SWR LA-15 Stumped

[Westrek]

I have a SWR LA-15 that was brought in with the mains fuse blown. Q1 and Q3 were shorted and Q10 junctions were open. I could find no other failed semis, resistors or caps. Replaced those transistors and powered up slow and noticed no excessive current draw, but in a few seconds C8 started getting hot and smoking. I removed C8 as it was the only way to do any voltage testing. Rail voltages are correct, but I have full + rail on the output which would account for C8 overheating.

I've rechecked everything several times and have checked for solder bridges, shorted disc caps, but no luck. SWR LA-15.pdf

I've attached the schematic with voltage readings at various points.

Thanks in advance for any thoughts.

Mike

[MarkusBass]

What exactly means that you checked everything several times? Have you checked other semiconductors and resistors? It looks like Q1 and Q5 are not conducting; have you check Q5 and R3, R7, R6 resistors?
I would remove Q1 and Q3 and test the amp without any load. But first I would check for open resistors (especially R3).

Mark

[Jazz P Bass]

My first thought is the voltages around Q10.
That is a PNP transistor.
The collector should barely be on, not +62 volts.
But where is it coming from?
Solve that & the output should settle.
Also Q9's collector looks suspicious.
0.25 volts?

[Westrek]

Thanks for the responses.

Q9 and Q10 check good. (I have new ones on hand)D3, R24,28,32,19,21,18,17,26 all ok. Of course, if I remove Q10, it relieves the + rail voltages noted in unusual areas.
My test conditions are: Preamp disconnected at J2, no load attached, and input connector at J1 disconnected.
I've looked at this so much I've got the "deer in the headlights" look.

Thanks again for the observations.

[John G]

Hi,
If you have removed the Amp Chassis fron the box and wiring harness you will need to supply a ground to the cold pin on the power amp board input connector(input ground reference).
Without the input ground there is no reference and the input will drive the out put away from 'zero'.
Cheers,
John G

[Westrek]

Thanks. That very well could be the problem.

[Westrek]

That was the problem exactly. No ground reference at the input. Once the input cable was connected it was sweetness and light.

I thank all that responded...Another lesson learned!

Mike

[MarkusBass]

This is the way in which many amps are implemented, starting from Marshall, Ashdown, SWR and many, many others.

Mark

[Enzo]

I remember trying to figure this out on some big Yamaha power amp 25 years ago. PLugged the input cable back in and voila, it worked again.

[kynsi]

Sorry to revive an old thread, but I also have a broken LA15 and measured dc on the speaker-terminal with the preamp cable disconnected from the poweramp board. Is this 'normal'? The big speaker is blown up..

Hi,
If you have removed the Amp Chassis fron the box and wiring harness you will need to supply a ground to the cold pin on the power amp board input connector(input ground reference).
Without the input ground there is no reference and the input will drive the out put away from 'zero'.
Cheers,
John G

[Jazz P Bass]

How does it read with the preamp connected?

[kynsi]

How does it read with the preamp connected?

DC is gone.. Voltages seem ok for now. I'm just bummed I blew the speaker because the preamp was disconnected so I wan't to be sure the poweramp is ok before hooking up a new one.

[kynsi]

Attached the measured voltages with input connected.

[MarkusBass]

What is missing is the voltage on the output. Is it 0V? I suggest stopping for a moment and think about 0.03V on R4 and 0.041 on R3. What is the quiescent current? Why the voltage on R3 is positive? Shouldn't it be negative?

Mark

[kynsi]

What is missing is the voltage on the output. Is it 0V?

It's 0.063V
The current it now draws is 270mA from the mains. I biased using Enzo's method. 270mA is the minimum. Voltage is 230VAC.