[Milsurplus] MAK needs a good home

[mac]

John,

The MAK and MO-1 manual includes dire warnings about over-modulating  
and modulating without the PA loaded properly so as to avoid arc-overs  
and burning in the 12 pin power input connector.  The arc-over problem  
is real but not a consequence of modulation and/or loading.  As  
designed, the HV B+ line to the PA and modulators is broken by the  
contacts on K-102 on key-up (i.e., opening the PTT switch on the  
mic).  Problem is, the 10H modulation reactor down on the power supply  
has something like 200+ma flowing through it and all this energy  
(inductive kick) has to go somewhere when the relay contacts open and  
the load represented by the PA and modulator is suddenly removed.  In  
the original MAK (and MO-1) design no place to go except to flash over  
to the chassis somewhere and I observed just this happening through  
the insulation on the lead to the PA plate cap and the same thing  
happening down on that closely spaced 12 pin connector would be a very  
likely occurrence as well.

Serious design error here.  A similar kind of HV switching is done in  
the TCS transmitter but an RC snubbing circuit is included to absorb  
the kick.  Same circuit can't be used on these sets because of the  
high level audio riding on the line along with the HV.

Came up with the following modification to solve the problem.  Rewired  
the plates of the modulator tubes so that they are connected to the  
incoming "hot" HV and audio line at all times (i.e., NOT through the  
relay contacts in K-102).  OK to do this because the HV dynamotor down  
in the PS is also turned off on key-up.  Bypassed the modulator  
screens (also fed through K-102) to ground with a .47 mfd cap in  
series with a 47 ohm, 2W resistor.   This delays the turn-off of the  
modulator tubes on key-up so they can absorb the inductive kick from  
the modulation reactor.  Included the resistor to limit the inrush  
current to the cap when the rig is keyed (may not be needed).  Scope  
now shows no key-up spike at all on the incoming HV line where I was  
seeing a kick of 2KV+ initially.

Note: These are rare radios and some on these forums would take  
serious issue with the surgery described above. Guess I've always been  
in favor, though, of reasonable changes as necessary to allow a radio  
to continue to operate as opposed to putting it on a shelf and just  
looking at it.    By the same token got no problem with a heart bypass  
or an artificial knee or hip in this old carcass if that's what it  
takes to keep me going a while longer.  :^)

Dennis D.  W7QHO
Glendale, CA

**********************

On Aug 14, 2012, at 11:16 AM, John McCarty wrote:

> Dennis;
>
>   Sorry if I asked you this before (my bad memory), since I'm the  
> owner of this MAK now,
> what's the design fault.
>
>
> 73 + tnx
>
>
>
> John n9hrt
>