K27100 kaputt

[Raimon-3]

Hi Friends
I have got a problem with my K2/100 and need your help.
I built K2, KPA100, KSB2 and KAT 100 which worked OK for years from the
first day until now.
Recently I upgraded the KPA100 with KPA100UPKT, it passed all tests (I
did not realign it), and it worked perfectly for two weeks.
One day I worked in the morning without any trouble, but when in the
afternoon I fired it up again, the KAT100 was unable to tune: relays
clicked, diodes did not light, tune was not obtained.
I disconnected KAT100 and observed that K2/100 was not working right.
Connected to a dummy load, power and swr readings in the LCD display are:
    Setting  9 w         Read  9.8 w   swr  1.2.1
    Setting 30w         Read  27 w    swr  1.8.1
    Setting 90w         Read   64 w   swr   2.4-1     first appears HiCurr
The bare K2 (disconnected from KPA100) shows no problem. Connected to
the same dummy load and to an OHR qrp wattmeter, power indications agree
in both LCD and wattmeter and swr is negligible.
So I repeated all tests made  when I built the KPA100, all were all
right but D13  cathode voltage was 0 v. instead of 90 to 150 v. (K2 and
KPA100 connected only with the ribbon, power supply only to the K2).
Resistance between D13 cathode and ground is >10k.
Then I connected again K2 and K100, power supply to the KPA100, and
checked voltages of diodes D11 to D14. I remarked notable divergencies:
Diode                     D11          D12         D13          D14
anode Tx 9 w           ---             ---             ---            
13.3 v        
anode  Tx 90w          ---           -----          -2.2 v        -2.2 v
cathode  Rx  9 w    13.4 v       126.2 v    13.4 v       126 v
cathode Rx 90 w     3.4 v           -----         13.4 v        ----
cathode Tx  9 w      13.4 v        123.4 v    13.4 v      122.8 v
cathode Tx 90 w          ---          126.9 v      -1.4 v     127.6 v
Where not reported, read and standard values did not differ substantially.
I suspected 90-150 v bias voltage, so I checked the involved diodes.:
    anode        cathode
D1    23            108        
D2    23.7        70.2
D3    ----            -----
D4    ----            ----
D5    16.2        31.8
D6    -----          16.5
D7    -1.5        -1.4
D8    -1.7        -----

Where not mentioned, read and standard values did not differ substantially.
Should I conclude that the problem is here? And if yes, how to find the
defective  component? Or should I look anywhere else?
I did not find anything similar in the reflector.
Your help will be greatly appreciated!!
I do'nt know whether the trouble was caused by a failure of the KAT100.
Sorry for the long explanations, but I hope they will help.
My electronics knowledge is rather poor.
73
Raimon   EA3EZO
[hidden email]

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[Don Wilhelm-4]

Raimon,

The KPA100UPKT changes the diode voltages in the T/R switch.  There is a
new chart for diode voltages in the KPA100 manual on the Elecraft
website that you should print and add to your older manual.  I am having
trouble following the columns for your measured voltages so I offer no
clear response to those.  Please re-check with the new chart.

If your KPA100UPKT did not contain a replacement for C31, you should
increase the value to .22 uf.  Elecraft will send you a .22 uf 65 volt
capacitor if you send a note to [hidden email], or you can replace
it with 0.2 or 0.22 uf capacitors that you obtain locally.  I have
paralleled 2 0.1 uf capacitors for use at C31 and that works fine.  Any
voltage rating above 25 volts will work.

Without the change in C31, the updated KPA100 can oscillate at about 660
kHz - not all KPA100s show that behavior, and some work initially, but
then later fail.  The .018 uf bypass capacitor at C31 is no longer an
effective bypass after the new design T/R switch components are added.

73,
Don W3FPR

raimon wrote:

> Hi Friends
> I have got a problem with my K2/100 and need your help.
> I built K2, KPA100, KSB2 and KAT 100 which worked OK for years from
> the first day until now.
> Recently I upgraded the KPA100 with KPA100UPKT, it passed all tests (I
> did not realign it), and it worked perfectly for two weeks.
> One day I worked in the morning without any trouble, but when in the
> afternoon I fired it up again, the KAT100 was unable to tune: relays
> clicked, diodes did not light, tune was not obtained.
>

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