[Elecraft] [K2] How much AF volts at 1kHz to get full RF power using KSB2?

[Gareth - M5KVK]

I’m rapidly getting nowhere with this problem. I need to understand how the
overall circuit works and what I should expect to see on the ‘scope.

My understanding is that RF Power is basically under the control of a
feedback loop controlled by the VRFDET circuit on the RF board.

The front panel PWR OUT is simply an input to Control Board U8, which
generates V PWR via U8. V PWR sets the target level in the ALC block and
affects V ALC. V ALC goes to Q24 and controls the o/p level of the BFO,
which then flows through filters etc to the PA, where VREFDET is generated.

In CW mode, that’s all there is, and it works. Varying PWR OUT varies the
RF power generated. The problem comes in SSB mode.

With the KSB2 installed, and in CW mode, the BFO signal goes in and out of
the KSB2 via Q3. All OK. In SSB mode, the KSB2 has an additional ALC
circuit block that uses VRFDET, ALC THR and PWR CTRL to throttle the SSB
generated on the KSB2 which then goes back out via Q2. This doesn’t work.

In SSB mode, the AF mixes in the balanced modulator but changing PWR OUT
has no effect. If my understanding is correct, the problem would seem to
lie in one of two places: a) the ALC circuit block on the KSB2 or b) the AF
compressor circuit.

I want to remove the Mic as a source of variability but I don’t know what
AF voltage should cause 100% of the set o/p power.

73

Gareth - M5KVK

On 1 October 2014 22:10, Gareth - M5KVK <m5kvk at m5kvk.org> wrote:

> Thanks for the help Don.I really appreciate it.
> Apart from finding that R7 was not fitted (how many times had I looked at
> the board?), I can see nothing untoward. Fitting it made no difference.
>
> I note that these were the ones changed in the Rev E errata. Relevant?
> Mine is a Rev D board and (e.g.) R9 is 10k rather than 3.9k as per the Rev
> D manual.
>
> The receive audio is slightly down with OP1 but not a lot. Diodes are all
> the right way around and T2 appears to be OK, no shorts and buzzes through
> OK.
>
> I did see a suspicious poor solder joint with R69 on the RF board, but
> again it made no difference.
>
> On 1 October 2014 18:16, Don Wilhelm <don at w3fpr.com> wrote:
>
>>  Gareth,
>>
>> The PWR CTRL on the KSB2 does not do much at all, so what you are seeing
>> is about right.
>> The power control is done from the VRFDET feeding the base of Q1.  It
>> causes Q1 to conduct and alters the voltage to the base of Q2.  The SSB
>> signal is 'throttled' after the signal is generated and passed through the
>> OP1 filter.  The BFO injection should not change for SSB operation like it
>> does for CW mode.
>>
>> Check resistors R4 and R9 for proper values and good soldering.  Check
>> the orientation of D2.
>>
>> If you are still not getting adequate signal at D4, then you may have a
>> problem with T2 or a solder bridge on one of the capacitors in the filter
>> area that is dragging the signal down.
>> How does the OP1 filter behave in receive.  If the RX signal is
>> attenuated more than with the variable filter, that problem may be related
>> to your TX problem.
>>
>> The VRFDET signal level is a level determined by the actual power output
>> of the K2 - it comes from the wattmeter forward power detection in either
>> the KAT2 or the KPA100 - without either of those options, it comes from the
>> RF Detector D9 on the RF Board.
>>
>> 73,
>> Don W3FPR
>>
>>
>> On 10/1/2014 11:17 AM, Gareth - M5KVK wrote:
>>
>> Don, Yes, D4 seems to be OK
>>
>> I've done some further probing and I'm wondering if the problem lies in
>> the ALC circuit. Basically, the power doesn't vary as I change the POWER
>> control.
>>
>>  Scoping U2. There is data on pin 8, but only the second nibble changes.
>> ALC THR goes up as I change POWER, but PWR CTRL stays stuck at about 5.6V
>> regardless of where POWER is set or what AF there is.
>>
>>  What is the purpose of PWR CTRL? Should it be changing?
>>
>>  Gareth - M5KVK
>>
>> On 29 September 2014 21:45, Don Wilhelm <w3fpr at embarqmail.com> wrote:
>>
>>> Gareth,
>>>
>>> Is D4 mounted with the correct orientation. - the line on the silkscreen
>>> points to the cathode end.
>>>
>>> 73,
>>> Don W3FPR
>>>
>>>
>>> On 9/29/2014 4:16 PM, Gareth - M5KVK wrote:
>>>
>>>> Thanks, Don
>>>> I'm getting more or less the same as you except for the RF out on U5
>>>> pin 4.
>>>> I'm seeing the BFO of about 400mV pp with no AF, and then going up to
>>>> 800mV
>>>> pp when I whistle into the Mic.
>>>>
>>>> Doh!
>>>>
>>>> I thought I had the carrier balance right, but obviously not. Switching
>>>> on
>>>> the SSBA=BAL shows a bif signal on U5 pin 4. I've adjusted that down to
>>>> almost zero on the 'scope, but I'm still not getting much RF out.
>>>> 73,
>>>>
>>>>
>>>
>>
>>
>