Sub amp not working - please help!

[SimontY]

Hi everyone,

I'd like to ask for advice regarding my broken subwoofer plate amp - which [unfairly :-) ] is just not working! The fault comes after modding by adding 4x10,000uf caps, each of which was wired in parallel to an original corresponding 4,700uf cap.

The fault is this:
Power comes on, controls seem to work ok i.e. turning up the volume increases noise from the drive unit. However, the signal from the hi *or* low-level input doesn't seem to be getting through.

I am convinced the fault is not specifically with my caps, but with something I've knocked or somehow upset - I just don't know what!

Has anyone else had a similar problem, or any clue as to what might be causing this?

Thanks,
Simon

ps - I can post pictures of the boards if anyone would like

[Tube_Dude]

See if the diodes or rectifier bridges are not burned with the stress ...from the dramatic increase of filter capacitance...

regards

[Peter Daniel]

I would say check the bridges first, but so far in my experience blown bridges resulted in blow fuses as well, So check fuses too.
Nice avatar, BTW

[SimontY]

Thanks for the suggestions, I will check all around for blown components, certainly tomorrow - it's 3.30am here

However, can we assume for now nothing has blown or melted, as I'm fairly sure everything *looks* fine?

Also, and I should have said this originally, but a few days before this mod I put in a 22nf film cap across each 4,700uf original, and I had exactly the same problem after doing that. The only difference was...I disconnected and left the pre-amp board out, went to bed, and reconnected it the next day, and it worked!!!

This time though, no joy, no nothing... at least below ~50hz

I have put a multimeter across the caps after disconnecting the power, and they have power stored in them, I don't know if this helps - to be honest, I'm a beginner with this type of mod!

-Simon

ps - I'm glad you like the avatar Peter, I was actually thinking of losing it - 'cause it looks so weird! Those caps there are 8uf 600v LCR (iirc), and they sound lovely

[Peter Daniel]

If you have voltage across the caps it means that bridges are fine. And if increasing volume brings more noise it seems like output stage is working too. What about your source, or connection between source and amp. Is it working fine?

[SimontY]

Connection to it is definately fine, I've tried one or both channels of hi-level and even connected the ends to a normal speaker - it was fine of course. I also tried the low-level from the LFE channel on my AV amp.

All the connectors seem fine also, before taking them off the 1st time I marked them with an indellible pen to be sure of putting them back right.

It seems as if the problem is the connection somewhere between the input board and the pre-amp board, *or* some kind of protection circuit, or something!

[Peter Daniel]

Originally posted by SimontY


It seems as if the problem is the connection somewhere between the input board and the pre-amp board, *or* some kind of protection circuit, or something!

It seems like that. The best would be to use test signal and a scope to check it. If you don't have a scope you might try using piece of wire, while holding it in your hand, and by touching different areas of a circuit listen to a noise in a speaker. Try to decrease the volume as not to cause any damage to speakers and in this way you might be able to locate wich stage in your circuit is not working.

[SimontY]

Thanks again, that's an idea I will try tomorrow. Just for fun, I suppose, here is a pic of the 'power amp' board, with the pre-amp board not connected.

-Simon