Repair of Crown Micro-Tech

Creative1

Member
2010-12-07 1:39 pm
I have an old Microtech 1200 that's been sitting around for a while...

Looks like Channel 1 is toast. The ODEP light doesn't light, and I get 83/84 VDC on the speaker outs. I assume that one or more of the output transistors has shorted.

It looks like the parts are cheap, $2.50 ea for the generic parts, or for geniune Crown parts, $4.83 for C6492 and $4.54 for C4751.

So 2 questions:
1) Is it really worth buying the genuine Crown parts? Some posts I've read said that the generics won't work and that the Crown parts are graded and better quality.

2) And, should I replace all the output transistors? All 6 C6492 and 6 C4751 on the channel?

I know I need to look for other damaged parts, I haven't had a chance yet.
 
Crown parts not needed, unless it is a unique to Crown part.

The output stage will run fine with just one single transistor per rail. Of course you can't run
it into a load at full power like that, but it can help to find bad outputs... all outputs should be tested out of circuit. Usually one or more get blown.

Crown's devices are matched. Not perfectly, but they fall within a reasonable range, there is usually a stamp on them with a number... So, it's not a bad idea to change them all IF you are going to run the beast hard, like in a PA/SR situation where things get very hot. Mismatched outputs can cause "current hogging" which causes them to turn into silicon fuses.

Check drivers too...

The Micro Tech is somewhat more difficult to repair due to the somewhat wild and wooly PCB... a schematic is a must on this amp...

You might get lucky, remove the outputs, look for the blown ones, check the drivers for shorts, and put in at least one that is known to be good, fire it up. If it works then you just need to replace the outputs, or the one that is blown...

_-_-bear
 

Creative1

Member
2010-12-07 1:39 pm
Crown parts not needed, unless it is a unique to Crown part.

The output stage will run fine with just one single transistor per rail. Of course you can't run
it into a load at full power like that, but it can help to find bad outputs... all outputs should be tested out of circuit. Usually one or more get blown.

Crown's devices are matched. Not perfectly, but they fall within a reasonable range, there is usually a stamp on them with a number... So, it's not a bad idea to change them all IF you are going to run the beast hard, like in a PA/SR situation where things get very hot. Mismatched outputs can cause "current hogging" which causes them to turn into silicon fuses.

Check drivers too...

The Micro Tech is somewhat more difficult to repair due to the somewhat wild and wooly PCB... a schematic is a must on this amp...

You might get lucky, remove the outputs, look for the blown ones, check the drivers for shorts, and put in at least one that is known to be good, fire it up. If it works then you just need to replace the outputs, or the one that is blown...

_-_-bear

Thanks a bunch. Sounds like I'll have to start de-soldering to start testing...

Yeah, I'm aware that the PCB design is a little....... WEIRD.


I'm tempted to replace all the outputs, since they're cheap.. I guess I'll see if it's just the outputs or not.