Grabbed this for $50 and thought I'd have a go at fixing it. I can understand about 20% of the schematic below and have a Fluke digital multimeter. I've fixed a half-dozen NAD amps with help from people here (anatech) and it's been a beautiful experience.
Upon powerup there are no lights or sound or fan, only lots of heat within a few seconds from the big 17W resistor near the 110V AC point. External fuse and power switch are ok, and there's 120v AC going into the thing that looks like a small spare tire. Heh. Anyway, the schematic has that resistor valued at 23W, but either way it's really hot. That tells me there's a short somewhere, right?
It does not appear anyone else has been in there, and there are no burns or anything obvious, so before I really start tearing it down to get to the bottom of the board and measuring (and ordering) parts I thought I'd ask here to see if I'm wasting too much of my time. I did a quick continuity test on the top surfaces of all 16 TO-3 cans and they all seem to be connected (low resistance).
My next step was to check voltage at what you guys call the "rails" and see if they are +/- 95v DC per the schematic, but I'm not sure exactly where to do that and whether it will tell me anything.
There is a Crown service center in town, but what fun is that?
Thanks in advance.
Upon powerup there are no lights or sound or fan, only lots of heat within a few seconds from the big 17W resistor near the 110V AC point. External fuse and power switch are ok, and there's 120v AC going into the thing that looks like a small spare tire. Heh. Anyway, the schematic has that resistor valued at 23W, but either way it's really hot. That tells me there's a short somewhere, right?
It does not appear anyone else has been in there, and there are no burns or anything obvious, so before I really start tearing it down to get to the bottom of the board and measuring (and ordering) parts I thought I'd ask here to see if I'm wasting too much of my time. I did a quick continuity test on the top surfaces of all 16 TO-3 cans and they all seem to be connected (low resistance).
My next step was to check voltage at what you guys call the "rails" and see if they are +/- 95v DC per the schematic, but I'm not sure exactly where to do that and whether it will tell me anything.
There is a Crown service center in town, but what fun is that?
Thanks in advance.
The softstart circuit isn't working by the sound of it. The relay for this might be faulty or the circuit controlling the relay might have a fault.
Make yourself a lead with a bulb in series with the Live/Line/Active wire of the mains. This will save a lot of possible damage and also alert you that things are not right.
You need to be really, really careful as the fault sounds like it's in the primary side of the power supply. One wrong move and you can easily end up causing a fire, or getting a nasty potentially fatal shock.
Make yourself a lead with a bulb in series with the Live/Line/Active wire of the mains. This will save a lot of possible damage and also alert you that things are not right.
You need to be really, really careful as the fault sounds like it's in the primary side of the power supply. One wrong move and you can easily end up causing a fire, or getting a nasty potentially fatal shock.
Thanks all, and you are right as the relay makes no click at all so I will start there.
And please pardon my attempt at humor about calling the transformer a "small spare tire" as I know the purpose and the danger of transformers.
I have brand new leads on my Fluke to prove it. Heh. (There I go again).
And please pardon my attempt at humor about calling the transformer a "small spare tire" as I know the purpose and the danger of transformers.
I have brand new leads on my Fluke to prove it. Heh. (There I go again).
The light bulb test should be the very first thing that's attempted with this or any other amplifier. Bypass the soft start and plug 'er in through the light bulb - you'll get a lot of information in a few seconds. If it stays on, then the soft start isn't starting for a reason - perhaps a good one. If the bulb goes dim after a few secs, you should know one or two quick voltage measurements later what's wrong with the soft start.
Probably shorted output devices/filter caps/rectifier. Looks like the softstart is powered from the same transformer as it's soft starting. If the output is shorted it won't turn on.
Good foresight by Crown adding those PTC resistors protecting the soft start resistor. Many amps burn up/explode their soft start resistor if something like this happens.
Good foresight by Crown adding those PTC resistors protecting the soft start resistor. Many amps burn up/explode their soft start resistor if something like this happens.
Making some progress: I replaced the eight shorted power transistors (almost $4 USD each!) in the one channel and a couple of the drivers, and now at least the front power LED comes on. The soft start relay does not click, and the resistor starts to get hot, but not as quickly as before. Light bulb test is dim for a half-second, then full brightness. I assume I need to continue to look for additional shorts. The rectifier seems fine with my diode testing, but the eight big caps are glued in and I was hesitating to take them out to test them, but I guess that's the next step. I have a capacitance feature on my meter but it doesn't seem to measure these in-circuit caps.
If the short is in one channel, is there an easy way to "unpower" the other channel and see what the bulb test result is?
If the short is in one channel, is there an easy way to "unpower" the other channel and see what the bulb test result is?
Ola gang...
Start at the begining verify the power transformer is working check to see if there is ac voltage at the bridge rectifier in and dc at the bridge output and ck both sides from 0 to plus and 0 to minus checking for ac in the dc side(ripple) can be informative(at the filter caps. Most amps that have protective circuits also have low voltage dc in the p/s as well. A schematic to peruse would be nice for discussion purposes....
good luck, Elwood
ps Anatech is a good source for wisdom in older amps...
Start at the begining verify the power transformer is working check to see if there is ac voltage at the bridge rectifier in and dc at the bridge output and ck both sides from 0 to plus and 0 to minus checking for ac in the dc side(ripple) can be informative(at the filter caps. Most amps that have protective circuits also have low voltage dc in the p/s as well. A schematic to peruse would be nice for discussion purposes....
good luck, Elwood
ps Anatech is a good source for wisdom in older amps...
I went back and tried the bulb test again and the relays clicked and the bulb dimmed. All the LED's lit at startup, then just the green power light, as it should do. Weird. So I hooked her up to a CD player and speakers and the right channel (with the new transistors) played great, but when I turned up the volume on the left the whole unit went completely dead. Now that big resistor gets real hot again, no LED's at all. I measure 120V going into the transformer and +85V and +82V coming out, which I tested by switching the amp on briefly for each test. At the rectifier itself it got weird, getting 75 AC or 85 AC at all four corners, and no DC at all, or under one volt. I pulled the rectifier and it fails the diode test on one or two of legs.
So, rectifier is bad, a KBPC5006, 50A 600V. Mouser is out of them for the next 8 weeks and Digikey doesn't stock it so I ordered it off eBay from Liberty Electronics. Will update when it arrives and gets installed.
Thanks to everyone for the help so far.
So, rectifier is bad, a KBPC5006, 50A 600V. Mouser is out of them for the next 8 weeks and Digikey doesn't stock it so I ordered it off eBay from Liberty Electronics. Will update when it arrives and gets installed.
Thanks to everyone for the help so far.
timbert said:
Its really down to logical testing.
1/ Disconnect transformer from rectifier and make sure it outputs correct volts.
2/ Add rectifier to circuit but disconnect from caps etc and check it rectifies OK.
3/ Add caps to cct but disconnect rest, check for correct DC volts.
4/ Add all but o/p transistors and feedback VAS output into LTP feedback. Check driver cct works OK with a signal in. Especially check bias cct.
5/ Check output transistors before adding back into cct.
You should have at some point found the problem.
Thanks for this discussion, it has been very useful. In my case I have no LEDs lighting up, indicating no voltage at all coming out of the transformer. After checking the schematic above (thanks!) I checked the voltage across the 23W resistor and got the full line voltage (240V). I disconnected the amp and checked the temperature of this resistor - it was cold. I measured its resistance at infinity. There is no sign of local heating, so it is possible the resistor spontaneously failed, but could this be the last in a chain of events - e.g., internal fault causes large current flow, the PTC resistors hang on for a while and then fail, and finally the 23W resistor is cooked?
I posted about these amps before. when they blow outputs, it pretty much blows every transistor on the board. just replace them all, even if they check good, just replace them. they have been weakened. check all resistors for opens even if they don't look burnt AND check the speaker relays. 9 times out of 10 when they blew they melted the speaker relays. they used really cheap cheap relays that get so hot they bend inside and short out.
Zc
Zc
I bypassed the 23W resistor with a lamp. The amp appeared to fire up - green LED, fan started, but relays didn't click in, and lamp stayed lit, although dimly. I will go through the process described above, but it is not looking good! Funny, I always thought Crown amps were indestructible; I have had Yamaha, Perreaux (NZ), and other PA amps, as well as Technics, Pioneer, Denon etc HiFi amps, and never had one that suddenly blew up. Has Crown gone downmarket?
YES crown has gone down market!! the Macro Techs are still fantastic amps but Crown has been struggling to compete against the Behringers etc. So they tried outsourcing some amps and came up with the Crown CE model disaster. horribly designed amp as far as a physical layout goes. SMD components on the bottom of the PCB with a giant heatsink mounted on top. vibration causes the board to flex and the SMD's pop off and the thing dies....15++ revisions to the model etc...crown gave up and instead outsourced to C-Audio of Europe for the XLS series. better physical layout. but not a very rugged amp. if used as a studio monitor amp etc its fine but beat on it at all and it dies.
From what I have seen on the service end. all the new crown models have service issues and they have gotten SO complicated that they are unserviceable. crown just swaps boards.
QSC seems to be leading the pack for reliability these days. Long gone are the days of the massively overbuilt brutally tough amps like CREST and Macro Techs or even the avg amps like the BGW's, Peavey's etc. you just don't get that same level of quality anymore. granted they were not the greatest sonically but you could beat the living tar out of them night after night after night and they would just sail along!
and to a degree, you get what you pay for. a $249 Behringer or $349 crown/qsc just isn't going to hold up like one of those old tour class amps that I'm sure retailed for $1,000 or more back in the day!
From what I have seen on the service end. all the new crown models have service issues and they have gotten SO complicated that they are unserviceable. crown just swaps boards.
QSC seems to be leading the pack for reliability these days. Long gone are the days of the massively overbuilt brutally tough amps like CREST and Macro Techs or even the avg amps like the BGW's, Peavey's etc. you just don't get that same level of quality anymore. granted they were not the greatest sonically but you could beat the living tar out of them night after night after night and they would just sail along!
and to a degree, you get what you pay for. a $249 Behringer or $349 crown/qsc just isn't going to hold up like one of those old tour class amps that I'm sure retailed for $1,000 or more back in the day!
The Perreaux and the Peavey I had in my last band did at least a 1000 gigs without a whimper, and are apparently still going strong another 14 years on. But yes, I paid $900 for the Perreaux in 1984, and something like $600 for the little Peavey (I think 130W/channel). The Yamaha I bought 10 years ago cost AU$1300, and it has also done many long road trips and gigs without murmur. So I guess you get what you pay for.
After I fix the Crown I think I will invest in a QSC! And use the Crown for decoration.
After I fix the Crown I think I will invest in a QSC! And use the Crown for decoration.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.