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
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
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
Craig
Last edited:
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
- 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
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
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
Craig
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.
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.
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
Attachments
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
Craig
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
Zc
llwht - I didn't know your on a Mac, no way that I know of to determine that from DIY website, but iMacs are great machines, if you have any issues or questions, just ask.....
Here is the update of the output transistors. I used a Fluke 87V and used the diode symbol setting. One way the reading would start low then go up, when the leads were reversed then I got a steady state reading, which is listed below.
Black Green
b-e b-c b-e b-c
1 .025 .486 .025 .452
2 .025 .444 .025 .451
3 .126 .444 .127 .495
4 .025 .444 .025 .452
5 .025 .445 .025 .452
6 .646 .645 .641 .636
7 .743 .713
8 .534 .537
Mag
Here is the update of the output transistors. I used a Fluke 87V and used the diode symbol setting. One way the reading would start low then go up, when the leads were reversed then I got a steady state reading, which is listed below.
Black Green
b-e b-c b-e b-c
1 .025 .486 .025 .452
2 .025 .444 .025 .451
3 .126 .444 .127 .495
4 .025 .444 .025 .452
5 .025 .445 .025 .452
6 .646 .645 .641 .636
7 .743 .713
8 .534 .537
Mag
Attachments
llwht - I didn't know your on a Mac, no way that I know of to determine that from DIY website, but iMacs are great machines, if you have any issues or questions, just ask.....
Here is the update of the output transistors. I used a Fluke 87V and used the diode symbol setting. One way the reading would start low then go up, when the leads were reversed then I got a steady state reading, which is listed below.
Black Green
b-e b-c b-e b-c
1 .025 .486 .025 .452
2 .025 .444 .025 .451
3 .126 .444 .127 .495
4 .025 .444 .025 .452
5 .025 .445 .025 .452
6 .646 .645 .641 .636
7 .743 .713
8 .534 .537
Here is the update of the output transistors. I used a Fluke 87V and used the diode symbol setting. One way the reading would start low then go up, when the leads were reversed then I got a steady state reading, which is listed below.
Black Green
b-e b-c b-e b-c
1 .025 .486 .025 .452
2 .025 .444 .025 .451
3 .126 .444 .127 .495
4 .025 .444 .025 .452
5 .025 .445 .025 .452
6 .646 .645 .641 .636
7 .743 .713
8 .534 .537
Attachments
Looks like 1,2,4 and 5 are outputs, 3s are the drivers, and 6s are predrivers. The low readings are probably the 30 Ohm resistors (R9&10) between the drivers and outputs, exactly like I said you'd get. Use the Ohms setting on your DVM to see if it comes up at about 30 Ohms. You need to check the emitter/collector for low readings on all of the transistors you listed. If they are shorted it will read low in both directions. From the readings you listed so far, so good. Also check D5 and D6, these are directly connected to the output and each rail. They are to protect the output transistors from inductive loads putting signal back into the amp.
Craig
Craig
I forgot ask, are transistors shown in the pics for one channel or one side is A and the other B. If that is the case you'll need to pull the modules out to get to the bottom of each. Looks like the heatsinks are separate for each channel, makes more sense thermally anyway. So you measured the PNPs for one channel and the NPNs for the other channel. Since you measured +67 VDC on the output look for a bad NPNs.
Craig
Craig
Craig,
Are you saying that there are transistors on the bottom of the big heat sinks, and thats why I must remove them to check on the bottom ?
Also does it matter where the neg lead from the Fluke is placed when reading the ohm of the b-e, b-c, e-c readings ? One way the ohms start at one reading then climb, if I switch the leads around the readings then reads a steady state reading , and thats the readings I provided. I assume I've been doing this right.
Mag
yes there are transistors on the bottom of the aluminum heatsinks. you will have to pull the bad module out and check the bottom transistors too!
The first question I should ask is there a bottom cover that is removable? If not you'll have to remove the heatsink assys. to get to the rest of the output transistors. The way it looks in the pics each channel is on it's own HS assy. Testing the PNPs (green) put the black lead on the base and the red lead on the emitter and then the collector. Testing NPNs reverse everything. That will test a GOOD xsistor but to test for a SHORTED xsistor put one lead on the emitter and the other on the collector. If you get a real low reading both ways you have at least one shorted xsistor and since there are four in parallel they will all test shorted. Also there is an emitter resistor for each output xsistor that may or may not have gone open, R25-R38. What you are looking for is a way for the + rail to get to the output, that can be many things but is usually an output xsistor.
Craig
Craig
Black Green
e-c e-c
1 .450 .452
2 .444 .452
3 .443 .459
4 .443 .459
5 .444 .451
6 .493 .499
7 .903
8 .107/.109 * this transistor read the same no matter which way I try to get a reading from the fluke.
And to answer you, there is no plate on the bottom to get to the units under the heat sinks, so I guess I'll need to remove the two back cards first to get to the heat sinks to remove.
The R9/10 both read 22 ohms, read in circuit, didnt remove a leg to read, in fact ALL readings were this way.
Couldn't physically find D5/D6, do you know which card I should be looking at, starting from back looking forward to the front....
Is there an easier way ?
Mag
e-c e-c
1 .450 .452
2 .444 .452
3 .443 .459
4 .443 .459
5 .444 .451
6 .493 .499
7 .903
8 .107/.109 * this transistor read the same no matter which way I try to get a reading from the fluke.
And to answer you, there is no plate on the bottom to get to the units under the heat sinks, so I guess I'll need to remove the two back cards first to get to the heat sinks to remove.
The R9/10 both read 22 ohms, read in circuit, didnt remove a leg to read, in fact ALL readings were this way.
Couldn't physically find D5/D6, do you know which card I should be looking at, starting from back looking forward to the front....
Is there an easier way ?
Mag
Get a piece of paper and write down all of the connections and start removing the heatsinks, it will be easier to test eveything and replace what's bad. DO NOT write on the unit, I hate it when people do that. The way that amp is built the HSs must come out fairly easily since there is no other way to work on it. Parts will be easier to locate to boot.
Craig
Craig
Bottom of Heat Sinks
Left HS - Green
b-e b-c e-c
1 .025 .457 .448
2 .025 .457 .448
3 .126 .489 .458
4 .025 .457 .456
5 .025 .457 .456
6 .640 .639 .490
Right HS - Black
b-e b-c e-c
1 .126 .496 .449 <----- I double checked these two
2 .025 .451 .444
3 .025 .451 .444 <----- I double checked these two
4 .025 .451 .444
5 .025 .451 .444
6 .886 .648 .495
7 .647 .732 .817
8 .707 .546 1.121/1.070
Left HS - Green
b-e b-c e-c
1 .025 .457 .448
2 .025 .457 .448
3 .126 .489 .458
4 .025 .457 .456
5 .025 .457 .456
6 .640 .639 .490
Right HS - Black
b-e b-c e-c
1 .126 .496 .449 <----- I double checked these two
2 .025 .451 .444
3 .025 .451 .444 <----- I double checked these two
4 .025 .451 .444
5 .025 .451 .444
6 .886 .648 .495
7 .647 .732 .817
8 .707 .546 1.121/1.070
Attachments
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