Crest LA901 red status light on CH A

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Read the red status lights can indicate;

power issues, over temp, or circuit protection

A. Can anyone state what the DC power supply voltage should be while reading with my new fluke DVM ?

B Over temp, is there any other over temp protection except for the one thats mounted on the primary AC transformer, if so where and whats the nominal ohms with one leg lifted. If thats not the only one then were is the other(s) and whats the reading it/they should be ?

C. Circuit protection, I'm assuming its talking about shorted outputs of the pre-amp, power amp and/or speakers/load.

Can one work back from the relays and disconnect connectors to help isolate ( get green led ) problem area. I have a schematic, but it looks somewhat incomplete.


Thx in advance


Mag

[llwhtt]

I just downloaded the schematics from the Crest site and the easiest thing to check with a DVM is DC voltage on the output of the amp, before the protection relay. The schematic that has the amplifier section is 21C1240. If you don't have that number or the amplifier schematic go to Crest site, service, LA901 and download it so we are all on the same page. When you have that look on the right side of the schematic and you'll find "out" and a inductor/resistor combo (L1/R59). With power "ON" carefully measure that point, you should have NO or very very little DC, less than 50mv or so. If it is close to 75VDC + or - you probably have some shorted output transistors. To answer one of your questions the power supply should be 75VDC, + and -. Check all of that and post your results and we'll go from there.

Craig

[Zero Cool]

Crest's are notorious for having bad capacitors that keep the amp in protect. often, there is nothing wrong with the amp and its the protection circuit itself at fault. so check all the small electrolytic capcitors

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- llwht;

thx for the info, do have that schematic and do see where you are talking about. Now I need to just find it; to save me time, if I was standing with the back of the unit facing me, where would I start ? The first card where the black square relays are ?

1 ohm 5 watt and the 2 micro henry inductor.....were art thou :-)

Posts to follow, thx for your help... really

- Zero Cool;

thx for the Cap info. Do they usually explode like the one that did in a Peavy SM115 monitor Im trying to fix ! No these two issues are not related, just stuff Im working on to help some friends......


So again thanks for the leads...


Mag

[llwhtt]

I have not seen that particular amp so don't know about physical locations. The inductor is going to be a heavy copper coil, possibly about 3/8" dia. and 1 1/2" long, hard to miss. Hopefully it is in a location you can get to. The point is to measure this and if it is OK you have a minor problem with the protection circuitry. Can you post any pics?

Craig

[Zero Cool]

the caps dont explode or show an physical signs of failure. but pull out a 220uf(example only) cap and measure it and if it measures less then 20-30% of its labeled value replace it.

I would check for DC at the output inductor. should be very low. few mv and definitely less then 100mv. if the offset is low. input a sine wave. if you see a clean sine wave at that point is almost always is bad caps in the protection circuits and front end supply circuits. small value EL caps.

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Originally Posted by llwhtt

I have not seen that particular amp so don't know about physical locations. The inductor is going to be a heavy copper coil, possibly about 3/8" dia. and 1 1/2" long, hard to miss. Hopefully it is in a location you can get to. The point is to measure this and if it is OK you have a minor problem with the protection circuitry. Can you post any pics?

Craig

llwht - The pics are attached below as you requested, I measured 67 Volts DC across L1 and ground. The ground point was one of the brass stand offs which is attached to the heat seats for the output transistors.

If this is the correct points to measure, then as you stated there must be shorted transistor(s). Once the caps bleed the voltage off ( 2 hrs) I will check each of the output transistors ohms between the base, emitter, and collector and look for one thats different.

Is there is a ohms range I should be looking for or is there a better way to check for shorts on them ?

Thx again for all the help to you all. If you have any question on any Apple product, I can help..... Thx again

Mag

[llwhtt]

Well you found that the protect circuit is working and why it is working. Is there a small diode symbol on your DVM? That's the position to test most semiconductor PN junctions, a good junction will read approx. .500. The big green transistors are PNP, the blacks are NPN. If you position the transistor so you can read the numbers the base is on the left, the collector is in the middle, and the emitter is on the right. Since you have one good channel use it to compare the readings. On the good channel you will probably notice that the base/emitter junction will read fairly low sometimes almost a short depending on the design of the driver/output stage. The base/collector junction should read normal.

Craig

[llwhtt]

How did you know I'm on an Apple (imac)?

Craig

[Zero Cool]

Make sure to check/replace the 2 associated resistors when you find the shorted transistor. there should be a large square low value white block resistor and a smaller round resistor for each output device as shown in the middle photo.


Zc