Cathode of D7 or R108/R109 have one end connected to the left Q5.
For the other one, gate of Q6, or one end of C35 or R103 are connected to the right Q5.
If that helps at all?

 

Anything that's bigger than a grain of sugar is always helpful.

However I've just ordered needle probes and a basic USB microscope too. They will be here Friday, but I'll look tomorrow at the location of the components you suggest - fingers crossed they'll be at least a little larger!

Until tomorrow - again, thank you!

 

As it looks to be very difficult to break the 5.4 volt feed to the audio amplifier (And C202) I think we could achieve the same think by preventing the P channel mosfet Q6 from turning on. I think shorting out the pads where C35 would have been fitted would would do this.(The * by C35 on the schematic indicates that it is not fitted.) If the battery current dropped and the 900 hz tone disappeared it would show the fault was a short on the 5.4 volt rail.

Les.

 

I doubt Q6 is turned on. If it is then about 5V is being dropped across it and it would be very hot or blown up (which may turn out to be the problem yet). The voltages I have asked for will tell if Q6 should be on or not.

The low voltage that is on TP23 may be via R108//R109 and one half of Q5.

 

Hi Albert,
I don't think Q6 is permanently turned on. I think it starts to turn on which then causes the LM3488 to start shutting down (Due to an overcurrent condition.) I think at this stage Q6 starts to turn off at this stage and the process repeates at 900 hz. One simple test that might be worth doing would be to measure the resitance between the 5.4 volt test point and ground to see if there is a solid short possibly caused by a failed audio amp or C202.

Les.

 

Hi guys

Just wanted to let you know I've not forgotten this - just been a manic couple of days at work, and as I write, my needle probes and microscope haven't arrived yet.

Soon as they do, I'll have the weekend, and will be able to do the tests as you've instructed - and hopefully we'll be able to move forward on what we need to do to either bodge, or fix the box.

I am sorry for the delay, and, as ever, very grateful for your continued support.

R.

 

measure the resitance between the 5.4 volt test point and ground to see if there is a solid short possibly caused by a failed audio amp or C202.

Hi Les, Albert.

You might be onto something here - all other voltage rail TPs measure KOhm and even MOhm - the audio amp supply rail measures 5 Ohm to ground. And, when shorting the pads of C35* the oscillation does indeed stop. (I don't have enough hands to see if the current-draw is reduced, but my assumption is that it is. I really need to get some kind of bench set up. Did I mention I'm working from an armchair?!

I am going to try with the new probes to measure the other bits today. The microscope I bought is only good when literally sat on top of its subject - I envisaged something I could sit on a tripod and work underneath. Bah.

 

I haven't been able to get any more measurements, as there's been a couple of small family matters I've needed to attend to over the last week or two - sorry about that.

Hopefully, if you guys are still on board to help me out, I'll be able to test some more early next week.

If not, but you're still reading - thanks again for the help - we didn't quite get there, but we got close!

Here's hoping for one (or two) last push(es)!

Ross