problem with Warwick Sweet 15.2

[spy]

I'm trying to repair a Warwick Sweet 15.2 bass amp. The output stage was fried and both fuses at the power supply were blown. I replaced only the power transistors MJ11015 and MJ11016 but there's still a problem.

I have full output without load but with a load it's very low and the transistors are getting overheated too fast.

I always replace the drivers with the power transistors but I didn't have them in stock right now. Do you believe the drivers are the problem?

The bias at each output transistor is ~9mΑ. 2mV at each emitter resistor (0.22 ohm).

The schematic is over here,
warwick_sweet-15.2_sch.pdf

Thanks for any help

[Leo_Gnardo]

Drivers are always suspect when output transistors blow. In this amp I'd also have a close look at the Q107/108 pair. These look to be limiters for the output base drive voltage, and are supposed to go "short" if they see excess voltage on their bases - that voltage derived from the output tran's emitter R's. If Q107/8 were damaged when the outputs failed that may explain your symptoms.

[spy]

Thanks for your advice Leo. I found a pair of BC548/557 to replace Q107/8 but unfortunately I have the same symptoms.

[spy]

ok, I feel a little bit embarrassed. There's a switch next to the speaker output which disconnects the speaker but it's at the negative side. That's why the output is visible at the oscilloscope without load. Also, the switch is not at the schematic and it's labelled as "Horn Off"! There's a block diagram in the manual that you can see this switch.

Warwick Sweet 15.2 manual

What I can't understand right now is why the amp is getting hot faster when the switch is open?

[oc disorder]

I may be talking through my socks here but although the schematic doesn't show it.. I'm sure when I last saw one (combo)
it used a trs socket (aka stereo) for the output as the horn and woofer positives come up to the amp from the combo speaker in the one cable.
For the amp to have a horn switch in the chassis there must be separate outputs.Using a tip and sleeve jack will short the output.

[spy]

Now, I feel more embarrassed!

If there is any excuse, I don't have the cabinet right here, only the chassis.

And I think I need vacation.

[MarkusBass]

This is clearly shown in the Manual you posted - on page 10. This is quite unusual because it's so easy to make a mistake and use mono cable (which shorts the output of the power amp to the ground).

Mark

[spy]

Oh yes, it's pretty clear in the manual. Thank god there's a protection circuit at the output for that kind of mistakes.

[oc disorder]

Well don't .. you know too much. You would not be the first or last.. if we all own up to these "faux par's" !
I remember reading a great article by one of the great eq developers whom have various tone control designs named after them.
I can't recall the names at the moment.. but he said .. it's very easy to fool one self with subjective testing or some thing similar!
He went on to detail when they were testing something (he and distinguished colleagues) they got excited at discovering something
and wrote down a heap of observations and conclusions... but... later he discovered that something wasn't terminated properly
and all the results were a collective mirage. I'd love to find that article again the above is a recalled approximation.

[nickb]

The schematics show that with no speaker plugged in the output is shorted to ground. That can't be true, surely, please tell me it isn't so,,,

[spy]

Haha, no it's not true. I measured full output directly at the output jack with no speaker or load plugged in so it's not grounded. Obviously, there are mistakes at the schematic. One that I came across while measuring is, there's a joint between D119, D120 and emitter resistors R11, R12. I'm sure there will be more.