SWR 350 bass head not working?

[Jam]

Hi I have a SWR head not working it did power up but no sound but until today I blew the 10amp pig tail fuse inside the amp when I started to test it . Where do you get the pigtail fuse? Then what be a good point to start trouble shooting it? Thanks any info

[The Dude]

MOUSER LINK

Hopefully the fuse blew before anything was damaged. If you get it to hold a fuse, the first thing I would do would be to jumper the effects loop. Run a standard guitar cord from effects in to effects out and see if the amp works. You could just have a bad switching jack. Next would be cracked solder on the input jack(s). If the amp won't hold the fuse, we've got bigger problems.

[Jam]

Can you use a Ceramic fuse instead of glass?

MOUSER LINK

Hopefully the fuse blew before anything was damaged. If you get it to hold a fuse, the first thing I would do would be to jumper the effects loop. Run a standard guitar cord from effects in to effects out and see if the amp works. You could just have a bad switching jack. Next would be cracked solder on the input jack(s). If the amp won't hold the fuse, we've got bigger problems.

[The Dude]

Can you use a Ceramic fuse instead of glass?

Yep, as long as ratings are equal.

[mozwell]

I'm not sure if any of these are for the amp you have, but maybe they are close.
If you are not familiar with electronics & safety with electricity, don't go any further, send it to a tech if teh 10A fuse blows again.

Before powering up, measure the output transistors & driver transistors with a multimeter, on ohms & diode ranges.
Also check the bridge rectifiers, or main rectifier diodes on the power supply

What does the fuse connect to on the board, or what is it close to, speaker output ?

You could also disconnect the +/- rails from the power amp before you power the unit up again (if you remove a quick connect terminal, make sure you insulate it so it doesn't touch anything when it is removed)

Double check the speaker is not shorted, and also check the speaker lead is not shorted.

[Jam]

I got the fuse and got it solder in and tried to test the output transistors but all I was getting was open on the meter. So I turned the amp on and it didn't blow the fuse. That's all I did so far but before I opened the amp I tried the effects I and out and it still didn't work?

[The Dude]

Next I would run a signal into the effects return and see if you get anything. That will help us determine if it's a preamp or power amp problem. You can also take an output from the effects send into another amp and see if the preamp works. Either/Both of these will help determine which part of the amp has a problem.

[Jam]

I hooked it up to the speaker and when I turned it on the 7a fuse blew on the outside of the amp right a way?

[The Dude]

You obviously have a problem other than simply the fuse, most likely in the power amp section. For starters, install another fuse, unhook the speaker, fire up the amp, and measure for DC voltage on the speaker output. I'm betting you'll have some DC there. I'd also do a quick check of the output transistors Q9-Q12 and make sure none of them are shorted. Report your results and we can go from there.

[Jam]

The transistors Q9-Q12 I was getting 16 VDC I had my meter set on 20 VDC and I had it grounded to the amp chassis. I thought it should be around 75V ? or did I test it wrong? When you say the speaker output do you mean the back of the input jack where it solders? Thanks

[The Dude]

Yes, measure the DC voltage at the speaker jack itself. If you can't get to the pads or pins on the jack, plug in a cable or plug and measure it that way. If you do measure something there, remember to note whether it's positive or negative.

You should get +75V on the collectors of Q9 & 10 and -75V on the emitters of Q11 & 12. Are you sure you had a good ground? If you've removed any boards or something else that might carry a ground, it's possible your ground reference was no longer ground.

[Jam]

Ok when I did check the speaker out it was +16V but then I realized I forgot to plug in the J1 clip(has a blue red black) on the preamp board and when I turned it back on it blew the fuse so I unplugged it from the preamp and change the fuse and it didn't blow the fuse.

[The Dude]

It looks like that connector carries your plus and minus 75v to the output board from the power supply. The fact that you are blowing fuses when you plug it in tells you that there's a short somewhere on the output board. It's most likely you have shorted output transistor(s). You will need to (of course with no power to the unit) check your outputs for shorts. That said, there could certainly be more damage, i.e. shorted driver transistors, open emitter resistors, etc., etc..

You are eventually, after you get the short sorted, going to have to power it up and test it with all connectors seated including the one that causes the fuse to blow. So, I'd also recommend you build a light bulb limiter (search forum for directions). If you don't, you're just going to keep throwing parts at it until you've spent more than taking it to a shop.

If you're uncomfortable with any of this or unsure how to proceed, it's best to take it to an experienced tech at this point. You could easily do more damage and make things worse. You could also hurt yourself.

On the other hand, if you're ok with it, we'll do our best to help you sort it out.

[Jazz P Bass]

As an aside, please stop putting a 7 amp fuse in the unit until it is repaired.

This amp should be able to hold a 1 amp fuse for test purposes. (or a 1/2 A for that matter)

The 7 amp rating is for driving the amp with a speaker load to full power.

[Jam]

Ok thanks I will start testing with the amp off and when I test the output transistor to get a good reading do I need to unsolder the middle leg? The driver transistors are those the Q7 Q8 and the emitter resistors those are the the big white ones? I built a light bulb limiter a couple years ago but I was told they wont work on SWR amps? Thanks