KPA 100 initial test problem

[ON4WIX]

Hi all

I've just finished building a KPA100 for my K2 s/n 5361. This kit is revision C-5, with R4 (High Voltage Bias Supply) 39k instead of 100k and 1k across RFC 3.
Up to p43 in the manual all seems to have gone well.
During Initial Testing (p20 to 23) I did measure some slight deviations from the resistances and voltages mentioned in the tables, in particular the following voltages (table page 23)
U1 pin 1: 4.93V instead of 5.0V
U1 pin 20: 4.93V instead of 5.0V
U1 pin 27: 5.63V instead of 5.8V
U1 pin 28: 5.52V instead of 5.7V
U4 pin 11: 4.93V instead of 4.93V
I assume that these voltages are close enough not to raise any suspicion.

Then, during Alignment and Installation, I measured 11.8 ohms at the bases of Q1 and Q2, which is just below the 12-16 ohms mentioned in the table at page 41.

At page 43 I ran into some seriously wrong voltages. The voltages at the cathode of D13 and pin 8 of U4 are not stable and fluctuating in a more or less repeating fashion.
At D13 I measure values between 150V and 188VDC whereas this should be a steady 90 to 150VDC.
Same at U4 pin 8: cycling between -28V and -32VDC, this should be a steady -5 to -25VDC.
I've been looking at the reflector archives but can't find any mention about this phenomenon. I have checked all soldering, component values, toroid tinning but can't see anything out of the ordinary.

Any hints as to where I should start looking are very much appreciated.

73 and thanks
Glenn ON4WIX
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[Don Wilhelm-3]

Glenn,

Normally, a 10% variation from the voltages listed is insigificant, and your
Initial Testing measurements are all within that 10% range, so no problem.

I would also judge 11.8 ohms to be well within a safe margin albiet on the
lw side of the range.

The voltages you questioned on page 43, are related to the High Voltage Bias
Supply changes (the new R4 value) - and the negative voltage is derived from
the same source.  My only concern is that your voltages are fluctuating -
while a change is normal between transmit and receive, I would think a
fluctuation could be due to an improper solder connection and it would be
wise to check the xtal oscillator and the associated components including
the diode string and its capacitors.  The list of components is a bit
lengthy, so check the schematic to see which components to check.

I admit that 188 volts is a bit higher than usual - 150 volts is to be
expected with the R4 change.  You could try a value a bit higher than 39k
for R4 if you find you need to lower the HV bias voltage - your higher
voltage could be the result of a crystal with geater activity than average,
and could be 'tamed' by increasing R4 - perhaps 56K would be a good first
trial value.

73,
Don W3FPR

-----Original Message-----

I've just finished building a KPA100 for my K2 s/n 5361. This kit is
revision C-5, with R4 (High Voltage Bias Supply) 39k instead of 100k and 1k
across RFC 3.
Up to p43 in the manual all seems to have gone well.
During Initial Testing (p20 to 23) I did measure some slight deviations from
the resistances and voltages mentioned in the tables, in particular the
following voltages (table page 23)
U1 pin 1: 4.93V instead of 5.0V
U1 pin 20: 4.93V instead of 5.0V
U1 pin 27: 5.63V instead of 5.8V
U1 pin 28: 5.52V instead of 5.7V
U4 pin 11: 4.93V instead of 4.93V
I assume that these voltages are close enough not to raise any suspicion.

Then, during Alignment and Installation, I measured 11.8 ohms at the bases
of Q1 and Q2, which is just below the 12-16 ohms mentioned in the table at
page 41.

At page 43 I ran into some seriously wrong voltages. The voltages at the
cathode of D13 and pin 8 of U4 are not stable and fluctuating in a more or
less repeating fashion.
At D13 I measure values between 150V and 188VDC whereas this should be a
steady 90 to 150VDC.
Same at U4 pin 8: cycling between -28V and -32VDC, this should be a
steady -5 to -25VDC.
I've been looking at the reflector archives but can't find any mention about
this phenomenon. I have checked all soldering, component values, toroid
tinning but can't see anything out of the ordinary.

Any hints as to where I should start looking are very much appreciated.

73 and thanks
Glenn ON4WIX
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