Re: Help with strange audio fault #qcx

Ted 2E0THH

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
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

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