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K2 PA and 232 problem

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Bob Whelan

K2 PA and 232 problem

Hi

My CAT just stopped working and I have found that there appears to be a
low resistance on the 5v rail which supplies the PIC / 232 interface
chips. This pulls the 5V rail down to 1 v with the result that the PIC
and 232 interface chips are not operating and the 78L05 at U3 is rather
hot and bothered.
Before I get the PA board off the heatsink and start desoldering chips
has anyone any idea of checks I could do to localise the problem

73 and thanks for any info

Bob G3PJT
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Don Wilhelm-4

Re: K2 PA and 232 problem

Bob,

Other than cutting traces or removing chips, there is no easy way to
check for such a shorted condition.

I do suspect that the MAX1406 chip (U4) is the one to remove first
because it is the most likely culprit. The EL5146 chip (U6) is also on
the suspect list.

Before desoldering, remove U1 from its socket and see if the 5 volt
output is restored to normal.

If you do not have adequate desoldering tools, I suggest that you order
both ICs and a regulator, clip the leads on both ICs and remove the
leads one at a time - remove the excess solder with solder wick and
clear the hole with a round toothpick or a stainless steel needle. The
replacement ICs are much more inexpensive than the entire board, so it
is prudent to just replace the ICs wholesale rather than tearing up the
board in an attempt to preserve the suspect components.

73,
Don W3FPR

Bob G3PJT wrote:

> Hi
>
> My CAT just stopped working and I have found that there appears to be
> a low resistance on the 5v rail which supplies the PIC / 232 interface
> chips. This pulls the 5V rail down to 1 v with the result that the PIC
> and 232 interface chips are not operating and the 78L05 at U3 is
> rather hot and bothered.
> Before I get the PA board off the heatsink and start desoldering chips
> has anyone any idea of checks I could do to localise the problem
>
> 73 and thanks for any info
>
> Bob G3PJT
>

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Bob Whelan

Re: K2 PA and 232 problem

Hi Don
I was just about to send something to the reflector. Both the 1406 and
the 5146 had failed but I do not know why.
However the solution was :-
- find out which chip was crowbarring the 5v line. Pull the PIC - it was
OK in my case. Cut pin 16 on the 1406, check and resolder if OK (it was
OK in my case) Do the same with pin 7 of the EL5146 which showed that it
was that chip which had failed. Extract the 5146 by cutting the pins and
clean up the board with desoldering tape. Sacrifice the chip not the
PCB. Replace with a 8 pin socket as EL5146 is discontinued here. Use
almost any op amp with the same pinout except for pin 8 and clip off pin 8.
- I determined which channels of the 1406 were not working using a
prototype board type interface and then rewired the 1406 to use the
unused 232/TTL interfaces, only the pin5/12 circuit had failed in my case
- replace the 78L05 as its not clear how much abuse it had suffered.

73 Bob G3PJT

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David Cutter

Re: K2 PA and 232 problem

Nice work, Bob.

I've seen 78 series regulators die with over voltage but never over current
or temperature, they have excellent protection for those.

David
G3UNA

> - replace the 78L05 as its not clear how much abuse it had suffered.
>
> 73 Bob G3PJT
>
>
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Don Wilhelm-4

Re: K2 PA and 232 problem

In reply to this post by Bob Whelan

Bob,

Great work.

The EL5146 can be ordered from Elecraft, and if it has been discontinued
in the DIP package, the SMD equivalent will be supplied mounted on a
carrier board to adapt it for thru-hole use.
If you have the K60XV installed in your K2, the use of a non tri-state
op amp will interfere with the K60XV transceiver output power control.

I really don't know the cause of these failures, I just don't have
enough information about the circumstances of operation when the failure
takes place. I have seen a few of them, but it is a small percentage -
you were one of the unlucky ones.

73,
Don W3FPR

Bob G3PJT wrote:

> Hi Don
> I was just about to send something to the reflector. Both the 1406 and
> the 5146 had failed but I do not know why.
> However the solution was :-
> - find out which chip was crowbarring the 5v line. Pull the PIC - it
> was OK in my case. Cut pin 16 on the 1406, check and resolder if OK
> (it was OK in my case) Do the same with pin 7 of the EL5146 which
> showed that it was that chip which had failed. Extract the 5146 by
> cutting the pins and clean up the board with desoldering tape.
> Sacrifice the chip not the PCB. Replace with a 8 pin socket as EL5146
> is discontinued here. Use almost any op amp with the same pinout
> except for pin 8 and clip off pin 8.
> - I determined which channels of the 1406 were not working using a
> prototype board type interface and then rewired the 1406 to use the
> unused 232/TTL interfaces, only the pin5/12 circuit had failed in my case
> - replace the 78L05 as its not clear how much abuse it had suffered.
>
> 73 Bob G3PJT
>

Bob Whelan

Re: K2 PA and 232 problem

In reply to this post by David Cutter

Yes but as I don't know what caused this all in the first place , good
engineering practice told me that as it is almost zero cost and effort
to replace it whilst I had the PCB off the heatsink then that's what you
do!

73 Bob G3PJT

David Cutter wrote:

> Nice work, Bob.
>
> I've seen 78 series regulators die with over voltage but never over
> current or temperature, they have excellent protection for those.
>
> David
> G3UNA
>
>
>
>
>> - replace the 78L05 as its not clear how much abuse it had suffered.
>>
>> 73 Bob G3PJT
>>
>>
>