"I have been using my Variac set at about 12V so far"
You're only feeding 12V to the amp?
No wonder it doesn't work.
Use the lightbulb. Let it rip. It shoulk blink, and then settle down to a dull glow. If it stays bright, something is really fried. With a dull glos we want the supply voltages toi be about 2/3 of normal at idle. This will allow normal opperation on music to a couple of watts peak so that you can see it working.
After I fix an amp I let it run for half the day or more on the lightbulb. If the bulb starts flashing bright after several hours you will be glad its there, it just saved the amp from total destruction (again), and will run it enough for troubleshooting.
I gave my variac away many years ago.
You're only feeding 12V to the amp?
No wonder it doesn't work.
Use the lightbulb. Let it rip. It shoulk blink, and then settle down to a dull glow. If it stays bright, something is really fried. With a dull glos we want the supply voltages toi be about 2/3 of normal at idle. This will allow normal opperation on music to a couple of watts peak so that you can see it working.
After I fix an amp I let it run for half the day or more on the lightbulb. If the bulb starts flashing bright after several hours you will be glad its there, it just saved the amp from total destruction (again), and will run it enough for troubleshooting.
I gave my variac away many years ago.
djk said:
I'm still using the variac, because at about 12-15VAC I'm reading -8VDC on the output of the one channel. I have tried raising it a bit and the DC voltage on the output goes up with it.
I don't have much experience yet, but logic tells me that having several volts where I should be seeing mV is not good for the componants. When I first got the amp I plugged it directly into the wall and measured the voltage on the output. I was reading -64VDC on one channel. I haven't plugged it into the wall again, only through the variac. On the good channel I am reading about .2VDC or less.
I am in the middle of putting together the soundtrack for our Easter program right now so I wasn't able to work on the amp last night. Probably won't get back to it until Thursday. I really appreciate all the help you folks have given. I will try and take better measurements when I get it opened up again.
Blessings, Terry
Terry,
The Egyptians used the lightbulb in 4000bc, then the art of softening the supply was lost for 6000 years... then found again Eureka! One of the nicest and cheapest insrances for amplifiers... I have a Variac... don't use it... bulbs rock!
Disclaimer: You kow I am kidding about the Egyptians right?
The Egyptians used the lightbulb in 4000bc, then the art of softening the supply was lost for 6000 years... then found again Eureka! One of the nicest and cheapest insrances for amplifiers... I have a Variac... don't use it... bulbs rock!
Disclaimer: You kow I am kidding about the Egyptians right?
Hi K-amps,
Don't get me wrong, I am going to build one of those light bulb circuits. I like the idea very much. My point was that I have high voltage on the output, so in lieu of plugging it into 120V I have been using my variac. It lets me bring things on very slowly and watch both meters while I do that. Maybe the light bulb trick will do that as well. I don't know, I haven't tried it yet.
I've got a question about using the light bulb trick. One amp I had would blow the main fuse instantly when switched on. By using my variac, I was able to bring up the power just enough to find the problem without blowing the fuse. Will the light bulb trick allow the amp to power up without blowing the fuse if there is a big problem like that? That would be very cool if it does.
Blessings, Terry
Don't get me wrong, I am going to build one of those light bulb circuits. I like the idea very much. My point was that I have high voltage on the output, so in lieu of plugging it into 120V I have been using my variac. It lets me bring things on very slowly and watch both meters while I do that. Maybe the light bulb trick will do that as well. I don't know, I haven't tried it yet.
I've got a question about using the light bulb trick. One amp I had would blow the main fuse instantly when switched on. By using my variac, I was able to bring up the power just enough to find the problem without blowing the fuse. Will the light bulb trick allow the amp to power up without blowing the fuse if there is a big problem like that? That would be very cool if it does.
Blessings, Terry
Yes Terry,
Instead of blowing a fuse, the bulb will light up and "eat-up" all the current going to the amplifier and limit it to less than 1 amp or thereabouts. This will prevent turn on catastrophies.
If you have DC on the output, you can attach a dummy load, the bulb will again light up because the amplifier is trying to drive an 8 or 4 ohm load with 75 volts (or whatever the rails or voltage leak is at the output) and load up the bulb which will light up and prevent any components from burning up.
However if you do not have a dummy load, and there's a fault on the input/ VA stages, you could burn up those components as they cannot source enough current for the bult to light up and starve the amplifier of current, hence when troubleshooting, I tend to keep a dummy load on the outputs while using a bulb.
I just switch the amp on/off everytime I trace and fix a fault till the bulb finally goes dim... then I know I am close or done fixing.
Instead of blowing a fuse, the bulb will light up and "eat-up" all the current going to the amplifier and limit it to less than 1 amp or thereabouts. This will prevent turn on catastrophies.
If you have DC on the output, you can attach a dummy load, the bulb will again light up because the amplifier is trying to drive an 8 or 4 ohm load with 75 volts (or whatever the rails or voltage leak is at the output) and load up the bulb which will light up and prevent any components from burning up.
However if you do not have a dummy load, and there's a fault on the input/ VA stages, you could burn up those components as they cannot source enough current for the bult to light up and starve the amplifier of current, hence when troubleshooting, I tend to keep a dummy load on the outputs while using a bulb.
I just switch the amp on/off everytime I trace and fix a fault till the bulb finally goes dim... then I know I am close or done fixing.
djk,
I have read many of your posts found in various threads here and have come to understand the depth of your experience. At this juncture in this thread's discussion it would be helpful to me, and perhaps to others, if you would tell us more about the use of a variac. Can you provide and example of an amp that will not work if brought up slowly with a variac? And, can you tell us why this is so?
I am thinking it might be an amp with integrated circuits used to sense various operating conditions or provide voltage regulation. Am I warm in my reasoning?
Thanks
I have read many of your posts found in various threads here and have come to understand the depth of your experience. At this juncture in this thread's discussion it would be helpful to me, and perhaps to others, if you would tell us more about the use of a variac. Can you provide and example of an amp that will not work if brought up slowly with a variac? And, can you tell us why this is so?
I am thinking it might be an amp with integrated circuits used to sense various operating conditions or provide voltage regulation. Am I warm in my reasoning?
Thanks
I have repaired at least a dozen of these. they are VERY stable amps. each one i have brough up on a variac without problems. I have even had these amps running on 20V dc rails!
If you have DC offset, you most likely have bad driver transistors. these go all the time. if those test good, then start yanking mosfets as these do fail!!!
The mosfets are attached directly to the heatsinks! the heatsinks are isolated from everything. the case of the fets and the heatsinks form the main output line. this is speaker level before the output inductor.
Also, check to see that the power supply rails are near the same voltage.
You can pull out all the mosfets and tie a 1K 1/2 watt resistors on both sides of the 180 ohm resistor between the drivers to the output line. this will allow the amp to run without the output devices to a healthy level. those driver transistors are rated for 1 watt!
Without the mosfets you should be able to get a good clean trace with minimal offset.
Start checking mosfets. none will be shorted. but the gate resistance will be all over the place when compared to the others. some will be in the M ohm range those are typically good, others will be in the 100 ohm range and those are typically bad.
Also the inputs to the fets have zeners and 1n4004 diode to limit gate voltage in gross overdrive situations. make sure none of those have shorted. also the fets have 1n4004 diodes across them. make sure those havent shorted.
These are fairly simple, and very robust amps. however the solder traces leave a lot to be desired. they burn off easily.
Email me if you still cant figure it out.
tetech2@doitnow.com
Zc
If you have DC offset, you most likely have bad driver transistors. these go all the time. if those test good, then start yanking mosfets as these do fail!!!
The mosfets are attached directly to the heatsinks! the heatsinks are isolated from everything. the case of the fets and the heatsinks form the main output line. this is speaker level before the output inductor.
Also, check to see that the power supply rails are near the same voltage.
You can pull out all the mosfets and tie a 1K 1/2 watt resistors on both sides of the 180 ohm resistor between the drivers to the output line. this will allow the amp to run without the output devices to a healthy level. those driver transistors are rated for 1 watt!
Without the mosfets you should be able to get a good clean trace with minimal offset.
Start checking mosfets. none will be shorted. but the gate resistance will be all over the place when compared to the others. some will be in the M ohm range those are typically good, others will be in the 100 ohm range and those are typically bad.
Also the inputs to the fets have zeners and 1n4004 diode to limit gate voltage in gross overdrive situations. make sure none of those have shorted. also the fets have 1n4004 diodes across them. make sure those havent shorted.
These are fairly simple, and very robust amps. however the solder traces leave a lot to be desired. they burn off easily.
Email me if you still cant figure it out.
tetech2@doitnow.com
Zc
karma said:
Yes fan is 2 speed, runs at slow speed under normal conditions. when really kicked in the PCR circuit steps into high gear and the fan kicks on full! then calms back down after the input or the temp back down.
I love these little amps! they seriously kick butt!!!! and they did the driver section RIGHT! just my .02c but they realyl do sound good. a little lacking on the ultra low end however. have been wanting to play with the input cap to see if it made any difference. etc...
Zc
Hi Zc,
I have been hoping you would show up. I have read many of your posts about the Soundcraftsmen amps.
I bought new parts and replaced all of the 2N3440's and MM4003's. I think those are the drive transistors you refer to no?
They all had missing legs. Two of them were just laying in the chassis. Most of them only had one lead still attached. Not sure if they corroded away or burned. Looked like corrosion though. There are rust spots on the cases of the MOSFETs. This thing has had some moisture in it.
The PSU measures good. I'm thinking it is probably the MOSFETs. I pulled every componant except for the MOSFETs that looked like it lead to the output and still there was voltage on it. I will post back when I get it apart again. Should be jsut a couple of days.
Blessings, Terry
I have been hoping you would show up. I have read many of your posts about the Soundcraftsmen amps.
I bought new parts and replaced all of the 2N3440's and MM4003's. I think those are the drive transistors you refer to no?
They all had missing legs. Two of them were just laying in the chassis. Most of them only had one lead still attached. Not sure if they corroded away or burned. Looked like corrosion though. There are rust spots on the cases of the MOSFETs. This thing has had some moisture in it.
The PSU measures good. I'm thinking it is probably the MOSFETs. I pulled every componant except for the MOSFETs that looked like it lead to the output and still there was voltage on it. I will post back when I get it apart again. Should be jsut a couple of days.
Blessings, Terry
djk said:
chris ma said:
I agreed with one building experience that the Variac brought up slowly the amp did not work. The amp was the BrianGT first GC group buy version. As to why I do not have any clues.
The Butcher
First off, I said I would build the light bulb gadget. I'm not trying to ignore anything. I bought and started using the variac due to several suggestions by many experienced folks on this very forum.
Secondly, I am not trying to get the amp to "work" on the variac. I'm not connecting speakers to this thing until I get that high voltage off of the output. I have been using the variac so that when I see 7 or 8 volts on the output, I know I still haven't fixed the problem and yet I feel safe that I haven't burned up any more parts. If this logic is flawed then I will stop doing it. In my mind, it seems as though even with the light bulb in the loop I would still get 120V to my transformer. Is this not true? Would I actually see less than 8VDC on the rails by using the light bulb trick? Are you guys saying that with the light bulb in the loop that there is no way that any of the components can get too much voltage? I'm trying to wrap my mind around this but for some reason I’m not seeing something here. Maybe it's just too far over my head to understand. I'm just trying to keep from damaging stuff while I search for the problem. Is there chance that using both the variac and the light bulb device would allow me to work with the lower voltages and have the added safety of bulb in the circuit? I feel like the chances of me getting shocked are much less with 8VDC than with 65VDC or so.
Please, help me understand this better.
Thanks, Terry
Terry,
It's the current that will kill components not necessarily voltage.
The bulb limits current, the variac limits voltage.
Go ahead use both if it makes your mind as ease, as Zerocool said, he has repaired the PCR's and used a variac with little or no ill effects.
The bulb will let your amp get a max of 80 volts ac ONLY if everything is working ok, otherwise it will drop the current (and hence voltage) to much lower levels. With a Variac, you get a relatively constant current with no auto failsafe like the bulb.
As I said earlier, use a 4 or 8 ohm dummy load at the outputs, that will decrease the power delivered to the amp if there's a fault and in such a scenario you will not see over 70-80 volts on the amp unless it is working fine, or you have removed all semiconductors.
It's the current that will kill components not necessarily voltage.
The bulb limits current, the variac limits voltage.
Go ahead use both if it makes your mind as ease, as Zerocool said, he has repaired the PCR's and used a variac with little or no ill effects.
The bulb will let your amp get a max of 80 volts ac ONLY if everything is working ok, otherwise it will drop the current (and hence voltage) to much lower levels. With a Variac, you get a relatively constant current with no auto failsafe like the bulb.
As I said earlier, use a 4 or 8 ohm dummy load at the outputs, that will decrease the power delivered to the amp if there's a fault and in such a scenario you will not see over 70-80 volts on the amp unless it is working fine, or you have removed all semiconductors.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.