Logitech Z5500 - distorted digital audio

[seanc]

Hi all,

I've been given a Logitech Z5500 set and reported that it sometimes had distorted/garbled digital audio and also that the slow-blow fuse in the amp blows once in a while.

I've opened up the controller and most things look fine, although I'm suspicious of a few solder joints.
I was going to re-cap, but after a quick PM with Th3_uN1Qu3 he suggested that might not be worthwhile.

Rather than bombard him with PMs, I thought I'd start a thread. He suggested some resistors might be out of tolerance, but I'm not sure where to start.
Attached front & back photos of the control pod, if they're not good enough, let me know.

Thanks,

Sean.

[seanc]

Re: Logitech Z5500 - distorted digital audio

Bump, anyone have any thoughts or worked on one of these?

[seanc]

Re: Logitech Z5500 - distorted digital audio

Hi,

So I've had chance to explore this more.

I mistakenly referenced resistors in my original post, rather than diodes...

For a while I couldn't get the set to power on at all, so I managed to pick up another control pod from eBay with a faulty cable (with the wrong PID, so it doesn't support surround sound). This seemed to wake the set up, once i went back to the original control pod, it was working, mostly.

The original control pod started out by only playing sound at very specific volume setting and the settings menu text was just displaying strange characters.
The next time I went to test it, it was all dead again, fiddling around with the eBay control pod seemed to bring it back to life again.

So it seems I have a power issue somewhere... I suspect it's the original control pod.
In another thread a user was going to replace the diodes with BAV103, I can see no markings or way to work out what the values of the original are. Are the BAV103 a good choice?

Thanks,

Sean.

[budm]

Re: Logitech Z5500 - distorted digital audio

https://www.badcaps.net/forum/attach...4&d=1501103871
May be the two diodes that Th3_uN1Qu3 talks about are D700, D701? May be you should verify that with him.

[seanc]

Re: Logitech Z5500 - distorted digital audio

OK, so to finish this one off....

In the absence of knowing what the specs of the diodes D700 & D701 were, I swapped them from the known good control pod to the bad one, no change in issue.

When I had the second control pod apart, I noticed the ST LM317D2 transistors were much bigger than the puny looking LM317 in the broken control pod. So being that these seemed to be part of the power circuit, I swapped them out of the working unit into the broken one. Problem solved!
One (or all) of the older transistors must have been failing, I wonder if Logitech changed to the larger LM317D2 in later designs because of superior reliability. They do seem to get quite toasty.

[Th3_uN1Qu3]

Re: Logitech Z5500 - distorted digital audio

Glad you got it fixed in the meantime. The LM317 is not a transistor but a voltage regulator, and indeed, it is put under a lot of stress in this circuit. The very reason those diodes are there is to limit the dissipation of the LM317 by dropping some voltage. A resistor would have been a better idea.

[seanc]

Re: Logitech Z5500 - distorted digital audio

Glad you got it fixed in the meantime. The LM317 is not a transistor but a voltage regulator, and indeed, it is put under a lot of stress in this circuit. The very reason those diodes are there is to limit the dissipation of the LM317 by dropping some voltage. A resistor would have been a better idea.

Ah yes, voltage regulator - I should know that, my mistake. Transistor was the first word I could think of.

Either way, thank you for your help, without you I'd be nowhere. Hopefully the "thicker" LM317 will hold up. I'll keep the butchered control pod for parts or fix it at a later date if I come across another broken set.

[seanc]

Re: Logitech Z5500 - distorted digital audio

I thought I was done with these.
Flipped on the sub woofer today and the pod was dead.
Did it again and the LED came on dim, a few more times and it came to life....

Starting to wonder if I might really need to look in the sub.

[momaka]

Re: Logitech Z5500 - distorted digital audio

Starting to wonder if I might really need to look in the sub.

Or the whole circuit itself and rebuild/re-engineer whatever looks dodgy.

Seems like these turn-of-the-millennium woofer systems are just built like crap. I just finished working on a JBL S412PII amp myself and some of the design stupidities I saw in it were mind-boggling.

[Th3_uN1Qu3]

Re: Logitech Z5500 - distorted digital audio

Starting to wonder if I might really need to look in the sub.

The control pod is being fed 8V from the sub, which is then brought down to 5V by those diodes and the LM317. I have repaired one with issues on the 8V rail, it was cold solder joints after someone else had been inside and replaced the bridge rectifier for that rail, but did not solder it properly. Take the board out of the subwoofer and have us some pictures.

[seanc]

Re: Logitech Z5500 - distorted digital audio

The control pod is being fed 8V from the sub, which is then brought down to 5V by those diodes and the LM317. I have repaired one with issues on the 8V rail, it was cold solder joints after someone else had been inside and replaced the bridge rectifier for that rail, but did not solder it properly. Take the board out of the subwoofer and have us some pictures.

Will do!
I've only been in there to have a look, not done any work.
Interestingly, I powered it all on again, pod came to life OK and then the sub started making popping noises. Also noticed a 50hz hum from the sub on when on the 'direct' input from the side of the pod.

[seanc]

Re: Logitech Z5500 - distorted digital audio

An update.

Original control pod (PID 52x) I could not get out of its coma. It persisted with dim red led or other strange things if it did turn on.
Butchered control pod (PID 8xx) i put back the lm317 and it continued to function fine all the time, but missing features since it's not in a compatible range.
3rd control pod, purchased dead but another PID 52x. Prodded around and found D701 had no continuity in circuit. Took it out and replaced with the diode from the original control pod, working!

Just got to fix the hum on direct input now, other things I've read suggest checking the caps/soldering in the sub which I'll have to do.

Thanks for the advice everyone who has participated. Maybe I'll solve the original control pod another day.

[Th3_uN1Qu3]

Re: Logitech Z5500 - distorted digital audio

Those diodes really are under a lot of stress. Sadly, larger packages will not fit that footprint as the circuit is squeezed together quite tight there.