Power suppy TL494 waveform

[jol50]

Have an A8000T audiobahn mono class AB. PS is toast, replace all IRFZ44, everything tests/looks good. So I fire it up on 2.7 ohm resistor and it draws 3A, resistor gets hot and its at about 8v. I get +/-4v on the rails and no 15v supply, it will not start up. Nothing in the amp gets hot. Check rail caps, transformer and so on, find nothing so maybe its the PS.

Put scope on PS, probe on each side (2x5 mosfets) clipped on a gate, and I get the same wave on both? Pretty sure there is no setting on the scope that would do that? They are not summed. Plus the wave is not right, it is up and flat, then drops with the curve at the bottom, but then goes right back up like both waves are together in one. There is no flat bottom. Have not tested at the IC yet resistor was too hot but not seeing anything connecting two sides? Can the IC fail this way? Am I missing something here? All the gate resistors were within 1 ohm of 100, nothing discolored, only the mosfets melted to the clamps a little as I've seen in a few of these amps.

[Perry Babin]

The PNP driver transistors have likely failed.

Compare the waveforms on pins 9 and 10 of the 494 to the waveforms on the emitters of the PNP driver transistors. If the waveform on 9/10 and on the bases of the driver transistor are square but the waveform on the emitter has the curve, the driver transistors are defective (assuming that the collector of the PNP driver transistor has a solid connection (~0 ohms) to the ground terminal of the amp).

[jol50]

Looks like its not the drivers, maybe, I see same waveform on pin 9 & 10. Drivers are A1023s in parallel with pair of opposite diodes, 1K ohm across both at diodes, same base to ground, 218 ohm ground to the gate resistors same as the pull down resistor shows would have to pull those, but everything is identical side to side. So I'll guess the 494 or something affecting it unless both drivers are shorted C to E the same. I could not get a uf read on the cap on pin 8 to ground, it is also connected to pin 11 then resistor to diode to ground. Could not get a read on that diode either. Timing pin signal looks normal. I'll pull it out of the sink and pull some legs to measure the pin 8 items/drivers.

Solders are good on drivers, I can see a very tiny amount of darkening between them but solder still shines. Yes I read you said the PNP are typical to go bad, lol. The waveform is smooth/flat top unlike yours, but your example is for PNP/NPN not PNP only. So mine has a weak pull down (like a backwards J) but never stays down, it rises stright up and stays up flat. I can photo it if you want.

[Perry Babin]

If the output of the 494 isn't square, you can't expect anything beyond that point to be square. On the two 1k resistors next to the drivers, do you see the malformed square wave on one end of each one and absolutely no signal on the other end?

[jol50]

Originally Posted by Perry Babin

If the output of the 494 isn't square, you can't expect anything beyond that point to be square. On the two 1k resistors next to the drivers, do you see the malformed square wave on one end of each one and absolutely no signal on the other end?

Yes, also tried the fluke and 4.29v on each 0v on ground side.

[Perry Babin]

Pull the PNP drivers and check them thoroughly. If you're unsure how to check them, go to the bipolar section of the following page, click the link and post the screen caps of the test results. Test them both on diode check and on ohms.
Basic Amplifier Repair - Transistor Test Applet Link

While the drivers are out, check the output on pins 9 and 10 of the 494. Is is square?

[Perry Babin]

Earlier you stated that the power supply had 2*5 FETs. Are you sure that's correct. The ones I have photos of have 2*4 FETs.

[jol50]

Who would desecrate this forum by calling a rectifier a mosfet? Yes 4 mosfets a side plus one rectifier. I must have been tired or blind. I've worked on more than a few of these, have another one I looked at to help with this one, and just put new IRFZ44 in this.

I took the 1023s out: tested good on the diode test, tested good ohms test, forgot to write it down.

Checked the 494 and still had the same waveform...so I found my bag of new 494s and put one in and the 1023s back in. Crossed fingers and it works great so far, so the bad 494 was putting out the same strange wave to both sides (maybe not a surprise to some of you but I never saw that before).

Now it shows a fairly good waveform; they are opposites, there is a little overlap at the end of the drop it curves a little at the bottom, about <2v difference where they cross. I ran it out of the sink for 15min and detected no heat in the IRFZ44s, about 1.25A idle draw, only 10mv offset, 55.4v rails on 13.7v. I might be good if the audio section works properly.

Appears maybe the 494 failure may have released the magic smoke? Thanks for the help he is wanting it back asap.

[Perry Babin]

When you previously looked at the drive signals for the gates, did you use both channels of the scope?

If so, were they both in phase (positive pulses vertically aligned)?

[jol50]

Yes, used both channels, they were identical. Zero'd they appeared as one.

Now you have me wondering if the pins on the 494 were somehow bridged, I tested the 494 and it has infinite ohms from 9-10 and 8-11, maybe I have an old board I can run it in. I did visually inspect both sides of the board for solder or debris, but can not recall if I checked ohms between the two sides. The amp is well used so a solder issue seems unlikely. I ran the scope summed and the waveform is similar in shape, but the size is not. The bad wave was slightly shorter and near half as tall as the summed wave I get now. Plus the bad wave was smooth, however now the transformer/etc is working and not knowing how much that affects the size of the wave. The summed wave has a little peak above and before the flat top, and some noise in the trough. The curve at the bottom is different and jagged, the bad wave was smooth with more curve and nothing jagged anywhere.

I changed nothing on the scope after replacing the 494 except it would not trigger, it is touchy, moved it a little and it came back looking good all other settings untouched. There is no way the probes touched, clipped the diodes outside the 1023s. With the bad 494 both waves were identical in timing and voltage. But not worked with 494 enough to know if external forces can cause it to change output like that.

I think the bad 494 was running a little faster the waveform was a little shorter in length, that is my guess as to why the trigger was off on the restart.

Scope is set to 5uS, half wave now is about 13 (small) divisions or 5.2 whole full cycle. 13*2=fullwave=26/5smalldiv=5.2fulldivisions*5uS=26uS, so 1/.000026= 38Khz? (very roughly, if I did it right)