Plexi 100: crackling and hiss, small booms...

[Twistingcrow]

Hi!
Here it is: my plexi 100 had been working fine since January. Yesterday it smoked then turned off. The 4A fuse has blown and so has a cap. Big black stuff on it. Of course it is now out of question of playing it!

ANY help, any thoughts about what could have caused this will be GREATLY appreciated!
Thanks!

Here are a few pics, let me know if you need any info to guide you through troubleshooting. I have built a dozen amps and it's the first time that I have to live through this... I'm totally panicked.

[Twistingcrow]

The black stuff on top of the cap is like black plastic that is hardened after having melt... If that helps...

[toner]

The first mains electrolytic filter cap exploded (obviously) and took the mains fuse out. You're lucky it didn't spray all over the insides. Are they JJ brand? I've read those can be unreliable compared to some others.

As for the cause, maybe there was a voltage spike that overloaded it. I've never had a cap fail so I'm not sure of possible causes. You should probably also replace the other mains cap above it. Who knows what might have hit it. Might what to check the power switch, etc. also.

[Twistingcrow]

The first mains electrolytic filter cap exploded (obviously) and took the mains fuse out. You're lucky it didn't spray all over the insides. Are they JJ brand? I've read those can be unreliable compared to some others.

As for the cause, maybe there was a voltage spike that overloaded it. I've never had a cap fail so I'm not sure of possible causes. You should probably also replace the other mains cap above it. Who knows what might have hit it. Might what to check the power switch, etc. also.

The caps are/were the stock CE caps provided with the kit, the ones with the white labels on them. I think I'll buy four F&Ts to replace those four... Damn, they're not cheap...

Thanks a lot for your help toner!

[Twistingcrow]

Mmm, I'm trying to find info about the CE caps the plexi 100W kit provided and I can't find any... Are they considered any good or are they low-cost (and low-quality) alternatives to the JJ or F&Ts?
Thanks.

[toner]

I don't know about the CE's but I've had no problems with F&T. ValveStorm has them for $9.95 - http://valvestorm.com/Products/Componen ... ectrolytic" onclick="window.open(this.href);return false;

Might be cheaper somewhere but I think I paid $15 for mine a few years ago.

[Twistingcrow]

I think I'm gonna change the whole 6 caps while I'm at it... I don't trust those CEs anymore... Thanks again toner!

[syscokid]

That's the first time I've heard of the CE's failing like that. George wouldn't supply those in his kits if he felt they were inferior just as a cost-cutting decision. That being said, I would go with the F/T's...and suggest purchasing them through Valvestorm...a Metro family member...

[Twistingcrow]

That's what I did. I actually bought the 6 caps, F&T brand from Valvestorm along with the two 2 watt resistors.

[Twistingcrow]

Hi!
Back here with some more news and troubleshooting...
So I have changed the four central 50+50uF caps for F&Ts. Obviously I changed the 4 amp fuse as well. The amp works and I can now play with it. BUT:

1. I hear noises as soon as I flip the standby switch ON. Like crackling noises and relatively loud hiss, they may be the new F&Ts breaking in but I'm not sure...

2. The Bass and Treble pots are very hissy all along their course. At times I can hear small explosive sounds when I turn them and I once saw a blue light in one of the power tubes when a small explosive sound occurred. When I stop turning them then the amp is all quiet except for problem #1...

3. I checked all the tubes and they're all good.

4. I changed the Bass pot to see if it wasn't the pot that was faulty but I have the same problem with the brand new one...

5. The presence, middle and volume pots are OK.

So if you have any idea as to what I could do to change that, I'm all ears...
Thanks!

[toner]

I doubt the filter caps have anything to do with the hiss. Maybe the crackling when flipping standby.

This is just a *guess* but may be worth checking:
You may have a tone stack cap leaking DC that can cause scratching or maybe hiss noise when turning the treble and bass pots. Check for DC voltage on the outer lugs (1 and 3) of the treble pot (meter common grounded). There shouldn't be any there. If there is, trace the wire back to the connected cap and try replacing it. Again, this is only a guess because I haven't experienced this.

[Twistingcrow]

Thanks toner... I'm glad you're here... I'll try what you say but tell me: do I have to run through this test with the amp turned ON or OFF or in Standby mode?

I have changed both .022 caps going to the bass and treble pots. Same problem... OMG, I don't know what's wrong in this amp... It just makes a loud boom when I flip the standby switch to off... And booms when I turn the bass and treble pots.

Help! George are you there please?

[toner]

...And booms when I turn the bass and treble pots.

That is really weird. Sorry the other idea didn't work out.

Maybe some other component was damaged when the cap died. ?

[Twistingcrow]

So I have done a couple of of other things:

1. Replaced the presence cap.
2. Refluxed a few solder joints.
3. Changed the treble pot.
4. Checked the board components with my MM.

I don't get booms anymore but the amp is still hissy and turning the bass and treble pots still makes way more hiss than it should. I really don't know what else I could check, I'm totally stumped...

[VelvetGeorge]

How do you know the tubes are good? This has tube written all over it. I'd start there.

When the cap failed many other parts were also stressed. Dont take anything as good until prove it. Culd be hard to troubleshoot, but you'll find it.

Those CE caps tested better than JJ's on my cap tester. First time I've seen one blow up. Sorry for that.

George