Woofer squeezing

[Behemot]

I got this woofer, there is huge capacity on the input and each amplifying board, despite that it started whistling quite noticeably I scoped quite high saw-teeth like waves of several tens of mV ripple on both + and - rail, combined it is closing to 100 mV. Closing hand between transformer and amplifiers makes it worse but that's probably just it behaves like antenna.

Capacity seems to be fine according to meter so the problem is not here. I noticed burned resistors, probably some low value but they do not measure anything in circuit, possible opened. From the remaints it does not make any sense, it seems to be black/black/brown (or carbonized red)/gold which does not seem to be anything according to calculators as black is 0. Can the resistors cut the spikes, helping to smooth the voltage?

[budm]

Re: Woofer squeezing

Those look to be bleeder resistors for the capacitors, there should eb two of them each one connected across the +/- legs of the cap.
0.100V ripple for linear unregulated power supply is not that much at all.

[Behemot]

Re: Woofer squeezing

But there is 13 mF of capacity per rail and as long as it is 100 Hz, the woofer can play it quite well. I measured that on cables going into the speaker itself.

[Th3_uN1Qu3]

Re: Woofer squeezing

That ripple isn't your problem in itself. You have a ground loop somewhere in the signal section that amplifies it and makes it appear in the speaker.

[Behemot]

Re: Woofer squeezing

May it be just that the amplifier is behaving like antenna, catching the ripple from transformer? As I said, it got much worse when I've put hand closer between the boards and transformer. As there si metal cover on the transformer, I've been thinking, what about swaping 2pin power cable for 3ping, adding some aluminium foil on the transformer and grounding it with middle pin?

[joshnz]

Re: Woofer squeezing

whistling tells me khz oscillation.
hum is frequencies less than soothing like 200hz.

so do you have whistle or hum?

[Behemot]

Re: Woofer squeezing

Couldn't find the right word, yep, would be hum than. I see 100Hz waves = rectified 50Hz line.

[Behemot]

Re: Woofer squeezing

Tried touching the metal ring on the transformer with grounded crocodile and 60 % of the hum went away. So I will proceed with changing 2pole cable to 3pole and grounding that thing (together with the whole back metal), that should severe the ground loop hopefully. I think it is supposed to be grounded through the audio cinch jacks but if you use it with not-properly grounded device, here it comes…

[Andrew F. Ali]

Re: Woofer squeezing

If it is 'HUM' then most likely Filter Capacitors in the Power Supply may be losing value. I suggest removing them from the board and testing out of circuit.

[Behemot]

Re: Woofer squeezing

Those are the huge 4,7 mF caps and all test fine. Also the additional caps on the boards itself test fine, alltogether with ~13,5 mF capacity.

[Behemot]

Re: Woofer squeezing

Hmm, it is OK without input connected, but with input it seems the ground loop is even worse…connected to phone, it is OK so the problem lays here.

I will try to completely disassemble it, check caps on the boards and clean that bloody glue, it is quite carbonated and maybe carries some signals. Damn how I hate audio equipment, something gets catched somewhere and whole bunch of problems is created. It's like expeling ghosts.