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Fixing and troubleshooting this MOSFET amplifier
Fixing and troubleshooting this MOSFET amplifier
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Old 3rd December 2019, 05:46 PM   #101
aparatusonitus is online now aparatusonitus  Croatia
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Quote:
Originally Posted by obiwankenobi View Post
Of course this version needs a fresh new PCB
I believe I was able to design a new schematic so that the modifications to the original Goldmund schematic/pcb are minimal...anything added can hang in the air or placed on a small test pcb. Eg. the complete arrangement around the Vbe multiplier replaces one resistance (R7 in your first post), so you need to set/solder everything up on a small pcb and fix it to the main heatsink along with the output M-fets via the screw and tab on the bias transistor (Q15 on my last schematic) and wire with 2 wires where R7 use to be...done! Note that all small transistors can be placed on top of the pcb without any heatsink, DC offset trimer U1 can be solder to Q10/Q11 collectors and resistors R11/R30 via solid wire etc...be creative it's all about diy.
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Old 5th December 2019, 01:30 AM   #102
obiwankenobi is offline obiwankenobi  Argentina
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After the fireworks obtained from the copied board I was working on, and as I told you before, I discovered a single fatal error. I placed a 33 Ohms R2 instead of a 330 Ohms and everything went wrong. A couple of days ago, I started to build a fresh new PCB, complete with Exicon MOSFETs. I am talking of the original schematic, not any of the mods we have been talking over the last days. In this original schematic version I replaced almost all the transistors by Zetex ZTX658 and ZTX758, except for the 4 x BC546 at the front end, up and down the dual JFET 2N5565.

The good news:
Transistors were placed on the bottom side of the PCB and heat sinks fixed with epoxy on. Q9 and Q12 in thermal contact same as Q10 and Q11. You may try to see this on the pictures. Oscilloscope on output, no speaker connected, Variac for mains, voltmeter across 0.1 Ohms quiescent current monitor, the amplifier turned on, no heating, no oscillation, almost 0mV DC at the output. With input grounded, 0mA quiescent current on output MOSFETs. After this, signal generator, still no speakers, and a light load, obtained 110 Vpp clean sine @ 1kHz before clipping.

The bad news:
Small speakers 8 Ohms connected at the output, small signal input. Unstable, low frequency motor boating noise at, with HF oscillation on top, crossing distortion. Horrible. Still no heating of any kind. Please remember we are talking about the first original schematic here uploaded.

Before devoting myself to aparatusonitus last schematic, I would like to give this original version amp a quick last chance and try just 2 of the modifications proposed. I need your opinion about this. It would take some few minutes and may calm this nervous temperamental Exicon MFs amplifier and put it to work. I think I am near that

Modification #1: eliminate C17 and C8 100pF caps of the original PCB schematic (shown) and place just C19 15pF cap of aparatusonitus last schematic (shown)

Modification #2: add C1 330pF , R42 100 Ohms, C4 100nF from the aparatusonitus last schematic (shown), to the actual amplifier. Just that.

I have a non heating, no fireworks, almost working amp. I think I should try these. What do you think ? After this test, whether it works or not, the full aparatusonitus modification comes. This needs a new Mouser order and 3 or 4 days delay. Hitachi MOSFETs didn't arrive yet, I paid customs import fees though. But I still need you to help me make the old amp work. My intention is to draw a new PCB with all of the necessary mods, heat sinks, etc. using Exicons.
Attached Images
File Type: jpg IMG_20191204_201332678_HDR.jpg (422.2 KB, 110 views)
File Type: jpg IMG_20191204_201401004_HDR.jpg (554.3 KB, 112 views)
File Type: jpg IMG_20191204_201520828_HDR.jpg (505.2 KB, 109 views)
File Type: jpg IMG_20191204_201543625_HDR.jpg (528.5 KB, 103 views)
File Type: jpg Captura_001381.jpg (433.3 KB, 103 views)
File Type: jpg Last mods.jpg (421.2 KB, 53 views)
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Old 5th December 2019, 05:36 AM   #103
aparatusonitus is online now aparatusonitus  Croatia
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[QUOTE=obiwankenobi;5997420]Before devoting myself to aparatusonitus last schematic, I would like to give this original version amp a quick last chance and try just 2 of the modifications proposed. I need your opinion about this. It would take some few minutes and may calm this nervous temperamental Exicon MFs amplifier and put it to work. I think I am near that [QUOTE]


The original schematic with modifications #1 & 2 can only work by pure luck...the thing is much more complex than it seems at first glance and includes parameters such as open loop gain, close loop gain, loop gain, etc., etc...
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Old 5th December 2019, 07:58 PM   #104
obiwankenobi is offline obiwankenobi  Argentina
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I know, dear aparatusonitus, of course. I will test my luck anyway. Whether it works or not, I will make the next step which is designing a new, fresh PCB for your last schematic. You have done an effort, calculations and wise advising. I could retrieve some by sharing this new version.

Anyway, remember the original sense of this thread. I need to repair a friend's amplifier. At the moment, I have an original working board with Hitachi-s and a new copied PCB that works bad with Exicon-s. Being the MOSFETs the only difference, there must be a simple way to stabilize this amp.

In the meantime I will place an order for the components of you last schematic. Could you please check if you have some last minute addition? I will do some "glue electronics" as you suggested over a copied PCB. Let's see if everythings works fine and if so, the new board will be designed.

Hitachi-s MOSFETs arrived today from Canada. Please, which is the simpler method to correctly measure and see if some matching is possible ?
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Old 5th December 2019, 09:24 PM   #105
aparatusonitus is online now aparatusonitus  Croatia
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Quote:
Originally Posted by obiwankenobi View Post
Could you please check if you have some last minute addition? I will do some "glue electronics" as you suggested over a copied PCB. Let's see if everythings works fine and if so, the new board will be designed.
I'm to tired to suggest anything useful...tomorrow maybe.

Quote:
Originally Posted by obiwankenobi View Post
Hitachi-s MOSFETs arrived today from Canada. Please, which is the simpler method to correctly measure and see if some matching is possible ?
Read carefully Matching L-Mosfets
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Old 5th December 2019, 09:27 PM   #106
obiwankenobi is offline obiwankenobi  Argentina
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Thank you ! This is also a hobby, you don't need to be in a hurry. Let's enjoy all this. I am learning and I thank you and Rick too.
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Old 6th December 2019, 06:14 AM   #107
aparatusonitus is online now aparatusonitus  Croatia
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The image shows the alternative values for the individual components (as well as the required dissipation) if further fine tuning is needed.
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File Type: png Goldmund 330 Clone Exicon sch alternative.png (63.1 KB, 40 views)
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Old 6th December 2019, 06:23 AM   #108
obiwankenobi is offline obiwankenobi  Argentina
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Thank you aparatusonitus , it is now 3:20 am here, I just finished a CPU design, working late. I'll take a real look tomorrow if you don't mind. Could you send me the .asc file ?
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Old 6th December 2019, 09:22 AM   #109
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As requested...
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File Type: asc Goldmund 330 Clone Exicon.asc (18.7 KB, 9 views)
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Old 6th December 2019, 12:47 PM   #110
obiwankenobi is offline obiwankenobi  Argentina
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Thank you aparatusonitus
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