Sunfire True Subwoofer MK IV repair

tonebells

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I picked up a Sunfire True Subwoofer MK IV for free as it had a bad hum. Most of what you read says replace all the caps and you are on the way. No joy. It went from bad hum to dead short after recap. After many searches I found a few clues as to the issue. Here is a list of what I found.

Q12 2SC4793 Tested good but replaced as it was already removed

Q13 2SC4793 Unknown/faulty

Q14-Q17 PNP 2SA1302 = MJL1302AGOS-ND NPN 2SC3281 = MJL3281AGOS-ND one of each shorted replaced all

Q18 2SA1837 Unknown/faulty

Q19-Q24 Mosfets IRF640PBF two shorted one on each rail replaced all

D10-D13 1N4148 all four shorted replaced all

JP1-JP2 0ohm resisters smoked

I hope this helps others. I have pdfs of the info I found on other forums if anyone whats to read them as well.

Tony
 
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I downloaded the schematic, want to verify this is the right one for the output amplifier.

Do you mean it's working now after you change all the stuffs in the first post?

This amp is the kind that they modulate the top and bottom rail with the input signal. The rail voltages track the output signal. This is used to keep the voltage across the signal amp small, just large enough to keep the signal amp operates in normal mode. This has a advantage that the signal amp see low rail voltage all the time and afford to run higher bias current and use less output transistors. From what you described, seems like the rail ( they called the down converter) has problem that put out too much voltage. You should probe the voltage on both rail and see how good they are in tracking the signal.

The middle amp is simple to fix, but if the rail puts out too much voltage, you're going to burn all the new transistors. So verify that you said the sub is working now first.

Also, feel the heatsink of the signal amp, is it getting very hot?

Seems like the two down converter circuits use an oscillator U3B and U3A to create a modulating square wave to modulate the circuit to generate the rail voltage. I am not sure I am comfortable to help you.
 

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"I downloaded the schematic, want to verify this is the right one for the output amplifier." That's the one.

"Do you mean it's working now after you change all the stuffs in the first post?" Yes.

"but if the rail puts out too much voltage, you're going to burn all the new transistors." I emailed Bob Carver with some questions and that is exactly what he said.

"Also, feel the heatsink of the signal amp, is it getting very hot?" I am not sure I know where that is.

I will go through and check voltages when I have time just to make sure there is not something I overlooked.

Thanks Again

Tony
 
Q13 to Q17 should be on a heatsink......some aluminum with fins. Do you have a scope? You monitor JP1 and JP2 for the voltages. If you don't have a scope, it's going to be hard to troubleshoot.

I don't even know how to call the part that I posted, I called that the signal amp as it's the one that amplified the signal to drive the speakers.

How much you know about circuits? If you can understand the schematic, the top and bottom down converter circuits should be a mirror of each other. My first assumption is only one side burned, the other side still working. You should be able to use the good side as reference to chase the signal around to compare the bad side.

Do not.......do not hook up speaker or any load to prevent the signal amp to blow again.


I know the transistors used, the C3281/A1302, C4793, C1837, I use all these in my amp. They are all supposedly able to handle the full +/-90V even if both down converters burned and put out max voltage. It's the heat that burn them. If you don't put the speaker or load, current draw should be minimal. Then you should be able to take your time to poke around.

I am not familiar with the down converter and I don't have time to really get deep into it and study. But if you see anything obvious, I'll monitor this thread and try to make some sense out of it.

Do post the full schematic here so someone else can join in to help you.
 
All of the large transistors or attached to the aluminum back panel. There was one of each transistor and a fet in each rail burnt. I let it play for about an hour and did not notice anything getting hot. The part of the amp you posted is call the Drive Amp on the schematic. I do have a scope. I am not shore I would know what the voltage should be doing. It is a good tip to know that without the speaker you can check thing and not burn it up again. I thought there should be a way to trouble shoot and not damage stuff.

Tony
 
All of the large transistors or attached to the aluminum back panel. There was one of each transistor and a fet in each rail burnt. I let it play for about an hour and did not notice anything getting hot. The part of the amp you posted is call the Drive Amp on the schematic. I do have a scope. I am not shore I would know what the voltage should be doing. It is a good tip to know that without the speaker you can check thing and not burn it up again. I thought there should be a way to trouble shoot and not damage stuff.

Tony
Another way if you don't feel comfortable to remove the speaker is to turn the volume very low or not playing any music. That will not draw extra current and you should be safe.

It is a good sign the back panel is not hot. You might be lucky enough that the down converters are ok.

You know where JP1 and JP2, those are the two points you can measure. Use a scope to look at the two points, note the voltages when there is no music playing. Then play some music and you should see it's moving with the signal to a certain extend. More importantly, you compare signal on JP1 and JP2 and see whether they looks similar ( of cause opposite polarity).

Hopefully Bob Carver replies and give you more instruction what to check also.
 
"Hopefully Bob Carver replies and give you more instruction what to check also." Bob is very kind as are you. He got me to this point. I think it is all set to go. I just need to double check when time allows. I mostly posted here as most of what I found when I started researching led to dead ends. I like to learn and hope it helps someone else.

Tony
 
Hi all. I know this is a little out of date but I was wondering if anyone can let me have the schematic for this amp as the link near the beginning is not functional.
Thanks.
 
Interesting that the schematic posted by Ray G is for an NHT Sub Two i, and that it appears to be exactly the same as the schematic for the Sub One i (save the model number and the claimed watts). Both are available on NHT's site, at the bottom of the Downloads page (https://www.nhthifi.com/pages/downloads).

I have a Sub One i. No hum, but is not fully functional presently. While it does give a thump when the power is turned on, it fails to "turn on" when presented with a signal (no sound, no signal light). I started to trace the signal into the auto-on circuit (top-left of schematic) and got as far as the quad op-amp module (MC34004) but ran out of time and haven't gotten back to it. It looks like a well built plate amp, though.
 
Hey folks. I've been loitering around here for a while. I have a Sunfire True Subwoofer that's been out of action for nearly a year now. I finally feel I have the time and the inclination to repair it. I'm having trouble getting hold of a schematic though. For some weird reason thecarversite.com says, "banned" which is a bit ominous. I think it might be because I tried to log in before getting final approval from admin.

I'm not even entirely sure what model/Mk it is. So far all I have done is check the voltage from the big caps which is fine. It powers up ok and comes out of standby when it receives a signal, but it just sounds like a someone farting into a rubber glove. I've taken the amp out and took it to work. I'm an led r&d engineer so I've got access to a couple of semi tame electronic engineers, but of course they just hiss at me and demand schematics.

If someone were to point me in the right direction I would be extremely grateful. I have made plenty of rather exhausting searches for a complete schematic, but have fizzled out.
 
I'm running into the same problem with mine. A schematic would be a great help.

Turns out mine is an original and the pre-amp design was abandoned after a month. I have a schematic for the output if you want. It's at work though so, will be in the morning NZ time.
 
Hi everyone, I have a MKII board with an odd symptom. I've replaced all electrolytic caps and the sub is working fine up to medium volume. After a 10 seconds or so at volume that causes around 3/4" peak to peak excursion at low frequency, the signal stops and a medium volume hum occurs. Removing the signal for a moment will correct the the condition, then it will reoccur after another 10 seconds after the signal is returned. If the volume is reduced it will run fine. Amp gets slightly warm on the back panel, but cooler than my other subs. Also, it's not related to the volume pot position. Different combinations of pot position and input level that achieve the same output level cause the condition. Before I start swapping the transistors I wanted to see if anyone have a theory of what might be happening. Thanks!
 
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