Ashdown EVO II 500 - Bass amp repair

[saint gillis]

Hello, just to warn anyone who's going to repair any of these that the 2 heat-sink blocs for the power transistors are connected through the collectors directly to the power rails.
So you have more than 120VDC between them, a real little electric chair, be extremely careful, and if you work with the amp unloaded it will take time to discharge.
Cheers

 

[CJ]

Oh boy, let me guess how you found that out,

I think peavey or acoustic might have put out a few products like that.

 

[Ike Zimbel]

The ATI 8MX-2, 8 channel pre-amp has +/-48vdc rails with the cans/heatsinks with those voltages present on them, so 96vdc when you touch both. And yes, I found that out the way CJ is implying!

 

[user 130681]

Somewhat like most QSC, commercial grade Crown and many more.
Engineers that work on these are supposed to know that. It is not an issue if you are manufacturer trained or at least can read a service manual.
Time to wise up ... maybe.

 

[CJ]

No, time for engineers to put the collectors at zero volts.

 

[user 130681]

I state again, if the "tech" hurts him/herself whilst repairing stuff, that means the "tech" is not qualified and has no business playing in big boys toys.

 

[saint gillis]

I found out slightly touching with my right hand one of the heatsinks with the power outlet removed from the amp. Maybe I was a bit tired. I still agree with CJ. And even if Jon S is also right in a way, if someone touches one heatsink with his left hand and the other with his right hand the level of risk is insane, we should also try not to kill the unqualified techs !

 

[CJ]

First troll I have seen in here in about 20 years congratulations jon!

 

[radardoug]

I got bitten the other day by an old tape machine, and thought I was miles away from any mains. Just shows you have to be ever vigilant!

 

[Nick Nack]

I state again, if the "tech" hurts him/herself whilst repairing stuff, that means the "tech" is not qualified and has no business playing in big boys toys.

First troll I have seen in here in about 20 years congratulations jon!

I don't think that's trolling. Forthright maybe. Surely it's not the engineer's job to make everything idiot-proof, else we'd have non-inflammable petrol and alcohol-free vodka.

 

[JohnRoberts]

That is a bit harsh... this is how we gain experience.

In many older power amp designs using metal TO-3 power devices that routinely have the collector bonded to the metal case there is a strong motivation to lose the mica washer between the TO-3 and metal heatsink. This reduces the thermal resistance between junction and sink. It also reduces the assembly labor and cost associated with insuring that the power devices are insulated from the metal heatsink. These "hot" (in more ways than one) heatsinks must be insulated from the chassis but this can be easier and cheaper than insulating multiple power devices.

I have never done this but I have seen it done inside amps from several major manufacturers.

Agency safety constraints are mainly focussed on preventing end users from touching energized parts or high temperature components. The concern about repair technicians are things like labelling internal fuses so a tech does not inadvertently replace a blown fuse with the wrong value defeating the protection against overheating capability.

JR

[edit - those of us old enough to remember working on old TV sets, they had power cord interrupts so you couldn't remove the back cover to get inside, without removing the line cord. Back then we bought "cheater" cords that allowed us to power up the TV sets with the protective back removed. Repairs back in those days were routinely DIY, removing tubes and testing them at the local drug store. Hot chassis designs offered more opportunities to learn about electricity. /edit]

 

[abbey road d enfer]

Vey harsh comment IMO. Accidents happen.
If all repairman that hurted themselves in the course of their job were to cease their activity, there would not be much left.
Indeed, once is bad luck, twice is neglect, thrice is foolishness.