Failed thermistor

[Mushypeas]

Evening all. Not sure how stupid I have been but here's the story.

 

The powered desk's amp from the old sound system had blown both of it's fuses from insanely low impedance, caused by an over excited engineer. We had a refit of the sound at the end of last season. as I decided that I didn't want to have anything like that happen again. And so we were left with this spare powered desk, a PMD-12 from JBSystems no less, but it has taken me to this weekend to order the right fuses and replace them.

 

There was absolutely no other fault of any kind with the desk at any other point and has worked fine on several occasions plainly as a mixing desk.

 

On opening up the case and the bottom amplifier compartment I was easily able to locate the fuseholders and replace the fuses. I sealed the case up connected the kettle lead and switched it on. The mains fuse in the plug blew. 5A just as the back of the mixer instructs.

 

My immediate thought was I had grounded/sliced a mains wire while shimmying the metal covers around. I opened it up again check all the mains cable found nothing. I ensured the plugs were all secure and checked for any loose screws. Nothing.

 

I then did something that in hindsight seems insane. Case sealed once more I replaced the fuse in the plug top put it back in switched it on and heard a pop and some smoke emanating from a vent. After much swearing I checked any residual had been discharged and opened her up to see that the second thermistor had failed.

 

My question:

Is it most likely down to the age of the thermistor that it failed? The desk must be at least 8 years old maybe more, it was definitely here before my time which started 3 years ago. It had also had no form of maintenance, and the vents in the amplifier compartment were obstructed from being stunk into a hole on the desk, so dust and crap was rife throughout it's innards.

If not, did I make a stupid and naive mistake?

 

May be worth mentioning that after both overcurrents the quick blow fuses remain fine, after all they are rated to 15amps and the mains fuse is at 5.

 

Regards

Luke

[Jivemaster]

Far too many amps will die under adverse conditions, and then clear their fuses. SO a repair will mean component level diagnosis, or bulk replacement. Usually a specialist repairer is a good plan.

[Brian]

... to see that the second thermistor had failed.

Which second thermistor?

 

I can's see anywhere in the post which mentions the first thermistor.

[Mushypeas]

 

 

I can't see anywhere in the post which mentions the first thermistor.

 

Sorry, the first thermistor, labelled NTC 1 appears to be fine. The 'second' thermistor labelled NTC 2 popped.

 

Thanks for that Jive, I agree on the specialist idea, but I'm starting to wonder whether I could make my own power supply just for the mixer. All the supply voltages are clearly labelled, or is that another insane idea?

 

Anyhow thanks both. It may be time to retire this for good.

 

Ta

Luke

[KevinE]

The NTC's will sometimes blow if there's a short in the amp. I suspect you've blown an output channel. It is a toss-up whether the internal fuses, the plug fuse or the NTCs blow at this point. You've obviously run through all of those options!

The internal fuses are internal for a reason...they aren't meant to be replaced by the user but only by an engineer who will check for other internal damage before replacing them and applying power.