SWR California Blonde goes insane!

[Sowhat]

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?

All good checks and this gives me some additional things to check which I have not done yet. I'm sure it will be the last thing I check that turns out to be the problem... it always is.

[The Dude]

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]

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]

I have a 2 amp push button breaker wired to clip leads. I use it all the time.

[Jazz P Bass]

A word of caution on circuit breakers.

They are only 'good' for so many 'hits'.

[Sowhat]

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]

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.

[ATTACH=CONFIG]48851[/ATTACH]

Somehow between it being on the bench and it being in the cabinet something bad happens, but what?

Check your screw lengths out of the chassis and see if there's a possibility of one them touching something it shouldn't. Look inside the cabinet to see if any shielding is loose also.
nosaj
nosaj

[glebert]

Does it power up fine on the bench directly into mains (no variac) and load connected?

[Sowhat]

Check your screw lengths out of the chassis and see if there's a possibility of one them touching something it shouldn't. Look inside the cabinet to see if any shielding is loose also.
nosaj
nosaj

I have not had a mounting screw re-installed since the beginning of this saga, I never get that far. I did notice that the HF driver + terminal was bent in such a way as to possibly touch the magnet but that should not make a difference, the magnet has no ground potential... a least I don't think it does. Didn't take a look at the cabinet shielding, if there is any, maybe that's a problem?

[nosaj]

I have not had a mounting screw re-installed since the beginning of this saga, I never get that far. I did notice that the HF driver + terminal was bent in such a way as to possibly touch the magnet but that should not make a difference, the magnet has no ground potential... a least I don't think it does. Didn't take a look at the cabinet shielding, if there is any, maybe that's a problem?

It's usually some type of foil, which could possibly short something.
nosaj

[Sowhat]

Does it power up fine on the bench directly into mains (no variac) and load connected?

Ahhh... I don't know. I always go through the variac when on the bench, firstly to breaker at 5 amps and have the ability to monitor voltage and current, I don't want to work on stuff drawing more than that and secondly just because that's my closest available socket in relation to the amp jig.

[J M Fahey]

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 itīs a "design feature" so donīt 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) īplug 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 youīll 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 youīll lose all sparkle (the main reason they used that weird Tweeter there)

https://www.ebay.com/itm/SS-Audio-Tw...-/152011150540

[Sowhat]

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 itīs a "design feature" so donīt waste time chasing it.

Just to retrace steps, basically to discard non problems :

Yeah, I dumped the limiter a while ago because that was obviously causing mischief. On the dummy load it works good and once I gutted the speaker network out of the cabinet and put it up on the bench with the amp, it works good as a whole system as well. The exact same system installed in it's cabinet just 10 minutes before made a loud 120 cycle-ish buzz through the speakers and then popped the fuse. Sounds like a short to me. The only thing I have not done on the bench is slamming on the mains... up to this point I have been ramping up slowly and monitoring for current spikes because I'm not using a limiter, I don't need any fireworks. In the cabinet I used the amps power switch plugged straight into the mains and that's the location that blows fuses, not the bench. I certainly need to test this tomorrow morning.

[Sowhat]

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]

makes me look like a genius, but we all know the truth about that.

Yes we do. You're the one who knows where to put the chalk mark. Collect your genius award!