| Electronics > Repair |
| Audio… (Amps, THD, channel separation) with analog equipment (scopes, etc.) |
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| Coordonnée_chromatique:
--- Quote from: CaptDon on December 29, 2024, 05:36:34 pm ---120vac line voltage is around 340vpp. As to your amplifier 120vpp equal 60 vp x .707 = 42.5vrms and that becomes 225 watts rms with no head room or around 22.5 watts with 10dB of head room all assuming an 8 ohm load. --- End quote --- Excuse me but could ou describe your numbers for idiots like me ! 10dB is the headroom of the : amplifier gain ? |
| bdunham7:
--- Quote from: Fried Chicken on December 29, 2024, 04:03:37 pm ---The Amp was working, but something was off and it got progressively worse after the recap (that included the filter caps)... that I didn't notice until I hooked up the Sony and realized what real sound is. It's textbook not enough power... I also have a hard time believing the filter caps would go bad and cause that, but that's my immediate instinct --- End quote --- Since you only have two channels of scope to work with, try this: Set up your amp with your function generator putting out a 1kHz 500mVRMS signal feeding both LINE, CD or AUX inputs of your amp. Use a Y-cord or similar, an actual parallel connection. Connect your 8R resistors to the output and then your scope to the power rails. Adjust your scope with the same V/div setting on each channel and the vertical positions so that you have both traces on screen. Then slowly turn up the volume and see what you get on the scope. Post a photo if you can. You can use your DMM on one of the 8R resistors to make sure you don't go over the maximum output (what is the amp rated for??). Then do the same thing with a 40Hz signal. I looked up your M-65 amplifier and it has a slightly unusual setup with all 4 of those 33mF caps are in series. When I get a chance I'll have a look at the schematics and see if I can come up with some other things to test. This is a pretty powerful amplifier so there's definitely something amiss. Unfortunately this happens sometimes with recaps where an error is made. The fact that it affects both channels seems interesting though. One additional question--is the Auto Class A turned off? It should be for now. |
| Fried Chicken:
--- Quote from: bdunham7 on December 29, 2024, 07:28:55 pm --- --- Quote from: Fried Chicken on December 29, 2024, 04:03:37 pm ---The Amp was working, but something was off and it got progressively worse after the recap (that included the filter caps)... that I didn't notice until I hooked up the Sony and realized what real sound is. It's textbook not enough power... I also have a hard time believing the filter caps would go bad and cause that, but that's my immediate instinct --- End quote --- Since you only have two channels of scope to work with, try this: Set up your amp with your function generator putting out a 1kHz 500mVRMS signal feeding both LINE, CD or AUX inputs of your amp. Use a Y-cord or similar, an actual parallel connection. Connect your 8R resistors to the output and then your scope to the power rails. Adjust your scope with the same V/div setting on each channel and the vertical positions so that you have both traces on screen. Then slowly turn up the volume and see what you get on the scope. Post a photo if you can. You can use your DMM on one of the 8R resistors to make sure you don't go over the maximum output (what is the amp rated for??). Then do the same thing with a 40Hz signal. I looked up your M-65 amplifier and it has a slightly unusual setup with all 4 of those 33mF caps are in series. When I get a chance I'll have a look at the schematics and see if I can come up with some other things to test. This is a pretty powerful amplifier so there's definitely something amiss. Unfortunately this happens sometimes with recaps where an error is made. The fact that it affects both channels seems interesting though. One additional question--is the Auto Class A turned off? It should be for now. --- End quote --- Yeah Auto Class A is off except for testing with the service manual. I did a recap from a kit from a guy in Canada off a recommendation of the audio forum members. Here is a link to the thread, but don't feel obliged to read through all that. There are a few people that are quite enamored with these amps, here's a guy who has single-handedly created a cult following through publications on his website. (it worked for me). The 120dB of the amp is real, the thing is silent. If you hear any noise coming from the amp, you better be very very careful what source material you're about to start on it, b/c it doesn't mess around. I'll get around to the setup, but I have other things in the way before I can re-do this. I already had everything set up as described: It wasn't until I hooked it up again but compared it to the sony I had laying around, that I realized I had missed the forest for the trees and something else is going on. Unfortunately I'm not an electronics guy. My guesses for what causes low/shitty power are: 1. filter caps, 2. volume level pots, 3. some circuitry controlling rail voltage? I don't think it's the volume pots, b/c I've noticed additional inconsistencies and weirdness (like transistors not reaching threshhold voltage?) until a certain volume is reached or something. My guess remains with the filter caps, but removing them is a bit of a hassle. |
| bdunham7:
--- Quote from: Fried Chicken on December 29, 2024, 07:42:14 pm ---Unfortunately I'm not an electronics guy. My guesses for what causes low/shitty power are: 1. filter caps, 2. volume level pots, 3. some circuitry controlling rail voltage? --- End quote --- Hopefully we can fix that! 1. Filter caps seem unlikely as their function is pretty straightforward 2. Volume pots are unlikely to affect both channels and 3. The rails are unregulated on these amplifiers. Check the power rails with a scope as I mentioned and that should remove all doubt. |
| vk6zgo:
--- Quote from: CaptDon on December 29, 2024, 05:36:34 pm ---120vac line voltage is around 340vpp. As to your amplifier 120vpp equal 60 vp x .707 = 42.5vrms and that becomes 225 watts rms with no head room or around 22.5 watts with 10dB of head room all assuming an 8 ohm load. --- End quote --- There is no such thing as watts rms. You can have rms volts, & rms amps, but when you multiply one by the other, you get "average power". |
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