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raise0 23rd November 2007 04:16 PM

HEXFET amp - please help me fix it.
2 Attachment(s)
Hi! This is my first post and Iím coming in full speed. Some years ago Iíve build the Giesberts HEXFET amp (elektor). Iím very happy with its performance. I think it has a quite clear sound overall, although I had the usual problems with oscillations, burned HEXFETs and resistors going up in flame! But I was always able to solve my problems. Mostly by replacing components of the output stage, along with some luck ;)

But now I canít find my way out! A year ago the left channel decided to blow my SEAS speaker. I replaced all the burned up resistors (donít remember which) along with all the transistors and FETs. I turned it on and Boooom! R27 (I think) was gone. I replaced it and turned it on once again, but nothing! I was stuck with 100mA (regardless the setting of P2) and zero output. I rechecked my PCB connections, but didnít come across anything suspicious! So in despair, I let it be!

This was until some days ago. I really miss my old amp and I want it working again! So I tried to take a look, once again. It only took me about 10 months to find out that my relay had a fault. So bypassing it I gave it another try and guess whatÖ I have sound! Problem solved. Not quite. It is distorted! But it is alive and with a high levels of amplification, meaning that my HEXFETS are just fine. Iím sure of that because I also tried them on the right channel board and went ok. This is what i see:

1. Amp current stuck at ~100mA (instead of 230mA), regardless adjustments of P2.
2. DC offset just fine ~1mV.
3. D1 and D2 stay on after amp goes off for 15sec. on the right working channel but on the left channel they are turned on for more than 40sec.
4. HEXFETís are ok since they provide hi level amplification.

Iíve read what egberttheone and Zimo did and Iím willing to give it a try too. Although I doubt their mods will do me any good. My amp has loud distorted sound without burning any of the circuit components. Why the hell canít I change the 100mA current? Is it a failure of T7? This was a working amp for over a 3 year period with daily usage. Iím ashamed to have it laying down dead.
Anyway my equipment is only a digital millimeter and an analogue current meter at 500mA. I have also been messing up too much with the PCB and lots of the traces are ďreplacedĒ by wires. So components are not easy to go in and out. Iím so close but still so far! Sorry for my English and my long post. Just wanted to give you a solid background of my situation. Wonít be checking in for 2 or 3 days cause Iíll be on a trip.

Thank you for your time!

east electronics 23rd November 2007 05:47 PM

may be trimmer is gone and or R19 ..... also you can e mail me privetly and take a look at it in my lab together

regards sakis 6944879399

radule 24th November 2007 11:45 AM

Check the fuses.
This amp producing sound even if one of the fuses is broken,
but distorted sound, only one half wave going to output.

radule 24th November 2007 12:14 PM

Check one more time T13, and F2.

If R27 is broken, it means that gate to drain of the T13 was be shorted.

Stability of this amp also depands from the value of the fuses,
with F4A or F5A, resistance in source is smaler, and amp became
more stable.

juma 27th November 2007 09:52 PM

This amp caused troubles to many diyers. Read these threads, it might help you:

raise0 30th November 2007 10:01 AM

Thanks for all your replies! Just got back and I am glad for your interest to my problem. Here are some quick replies for you too.

sakis >>> R19 and trimmer P2 seams fine. Iím only using PIHER trimmers. I wish I could also be proud for my matsushita relays, which are currently BOTH dead. Elektor suggested Siemens.

radule >>> I tried both T13 and T12 on the right channel pcb board and worked just fine. I am aware of the situation in which distorted sound is produced due to a broken fuse. But I didnít knew that the fuse value has such a great effect on the resistance in source. However Iím not sure I want to go so high.

Juma >>> Before I post I read both threads that you mentioned thoroughly. But I was hoping to a more "magic" solution!

It seems there is only one way to go on. Replace T7 (maybe reason for weakness of current adjustment) along with T8,T9 (just to be safe) and T10,T11 (may have caused earlier R24,R27 explosions) Apply the suggested changes from the elektor revision of 2002 (DC Protection for the IGBT Power Amp by E. Potters, 024029-1).

Iím also thinking of 6.8V/1W Zener to protect HEXFETís as egberttheone did. Do you think I could leave them on permanently? Will they degrade sound performance?

So Iím going to get all i need and see what i can do. This is going to be tricky since my right arm is in cast (2 broken bones). But this is not going to stop me, although it may slow me down a bit. Any thoughts till then are welcomed. I thank you all.

raise0 29th March 2008 03:10 AM

I Fixed it, FINALLY.
:wave2: Hi! My arm is free again, after 6 months of immobilization. So last night I tried to continue my work on this hexfet amp. I started by replacing resistors and caps as suggested by elektor mag. and other members from this forum. Placed some decoupling caps, changed fuses to 5Amps, replaced trimmer P2 and T7 and R3, R4 to 33ohms. But while I was soldering the 20K resistors to the collector of T8 and T9, I FOUND IT. :magnify: My PCB after a lot of part replacements lost a lot of traces. So my circuit slowly transformed, into a point to point connection. Something went wrong during that and I couldnít find. It took several months but I saw the base of T7 connected to feedback loop (where R25,R28,R30,R31 meets) :h_ache: I stopped everything, fixed that trace and I powered up the ampÖ :xfingers: 220mA and 4mV to output! FINALLY Öwaited 20~30 minutes and connected speakersÖ IT WORKS! :D Cannot believe I was so stupid and BLIND before. :headbash:
I am not sure what this connection did to the circuit and which part was ďcancelledĒ. I mostly get it, but not quite sure. :scratch2: Any comment on that is welcomed. AnywayÖ Although it works now I will complete all the following mods to both circuits.

F1, F2 = 5A Fuse
100nF decoupling caps
27N over R31
20kohms resistors from collector T8 & T9 to ground
R3, R4 to 33Oohm
R20 to 1k8
R17, R18 to 390ohm
R25, R28 to 22ohm
R26, R29 to 120ohm

Furthermore I will place heatsinks on T8 and T9Ö They get extremely hot even with no signal applied. :hot: Probably they are oscillatingÖ Also BOTH transformers make a slightly audible sound. That has no effect to output, but it wasnít there some time ago. :confused: Any ideas on what caused that? Or is it just random. StillÖ both of them simultaneously? Will post again with the results, but not to soon. I plan to listen to it while Iím working on my other amp project. Here are some pics whith my mistake! The second one is a moving gif. Check it out.

meanman1964 12th February 2009 10:07 AM

Did you use the IRFP240 and IRFP9140 fets aswell?

EDDELARUE 12th February 2009 08:55 PM

2 Attachment(s)
Hi all,

EDDELARUE 12th February 2009 08:59 PM

Please forget C50 ( it is for my simulations ).

BD135 -----> BD139

BD136 -----> BD140

Best regards


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