[Elecraft] KPA500 T/R Switch Board – D8 Identification

[Brian D]

Follow up on my recent problem. D8 and D9 changed (one was about 0.7 volts
at 1A forward current, the other about 1.2 volts.) It has had heavy use in a
couple of contests since with no problems.




Brian D via Elecraft <elecraft at mailman.qth.net> wrote:

> D10 S/c gave me the same problem. several years later it has recurred but
> is intermittent this time. I'm about to investigate suspect heat has
> affected a dry joint this time.
> 
> If yours is an early amp then there are modifications to bring it up to
> current spec. I seem to have lost teh details of that.
> 
> 
> 
> "Earl via groups.io" <earl.ab9tx=gmail.com at groups.io> wrote:
> 
> > Oliver it looks like D10 is cooked not D8?
> > 
> > On Sat, Jun 6, 2026, 06:32 Oliver Bross via groups.io <oliver=
> > bross.eu at groups.io> wrote:
> > 
> > > Hi all,
> > >
> > > I’m troubleshooting an Elecraft KPA500 that has the following symptom:
> > >
> > >    - TX output is normal (500+ W)
> > >    - In STBY, receive is normal
> > >    - In OPER, receive is attenuated significantly on all bands
> > >    - Elecraft support suggested an internal T/R switching issue
> > >
> > > While inspecting the T/R Switch Board, I found D8 appears damaged and
> > > may need replacement.
> > >
> > > From the KPA500 schematic I have, D8 on the T/R Switch Board is
> > > labelled *S1M*. D7 and D9 in the same area are also shown as S1M.
> > >
> > > Before I replace it, can anyone confirm:
> > >
> > >    1. Is D8 indeed an *S1M (1A, 1000 V SMA rectifier)*?
> > >    2. Were there any board revisions that used a different diode?
> > >    3. Has anyone seen a failed D8 cause receive attenuation in OPER
mode
> > >    while TX remains normal?
> > >    4. Are there other components on the T/R Switch Board I should
check
> > >    at the same time (relay contacts, PIN diodes, switching network,
> etc.)?
> > >
> > > I’ve attached a close-up photo of D8 and can provide the relevant
> > > schematic section if needed.
> > >
> > > Thanks and 73,
> > >
> > > OliverOM0RX
> > > 
> > >
> > 
> > 
> > -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent
> > to this group. View/Reply Online (#3570):
> > https://groups.io/g/elecraft/message/3570 Mute This Topic:
> > https://groups.io/mt/119676095/324488 Group Owner:
> > elecraft+owner at groups.io Unsubscribe:
> > https://groups.io/g/elecraft/leave/1119757/324488/822510839/xyzzy
> > [groups at planet4.co.uk] -=-=-=-=-=-=-=-=-=-=-=-
> > 
> > 
> > 
> > 
> 
> 


-- 
Brian D 
G3VGZ G8AOE G3T
IO94im