My P68 sub amp just went poof - help me fix/troubleshoot it! - diyAudio
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Old 15th December 2002, 01:58 AM   #1
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Unhappy My P68 sub amp just went poof - help me fix/troubleshoot it!

Hey all,

Just received my sub driver the other day, and got the box etc. all finished. Chucked it in, hooked everything up (Including previously functioning - certainly without a load P68 sub amp) and started running the thing. Produced a nice tone at 20Hz , but upon cranking it up the output stopped. A few seconds later there was a nice "POP" and cap C13 (100uF electro bypass rated 63V from +ve rail to ground) blew.

I've cracked it all open and cleared the smoke etc. and it appears (from LOOKING at the board) that Q6 is also blown - nice charred marks around it etc, and I have a feeling this may have gone before the cap - not that this is just a guess. The +ve rail fuse also blew. Everything else looks fine. I don't have any way of testing transistors etc. in circuit (And don't really know how to do it out of circuit either) but everything looks like it's kosher.

One thing that MAY have caused it is the driver transistors (Supposed to be MJE350's) are marked KSE350. I don't know if these KSE350's are to blame or not - Q6 was one. I've ordered MJE350 replacements.

If anyone has any idea at all as to what may have caused it to blow please let me know! It was running into a 4ohm load at the time of death. (Although at 20Hz the impedance is more like 6ohms)

P68 info/schematic

Photos of my sub amp (Before it blew!!)
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Old 15th December 2002, 02:42 AM   #2
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KSE350 are second-sourced MJE350s from Fairchild Semiconductor. While they may not be of as high quality as the OnSemi devices, they're certainly fine, and will have no problems replacing the MJE350 in this application (I use the KSE3x0 devices myself with no problems.)

One thing I can think of is that the load of the DPL12 is highly reactive at ~20hz, as its free-air resonance frequency is between 16 and 18hz (my samples measure 17.5hz average, out of four)

In theory, the amp should handle a reactive load just fine, but know that that particular frequency might stress it more than others.

What were your rail voltages, by the way?

(loaded and unloaded)
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Old 15th December 2002, 07:12 AM   #3
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The rails are at +/-58V (40-0-40 transformer rectified and filtered with 6000uF per side) at no load - reduces to +/-54V or so under load. This was measured a while back.

Thanks for the info on the KSE350 - I guess I'd better look a bit more closer. I'm a little perplexed as to why the cap blew in particular. It was rated for 63V so SHOULD have been fine. I've tested the amp (with no load) for several hours etc. before and everything was fine.

I've ordered replacements and will probably receive them within a couple of days. I'll definitely post here when I've replaced them to let ya'll know how it goes.
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Old 15th December 2002, 07:55 AM   #4
halojoy is offline halojoy  Sweden
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Default Re: My P68 sub amp just went poof - help me fix/troubleshoot it!

Quote:
Originally posted by Jonathan M
Hey all,

Just received my sub driver the other day, and got the box etc. all finished. Chucked it in, hooked everything up (Including previously functioning - certainly without a load P68 sub amp) and started running the thing. Produced a nice tone at 20Hz , but upon cranking it up the output stopped. A few seconds later there was a nice "POP" and cap C13 (100uF electro bypass rated 63V from +ve rail to ground) blew.

I've cracked it all open and cleared the smoke etc. and it appears (from LOOKING at the board) that Q6 is also blown - nice charred marks around it etc, and I have a feeling this may have gone before the cap - not that this is just a guess. The +ve rail fuse also blew. Everything else looks fine. I don't have any way of testing transistors etc. in circuit (And don't really know how to do it out of circuit either) but everything looks like it's kosher.

One thing that MAY have caused it is the driver transistors (Supposed to be MJE350's) are marked KSE350. I don't know if these KSE350's are to blame or not - Q6 was one. I've ordered MJE350 replacements.

If anyone has any idea at all as to what may have caused it to blow please let me know! It was running into a 4ohm load at the time of death. (Although at 20Hz the impedance is more like 6ohms)

P68 info/schematic

Photos of my sub amp (Before it blew!!)
we are busy working on your case

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Old 15th December 2002, 02:47 PM   #5
paulb is offline paulb  Canada
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Default Re: My P68 sub amp just went poof - help me fix/troubleshoot it!

Quote:
Originally posted by Jonathan M
...A few seconds later there was a nice "POP" and cap C13 (100uF electro bypass rated 63V from +ve rail to ground) blew.
That must have been exciting.

Odd that a bypass cap blew. I would normally suggest maybe it was in backwards, but you've said you tested the amp so it would have shown up before. And anyways I'm sure you've checked that.

What kind of testing have you done before you tried the subwoofer load? Has it run before at high power, 20 Hz into a dummy (resistive load)?

I think ThingyNess may be right about the reactive load. I wonder if bypass diodes should be added from output to each of the rails, as I've seen in many other amps.

And...have you asked Rod?
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Old 15th December 2002, 03:52 PM   #6
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Quote:
If anyone has any idea at all as to what may have caused it to blow please let me know! It was running into a 4ohm load at the time of death. (Although at 20Hz the impedance is more like 6ohms)
Hi Jonathan,
Are you sure there wasn't a short on the output?
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Old 15th December 2002, 08:53 PM   #7
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Thanks for the replies/suggestions guys!

I don't know if exciting is the word - it made a nice pop and a goodly amount of smoke etc. was produced (From the cap mainly - the transistor burnt up and had a nice plastic smell to it)

I'm 99.9% sure that the output wasn't shorted. It has DC protection on the output and I assume that this kicked in before the thing went bang. I've checked and rechecked the protection circuit and the output from the amp is not being shorted under a fault condition.

As for the cap - that's a bit of a mystery. One would assume that it would take a bit of a large amount of voltage to blow it up - unless it was a negative potential.

I have identified (and fixed) a dodgy signal detection circuit which may have caused mains to be disconnected and then reconnected immediately. This would have occured after the DC protection circuit kicked in. I don't know what could eventuate from power being removed then immediately reapplied - but it COULD have caused the +VE rail fuse to blow.

Once the +ve rail was gone, it does open up a slight possibility for a negative potential across C13 - but this is a HUGE stretch.

Cheers
Jonathan
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Old 16th December 2002, 07:15 AM   #8
Duo is offline Duo  Canada
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Are you sure the cap wasn't in backwards? Perhaps if it was, it stood up for a while and then blew, letting AC go through the transistors. Is the cap on the same rail as the transistor that blew up? Or the other? Maybe the cap had a manufacturing defect...
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Old 16th December 2002, 08:46 AM   #9
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Quote:
As for the cap - that's a bit of a mystery. One would assume that it would take a bit of a large amount of voltage to blow it up - unless it was a negative potential.

I have identified (and fixed) a dodgy signal detection circuit which may have caused mains to be disconnected and then reconnected immediately. This would have occured after the DC protection circuit kicked in. I don't know what could eventuate from power being removed then immediately reapplied - but it COULD have caused the +VE rail fuse to blow.
There are two cases when the cap blows: as is in backwards or the voltage exceedes its voltage is rated for. Becouse you tested your amp before the load was connected to, than the cap wasn't in backward , I think.
Maybe when your protection circuit taken an action, you lost the ground connection and there was rail to rail voltage on the cap.
I would disconnect all protection circuits and than start again.
Regards,
Jarek
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Old 16th December 2002, 09:54 AM   #10
trwh is offline trwh  United Kingdom
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"It has DC protection on the output and I assume that this kicked in before the thing went bang."

No, the DC protection circuit is to prevent damage to the speakers _after_ a fault in the amp has occurred.

If you were driving the amp hard into a very rective load, maybe the reactance of the speaker threw back a higher voltage than it was being fed, thus blowing the cap which was already operating near to its rated voltage. Just a guess - is this possible?

Another stab in the dark - 6,000uF reservoir capacitance isn't much for an amp of this size (especially with a 4 ohm load). Maybe the decopling capacitors were exceeding their ripple current rating.

Hope this helps,
Tim.
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