Good Day everyone,
I'm new here but I'd love some assistance, I'm totally out of ideas. I have an amplifier (A clone/ chinese copy/ remake of a crown macrotech 5002 vz) that I'm currently using for bass. Here's the issue about 2 months ago it started overheating while playing and going into thermal protect (So hot that the back of the amp will burn your hand) . So, I've read a lot and diagnosed a lot. With no speakers or input signals connected, it stays cold. With input signal and speakers connect it runs cold on low to moderate volumes. Only when playing at loud volumes the amplifer overheats terribly. I know i'm not pushing the amp past it's limits as in the past I've used it on heavier duty and it worked fine and stayed cold. Also, I've noticed, that on 8 ohm load it takes longer to overheat (Approximately 1 hour) 4 ohm load it overheats in approximately 15 minutes. I usually run 4 ohm load on the amplifer with no issues
Some of the things I've done:
Checked speakers and cables, even bought brands new speakers just incase, no difference
Checked bias and adjusted and readjusted( lowered and even raised it, no difference
Checked all pcb's for any visible damages, none
Checked output transistors (If you all suspect that failed output transistors may caue this, I'll recheck them)
Checked pre drivers, they seem good
The amp has independent transformers, power supply sections and output modules. only the pre amp section links both boards (Possibly) but the problem affects both channels equally and in the same manner. Its not different for each channel, it affects both channels once loaded the same way
I cannot understand this problem and I've reached a point I don't know what else to check or fix. Any help or advice would be grately appreciated. Thank youuuuu!!!!
I'm new here but I'd love some assistance, I'm totally out of ideas. I have an amplifier (A clone/ chinese copy/ remake of a crown macrotech 5002 vz) that I'm currently using for bass. Here's the issue about 2 months ago it started overheating while playing and going into thermal protect (So hot that the back of the amp will burn your hand) . So, I've read a lot and diagnosed a lot. With no speakers or input signals connected, it stays cold. With input signal and speakers connect it runs cold on low to moderate volumes. Only when playing at loud volumes the amplifer overheats terribly. I know i'm not pushing the amp past it's limits as in the past I've used it on heavier duty and it worked fine and stayed cold. Also, I've noticed, that on 8 ohm load it takes longer to overheat (Approximately 1 hour) 4 ohm load it overheats in approximately 15 minutes. I usually run 4 ohm load on the amplifer with no issues
Some of the things I've done:
Checked speakers and cables, even bought brands new speakers just incase, no difference
Checked bias and adjusted and readjusted( lowered and even raised it, no difference
Checked all pcb's for any visible damages, none
Checked output transistors (If you all suspect that failed output transistors may caue this, I'll recheck them)
Checked pre drivers, they seem good
The amp has independent transformers, power supply sections and output modules. only the pre amp section links both boards (Possibly) but the problem affects both channels equally and in the same manner. Its not different for each channel, it affects both channels once loaded the same way
I cannot understand this problem and I've reached a point I don't know what else to check or fix. Any help or advice would be grately appreciated. Thank youuuuu!!!!
Hi DJ Avii,
At this point you need to see if the amp is oscillating under load. Some circuits have a low value resistor between chassis (the case) ground and the circuit common. If this goes open the amplifier may become unstable.
Given it isn't the real deal, that complicates things and the amp may not be as stable as it should be also. You seem to have some knowledge and equipment. The other thing to check is to see if you aren't getting a DC offset at higher power levels, but the DC protection should still work. It only shuts down due to temperature from what you say.
At this point you need to see if the amp is oscillating under load. Some circuits have a low value resistor between chassis (the case) ground and the circuit common. If this goes open the amplifier may become unstable.
Given it isn't the real deal, that complicates things and the amp may not be as stable as it should be also. You seem to have some knowledge and equipment. The other thing to check is to see if you aren't getting a DC offset at higher power levels, but the DC protection should still work. It only shuts down due to temperature from what you say.
Hey anatech, Ok so i checked the dc on the outputs under no load, it was less than 10mv for each channel. Thanks, i do have a fair knowledge just not very much practical experience, with regards to checking if the amp is oscillating under load, how would i go about that?
"Some circuits have a low value resistor between chassis (the case) ground and the circuit common. If this goes open the amplifier may become unstable" any pointers to locating this?
Also, yes when the temperature builds to those extreme levels when under duty, the amplifier's protection turns on, cutting all output until it cools
"Some circuits have a low value resistor between chassis (the case) ground and the circuit common. If this goes open the amplifier may become unstable" any pointers to locating this?
Also, yes when the temperature builds to those extreme levels when under duty, the amplifier's protection turns on, cutting all output until it cools
Also sorry if my replys are taking a while, I'm replying immediately but as i'm new here i think my posts have to be approved firstly before you all can see them so it's taking a while ( I'm see a notification saying "Your post is invisible to others until a moderator approves it. After a few quality posts you will be able to post freely.")
Hi DJ Avii,
Yes, you should be released soon.
Unplug any input connectors / cables. Use an ohmmeter and measure between chassis ground and an RCA or 1/4" jack outer connection, do both. You should measure either a short (not a great plan), or somewhere between 10 and 100 ohms. Also measure between the two RCA shields. If it's open, you have a burned out resistor. I don't know exactly where they might install it in that model. Typically near the input circuit and chassis common. It isn't uncommon for each channel to have it's own resistor.
For oscillation, run the amp up to where it starts drawing too much current with your oscilloscope already connected to the output on one side, scaled to handle the voltage. To draw a lot of current you will have a large high frequency appear if that is the problem. Also look at the zobel network if it has one. That is the resistor - capacitor across the output and ground. It may be on the PCB, or right on the speaker terminals. If it goes open, your amp will be unstable. The resistor should be overheated if you have high frequency oscillation. They are not normally "cooked".
Yes, you should be released soon.
Unplug any input connectors / cables. Use an ohmmeter and measure between chassis ground and an RCA or 1/4" jack outer connection, do both. You should measure either a short (not a great plan), or somewhere between 10 and 100 ohms. Also measure between the two RCA shields. If it's open, you have a burned out resistor. I don't know exactly where they might install it in that model. Typically near the input circuit and chassis common. It isn't uncommon for each channel to have it's own resistor.
For oscillation, run the amp up to where it starts drawing too much current with your oscilloscope already connected to the output on one side, scaled to handle the voltage. To draw a lot of current you will have a large high frequency appear if that is the problem. Also look at the zobel network if it has one. That is the resistor - capacitor across the output and ground. It may be on the PCB, or right on the speaker terminals. If it goes open, your amp will be unstable. The resistor should be overheated if you have high frequency oscillation. They are not normally "cooked".
Ok thank you, will do the checks with the inout, checking with the "hot" signal of the 1/4 input right? also I may not be able to do the oscilloscope test, i need to get a new oscilloscope 🙁. But i'll do those test in a bit and update
Nope. You are testing between the grounds. The hot connections should measure open (coupling cap) or in the tens of K ohm. Maybe a couple hundred K ohm.
An old scope will work fine. Oscillation is typically less than 250 KHz.
An old scope will work fine. Oscillation is typically less than 250 KHz.
As well as the excellent suggestions above, you may want to check the bias current, it's possible a small resistor has failed in the driver.
ok so i did all the checks as suggested, the inputs test fine as you described, rca to rca shield is not open(shorts), the chassis ground to each 1/4'' jack out connection is short (also referenced this with my qsc rmx 5050, same readings). Also double checked the resistor and capacitor on the speaker terminal board connecting hot to ground (One for each output), those test within the stated values and are good, rechecked all resistors on the input board and power supply board. Only the scope part i'm not able to check, i don't have any accessible atm. i have to purchase a new one 🙁
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I also went back to the other power supply board and rechecked all the capacitors. Upon reaching the five 100uf 25v, i noticed that all of them gave lower readings, around 80's uf and one in particular had an extremely high esr value as seen in the pictures. Also if i remember correctly, an amplifier oscillating would damage high frequency drivers such as tweeters? Well the other day during testing, this amp did blow out a bullet tweeter even though it was ran off an external crossover(rane ac23b) and had a crossover (capacitor and resistor) on the tweeter itself. Leads me to believe the amplifier is oscillating? Just off what i can check without a scope
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Is this a home brew, or did you buy it somewhere? It doesn’t even look close to a real 5000VZ internally. The original can and will overheat if it stays “stuck” on high rail all the time, instead of running up there only on demand.
Hi wg_ski,
It's Chinese clone I think, stated earlier.
Hi DJ Avii,
If the amp is oscillating it will normally burn the zoble resistor, your eyes can see this if it happened. If the frequency is high enough, the inductance of the tweeter VC will keep excessive current from flowing and it may not burn it out. It could, it's all a matter of degree here and depends on the frequency, inductance and level of oscillation. That and how much heat the VC can dissipate safely.
You absolutely require an oscilloscope to service audio, period, and of story. I would highly recommend an analogue scope, used is great. You can pick up good 100 MHz, dual trace scopes cheap. A scope > $2,000 can be had for $250 in Canadian dollars. Digital scopes have a lot of gottcha's, and they can lie to you easily. Cheap digitals aren't work much on the service bench. You don't want to know how much my new digital scope is worth, and I still keep an analogue in the wings. A digital does some things well, but nowhere close to everything and not with great accuracy.
Some of those caps can cause instability depending on where they are and the issue with them. an 80 uF that should be 100 uF isn't a big deal. Measure OUT OF CIRCUIT only. Also, your tester may or may not be accurate and it also depends on the test frequency. A decent capacitor tester (LCR meter) is >$1,000. A really good used one (very old) is >$2,000. So that should tell you something about the less expensive ones. They are good to a point, but do not expect accuracy or the full story.
Your audio to chassis connection is at least not open, and that is a good thing.
It's Chinese clone I think, stated earlier.
Hi DJ Avii,
If the amp is oscillating it will normally burn the zoble resistor, your eyes can see this if it happened. If the frequency is high enough, the inductance of the tweeter VC will keep excessive current from flowing and it may not burn it out. It could, it's all a matter of degree here and depends on the frequency, inductance and level of oscillation. That and how much heat the VC can dissipate safely.
You absolutely require an oscilloscope to service audio, period, and of story. I would highly recommend an analogue scope, used is great. You can pick up good 100 MHz, dual trace scopes cheap. A scope > $2,000 can be had for $250 in Canadian dollars. Digital scopes have a lot of gottcha's, and they can lie to you easily. Cheap digitals aren't work much on the service bench. You don't want to know how much my new digital scope is worth, and I still keep an analogue in the wings. A digital does some things well, but nowhere close to everything and not with great accuracy.
Some of those caps can cause instability depending on where they are and the issue with them. an 80 uF that should be 100 uF isn't a big deal. Measure OUT OF CIRCUIT only. Also, your tester may or may not be accurate and it also depends on the test frequency. A decent capacitor tester (LCR meter) is >$1,000. A really good used one (very old) is >$2,000. So that should tell you something about the less expensive ones. They are good to a point, but do not expect accuracy or the full story.
Your audio to chassis connection is at least not open, and that is a good thing.
Thank you anatech. I'll try to order a new scope and in the future an lcr meter. I really wish I had one atm. Previously I use to use a friend's one but he sold it. So I'll have to get my own. And yes on the bright side the audio to chassis is not open. I'll switch out those capacitors in the meantime I order an oscilloscope. So so far we still are in the process of figuring out 😩
Buy a used analogue oscilloscope. Philips should be good, a Tektronix 2235 or similar would be good. Don't buy the fancier, feature rich models, they will only bite you. You want a good basic 'scope.
Small electrolytics going bad and developing high ESR is par for the course with cheap off brands. Amps can start oscillating if they lose their bypass/decoupling capacitor(s). Even a cheap ESR meter tells you that one is toast. It may not be able to tell you if the big reservoir caps are in spec, but seeing how much the rail voltage sags under load will give a good indication.
To me, the amp looks like it was made from a kit. The original design was TO-3 outputs not flatpacks, and the chassis is jam packed where you can’t service any of it without taking every single screw out, unhooking every wiring harness, and completely disassembling it on the bench. There is some wide open space in this one - even being able to get to the induvidual halves of the grounded bridge.
To me, the amp looks like it was made from a kit. The original design was TO-3 outputs not flatpacks, and the chassis is jam packed where you can’t service any of it without taking every single screw out, unhooking every wiring harness, and completely disassembling it on the bench. There is some wide open space in this one - even being able to get to the induvidual halves of the grounded bridge.
For those with digital scopes, I would say the biggest aspect for this type of trouble shooting is that the slow speed settings will often hide RF, so you when you can see the audio or test signal, whizz up the timebase to look at what's really there in the RF.
I.e. you need to zoom in to see stuff that on an analog scope will be visible as a fuzz on the screen.