Re: Help with strange audio fault #qcx


Bill Cromwell
 

Ted,

Do not intentionally short the battery terminals out. If you do be sure to wear adequate eye protection. Have a new fuse on hand and the present fuse between your crowbar and those battery terminals. Batteries can and do explode when the terminals are shorted out.

73,

Bill KU8H

On 1/14/19 8:37 PM, James Daldry W4JED wrote:
Hi, Ted
On the top side of the board, check the resistance from pin 5 of IC10 to ground. It should be around 10K. Now short the battery terminals out. It should now be about 5K. If it fails these 2 tests you have a connection problem at pin 5.
If you connect your voltmeter to pin 5 (top side, not bottom) and then connect the battery it will probably start at 5-ish volts and gradually drift upward. That's your 5 seconds of audio. One end of R39 should be zero ohms to battery negative and the other end zero ohms to pin 5 of IC10, measured on the top of the board. I keep stressing "top of the board" because there may be a bad solder joint or an open piece of foil at pin 5. The break may be invisible. It's possible that R39 is open but so unlikely that I would exhaust all other possibilities before changing it out. Oh, the other foil that could have a problem is from R39 to ground.
Jim Daldry W4JED
On 1/14/19 2:01 PM, qrp@... wrote:
Just a quick update.

I am still struggling to track this fault down but I do now have some data from my latest investigation.
Do note that this QSX40 set aligned perfectly and was working fine for 2 weeks.

The fault is no receiver output. However if I turn it off for an hour, it fires up again for about 5 seconds and then fades out. This is repeatable. If I fire it again instantly, the fault hasn't cleared and stays silent.

I have been around every solder joint with the iron, not very scientific but sometimes it is a quick fix.

So I though I would start at the beginning with a factory reset and realignment.
8.7 Peak BPF worked fine and produced its normal deafening output.
However 8.8 I-Q Bal, 8.9 Phase Lo and 8.10 Phase Hi are all at 0
Given this alignment needs audio from the output stage, this I think is a fairly good pointer.

IC9 Voltages
1) 4.78
2) 4.78
3) 4.45
4) 0
5) 2.44
6) 2.45
7) 2.45
8) 12

And now IC10, these voltages are clearly wrong
1) 11.06
2) 11.06
3) 11.09
4) 0
5) 5.88
6) 5.36
7)11.10
8) 12

I have been over and over the PCB looking for shorts and dry joints around IC10 and IC09 to no avail (that doesn't mean they don't exist)
The weird part is that after leaving the set off, after time it will always fire up for a few seconds before fading away.

I am going to get my supper and then have another bash but any pointers at all would be most gratefully received.

73 Ted M7ECH

On Thu, Jan 10, 2019 at 02:13 PM, <qrp@...> wrote:

Just before I remove the QCX40 from its enclosure to fault find, I
thought I would share it in case anyone else had ever experienced
a similar issue.



My QCX40 rev4 has been performing beautifully for about a month
now since it was built.
It is installed in a bespoke enclosure with a alternative volume
control with a power switch.

The fault first manifested itself about 2 weeks ago, I had left
the set on all day but when I went to listen out later in the
evening, there was no audible output.
I had some work to complete so I turned it off for a few hours but
when I switched it back on, all perfect again!
It remained fine for about 5 minutes and then it just went quiet
again, not instantly but as though a volume control was being
turned down.
It was now late so I left it overnight with a view to fault find
it in the morning.
Of course in the morning all fine again!

It has been fine until a few days ago when the fault returned.
If I turn it off for an hour, it fires up again fine but then the
gremlin in there turns the audio down after about 20 secs. This is
fairly repeatable. If I fire it again instantly, the fault hasn't
cleared.

A very quick check at low power during fault conditions
demonstrates it is still able to transmit on the selected frequency.
I am suspecting a capacitor issue in the audio circuitry or
earlier in the signal path.

I just wondered if anyone has experienced anything like the same
issue?

Many thanks in advance.

73s Ted M7ECH
--
bark less - wag more

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