You are saying, when rca ground is connected to 12v ground, idle consumption increases and output gets hot. Try to disconnect inputs of power amplifiers itself , maybe just desolder a dc blocking capacitors in signal path. Looks like oscillation or heavy dc at outputs. Did you tried that without loads and speakers connected?
Something else i've noticed....connecting rca ground to the main gnd introduces a lot of noise everywhere....rail voltages, +15. -15volts and so on. There is like a small wave riding on top of those voltages.
Did you tried that without loads and speakers connected? - tried both. Connecting a speaker does not change anything, except at the moment I ground the RCA shield to the main gnd there is rail to rail popping sound at the outputs. I use a heavy duty subwoofer so if this was a small speaker maybe it would have been damaged.
Can't see any DC at the outputs....
Forgot to mention - Main gnd and the speakers (secondary) gnd are connected together. So only the RCA gnd is isolated.
Did you tried that without loads and speakers connected? - tried both. Connecting a speaker does not change anything, except at the moment I ground the RCA shield to the main gnd there is rail to rail popping sound at the outputs. I use a heavy duty subwoofer so if this was a small speaker maybe it would have been damaged.
Can't see any DC at the outputs....
Forgot to mention - Main gnd and the speakers (secondary) gnd are connected together. So only the RCA gnd is isolated.
Speakers ground is connected to 12v ground ? Thats very strange, in all amplifiers, which i have seen, dc-dc converter's output is isolated, creates +-25-35V , and rectifier output gnd is speaker gnd , and also rca ground , and there's no connection to 12v ground. Check output devices and rectifier diodes ,maybe one of them shorting to main case ( damaged thermal insulator).
The diagram shows the primary and secondary grounds directly connected. The input ground is isolated by the transistors at the RCA end of the circuit. They act as a differential input.
Did you ground the RCA shield for the input that you're using to drive signal into the amp?
Did you ground the RCA shield for the input that you're using to drive signal into the amp?
Yes I did...I've also tried with the other channels too...it does not make a difference, even without the RCA/signal once there is connection between the rca shield and the main gnd amp goes in the 3A idle current mode....Amp is out of the case, outputs are sitting on the big aluminium bar.
It is weird that the other two identical amps make absolutely the same thing....
"The input ground is isolated by the transistors at the RCA end of the circuit." - these transistors does not have the proper voltages according to the service manual....but i've already checked all the resistors in the circuit, changed the transistors itself, changed the Q519...same. Actually once I ground the RCA shield to the main gnd, then i have the proper ones....
It is weird that the other two identical amps make absolutely the same thing....
"The input ground is isolated by the transistors at the RCA end of the circuit." - these transistors does not have the proper voltages according to the service manual....but i've already checked all the resistors in the circuit, changed the transistors itself, changed the Q519...same. Actually once I ground the RCA shield to the main gnd, then i have the proper ones....
Sometimes between grounds is resistor installed, to prevent direct connection, and ground loop. It may be several ohms , in one of amplifiers it's 1k. You may get incorrect voltages measured, as ground is not a ground ,if not connected together.
Not every amp uses this type of secondary gnd with a small film cap and a resistors between 100~1000ohms. Some do have direct connection. Some of the half bridge class D amplifiers have direct connection between the speaker gnd and the main one. This amplifiers uses the same principal...but almost in every case of direct gnd connection, the input section or the pre-amp is being isolated in some fashion. Firstly I was thinking that somebody try to input hi-input signal thru the RCA but that seems not to be the case....and i'm not familiar with this type of a pre-amp isolation....and having 3 amps with the same issue...there should be something....
According to this schematic ,amplifier do not look like D class one .Normal AB class amplfier .If this schematic you posted matches what you have in your hands , then ok .Q101 ,Q201, Q301,Q401 can't operate without RCA ground connected to mains ground .Try to desolder temporarily C111 ,C211 , C311 ,C411 to test just power amplfier behavior ,to connection of RCA ground to mains ground . With these desoldered , signal path is breaked , but internal ground would remain ,if burned trace you mentioned fixed properly ,so amplifier should not respond to ground manipulations ,there would be no sound too .
ximikas - done that, desoldered C111 ,C211 , C311 ,C411 and still no difference. Still doing the same, idle current jumps to 3A, BJT outputs are heating in the same manner.
Yeah it is class AB amplifier, but the ground connection between speakers gnd and main gnd is in direct short as it is in many class D amps, that what I was trying to say.
Yeah it is class AB amplifier, but the ground connection between speakers gnd and main gnd is in direct short as it is in many class D amps, that what I was trying to say.
If removal of those capacitors have no effect, problem is somewhere in output stage or bad power filtering i think. What about opamp power filtering capacitors? Main rectifiers capacitors , did you tried to desolder them and check esr and capacitance ? Device looks like older design, they all are old and can be bad.
As I have 3 amplifiers of those, one of them i've already recapped thinking the same....but no avail, amps shows the same behaviour as before recapping. That's why the thread over here,
I think, if you get stong high frequency output at speaker output, and it's frequency matches that upc494 is operating at, then it's power supply problem, maybe broken ground . At best post pictures what your scope displaying when RCA ground is shorted with 12V ground.
Yes, at speaker output. Also ac coupled check ripple at main rectifiers positive and negative outputs.
