Infinity 475a car amplifier 75Wx4Ohm

[tibimakai]

I have picked up this amplifier for $10 from CL.
The amplifier is in protect mode.
I have tested all the heat sink mounted components and they all look good.
It uses a TL494 PWM IC which I think is the problem here.
There are one diode at each output and only one of them hows real voltage, 12.07V, the other is only 0.2V.
There is a way to check it to be sure that it's bad?
The IC is mounted on a small daughter board vertically, so it's very hard, or almost impossible to take measurements with it installed.
Diode D2 and D3 are on the main board.
I have never dealt with this IC in the past, any help would be appreciated.
Schematic is available here:

[budm]

Re: Infinity 475a car amplifier 75Wx4Ohm

The outputs of the TL494 E1 (PIN 9), E2 (PIN 10) are 180 deg. out of phase form each other to drive the Gates of those MOSFETs which are setup as push-pull output to drive that power transformer, only one set of the MOSFETS will be on one at a time. D2 and D3 are for the clamping circuit, if too much current flows through the DIODEs, it will bias on the transistors Q8, Q9 which will route the Gate drive signals to ground. You can at least compare the resistance readings on those two outputs, you can also look at those pins with the scope.

[tibimakai]

Re: Infinity 475a car amplifier 75Wx4Ohm

It's safe to remove the PCB from it's case/heatsink for measurements? (no signal)

[tibimakai]

Re: Infinity 475a car amplifier 75Wx4Ohm

I have replaced the TL494, just because it was so cheap, but it didn't change anything. It's still in protect mode.
There are two other diodes D2 and D3 on the little daughter board and I can't get with the scope to them. There are some other components in front of them.

[tibimakai]

Re: Infinity 475a car amplifier 75Wx4Ohm

Actually, I have managed to reach in there and I have got two nice sinuses at both diodes.
I have noticed that there is a +/- 29.24V at both sides of the amp, at the finals.
What else can it be?
The wire that I'm powering with it matters, how thick it is?
I'm using the wires from the Chinese power supply(GQ A-305D), basically I'm tightening the alligators into the wire slots.
I'm measuring the speaker outputs of all four channels, they all show 0.079V.
That is not to high? It shouldn't be under 0.01V?

[tom66]

Re: Infinity 475a car amplifier 75Wx4Ohm

Check lines PRO and DC which could be indicating a fault. They are on the schematic area for the final output transistors.

Incidentally I'm having a little giggle with them using TIP35 output transistors, I guess they aren't targeting audio fidelity (I'm no audiophile, but I can appreciate the difference between standard and audio rated, high linearity transistors...)

[tibimakai]

Re: Infinity 475a car amplifier 75Wx4Ohm

I'm noticing these designations, like PRO, VH and such.
What they mean?

[Agent24]

Re: Infinity 475a car amplifier 75Wx4Ohm

I'm noticing these designations, like PRO, VH and such.
What they mean?

PRO probably stands for protect, either used to show the protect mode status (active or not) or it carries the actual signal that will turn protect mode on or off.

VH would likely be some voltage rail.

Look a the schematics in your manual there, you will find the names.

Incidentally I'm having a little giggle with them using TIP35 output transistors, I guess they aren't targeting audio fidelity (I'm no audiophile, but I can appreciate the difference between standard and audio rated, high linearity transistors...)

It's a car amplifier, who cares for audio quality?

[budm]

Re: Infinity 475a car amplifier 75Wx4Ohm

VH+, VH- are the power supplies (generated by T1 windings pin 5/8/10) for the output stage driving circuits, see page 22~25, 28.
'PRO' is the protection signal which are generated by Q7A/B/C/D that monitor the output section of all 4 amplifiers, see page 22~25. The outputs are OR gated to Q10 which feeds Q3 which feeds pin2 of U1, page 28.

[tibimakai]

Re: Infinity 475a car amplifier 75Wx4Ohm

Thank you for all the info.
The question is, how much it should be the supplied power at the finals?
Right now, I'm getting +/-29V. It should be much higher then that, no?

[budm]

Re: Infinity 475a car amplifier 75Wx4Ohm

OK, this is an estimate (assume 100% efficiency for easy calculation), the power supply is putting out +/- 29V = 58V P-P, or about 20V R.M.S, so with 4 ohm load, the power will be 100W, but this amp is Class A/B so the efficiency may around 60~70%, so output is about 60~70W.
So +/- 29V should be OK, the caps are also rated at 50V (about twice the output voltage, so it looks right for you are getting).

[tibimakai]

Re: Infinity 475a car amplifier 75Wx4Ohm

The transistors are good, the power is there and it's still in protect mode.
What do you think I should be concentrating here?

[budm]

Re: Infinity 475a car amplifier 75Wx4Ohm

There are 4 amplifiers PROTECTION detectors Q7A~D, and the Temperature detector RT1, any one of those 5 detector can activate the shutdown.
It can be one of the amplifier has DC off set that set off the protection so you will have to find out which one by disconnecting the +VCC/-VCC feeding the amplifiers one at a time, or if you can disconnect the +VCC/-VCC from feeding all 4 amplifiers to see if the power supply will stay on, but make sure to put some load on the +VCC/-VCC.
Also check RT1 resistance.

[tibimakai]

Re: Infinity 475a car amplifier 75Wx4Ohm

Bud, do you think that is safe to do the measurements without the heatsink mounted? Basically the whole PCB pulled out of it's housing?

[budm]

Re: Infinity 475a car amplifier 75Wx4Ohm

For the Audio power amp sections will be OK since there will not be any speakers at the output and the Idling current on the output transistors will be very low, but you do need to have heatsink on the switching MOSFETs because it will be still driving the power transformer.

[tibimakai]

Re: Infinity 475a car amplifier 75Wx4Ohm

I will pull it out half way then. Thank you.

[budm]

Re: Infinity 475a car amplifier 75Wx4Ohm

So, the situation right now is that you will see the +/-29V at the power supply output, then the protection kicks in with 'PRO' LED lit up, correct?
BTW, I also spot en error of the schematic on CN5 PIN 4 and PIN 5 connection.

[tibimakai]

Re: Infinity 475a car amplifier 75Wx4Ohm

I can't get proper measurements with the board in it's place, so I can't even measure the PS outputs.
I have measured the 29V at the finals.
I still haven't measured the +/- 15V supply.
As I apply the power, it goes right into the protection mode. The power LED doesn't tun on at all.
Yesterday I was busy with a Kenwood nav unit(I have picked it up for $150 and it's already for sale for $350, no problems at all. I have done some software upgrades and tested all the functions.), I didn't have time to look at this amp, but today I will do some measurements.
The two LEDs are switched?

[budm]

Re: Infinity 475a car amplifier 75Wx4Ohm

The LED connections are OK in the unit, just the drawing is wrong. So the +/-29V is there even when the 'PROTECTION' LED is on? The power supply caps may maintain the voltage for awhile even after it goes into shutdown until they are discharged.

[tibimakai]

Re: Infinity 475a car amplifier 75Wx4Ohm

I have spent a half an hour with this amp, and I have measured a couple of spots.
The voltage is 25.4V now and not 29V as before, not sure why though.
I have measured a discrepancy at the output, at the resistors R25.
R25C(third amp from the top of PDF) measures 0.081V, while the other three R25s measure 0.041V.
I'm having a hard time finding the amps components on the board.
I still don't know where is the RT1D and other components. How this components should measure if it's bad or good?
I will take the board out and look for these components and while at it, I will check all the components on the channel C.
Tomorrow, I start checking the LG TV.