted to verify there was some voltage on the 
key with key up.  I'd never trust anything my DVM told me on these 
hollowstate rigs.  I was surprised that somehow the DVM being connected 
changed the behavior


----- Original Message ----- 
From: "Bill Stewart" <cwopr at embarqmail.com>
To: "Curt" <rhulett1 at comcast.net>
Cc: <heathkit at mailman.qth.net>
Sent: Monday, August 30, 2010 6:24 PM
Subject: Re: [Heathkit] DX-40 Keying trouble


> Hi Curt,
> What I'm suggesting probably doesn't make good sense, but once I was using 
> a DMM to make measurements on a HW-16 osc. ckt..never could get the 
> correct readings and seemed to load the ckt or at least have an effect, 
> making me think there was a problem...then chg'd to the old V-6 VTVM and 
> everything started working and measuring right & no more problems. Found 
> no pblms with the DMM.
> 73, Bill K4JYS
>
> ----- Original Message -----
> From: "Curt" <rhulett1 at comcast.net>
> To: heathkit at mailman.qth.net
> Sent: Monday, August 30, 2010 6:46:47 PM GMT -05:00 US/Canada Eastern
> Subject: Re: [Heathkit] DX-40 Keying trouble
>
> I'm aware that any electronic component, including a ceramic disc cap, can
> fail.  However would expect either a fail short or fail open, not
> intermittent.  Intermittents in my experience are active device problems (
> tube or transistor ) or poor connections or shorts.  This problem doesn't
> exhibit with vibration or movement of the transmitter.
>
> The behavior of the transmitter is that sometimes (but not every time) the
> keyline stays low enough to continue to turn on the oscillator / PA after
> key-up.  The chirp is always there, but the failure of the oscillator to
> turn off surfaced during a prolonged CW rag chew, so may be temperature
> related.  Swapping oscillator/buffer tubes and crystals makes no 
> difference
> in behavior.  With a DVM connected accoss the key ( to measure the voltage
> on key-up ) the problem disappears.  Disconnecting the DVM causes the
> oscillator to hang up again.
>
> I've ordered the caps, but plan some more voltage measurements and further
> testing before wholesale change of the disc caps.
>
> Thanks for all the advice, am surely happy this is a simple rig.
>
> 73, Curt KB5JO
>
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