US Amps AX-1500DE

[caraudioguy2100]

This amp uses C3198 and A1266 as the power supply drivers

Is there a good sub for these ?

Power supply fets are RFP70N06

 

[Perry Babin]

The BD139 and BD140 are commonly used as subs for most all of the through-hole driver transistors.

 

[caraudioguy2100]

Replaced all the power supply drivers checked the gate resistors replaces the 494.

Power supply fets still get hot within 10 secs .

Anyone have any ideas ?

 

[Perry Babin]

Post a photo of the power supply area.

 

[caraudioguy2100]

Computer is down due to no internet .

So here’s a couple of pics I took with cell phone

 

[Perry Babin]

I don't recognize it. Do you have a scope?

 

[caraudioguy2100]

Yes I have a scope

 

[Perry Babin]

Post gate pad waveforms. If you have a capacitor in the range or 0.01uF and 0.1uF, solder it across the gate and source pads to load the drive circuit, before taking the photos.

 

[caraudioguy2100]

0.1 cap across drain and source

Scope set to 2ms 2volta divide

These pics are of both banks .

Original fets were DFP70N06 with 47 ohm gate resistors

I replaced them with RFP70N06 with 47 ohm gate resistors

Sorry not sure why the pics flipped sideways

 

[Perry Babin]

Connect the cap across the gate and source.

 

[caraudioguy2100]

Sorry that’s what I meant it’s actoss the gate and source

As soon as I put 1 power supply get in each bank and remove the cap it draws excessive current when trying to power up

 

[Perry Babin]

I just realized that the frequency is WAY too low. At 2ms/div, that's less than 200Hz. It should be above 20kHz.

 

[caraudioguy2100]

Should I try replacing the 494 again ?

 

[Perry Babin]

Do you have a multimeter that measures frequency?

 

[caraudioguy2100]

Yes

 

[Perry Babin]

Measure the frequency at the gates of the FETs.

 

[caraudioguy2100]

I replaced the 494 again and the frequency at the gates measures 195hz

 

[Perry Babin]

Check or replace the timing capacitor and resistor. They are connected to pins 5 and 6 of the 494.

 

[caraudioguy2100]

Replaced the cap and resistor Amp powers up and idles fine now .

Now the 2nd problem I have is I have full rail voltage on the speaker outputs

 

[Perry Babin]

A couple of notes.

I'd suggest that you install that 0.1uf cap across the gate and source and look at the waveform. This is a good test for a drive circuit and you should know what the loaded waveform looks like. I think you can do this with the FETs in the circuit but if it looks like it's causing excessive current draw, do it on the next amp you repair the supply on.

I assumed that you had the scope set to the 'best initial setting' from page 73 of the site. That's why I initially didn't realize that the frequency was so low. There have been a lot of typos so I didn't really trust what was posted 100%, making some assumptions. When checking the PS waveforms, use 10uS. You would have immediately have caught the low frequency issue.

The reason I finally caught the low frequency was because the waveform wasn't distorted as much as I'd expect. I was going to post a waveform to show what you should see and that's when I remembered that you posted 2ms.

Also measure the frequency that it's operating at right now.

Post a photo of the audio section of the amp.