SWR California Blonde goes insane!

[Sowhat]

I got an SWR California Blonde in for repair with the complaint that it don't work… yeah! nice descriptive complaint. I Checked the external 3ASB power fuse and it was missing, cap and all, this could explain something here.

swr_california_blonde_sch.pdf



After replacing the fuse holder and fuse I powered it up on a limiter with no signal and no load and it came up just fine. I took this opportunity to check the rails and check for DC on the output and everything looked good so I decided to hook up the signal generator, dummy load and scope to take a look at the output signal. Unfortunately I never saw any output because when I powered up I got a high current draw (greater than 2 amps) and my limiters lilt up the whole shop. I disconnected the load and it powered up just fine… mmmmm?

Looks to be working good with no load but when a load is applied it goes insane! I don't know how to even approach this problem aside from shotgunning the whole amp, which is something I refuse to do. Are there any possible plans of attack out there to isolate this kind of misbehavior?

I forgot to mention that I had installed it back into it's cabinet where it promptly blew it's new 3 amp slo-blo fuse which brought it right back to the bench where the limiter story comes in.

[The Dude]

Were you trying to run the amp on a light bulb limiter? That will cause problems if so.

[Sowhat]

Were you trying to run the amp on a light bulb limiter? That will cause problems if so.

Yeah but I ran it straight on the mains too and the behavior was the same except going straight on the mains cost me a fuse.

[nosaj]

I got an SWR California Blonde in for repair with the complaint that it don't work… yeah! nice descriptive complaint. I Checked the external 3ASB power fuse and it was missing, cap and all, this could explain something here.

[ATTACH]48820[/ATTACH]

[ATTACH=CONFIG]48819[/ATTACH]

After replacing the fuse holder and fuse I powered it up on a limiter with no signal and no load and it came up just fine. I took this opportunity to check the rails and check for DC on the output and everything looked good so I decided to hook up the signal generator, dummy load and scope to take a look at the output signal. Unfortunately I never saw any output because when I powered up I got a high current draw (greater than 2 amps) and my limiters lilt up the whole shop. I disconnected the load and it powered up just fine… mmmmm?

Looks to be working good with no load but when a load is applied it goes insane! I don't know how to even approach this problem aside from shotgunning the whole amp, which is something I refuse to do. Are there any possible plans of attack out there to isolate this kind of misbehavior?

I forgot to mention that I had installed it back into it's cabinet where it promptly blew it's new 3 amp slo-blo fuse which brought it right back to the bench where the limiter story comes in.

What voltages do you get on the outputs with no load AC and DC. Kinda thinking oscillilation but curious about the voltages.
nosaj

[The Dude]

Agree with possible oscillation. Have you scoped the output with no load/no signal?

[Sowhat]

Agree with possible oscillation. Have you scoped the output with no load/no signal?

That could be, I didn't scope the output at no load/signal but I did read 0.01 something or another VDC, hard to tell values that low with my ancient meter and heavily damaged probes but I don't really see any DC on the output. As far as AC, didn't even think to look at that... maybe I should do that.

[hodgy]

Perfect, I have a Super Redhead that just came in with an almost identical issue. I'll be watching this thread and commenting if I find anything helpful!

[Sowhat]

Had to check back in and give an update. I'm bobbing between 5 different tickets at the time so I don't get a large patch of time to mull things over. I decided to ramp up on a variac and bypass the limiter seeing just where I exceed 3 amps. With load and signal attached I ramped up the voltage while monitoring the current draw and voltage. It rose up to about 1-1/2 amps at 120 volts AC!!! what! no blown fuse? I must be seeing things, not only that but I get a nice clean symetrical sine wave out of the outputs, nice and controllable... what's up here the amp seems to work just fine, I brought it up and down a dozen times with no variance, so... I slap it back into the cabinet, attach the crossover and drivers, flip the switch and a loud shorted something or another buzz roars out and then silence... there goes my second and last fuse!

There's got to be something wrong with the crossover or drivers but checking through all the components, everything looks good. I'm going to dynamically test the drivers tomorrow but they measure out just about what you would expect with a meter preliminarily, they look good. The crossover, which don't have much to it measures out to be doing it's job correctly... once again, mmmm..?

[The Dude]

The odd thing here is that you said it also blew fuses on your dummy load (post #1)? At this point, I might hook up a tube amp to the crossover/speakers in the amp and see if it works. I say tube amp because a tube amp will be able to withstand a short for a small period of time. You might blow up another solid state amp if there is indeed a speaker/crossover problem. Also, if you check the wires going from amp to crossover (speaker out of amp), what do you measure. Does it show anything shorted?

[glebert]

Seems like it is "good on the bench" and "fails in the cabinet"? I think these can have the known SWR problem of cracked transistor legs, any chance of something getting pulled loose when you reinstall in the cab?

[nosaj]

Had to check back in and give an update. I'm bobbing between 5 different tickets at the time so I don't get a large patch of time to mull things over. I decided to ramp up on a variac and bypass the limiter seeing just where I exceed 3 amps. With load and signal attached I ramped up the voltage while monitoring the current draw and voltage. It rose up to about 1-1/2 amps at 120 volts AC!!! what! no blown fuse? I must be seeing things, not only that but I get a nice clean symetrical sine wave out of the outputs, nice and controllable... what's up here the amp seems to work just fine, I brought it up and down a dozen times with no variance, so... I slap it back into the cabinet, attach the crossover and drivers, flip the switch and a loud shorted something or another buzz roars out and then silence... there goes my second and last fuse!

There's got to be something wrong with the crossover or drivers but checking through all the components, everything looks good. I'm going to dynamically test the drivers tomorrow but they measure out just about what you would expect with a meter preliminarily, they look good. The crossover, which don't have much to it measures out to be doing it's job correctly... once again, mmmm..?

I will say this ...it could always be how it was put back together. I have a gk 250 lunch box and if you don't put the short screws in the right place you short the one of the outputs. The crossover is just a passive device right?

Just for clarification, your nice clean sine wave was going to a speaker or a dummy load?

nosaj

[Sowhat]

I will say this ...it could always be how it was put back together. I have a gk 250 lunch box and if you don't put the short screws in the right place you short the one of the outputs. The crossover is just a passive device right?

Just for clarification, your nice clean sine wave was going to a speaker or a dummy load?

nosaj

I never put in the mounting screws before the second fuse blow, just slid it into it's cabinet hole and plugged things in. The nice clean sine was on a dummy load (4 ohms). I don't have a 2 ohm load currently, just 16, 8 and 4. Now that I'm home and having dinner, I keep thinking about what could possibly be amiss... some hidden short that only manifests itself when hooked up to the SWR speaker wiring? I have not tried the amp through my shop test speaker cabinet (which is 8 ohms) but I bet it will work OK, that is once I stop at my local electronics supplier and pickup a few more 3 amp fuses.

[nosaj]

I never put in the mounting screws before the second fuse blow, just slid it into it's cabinet hole and plugged things in. The nice clean sine was on a dummy load (4 ohms). I don't have a 2 ohm load currently, just 16, 8 and 4. Now that I'm home and having dinner, I keep thinking about what could possibly be amiss... some hidden short that only manifests itself when hooked up to the SWR speaker wiring? I have not tried the amp through my shop test speaker cabinet (which is 8 ohms) but I bet it will work OK, that is once I stop at my local electronics supplier and pickup a few more 3 amp fuses.

The dude or G1 has mentioned this before...the resettable circuit breakers you see on Traynors and wiring one in can save a lot of fuses till you work out the issue.

nosaj

[Sowhat]

The dude or G1 has mentioned this before...the resettable circuit breakers you see on Traynors and wiring one in can save a lot of fuses till you work out the issue.

nosaj

Those are real fuse savers but I don't have a 3 amp one around. I have one in my tube tester, my scope, my signal generator and even one grafted onto the side of my multimeter for current readings in which I always seem to pop the 10 amp internal fuse... what a chinsey idea to put a fuse in there, it should have been a circuit breaker.

[Sowhat]

The odd thing here is that you said it also blew fuses on your dummy load (post #1)? At this point, I might hook up a tube amp to the crossover/speakers in the amp and see if it works. I say tube amp because a tube amp will be able to withstand a short for a small period of time. You might blow up another solid state amp if there is indeed a speaker/crossover problem. Also, if you check the wires going from amp to crossover (speaker out of amp), what do you measure. Does it show anything shorted?

Maybe that's the impression I gave but it's not really what I said. The amp was on a limiter and showing in excess of 2 amps of draw with a load attached which is the limit of my limiter and it was full on. With the limiter fully engaged and the voltage showing somewhere around 40 volts full on I figured that was equivalent to well in excess of 3 amps bypassed to the mains. That's OK, I'm still scratching my head on that one in light of day 2's findings. Tomorrow, I'll get to the bottom of the speaker network by hook or crook and get this on put to bed.