SWR Workingman's 15 no power

[nosaj]

SWR Workingman's 15 no power I can hear a slight hum when turning on power.

Clipped 2 legs of transformer secondary getting about 94V

One Output transistor bad other really leaky.

My question is Can I remove output transistors to power up the amp with no signal with out damaging any other transistors?
i just want to see what else is still good.

nosaj

[nosaj]

Ok out of circuit output transistors test good.
R43 is burnt would that keep sound output from working?
nosaj

[nosaj]

sure is aggravating when your used to parts numbers printed on the PCB that correspond with the schematic.. There are none here or a layout so tracing is the only option.
nosaj

[DrGonz78]

My approach to those amps would be test bad parts and replace. If one transistor is cooked I am rebuilding the power amp for further testing. I have only ran a couple of amps without their finals and it never helped solve any problems. One of the amps I ran without outputs was a crate gt80. I think it was mosfets. My problem with that amp I was never able to figure. I was so desperate I even removed the output transistors.

Could be oscillations caused amp failure? Burning up R43 a bit on the way. Still I would replace the output transistors and any pre drivers. It took a pretty steady current draw as it failed.

[Jon Snell]

[QUOTE=nosaj;n982005]Ok out of circuit output transistors test good.
R43 is burnt would that keep sound output from working?
nosaj

In one word; No.
That network is to stop the amplifier from 'taking off' if a capacitive load is applied to it. (Piezo cells etc).
R43 will get hot if the amplifier is overdriven into distortion for prolonged periods due to square waves that have very many high frequency harmonics.
Consider this;
If OUT has +ve DC on it, so will the base of Q6, turning it off and causing the emiter of Q6 & Q7 to rise, making the collector of Q7 to rise due to bias of Q7. This causes Q8 to turn on, causing the collector of Q8 to fall pulling the base voltage of Q9 down and that causes the base of Q10 to rise pulling the collector of Q10 down. That reduces the +ve offset on OUT which reduces the aforementioned action. If this goes the other way; causing OUT to go -ve DC, the same but opposite will apply.


Edit; More info.
Consider this as well.
If AC voltage is not applied to Q1 base, via D7, R19 and C7, Q1 will not switch on, that will leave Q2 off leaving very low current feed for Q6 & Q7. That will let the output stage float. It is just a way of muting the amplifier at power on/off to reduce the thump in the loudspeaker but it could cause problems if not working properly.

[nosaj]

[QUOTE=Jon Snell;n982020]

Ok out of circuit output transistors test good.
R43 is burnt would that keep sound output from working?
nosaj

In one word; No.
That network is to stop the amplifier from 'taking off' if a capacitive load is applied to it. (Piezo cells etc).
R43 will get hot if the amplifier is overdriven into distortion for prolonged periods due to square waves that have very many high frequency harmonics.
Consider this;
If OUT has +ve DC on it, so will the base of Q6, turning it off and causing the emiter of Q6 & Q7 to rise, making the collector of Q7 to rise due to bias of Q7. This causes Q8 to turn on, causing the collector of Q8 to fall pulling the base voltage of Q9 down and that causes the base of Q10 to rise pulling the collector of Q10 down. That reduces the +ve offset on OUT which reduces the aforementioned action. If this goes the other way; causing OUT to go -ve DC, the same but opposite will apply.


Edit; More info.
Consider this as well.
If AC voltage is not applied to Q1 base, via D7, R19 and C7, Q1 will not switch on, that will leave Q2 off leaving very low current feed for Q6 & Q7. That will let the output stage float. It is just a way of muting the amplifier at power on/off to reduce the thump in the loudspeaker but it could cause problems if not working properly.

You know John i really appreciate the way you do your explanations it allows me to print the schematic and trace what you describe which in turn helps me being a visual learner.

nosaj

[Jon Snell]

[QUOTE=nosaj;n982052]

You know John i really appreciate the way you do your explanations it allows me to print the schematic and trace what you describe which in turn helps me being a visual learner.

nosaj

My pleasure and pleased to help.

Only took 24hours and I have remembered the name of the RC network, R43/C20 & R44/C21; Zobel Metwork.