Ampeg PF500 +15v missing

[KCman]

This amp started out with the Fault light on. I found Q10 to be shorted. I checked other components around it and they all seemed fine. I replaced the FET and fault light was off, but sound was cutting in and out.
I then found I had no +15v. -15v was there and + and - 82V was there.

I took L4 out of circuit to isolate better. Still no +15v. Any thoughts as to what to do next? I'm not that good at switching supplies, i've only been able to fix a couple.

Thanks

Ampeg PF500.pdf

[Jon Snell]

If Q10 has failed then Q11 will have suffered along with the gate side components, D39 and U1.
Removing L4 wil disable the +15v supply. However, there should be +15 ish across C45. If not replace the diode D14.

Be careful, there are signal grounds and power supply grounds, mix those up and you may get false readings!

[x-pro]

If Q10 has failed then Q11 will have suffered along with the gate side components, D39 and U1.

I'm sorry, what scheme is this on?

[Helmholtz]

The schematic link doesn't work.

[x-pro]

Post # 54
https://music-electronics-forum.com/...hematics/page4

[x-pro]

If Q10 has failed then Q11 will have suffered along with the gate side components, D39 and U1.

Not necessarily. Sometimes a single mosfet fails.

... If not replace the diode D14.

Also check pins 12 and 11 of the T3 transformer, and the connection and soldering quality.

[Jon Snell]

Not necessarily. Sometimes a single mosfet fails.



Also check pins 12 and 11 of the T3 transformer, and the connection and soldering quality.

That is what I call living dangerously.
Not sure about you but I don't like revisiting repairs on a repeat failure, very unprofessional. Replacing both ensures I can sleep without the worry of when, not if, it will fail again.

[x-pro]

For this purpose, a 4-hour test run of the amplifier is provided on sine wave under load and at 80% power.
I always do this.

[KCman]

Here is the progress of what has happened;
- Right away I replaced Q10 and Q11 and tested other components around those.
- Fault light was off now and I tried it with sound. It was cutting out a lot.
- I tested Fets and they were good still
- I tested rails and found the +15v was not there. I tested more than once.
- I pulled L4 out and tested at the positive of C45 and still no +15v
- I read replies and tested some more. This time the +15v was there.
- I put L4 back in circuit and I still have the +15v
- Tested with audio and it works now.

I didn't really do anything from the time it was cutting out and now, so I will have to keep checking it. I hate when this type of thing happens, I know it's still not right.

I'll start by touching up some solder. Also, sorry for the schematic file not working.

Thanks

[KCman]

How do I post a file? The one I put in the original message appears to be a link. Also, Where should I put a schematic I have that doesn't seem to be on here? I'm not able to click on Schematic Hell. Under requests?

[g1]

How do I post a file? The one I put in the original message appears to be a link. Also, Where should I put a schematic I have that doesn't seem to be on here? I'm not able to click on Schematic Hell. Under requests?

If you want to post a schematic not related to this thread, yes, schematic requests should be a good place. Use the 'upload attachments' button below the message box.
Not sure why the one in your 1st post of this thread didn't work, seems related to the corrupt files issue we had years ago.
Once the attachment uploads, I never click on 'insert', it just seems to confuse things. I just hit 'post reply' and the attachment will appear at the bottom of the post.

[KCman]

Ah, the one in this post I hit insert. OK Thanks