Crown Macrotech 1200 Fault

[smalljoshua]

Hopefully a quick one, but I have a feeling this could get messy.

 

I've got a poorly Macrotech 1200 in front of me at the moment. Channel 2 is working perfectly and outputs at full level with a full brightness OPEP LED. Channel 1 has a dimmed ODEP LED on the front of the unit and a correspondingly low output level. I have to attenuate Channel 2 by a fair bit to obtain the same (audible) output level.

 

The amp normally runs a single 4R sub on each side. The problems were first noticed on return of the amp after a DJ had driven both of the subs to destruction.

 

I've got a service manual for the amp, and I'm quite happy to open it up (provided it's safe) and repair it myself.

 

Any suggestions would be appreciated.

 

Josh

[adamharman]

Any suggestions would be appreciated.

 

Josh

 

I think you know Wrighty? He seems to be doing quite a good sideline in amp repairs recently. Sure I remember him talking about a macrotech with that sort of fault so definately worth ringing him. He's not on here much so probably won't see your post.

[KevinE]

Probably popped a couple of output transistors given what you've said.

 

The ODEP circuitry will reduce the gain of any channel that is under stress, either current or thermal, the idea is that it tries to keep the amp going when most amps will have shut down or tripped.

 

A quick prod with a multimeter at the relevant points will soon track the fault down.

 

The caps on the macrotechs can stay charged up for quite a while in there after power-off so keep fingers away till you're sure they're discharged.

[smalljoshua]

@adamhardman: I certainly do know Richard, and my plan was to send it to him if I was unable to fix it myself. I do however quite enjoy fixing things for myself so wanted to give that a try first.

 

@KevinE: I thought I'd popped something, wasn't sure what though. Would I find the points (and correct values) in the Service Manual? If not, where are these points and what should the reading be?

 

Many Thanks

 

Josh

[KevinE]

What I would do is this.

 

First of all, power off, make sure bus caps are fully discharged. Do not stick your screwdriver across them, there might be a big bang and you lose the end of the screwdriver. Use a 100W lightbulb on prods if theres much in there.

 

Do a visual check first, scorch marks, things getting brown or flash marks around components.

 

On the faulty side, do a cold check of all the output transistors. Compare with the good side if you're not sure.

If there's any transistors gone, they'll usually be short C-E, possibly the corresponding emitter resistors (large ceramics, one per transistor) will be o/c as well. If anything shows up as blown at this stage, there will usually be other things gone too, fusible resistors, diodes etc. Do not just replace the obvious and turn on, this might be an expensive hobby.

 

If the fault were so drastic as to short both NPN and PNP transistors then it usually trips the thermal breaker or 13A fuse, the amp may continue to run bravely if just PNP or NPN transistor(s) alone have gone.

 

If nothing shows, power up and look at the manual, look at the ODEP voltages for the faulty side and measure them. In my copy of the manual there is a misprint which shows the ODEP voltages as millivolts when they should be volts, or viceversa, I cant remember but it will be obvious when compared to the good side.

 

The ODEP volts must be in spec, I suspect they will indicate otherwise since the ODEP light is dim on the bad side. They dont have to be far out to cause ODEP protection. Read the manual's explanation of how the ODEP voltages are generated. Check the bias voltages, if these are very weird then the output stage is definitely faulty and it's back to cold checks on the output module.

 

The ODEP and Bias voltages are present on the test points on the top card, read very carefully how to measure them.

 

If nothing serious still shows up with the output stage then you can spin the top card round and swap the ribbon cables over, and run the amp with the drive signals reversed, to prove if the fault's on the top or bottom section. You will need to balance the top card on it's edge to get the ribbon cables to reach the opposite modules. Do not do this if the faulty side has serious semi blowup damage, it may damage the top card on the other side.

 

That's all I can say without enrolling you on a crash course in electronic servicing!

Take care, lots of high voltages and currents in there and if you slip with a probe you may suddenly increase your repair bill and lower your morale somewhat!

[smalljoshua]

Hi Kevin,

 

Thanks for the in depth reply. I'll get onto that this afternoon.

 

Cheers

 

Josh

[Wrighty]

Watch what you're doing, Josh. If you're not 100% confident with what you're doing, leave it well alone and get it to me. A failed repair attempt is often more time consuming and costly to fix...

[smalljoshua]

Well, it didn't take long, but I think I'll be admitting defeat on this one. Just had the cover off the top and I'm not sure I feel comfortable poking around in the unknown.

 

@Wrightly: PM Sent.

 

Josh