[All] [Generic] Broken Amplifier Help

[toxo]

I fitted some old amps in my car today. I have a 4 channel amp and a 2 channel amp. The 4 channel amp was to run the speakers, and it has a low-pass preout which I ran to the inputs on the 2 channel amp. The 2 channel would then run a dual voice coil sub. When I fired it all up for the first time it worked great, but as soon as I shut/slammed the bootlid I started getting mad hum from the sub, which built and built louder and louder, so I switched off, checked my wiring to see if anything had come loose, and switched on again. Same problem. I took the 4 channel amp out of the equasion and ran the output from the stereo to the 2 channel amp, which works fine. I did some more testing with just the 4ch amp and the sub, and found that I get the same building hum (which I think is DC voltage, but I didn't have my meter so I don't know if it's 12VDC or internal rail voltage). I get it from any of the 4 speaker outputs and it's obviously outputting it down the preout to thw downstream amp as well, so I've narrowed the fault down to the input stage of the 4ch amp.

I have previously replaced the transistors in the power stage in this amp (4 or 5 years ago I expect) but as the fault seems to lie with the input stage I'm not sure where to start. All the transistors test OK, there's no obvious shorts (it doesn't blow any fuses). The amp is a Soundstream USA 364 and I can't find a schematic for it anywhere.

Does anyone have any suggestions to help me find the fault?

[toxo]

Well, I fixed it... it took me a while but it turns out it's the microswitches. It's a 4 channel amp, and you can set it so that 1/2 and 3/4 have separate inputs, or all 4 channels share ch1/2 inputs. The switch for that is knackered, if you tap it it grounds out. There's also a 45hz boost switch for the low pass out which does the same, and the hpf/full range/lpf switch is knackered too! They are good quality microswitches with a ball bearing in so it must just be age - the soldering's fine.



I knew there was a guy on eBay who used to work for Soundstream, he sells loads of spare parts, so I asked him about switches. A few photos later and $12 including delivery and I have 15 switches (5 of each type)! I was just going to hit up RS or Farnell but I don't know what the switches do internally, one of the 3 way ones has 14 pins! So best to play it safe and get them from the states.

I've not used channels 1 and 2 yet (not for a long time anyway) so the switches on there might be knackered too, and I have another SS amp which I think might be suffering the same.

Oh if you were wondering why the PCB has red and white stripes, the PCB in the USA model amps are all done out like the american flag.