Home Cinema active Subwoofer malfunctioning

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D2+R5 = 0.55V approx upon startup, decreases with time to ... 0.38V roughly?
R8 = 0

Q2 is the right way in.
 
KeepItSimpleStupid said:
Q2, D2 or R5 has to be bad. I'll bet Q2. Maybe put the last batch back that worked and forget about matching.
Click to expand...

So, I changed both Q1 and Q2 with some beautiful new C2240s, closely matched (400 and 420HFe), and... Q7 E-C still stays at 0! Left with D2 or R5?

D2 reads 0.54V in diode mode (and AC etc off) one way, 0 the other (as expected that is)
R5 seems to indicate a resistance of 0.67k.
 
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Mmm. I can't seem to remember anything. While sleeping, I forgot about an issue. The biasing on Q7 will NOT be present UNLESS C4 is shorted. That's how they get away with not using a pot for a bias adjustment. Sorry about that.

So, check the voltage across Q7 and the variability with the new pot with C4 shorted.
 
Hello there! Back from some trips, busy with work lately, but I found a few minutes to do some testing at last.

Alas, no change with C4 shorted. Nothing is burning, but no voltage on Q7 E-C... Same conditions, Q10/11 still out.

 
Welcome back!

How in the world did we get to the point in which nothing seems to work?

Remember, way back when when you injected the 1.5 V battery signal and looked at the differential amp?

This doesn't make much sense because, i think the only change you really made was changing the transistors in the diff amp. (Q1/Q2) I think.
 
yeah I know it's rather confusing. no idea what's going on? what do you suggest I do?

I tend to be busier these days with work but i'm still keen on fixing it in the evenings when I can!
 
I'm busier right now too. How about quickly putting the outputs in; setting R(whatever) so it's 3.6K (memory) and basically see if it functions. Look at E-C with signal, but no speaker.
 
Hi!

Many weeks later, I've picked this again... I threw everything back in (only Q10 and Q11 were still out), and gave it a shot. Now, in auto-mode, it detects input fine, switches from red to green, but I get no output whatsoever. I've checked Q10 and Q11 voltages, roughly around the 30V mark, so something is happening; but they are not heating up whatsoever.

I mean at least nothing is burning at this stage, but who knows what I've bust now... Need to get back into the swing of things, any ideas of what I should check? As far as I can tell, transistors are fine (and closely matched), caps were upgraded, C4 was replaced too, although I did try shorting out C4 in the tests (for the variable resistor to kick in)...

If you are not too busy with other things at the moment, any ideas what I should look for and start measuring?

Hope you had a good summer !

KeepItSimpleStupid said:
I'm busier right now too. How about quickly putting the outputs in; setting R(whatever) so it's 3.6K (memory) and basically see if it functions. Look at E-C with signal, but no speaker.
Click to expand...
 
OK, saw your post. I'll be busy for the next couple of days. I'll have to get back into the swing of things myself.

So, at this point everything is supposedly complete, but not working? What mods are in place?
What's that voltage between the two bases (the bases that have been been shorted together)?

Check the resistance of the speaker?
 
So, everything is (I believe) back in place. The mods are the following:
- R14 replaced with a 3k + 500ohm pot; currently setup to 3.5k, close to its default 3.6k value
- C1: improved mylar type cap
- C10: ditto
- C4 has a NP cap, I've tried shortening it out altogether during the tests, since we have the variable R14 in there now.

Voltage at the base of C4 : 110mV
Speaker resistance is 3.8 Ohms - seems about right that does!

On the signal board, all the caps were replaced, and as explained before, it detects signal in auto-mode, so that works; but clearly something is stopping the power section somewhere... I'm just glad it's not burning this time for change, had my fair share of magic smoke in the spring...

How are things in the US? Hope you weren't affected by Irene?

 
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