I have a MCS 3233 receiver. It is a stereo 33w @ 8 ohm receiver. It has a single power suply for both channels. So it's nothing special. But it sounds great and I want to keep it running strong. The power supply is underrated and the transformer fries all the time, but it's never died on me, I use it ever night.
Problem is, I was trying to bias it. I tried going for 125ma on the .33 ohm emitter resistor. It has a MJ2955 and a 2n3055 pair of transistors for each channel. The right channel died, but nothing smoked. The current on the resistor disapeared when I was in the middle of ajusting the pot. I measured the voltages on the base and the emitter. Instead of being around ~2, and a few hundred millivoltes, they where close to rail voltage (plus and minus 32v). They mesured 31v. I really don't know what happened. I was under the impressing I broke a solder joint by accident and got an open circut. Did I kill the transistors? If so, what transistors will improve a push pull amp like this, that I could just mount on a heat sink and wire up?
Thanks,
Nick
Problem is, I was trying to bias it. I tried going for 125ma on the .33 ohm emitter resistor. It has a MJ2955 and a 2n3055 pair of transistors for each channel. The right channel died, but nothing smoked. The current on the resistor disapeared when I was in the middle of ajusting the pot. I measured the voltages on the base and the emitter. Instead of being around ~2, and a few hundred millivoltes, they where close to rail voltage (plus and minus 32v). They mesured 31v. I really don't know what happened. I was under the impressing I broke a solder joint by accident and got an open circut. Did I kill the transistors? If so, what transistors will improve a push pull amp like this, that I could just mount on a heat sink and wire up?
Thanks,
Nick
reply on "dead amp"
I think the emitter resistors have blown with the transistors.
Those pots used for biasing usually are of the cheapest kind and if they weren't used for many years they'll loose contact while turning them.The result will be that for a moment the bias current raises and destroys the transistors and the emitter resistors.
But it doesn't seem to be a real problem here because these types of transistors are cheap and easy available.... and don't forget to change the pots too.
I think the emitter resistors have blown with the transistors.
Those pots used for biasing usually are of the cheapest kind and if they weren't used for many years they'll loose contact while turning them.The result will be that for a moment the bias current raises and destroys the transistors and the emitter resistors.
But it doesn't seem to be a real problem here because these types of transistors are cheap and easy available.... and don't forget to change the pots too.
Safer bias
Things could be safer if a different arrangement was implemened on every Vbe.
First of all put a parallel resistor with the trimpot.
Second add a series resistor with the trimpot.
In such a way contact failure shouldn't be a deadly potential problem.
So if you change the pot you cold quite well do this mod.
On both channels.
Carlos
Things could be safer if a different arrangement was implemened on every Vbe.
First of all put a parallel resistor with the trimpot.
Second add a series resistor with the trimpot.
In such a way contact failure shouldn't be a deadly potential problem.
So if you change the pot you cold quite well do this mod.
On both channels.
Carlos
The resistors seem to be good without taking them out. They have not changed value, and have not smoked. So if I change all the power transistors, am I going to have to change the driver transistors also? I guess I should only work with the blown channel first. Where can I get matched transistors from? Radio Shack stoped carrying them a year or so ago and my local electronics store is jake. Maybe I can grab the phone book and start looking for a better store downtown.
The trim pots are 2K ohm. I'll get some replacements. I was reading the article on www.tnt-audio.com and they don't really mention anything breaking because of bad trim pots, he just says they sound bad.
It sucks not having a schematic. But the board for the power amp in this receiver is rather simple. I could trace it in a couple hours, but i've never traced anything before.
The trim pots are 2K ohm. I'll get some replacements. I was reading the article on www.tnt-audio.com and they don't really mention anything breaking because of bad trim pots, he just says they sound bad.
It sucks not having a schematic. But the board for the power amp in this receiver is rather simple. I could trace it in a couple hours, but i've never traced anything before.
Crap, ok.
I guess I should find a better store to go to. Or just find someone cool online to order from. I might need some alternative transistors. I think I'll keep the same output transistors. I don't have the power to do anything more anyways. Any good places to shop online, or to order from I mean (don't care if they have an online store)?
Another question, is it normal for an iron core transformer to get hot enough to burn yourself on, but not melt itself or short? The transformer gets freeking hot in this amp. But then again, it sounds good. It dosn't distort at all, it pretty much a set headroom. You hit it and you know you've gone to far. So it's not like I'm listening to a song, a bass note hits and it runs out of power. I hate amps that do that. haha, we got computer speakers at school that clame they have a 200W amp, get real, 200w on an amp that is probably built with a power supply that can bearly handle 50... Sounds like crap....
- Nick
I guess I should find a better store to go to. Or just find someone cool online to order from. I might need some alternative transistors. I think I'll keep the same output transistors. I don't have the power to do anything more anyways. Any good places to shop online, or to order from I mean (don't care if they have an online store)?
Another question, is it normal for an iron core transformer to get hot enough to burn yourself on, but not melt itself or short? The transformer gets freeking hot in this amp. But then again, it sounds good. It dosn't distort at all, it pretty much a set headroom. You hit it and you know you've gone to far. So it's not like I'm listening to a song, a bass note hits and it runs out of power. I hate amps that do that. haha, we got computer speakers at school that clame they have a 200W amp, get real, 200w on an amp that is probably built with a power supply that can bearly handle 50... Sounds like crap....
- Nick
No.... It's not normal and you should measure the AC secondaries so when you want to replace it, you know what the original was...... This transformer is dieing dude, and I dare to say "just plain unsafe" it will either start blowing fuses or catch fire. It neds changing.
Good luck,
Chris
Partsexpress.com
MCMelectronics.com
Those are good starts
Good luck,
Chris
Partsexpress.com
MCMelectronics.com
Those are good starts
If it's dieing it's been dieing ever since I've got it. It's never flaked out, only problem I ever have is a random radio communication getting picked up. I'll check out those sites. If the transformer dies I don't think it's worth replacing, i'd rather spend the money on a better receiver or amp and preamp.
Whoa there, Nicksmurf! Before changing any output transistors, I'd go after that pot. It sounds like it went open circuit and now the base of the driver transistor has been pulled up to the rail voltage. This is especially likely if the emitter voltage of the suspect output transistor is 600mV or so less than it's base voltage (the Vbe drop is still there, which means the emitter junction hasn't been melted!).
If you do go ahead and change the output pair, try the MJ802/MJ4502 pair instead. They seem to have the best intrinsic matching of any commonly available npn/pnp pair.
If you do go ahead and change the output pair, try the MJ802/MJ4502 pair instead. They seem to have the best intrinsic matching of any commonly available npn/pnp pair.
thanks jeffreyj, I'll change the pot first and see if it gets me anywhere. I wasn't using the correct screwdriver when I was turning it. I don't give a care about the transformer right now. If it blows the unit is hitting the trash. I'm not going to spend money on something I can replace for very cheep at a flee market.
screw it
screw the mcs reciever. I got another reciever for $25 at the flee market the other day. Only thing that was wrong with it was a dirty tape monitor switch. I just fixed it.
It's a Ward Montgumry Model 450. Looking at it's construction and model number it seems to be 50 watts per channel. I'll crank it tomorrow. Maybe i'll have to be a little easy on my vintage scott s-15 speakers, and my epi 120's too.
This thing has a dual power supply and an effects loop!!! cool
screw the mcs reciever. I got another reciever for $25 at the flee market the other day. Only thing that was wrong with it was a dirty tape monitor switch. I just fixed it.
It's a Ward Montgumry Model 450. Looking at it's construction and model number it seems to be 50 watts per channel. I'll crank it tomorrow. Maybe i'll have to be a little easy on my vintage scott s-15 speakers, and my epi 120's too.
This thing has a dual power supply and an effects loop!!! cool
I just glanced through all the messages and nobody informed you of "Checking" the outputs to see if you blew them. Normally if you blow the output transistors the fuses will blow because the outputs on every amplifier are tied to the B+. So if your not blowing fuses and the power resistors are okay, your output stage is okay. I've seen predrivers pile up and the amp just fail to work, but no visable damage.
I might still fix it. But since i've been advised that a transformer hot enough to burn you is bad, then maybe not....
I'm happy with my new reciever. I'm working out a couple problems. Think i'm going to try a 4 ohm load (two pairs of speaker). It should be very loud. I'm probably going to upgrade the 4 6800uf caps in it to something slightly larger. If I try to bias it i'm replacing the pots first.
I'm happy with my new reciever. I'm working out a couple problems. Think i'm going to try a 4 ohm load (two pairs of speaker). It should be very loud. I'm probably going to upgrade the 4 6800uf caps in it to something slightly larger. If I try to bias it i'm replacing the pots first.
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