Orange Terror Bass 500 PSU fault

[Timbo]

Hi,
I’m working on a Terror Bass 500 that appears to have a fault in the PSU. I initially tested it with a guitar and found that although it worked, the output was lower in volume than expected. I also noticed that the power lamp was not lighting up. At the time I thought that was probably just a bad lamp. After around 2 hours, I was suddenly woken up by loud feedback as the amp came up to full power. The power lamp came on at the same time.
Now, unfortunately, I can’t reproduce the fault. Power comes on fully within a couple of seconds of switching on. I will try again tomorrow to see if this is time dependent.
Does anyone have any suggestions?

orange_tb500_bass_terror_sch.pdf

[Jon Snell]

It won't be the power supply as that will either work or not, same with the power output stage being Class D.
Check the effects loop sockets.

[glebert]

Dirty effects loop wouldn't cause power light to not come on, but looking at the schematic the power light is not jumping out at me. Kind of hard to diagnose a problem when the amp is working, I would try chopsticking parts or giving the amp a couple of good raps on the bench to see if you can recreate the problem.

[mikeydee77]

Pretty sure from memory that the power lamp is from the 6v power. Maybe the 6v connection on the fuse onwards is unreliable or the heater windings are partially short and dragging that supply down.

By tapping the amp when powered up in various places you might discover whether it is an intermittent connection or bad solder connection issue.

[J M Fahey]

Yes.
Loose in socket or corroded F4 fuse or CN4 connector could explain those symptoms.

What type of pilot light is it?

A LED? ... an old school 6.3V filament bulb? ...

[Jon Snell]

The lamp is an LED running from the +15v rail.
I have had the regulators fail but they usually take out the Wickman fuses from the power supply (not common and depends upon the date of manufacture).
Could have two issues. Treat them separaely; fix the sound and then the LED issue.

[mikeydee77]

I think that is the active / passive switch with built in LEDJon Snell

[Jon Snell]

There are no other LEDs that can be on when opperating normally.
Protect LED should be off when running. The green LEDs are mounted on the PCBoard.

[Timbo]

Thanks for your help so far everyone. It’s now developed a more serious fault. It has the full rail voltage appearing across the speaker terminals and has blown my test speaker and smoked my power attenuator! I’m assuming one or more of the output FETs have shorted. I will report back when I’ve had time to look properly.

[mikeydee77]

Ouch! Hope you can figure out why it did that in case it is something to do with your test setup and will do it again if setup not changed.

[Timbo]

One of the output FETs was short circuit across the source and drain. I replaced that and it’s working again. I don’t know why that happened; there doesn’t seem to be a problem with my setup. Anyway, the original fault isn’t evident at the moment, so I’m giving it back to the customer with a big question mark, which I hate. One thing I did find, which may be relevant, was a lot of dust and debris inside the PSU compartment, evidently sucked in there by the cooling fan. That may have been causing the over temperature circuit to kick in.