Amcron (Crown) Micro-Tech 1200 help

[Eddie Wolve]

I recently have had a problem with this amp.

Does anyone have a link for a schematic for this amp? or have experience in fixing this amp to give me some idea's. Crown's web site seams to have every other schematic for their macro-tech and micro tech series, (what's the difference anyway??) but not this one.
In a nutshell, one of the OPT's went short, (which I have replaced) but I still have bad distortion coming from that channel (sounds like an amp with one of the supply rail fuses blown).

Unlike other amps which have a 'centre tapped' power supply, these ones seam to have a 'floating' ground and measuring the rails on the cases of the OPT's I get +84 on one side and -5 volts on the other...(bride rectifier is OK)

Any help appreciated.

Ed.

[MOER]

These are grounded bridge amplifiers

Steve

[The Saint]

Hi

I would say that you have some damaged driver transistors
In the Output PCB.
also look at the possiblity of more leaky output devices

Have a look at this link to get the schematic you require

http://www.crownaudio.com/gen_htm/legacy/legacamp.htm

[Eddie Wolve]

Thanks Anthony. I have already been at that page, and it doesn't actually have a schematic for the 1200, only a service manual which does not include the schematic. I have found a micro-tech 1000 diagram, which I assume has the same internals, but smaller transformers (etc) and I am trying to use that.

I will test the driver transistors and see how I go.

I have aldready seen you web site Anthony and am very impressed with some of your work... Im might give one of them a crack one day.

Cheers.

Ed.

[djk]

Crown made about 15 different amps using the same circuit boards.

The Macrotech 1200 is close enough for your use.

Unless you intend to dump it cheap, fix it properly. I always do all the outputs, drivers, pre-drivers, bias, and Vas.

[Eddie Wolve]

Thanks DJK.

I happen to aggree. I have just ordered heaps of MPSA42,43's, and MPSA19's...Also the drivers, MJE15030/31's..Not to mention about all the OPT's...(not great on the wallet!!)

I also found some blown tracks on the main PCB, caused by some shorting diodes (4004's and some 4148's)..I bit of a war zone really, but I will get there..

Cheers.

[djk]

All but the outputs are cheap.

For outputs I generally use MJ15024/25 that I buy in 100 lot, so they are only $2.50 each.

Consider adding the last set of outputs if you intend to keep it a long time, I do this on the 600s and 1200s (and change the emitter resistors to suit).

[unclejed613]

i do a lot of pro-audio repair, and i have the dubious honor of telling customers that their amp is NERTS (not economically repairable) when i see a lot of traces evaporated from a board (sometimes so rapidly that they don't even leave much in the way of burn marks!!!!!!)...... needless to say pro-audio equipment gets abused worse than home equipment, and i see a lot of hopeless amps.

the other tech where i work had one on the bench today that has a very strange design, in which the collectors of the output devices are all grounded, and the emitters "walk" the floating power supply rails, and couple the voltage changes to the speakers through the filter caps......

[Eddie Wolve]

I think Very Strange is an understatement for that amp!!.

I get confused enough on the Crown's 'floating ground' which have the emitters of the 'low side' NPN OPT's going to 'ground'...

[djk]

"a very strange design"

QSC, Behringer, Tapco, Alto, and many more I'm sure.