Ray,
I would say that Q21 does not have adequate base voltage for it to
conduct. The base voltage should be about 2.3 volts.
Check R72 and R73 and C143 for good soldering and correct values. Check
R75 on the bottom as well.
The base voltage is set by the voltage divider consisting of R72 and R73
and R75.
73,
Don W3FPR
On 12/25/2012 11:28 AM, Ray Spreadbury wrote:
Hi Don
Thank you for your reply.
I visually checked the components in the preamp circuit & reflowed the
solder of them all including the relay, T6 & Q21, but still got the same
results as before. The coil T6 wires have been put in the correct holes but
I can't check the wires were tinned correctly without removing it but I
think they were.
However, I did a dc voltage check on the transistor Q21 & found some
anomalies- the E,B & C should be 1.6vdc, 2.3vdc & 13.3vdc but with the
PreAmp OFF they are E 0.00vdc, B 0.00vdc & C 13.36vdc. With the PreAmp
switched on, they are E 0.00vdc, B 0.33vdc & C 13.17vdc. (For completeness I
checked them with the Attenuator switched on & they were the same as with
the PreAmp switched off.)
Does it look to you as though the transistor Q21, a 2N5109 is faulty?
Regards
Ray
-----Original Message-----
From: Don Wilhelm [mailto:[email protected]]
Sent: 24 December 2012 21:38
To: Ray Spreadbury
Cc: [email protected]
Subject: Re: [Elecraft] K2 #7340 PreAmp Problem
Ray,
Check every component in the Pre-Amp circuit, including the relay.
Do you have the leads of T6 well stripped and tinned? If so, check to see
that the leads are in the correct holes.
After that look critically at the solder pads for those components around
the preamp circuit - look at the schematic sheet 2 to identify those
components. You should see the solder flow out to an almost invisible edge
on both the solder pad and the component lead. It has been said that the
esolder connection should look like a mountain, but not a volcano - if there
is a ring around the lead or you do not have concave solder fillets, not
enough heat was applied to the connection.
73,
Don W3FPR
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