[1000mp] FT1KMP MK V Receive Break-in Problem: FIXED !

[Steven J Fraasch]

I fixed my S-Meter overshoot problem following replacement of Q6451: 
There was an undocumented, leaded 100 pF (?) cap added across Q6451
Collector-Emitter that I had re-installed on the replacement transistor. 
Being suspicious of something not documented in the schematic, I removed
it.  The S meter no longer overshoots going from transmit to receive.

It's possible that this cap went across something else, and I was off by
a lead, but both the B-E and C-E junctions are bypassed (C6455 and C6459
respectively); therefore, I don't know why another cap would have been
added.  My only guess is that C6459 is relatively large (.1 uF), has poor
HF bypass characteristic, and is located too far away from Q6451 C-E to
be effective; hence, a smaller RF cap was added to improve the bypass
characteristic, especially on the higher bands (rig again seems fine on
all bands without the cap).

Does anyone have a service bulletin pertaining to this extra leaded
capacitor ?  If you refer to the tuner-main PCB parts layout (Page 6N-4
in the manual) it is located between Q6453 and C6453.  Again, I am pretty
sure it went across Q6451 C-E.

BTW:  If you ever need to replace Q6451 (I noticed W7HV had to make the
same repair, this seems like a weakness in the MP and Mark V), you don't
need to take the tuner apart, nor remove any cables.  Merely remove the
top rear cover, remove the tuner cover, solder-suck the junctions of
Q6451, and reach in the side with a small screwdriver and pry the
transistor out.  You can shake the tuner to get the transistor out once
it falls thru.  Then, merely solder the transistor on the back-side of
the PCB.  I think that side makes more sense anyways, as the transmitted
E-field should be less intense on that side of the board.  Perhaps
locating the transistor on the RF component side of the board had
something to do with adding the leaded cap.

73,

Steve Fraasch, K0SF


On Sat, 21 May 2005 14:04:42 -0500 Steven J Fraasch <sfraasch at juno.com>
writes:
> I am a relatively new owner of a Mark V, having purchased a used unit 
> in
> March.  I applied the key click and roofing filter mod before I used 
> the
> radio, and was very happy with how the radio worked.  In fact, I 
> like it
> better than my IC781.  BTW: I measure the 10-90% rise/fall time to 
> be ~3
> ms after applying the INRAD key click mod.
> 
> About 3 weeks ago, I had left the antenna switch on to a high 40m 
> dipole
> while it was connected to my Mark V, which was turned-off.  While I 
> was
> away, a brief snow squall came thru.  When I came home, I noticed 
> that
> the receiver was dead.  I troubleshot the receiver, (and made a few
> "left" turns) but finally determined that Q6451 was shorted.  I 
> presume
> that the precipitation static had exceeded the VC-E breakdown 
> voltage of
> 100V, and the transistor failed.
> 
> I replaced the part last night with a NTE equivlaent, a NTE289A 
> from
> Mouser.  After replacement, the receiver came to life.
> 
> Yet, I still have a problem:  In break-in, the S-meter does not 
> recover
> in time during initial transmit, such that the receiver path still 
> seems
> to be open during an initial "dit" from the keyer.  The problem 
> seems to
> be that the receive path is neither "opening" completely, nor 
> quickly
> enough during transmit.
> 
> I have not attempted further troubleshooting; hence the email, but 
> my
> next check is to check RL6451 and insure that it is operating 
> properly. 
> An open D6453 could keep the relay open longer than it should.  
> After
> that, I'll look at the PIN diodes D1055 and D1056 in the RF unit and 
> make
> sure that they operate properly.
> 
> A couple weeks ago during one of the wrong "left" turns, I had also
> replaced Q6453, a self-biased PNP.  I had failed to take into 
> account its
> series 10K limiting resistor; therefore, I had concluded that the 
> B-E
> junction was bad when in fact it was operable.  I replaced it with a
> similar Digi-Key part, except the replacement part has a 10K B-E 
> shunt
> instead of the 2SA1563's 47K shunt.  Otherwise the base resistor is 
> 10K,
> same as the 2SA1563's.  The stiffer pull-up would not seem to cause 
> this
> problem; if anything, it would lengthen the rx open time, rather 
> than
> lengthen receive time.  Also, the circuit would seem to indicate a
> stiffer pull-up is fine.  Therefore, the fact that I don't have the 
> same
> exact transistor seems to be a ruse.
> 
> Any other suggestions ?
> 
> 73,
> 
> Steve, K0SF
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