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Old 12th June 2019, 10:03 PM   #11311
bullittstang is offline bullittstang  United States
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Quote:
Originally Posted by xrk971 View Post
Hi Bullitt,

Please mount the BD139 with circa 3in to 4in long flying leads (shrink tube the soldered legs) and bolt to the top of one of the IRFP240's. This will sense the thermal runaway much faster and reign it back in.

Other possible issue is that your MOSFETs are so mismatched that one of them is a current hog and all the current flows through it and thus it melts down. Use a transistor tester to measure Vgs on all of them. They will come out in ranges from 3.5v to 4.5v. If you have say two at 4.5v and one at 3.5v, that one is going to take the brunt of the current. Try to use reasonably matched MOSFETs (+/- 0.2 to 0.3 volts of Vgs)

Another possibility is that you have oscillation as voltage goes up, and this self-immolation occurs to the MOSFETs.

Do you have an O-scope? Look at output to see if it oscillates? Many times ultrasonic MHz oscillation and a visible sign of this is unstable bias as voltage goes up or time goes by. Unstable bias if you connect input source to audio in with volume turned down is also sign of oscillation.

Try changing C6 to 15pF to 22pF (silver mica is best, but NP0 or C0G is fine too) and see if that helps. Also try increasing C1 and C2 to 150pF or 220pF (220pF MKP film works here). If problem goes away, it is oscillation.

In the process of changing out one mosfet (blew again) and the BD139 (to mount on irfp240) I pulled out two vias and 3 pads came off.

So my plan is to take as many expensive parts off this board, test them and populate a new board. I will use new mosfets, transistors and diodes to make sure if one of those was the issue it won’t carry over.

I plan to mount the BD138 to the IRFP240 and take that out as a possibility. Will take your other suggestions as “next steps” if it still is not working correctly.

Thanks for the assist - still learning, so specifics are a big help.
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Old 13th June 2019, 11:47 AM   #11312
Astaro is offline Astaro  Russian Federation
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People! Tell me, please. Is it will be perfect for apex ax-14, to use MJW1302AG / MJW3281AG transistors?
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Old 13th June 2019, 11:49 AM   #11313
Astaro is offline Astaro  Russian Federation
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is it true that of all the Apex`s the best sound is not AX-14, but AX-17?
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Old 13th June 2019, 12:26 PM   #11314
HatlessChimp is offline HatlessChimp
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Quote:
Originally Posted by Astaro View Post
is it true that of all the Apex`s the best sound is not AX-14, but AX-17?
Pretty sure I read Apex say the A40 is the best
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Old 13th June 2019, 02:29 PM   #11315
xrk971 is offline xrk971  United States
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He also said FX8 was one of his favorite "small amps" - hence FH9, FH11, etc spawned from FX8. More transistors does not always sound better.
But he did say AX40 is his all-time favorite.

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Last edited by xrk971; 13th June 2019 at 02:31 PM.
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Old 13th June 2019, 02:36 PM   #11316
xrk971 is offline xrk971  United States
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Hi Bullitt,

Sorry to hear about this:
Quote:
In the process of changing out one mosfet (blew again) and the BD139 (to mount on irfp240) I pulled out two vias and 3 pads came off.
But, you can salvage it by using flying leads on even the power MOSFETs (use flexible high current silicone RC car wire). Solder to exposed trace remaining attached to board. We now always use 2oz copper and use via stitching of two identical layers on 2 sides to increase conduction and durability (JPS64's trademark layout style).

For example:
Click the image to open in full size.

I have yet to ever pull a solder pad off now.
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Last edited by xrk971; 13th June 2019 at 02:38 PM.
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Old 13th June 2019, 11:57 PM   #11317
lomdwrrrt is offline lomdwrrrt
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Default apex 555 stere0

Quote:
Originally Posted by RSK View Post
Only as my preference. I like to have indicator for fault and standby for the speaker protect. Built it and works great.
hello RSK could you share me apex 555 stereo pcb pdf
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Old 14th June 2019, 11:58 AM   #11318
bullittstang is offline bullittstang  United States
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Quote:
Originally Posted by xrk971 View Post
...Please mount the BD139 with circa 3in to 4in long flying leads (shrink tube the soldered legs) and bolt to the top of one of the IRFP240's. This will sense the thermal runaway much faster and reign it back in.

...MOSFETs are so mismatched that one of them is a current hog and all the current flows through it and thus it melts down.

...oscillation as voltage goes up, and this self-immolation occurs to the MOSFETs.

Try changing C6 to 15pF to 22pF (silver mica is best, but NP0 or C0G is fine too) and see if that helps. Also try increasing C1 and C2 to 150pF or 220pF (220pF MKP film works here). If problem goes away, it is oscillation.
So - I populated a whole new board, testing every component I took out of the old board and replaced if out of spec. I used all new BD139, KSA/KSC and IRFPs along with (1) of the 3 2N5551s that seemed a little low on gain.

The SAME issue persists - anything higher than ~50Vdc PSU on I can't get the bias to stay constant, It slowly moves upward +25mV (then I turn it off)

- on note, is that the other channel I can reduce the bias much lower, like down to 1mV, where this "problem" channel will not go lower than 8mV, regardless of PSU voltage (35, 40, or (48.5, which 60V PSU w 60W DBT in series - bias won't go below 12.5mV)

What I did differently this time:
-I mounted the BD139 with flying leads on top of the IRFP240
-I don't have a O-scope - so I can't test oscillation that way
-I don't have a curve tracer - only a cheap transistor tester, so I grouped the transistors by Hfe so the gain wouldn't be way off on any of them. Also confirmed with B&K multi-meter tester that has transistor function
-Oscillation - I will try increasing C6 and C1/C2 to higher values this weekend and hopefully will fix the runaway.
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Old 14th June 2019, 02:35 PM   #11319
hassan4119 is offline hassan4119  Iran
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hi mr apex....please see in the pdf file in below...i design in proteus a new version of apex ax17 with 2pair c5200/a1943
plz chek this and say to me any mistakes or say upgrate .tnx again
s3.picofile.com/d/8363557634/178a4c9c-7ece-4a37-a230-6465f1a18c95/apex_ax17.pdf
oh dear apex what u dont answer about my fh11? when i power on 2n5551 be hot

power of music not bad....but when in first i turn on the power i have hi hoom in speaker...i cheked all of mistakes....but every thing be ok.....fh11 is realy a good amplifier?
i fear that my apex ax17 too dont woek prpperly....Similar of apex fh11
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Old 14th June 2019, 02:36 PM   #11320
hassan4119 is offline hassan4119  Iran
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tnx i do that
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