KPA-100 QRP except on 80m

[Bob Towers]

I've just completed building the kit (bought 3 months ago from Elecraft)
and all went well until I got to Page 50 of the manual, the QRO transmit
tests. I could only get any power above 11 watts on 80m (I don't have
the 160m module). On all the other bands the output power to the dummy
load, measured on my power/SWR meter, got to 10-11 watts when K1/K2
tripped and then dropped to a few watts and would not increase at all.
On all bands the SWR into the dummy load was 1.1:1 or better.

I thought at first it might be the LPFs, but now I think it isn't

a) receive performance was the same across all bands
b) QRP maximum output was 9-12 watts on all bands
c) the KPA-100 relays tripped at the appropriate band changes eg 17/2 ->
10/12/15 -> 80.
d) the toroids in the LPFs were all soldered in right
e) the physical layout of the LPFs (toroids, capacitor values) is right.

I did a few voltage measurements:

                                    Bands         QRP (<11W)
QRO (>=11W)

T sense (pin 5 of U1)                80            0.00V              0.62V
    all other        0.00V              0.00V


For some reason, the power amp isn't switching on, on all bands except
80m. Presumably T sense is the trigger...

Any help would be much appreciated.

Many thanks

Bob Towers
MM0RKT

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[Don Wilhelm-4]

Bob,

What have you finished of the Testing and Alignment.  If you have not
yet done the PA bias adjustment, your KPA100 will have little or no
output as you describe.

Actually I had some trouble understanding your exact problem.
Do you mean that you have no KPA100 output on 80 meters, but it is OK on
other bands?  Or do you have no hi power output on any band?  Or, do you
mean that you have about 11 watts of power anytime the power request is
set above 11 watts?

It is not 'T sense' - that only controls when the fan comes on.

If QRP transmit is OK, then likely your Low Pass Filters are good -
check on all bands.
If you receive properly when set to 11 watts or higher, then at least
part of the T/R switch is working properly.

Calibrate your wattmeter initially at 9 or 10 watts (you can re-do it at
higher power later), but the wattmeter must work to control the power of
the K2 (and the KPA100).

If your problem is that you have some power output when the power
request is set above 11 watts, but not the proper amount, then check the
Scaling function of the wattmeter - RP1, R24 and R25 and Q10 and Q11.  
During hi power transmit, the drain of Q10 and Q11 should be near 0
volts and you should have 4 to 6 volts on the gates.

73,
Don W3FPR

Bob Towers wrote:

> I've just completed building the kit (bought 3 months ago from
> Elecraft) and all went well until I got to Page 50 of the manual, the
> QRO transmit tests. I could only get any power above 11 watts on 80m
> (I don't have the 160m module). On all the other bands the output
> power to the dummy load, measured on my power/SWR meter, got to 10-11
> watts when K1/K2 tripped and then dropped to a few watts and would not
> increase at all. On all bands the SWR into the dummy load was 1.1:1 or
> better.
>
> I thought at first it might be the LPFs, but now I think it isn't
>
> a) receive performance was the same across all bands
> b) QRP maximum output was 9-12 watts on all bands
> c) the KPA-100 relays tripped at the appropriate band changes eg 17/2
> -> 10/12/15 -> 80.
> d) the toroids in the LPFs were all soldered in right
> e) the physical layout of the LPFs (toroids, capacitor values) is right.
>
> I did a few voltage measurements:
>
>                                       Bands         QRP (<11W) QRO
> (>=11W)
>
> T sense (pin 5 of U1)                80            0.00V              
> 0.62V
>                       all other        0.00V              0.00V
>
>
> For some reason, the power amp isn't switching on, on all bands except
> 80m. Presumably T sense is the trigger...
>
> Any help would be much appreciated.
>
> Many thanks
>
> Bob Towers
> MM0RKT
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
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> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
>
>

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[Bob Towers]

Many thanks, Don, for your comments.

Here's my replies...

Don Wilhelm wrote:
> "Bob,
>
> What have you finished of the Testing and Alignment.  If you have not
> yet done the PA bias adjustment, your KPA100 will have little or no
> output as you describe. "
I've done SWR calibration, power calibration, power supply test, fan
test, external keying test, PA bias adjustment and power tests <= 11
watts. I've started the 20W power tests and that's where the problems
appeared.
>
> "Actually I had some trouble understanding your exact problem.
> Do you mean that you have no KPA100 output on 80 meters, but it is OK
> on other bands?  Or do you have no hi power output on any band?  Or,
> do you mean that you have about 11 watts of power anytime the power
> request is set above 11 watts? "
I have hi-power on 80m only. All the other bands (I don't have 160m)
only power up to 11W. Over 11W, K1 latches, but the power then drops to
around 1-2 watts. There is no increased bias to the PA finals.
>
> "It is not 'T sense' - that only controls when the fan comes on."
Thanks! I thought it was the control to switch the bias to the PA.
>
> "If QRP transmit is OK, then likely your Low Pass Filters are good -
> check on all bands. "
Yes, QRP transmit on all bands is OK.
> "If you receive properly when set to 11 watts or higher, then at least
> part of the T/R switch is working properly."
I'll check that one.
>
> "Calibrate your wattmeter initially at 9 or 10 watts (you can re-do it
> at higher power later), but the wattmeter must work to control the
> power of the K2 (and the KPA100). "
Yes, I've already done that via R26.
>
> "If your problem is that you have some power output when the power
> request is set above 11 watts, but not the proper amount, then check
> the Scaling function of the wattmeter - RP1, R24 and R25 and Q10 and
> Q11.  During hi power transmit, the drain of Q10 and Q11 should be
> near 0 volts and you should have 4 to 6 volts on the gates. "
I'll check that one, too.

A feature so far is that I only get the correct bias voltage (base of Q4
= 1.51V @20W O/P) on 80m. On all other bands, it stays unchanged at
0.52V when I move from <11W to >=11W (the point where K1 latches).
>
> "73,
> Don W3FPR "

73

Bob MM0RKT


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[Bob Towers]

Don

I've checked the bias voltages in TEST mode, both QRP and QRP, and
they're correct for all bands. So the fault is clearly not a logic one
associated with U1. So... the possibilities, I think, are

a) T/R circuit, but I don't know why that should be band-specific
b) LPFs, but QRP TX and RX is unaffected
c) ALC

The ALC could kill the QRO outputs on 40 - 10m, but an explanation of
how it does it above 11W would be much appreciated.

73

Bob MM0RKT


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[Bob Towers]

Don

Oops! I should have written

".... TEST mode, both QRP and QRO, and...."

Apologies

Bob MM0RKT

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