|Sowhat||5/11/2018 6:04 PM|
|SWR California Blonde goes insane!|
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.
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||5/11/2018 6:53 PM|
|Were you trying to run the amp on a light bulb limiter? That will cause problems if so.|
|Sowhat||5/11/2018 7:02 PM|
|nosaj||5/11/2018 7:30 PM|
|The Dude||5/11/2018 7:35 PM|
|Agree with possible oscillation. Have you scoped the output with no load/no signal?|
|Sowhat||5/11/2018 8:13 PM|
|hodgy||5/15/2018 11:14 AM|
|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||5/15/2018 5:08 PM|
|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||5/15/2018 5:25 PM|
|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||5/15/2018 5:32 PM|
|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||5/15/2018 5:58 PM|
Just for clarification, your nice clean sine wave was going to a speaker or a dummy load?
|Sowhat||5/15/2018 6:13 PM|
|nosaj||5/15/2018 6:16 PM|
|Sowhat||5/15/2018 6:29 PM|
|Sowhat||5/15/2018 6:50 PM|
|Sowhat||5/15/2018 6:55 PM|
|The Dude||5/15/2018 7:34 PM|
|I think it was George Carlin who talked about losing things. "You always find it in the last place you look".|
Of course you do, because you don't keep looking after you've found something.
|g1||5/15/2018 9:09 PM|
|Not sure if this is of any help, you may know this already, or it may not be relevant in this case. But it could have led to a 'false positive' over-current reading initially.|
Some amps won't start up right on a limiter with a load connected. The output will stick to one of the DC rails and the bulb will light up and stay lit. Then you try it with no load and it doesn't light up.
In those cases, start it up on the limiter without load, then connect the load while still powered on (with clips, not with 1/4 inch connectors).
In your case, it still blew the fuse when put in the cab without limiter, so obviously there is a problem. Just didn't want you chasing your tail on the bench with a test equipment induced fault.
|Randall||5/15/2018 10:19 PM|
|I have a 2 amp push button breaker wired to clip leads. I use it all the time.|
|Jazz P Bass||5/15/2018 11:04 PM|
|A word of caution on circuit breakers.|
They are only 'good' for so many 'hits'.
|Sowhat||5/16/2018 3:34 PM|
|I've just about had it with this amp, here's what's up now. The picture attached shows how I have the amp hooked up... just like it is as it would be in the cabinet, speaker, horn, crossover and all. The only thing different is that my fuse is currently that red jumper wire. I ramp this up on the variac closely monitoring the current draw and damned if the thing don't fire right up, no shorting buzz, no funny business, works fine, low current draw.|
Somehow between it being on the bench and it being in the cabinet something bad happens, but what?
|nosaj||5/16/2018 3:49 PM|
|glebert||5/16/2018 4:47 PM|
|Does it power up fine on the bench directly into mains (no variac) and load connected?|
|Sowhat||5/16/2018 5:25 PM|
|nosaj||5/16/2018 5:40 PM|
|Sowhat||5/16/2018 5:47 PM|
|J M Fahey||5/16/2018 7:48 PM|
|ALL SWR amps have the same problem: you turn them ON while plugged into a current limiter AND load connected and they stick to one rail. Always.|
G1 mentioned this earlier and I confirm it´s a "design feature" so don´t waste time chasing it.
Just to retrace steps, basically to discard non problems :
1) plug into limiter, no signal, all controls to 0 , NO LOAD : does it turn on fine?
2) check all voltages: rails (which of course will be somewhat low), DC at output (less than , say, 150mV), bias current (drop across emitter resistors)
3) connect load. Everything fine? Remeasure basic voltages.
4) ´plug straight into mains, no load.
5) same but now turn on with load.
For peace of mind repeat basic voltage measurements.
6) play it.
Look at scope screen to catch any oscillation , fuzzies, etc. which might cause instability.
Repeat connected to actual combo speakers, I think you´ll find it uses a Brazilian "Ferroelectric" LeSon Tweeter, a weird hybrid of piezo and magnetic.
If it shorts, impedance is wild reactive and power amp may become crazy unstable, or plain oscillating.
If dead, I guess Parts Express carries them or worst case replace it with a standard Piezo that fits (or enlarge hole) with a 47 ohm 2W resistor in series.
Easy load for any amp but you´ll lose all sparkle (the main reason they used that weird Tweeter there)
|Sowhat||5/16/2018 10:33 PM|
|Sowhat||5/18/2018 2:39 PM|
|We have a successful conclusion to this saga...|
I simply slipped the head into the cabinet and powered up with my hand hovering over the kill switch. All the way in I got a runaway short, max amps then, I slipped the chassis half way out and it came up just fine... slipped it back in and fireworks, slipped out nirvana. Well, I guess you know where this is going, what happens when I flex the chassis? BINGO! I can control the short. Long story short both the collector and emitter leads of Q4 (which is attached to the chassis heatsink) were cracked. Whoever it was who posted about cracked leads on that heatsink, give yourself a pat on the back from me.
I didn't even replace that transistor, I fired up the microscope and spot welded in some new leads... I love how I can do microscopic work now, makes me look like a genius, but we all know the truth about that.
|Leo_Gnardo||5/18/2018 3:39 PM|
|J M Fahey||5/18/2018 5:29 PM|