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Power Amp Repair - fuses

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spacebiscuit View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote spacebiscuit Quote  Post ReplyReply Direct Link To This Post Posted: 18 February 2024 at 8:30pm
Thanks and I do hear you regarding purchase from a reputable source. I was slightly put off by opting for the Exicon  because earlier in this thread it was suggested that I would need a mod - something I didn't really understand and thought would be beyond my skill set.

i fitted a replacement pair today, powered up via the bulb tester. After taking some voltage readings I added a source and my speakers and I was able to playback sound for about 25-30 seconds before the FETs got very hot. They are not screwed down to the heatsink as I initially wanted to make sure they weren't already toast. However even when idle without a source they tend to get quite hot quickly.

So now I'm wondering - does the heatsink work that well (on the good channel I couldn't feel any noticeable heat when touching the black from of the FET) that I simply need to screw them down or is symptomatic of another issue?

I'm not sure if this is a good sign or not, but relief to have those fuses tripping which feels like a small victory (I did power it directly to the mains without the bulb tester to confirm this).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Xoc1 Quote  Post ReplyReply Direct Link To This Post Posted: 18 February 2024 at 10:27pm
Definitely fix them to the heatsink with fresh thermal paste & check that there are no shorts if insulators are fitted
The Bias current should be checked which would probably involve wiring a DVM set on amps across one of the fuse holders with the fuse out.
But can't see any bias adjustment on the PCB in your pics.
Using the Exicons would not be beyond your rapidly increasing skill set. Could always help you with how to add the extra components to your PCB which might cost £1 or so!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote spacebiscuit Quote  Post ReplyReply Direct Link To This Post Posted: 19 February 2024 at 2:07pm
Originally posted by Xoc1 Xoc1 wrote:

check that there are no shorts if insulators are fitted

Going to screw them down, I have new paste and new pads. Can you educate me here - why would insulators cause a potential short?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 19 February 2024 at 6:25pm
Some amps are designed with rail voltages on the heatsink, so no insulators are used. That's usually done to aid cooling as devices are bolted straight onto the sink.

If the amp uses micas, then it means that the heatsinks are on the chassis, ie at earth potential. So any contact between the tab of the device & sink will end in tears.   Ouch



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spacebiscuit View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote spacebiscuit Quote  Post ReplyReply Direct Link To This Post Posted: 22 February 2024 at 8:49pm
Screwed the FETs down and used insulation pads and thermal paste - I have sound and believe the FETs are not getting too hot.

When I first power up (if amp has been off for a period) I sometimes get a bit of a hum that goes as I turn up the source volume and it eventually fades. I'm sometimes getting oscillation, in which case I power off and back on and it seems to disappear.

When the hum and any oscillation has gone I've let the source play for around 60-90 seconds, slowly increasing volume and the bottom end. The sound is great, nice, deep and rich, I didn't push it excessively, but enough to feel it working. Try to plan my next move....


Edited by spacebiscuit - 22 February 2024 at 8:51pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 23 February 2024 at 10:02am
Ok. Have you stuck caps on the FET legs? 10pf-100pf, soldered between Gate & Drain. See if that helps get rid of hum & oscillation.

But good to here that sound quality is satisfying. That makes all the work worth the effort.   Thumbs Up


EDIT:-  Or it could be a problem with the main filter caps. Are they ok?







Edited by Earplug - 23 February 2024 at 10:04am
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spacebiscuit View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote spacebiscuit Quote  Post ReplyReply Direct Link To This Post Posted: 23 February 2024 at 10:29am
I'll give the caps a go - why do I need the caps now when previously I didn't. I'll check all other caps too a little later.

First thing to day when I fired the amp up no source connected I had quite bad distortion which steadily rose, until i powered off. Powered on again same, but this time I turned the volume a touch and I got quite a bad flapping sound. Third time just sounded like a bad earth - humming was consistent and stable - not rising. As i said - as soon as a source is added and played seems to be fine.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 23 February 2024 at 11:27am
The small caps are good to "tame" the new FETs. You never know with differences in the devices and difference between the originals & new ones.

Bad/leaky filter caps can cause all sorts of (unpredictable) problems - like distortion. Well worth changing if there is any doubt.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote spacebiscuit Quote  Post ReplyReply Direct Link To This Post Posted: 23 February 2024 at 5:37pm
Do I need the caps on both the N and P-FETS - I have two pairs.

I've I'm going to pick some up better to go for 10, 100 or something in the middle? Also do they need to be high voltage, I've seen 50v and 1kv?


Edited by spacebiscuit - 23 February 2024 at 5:43pm
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 23 February 2024 at 6:15pm
Yes, best on all. Get whatever is available to you, like at your local component shop, etc. The actual value isn't that important. Maybe get a selection and try see if you can hear/note any difference. They shouldn't cost much.    LOL LOL LOL

50V is a bit low. Are there no 250V ceramic caps available? If nothing else, just get the 1kV, no problem.They are all relatively small & should fit without too much messing about. As I said, solder direct onto the legs of the FET.

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