> From: Colin Lamb <k7fm@teleport.com>
>
> The pi-network is really a transformer. So, a change in the output
> capacitance could be based
> upon a change in the input characteristics as well as the load. Is the
> voltage correct? Are
> both tubes the same color? Are any of the turns shorted?
The tubes glow equally when the plate amperage is high enough. No shorts are
evident. HV is
good. A previous owner changed the relay and shielded it. He moved some
circuitry from its
original position to the top of the load capacitor. (If you have broadband,
check out
http://www.geocities.com/w4kv/sb220 )
>
> If there is no logical explanation, it could be an illogical one. If a
> friend comes over and
you
> go out of the room, does it still tune the same?
Since the enclosure is open on the rig, I'm afraid to let anybody in the shack.
>Do you have any confessions you want to share with the group that might cause
>the amplifier to
act in such a manner?
A couple of weeks ago I "tested" the HV with a DVM and fried the DVM and
several components on the
Harbach PC board. Fortunately I was not a participant in the circuit, but
since the POW, I don't
recall what the position of the load capacitor was prior to the event.
>Have you had this effect on amplifiers at club stations?
They won't let me use the amplifier.
>Have you ever been rejected for a dxpedition, even though you put the money up
>front?
I plead the 5th.
>
> You can work backwards and play with the amp offline (unplugged and plate
> shorted). Put a
> resistor equal to the plate impedance from plate to ground, then use a MFJ or
> Autek (or other)
> bridge on the output and see where you get 50 ohms. Is the plate tuning
> control where it should
> be?
I'm going to move the stuff from the load capacitor first.
> In the end, if you cannot find the results you expect, you need to keep
> asking dumber and dumber
> questions.
I find that quite easy.
>
> Colin K7FM
R. Measures wrote:
>>> Two possibilities: The load R is not 50-ohms -- or the bandswitch has
>>> some vapourized contacts due to previous arcing. This is more likely
>>> to occur to the 15m and 10m contacts.
>>> The load R's ohms can be measured with a DMM.
>>
>> I had previously checked the dummy load with DMM (47.8 ohms)
>
>Close enough
>
>> and with a MFJ 239 SWR meter (50 ohms
>> and 1:1 swr). There is no change with switching the bandswitch.
>
>To find missing / vaporized contacts, one must visually inspect all of
>the contacts.
>
>> I have polished the contacts
>> with a bit of grocery bag paper. No change.
I also checked the switch, front and back, and they appear intact.
I also took the "unmodified" amplifier apart and replaced the PC board with the
Harbach mod. It
now loads properly with the capacitor dial at about 1:30
Thanks
John
w4kv
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