Rythmik Audio 250 basic plate amp help needed please! - Page 2 - diyAudio
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Old 27th August 2006, 09:09 PM   #11
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Have you got any devices you can sub in just to test? Although I'd be wary of repeatedly soldering and desoldering stuff as it lifts the tracks on the PCB
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Old 27th August 2006, 10:57 PM   #12
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the voltage measurements are taken with, transistors in circuit but, transistors were removed for testing. this must be different topology because the red speaker wire connects to pc board, goes through an emmiter resistor which measures .3-.4 ohms straight to the outer pnp transistors emmiter? not used to what i am used to seeing either lol! as far as the fuse, i think it must be bundled up inside a large piece of heat shrink tubing behind the iec power connector on amp with lots of gooey crap all over it. needless to say, i don't want to look at it that bad since i know it must be good anyway lol! yeah, i'm used to seeing one pnp and one npn on each side. but this has two npn on one side, and two pnp on the other. maybe they are running the pairs on each side, in paralell? since they are rated 70 watts each and the amp is 250 watts rms? there is also two emmiter resistors per side. appreaciate your help! thanks
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Old 28th August 2006, 12:12 AM   #13
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My mistake.You are right it's an EF topology(emitter follower)and mono.That's why 1 pair each of same polarity.Still you should not have -68V dc at output.There's a fault somewhere.Continue checking.

edit- I presume it was assembled and tested when you received it? 2 emitters per side total 4?
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Old 28th August 2006, 12:27 AM   #14
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yes you are correct! 2 emmiters per side. but wow! it sure has my head scrambled lol! thanks again
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Old 28th August 2006, 01:15 AM   #15
singa is offline singa  Singapore
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Holy s**t! I just remembered if there's -68V dc at output your speaker coil may be fried.Disconnect the speaker and measure the voice coil resistance if 8 ohm should be about 6 ohm.Or use 1.5V battery and connect intermitently and you should hear clicks.

Also measure the output again without speaker load. Jeez I am sorry.
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Old 28th August 2006, 01:46 AM   #16
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oh, it's alright i haven't hooked it back up to the sub since i unhooked it. first thing i did was check the dc resistance of sub after i unhooked the amp and it was the same as when i checked it new. i also have a smaller 150 watt amp running on it right now working fine. thanks for the alert though! appreaciate it.
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Old 28th August 2006, 03:24 AM   #17
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It is likely with negative 68V at output,the pnp transistors are short.The correct procedure using diode test is:

p"n"p base is negative so black probe (-) for digital meter to C and similarly base to E and reverse probe again.You should get low resistance reading and high reverse.For n"p"n is the opposite.

The output should be zero (ideal) or only a few milivolts dc.

Good luck.
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Old 28th August 2006, 02:44 PM   #18
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hi singa, well i took out all outputs pnp/npn outputs+drivers and all checked good. now i noticed i have about 215-220 +mv. at speaker wires. all voltage tests so far have been with plug connector from preamp board with on/off/auto/crossover connected to power board but, always turned to off. now, if i unplug preamp board from power board the dc offset drops to 1.5-2 mv. would this mean maybe, problem in preamp board and then being amplified, by outputs? there are 5 4558 sil pack op amps on preamp board maybe i should check for shorted output to -12v supply etc. make any sense? thanks for any opinions.
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Old 28th August 2006, 05:45 PM   #19
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Default Rythmic Audio 250 basic plate amp help needed please!

Hi CC,
Did'nt know you had preamp board until now.If you are checking power pcb you should reference the ground of power pcb.1.5-2mV is very good that would give a all clear to your power pcb.But how did -68V arise?Did you install the insulation(mica or impregnated silicon insulator) properly last time?

It's possible or logical to suspect the preamp pcb now.You mentioned -12V ( is this a typo erro?) more like +12V or +/- 12V.
You're getting closer to finding the fault.A scope would be useful at this point.But will just have to use what you have.

Happy hunting.
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