hi, my sub amp made a strange noise (hard to explain) but, then quit producing music and just had a medium level hum wasn't real low like a 60hz hum but higher. i quickly, shut it down and looked it over real well and saw nothing obvious or burnt looking, and no smell. the amp appears to use 2sc5198/2sa1941 outputs. there is a pair of 5198s on one side, and a pair of the 1941s, on the other side. in between the two pairs is a MPSA13 Darlington. i checked all transistors on power board plus, the outputs 5198/1941. they all checked fine not quite sure how to check the darlington! now, when i plug unit in nothing gets hot, or smells but, i have almost full negative rail voltage of -68.8v at speaker wire outputs? any help would sure be appreaciated! could it maybe be the MPSA13? since collector feeds to one side and the emmiter feeds the other side? thanks 
i also checked all diodes and bridge rect. as well
i also checked all diodes and bridge rect. as well
hi fellas and thanks for your help! i checked at the bridge rect. and the ac in is around 100v. the pos. out of bridge is around +67-68 vdc in respect to ground and the negative volts out in respect to ground are also, -67-68v. dc i removed all output transistors and, drivers and checked for shorts and opens with diode check on my fluke and all tested fine? if the bridge rect. has correct output voltages and all output transistors have around 67-68v. dc +/- going to their collectors, i wouldn't have a blown fuse would i? or is it still possible. i've looked and still, haven't been able to find any fuse yet. when the amp ouit, it had a light buzz comming out sub so, i thought maybe a cap, diode or , rect. problem but so far, i can't seem to find anything. could an output be shorting under load when powered up? i've always thought either they were permanent shorted, or open when faulty. thanks very much for your help and any more, will sure be appreaciated! can't seem to find schematic sorry!
can't seem to find schematic sorry!
A dodgy output device can test OK with a diode check. I'm surprised there is no fuse anywhere. Yes as you say if there really is voltage at the output devices that part must be OK.
A blown cap could cause hum, but I would expect it to be bulged a bit so should be possible to spot with the naked eye. Try putting the drivers back in and just one pair of output devices and see if anything's different, just don't crank it with only one pair.
A blown cap could cause hum, but I would expect it to be bulged a bit so should be possible to spot with the naked eye. Try putting the drivers back in and just one pair of output devices and see if anything's different, just don't crank it with only one pair.
Without the schematic it hard to diagnose, but I suspect that the VAS transistor might be open circuit.
edit: just seen your edit. It looks like it uses a CFP/Sziklai output configuration then. Trace back to the drivers and their bases should give you more what voltages you are expecting to see.
edit: just seen your edit. It looks like it uses a CFP/Sziklai output configuration then. Trace back to the drivers and their bases should give you more what voltages you are expecting to see.
I'm confused,are you taking measurements with trasistors in or out of the pcb?There should be a fuse just follow the AC input to power switch and somewhere along you should see it,either a glass fuse or something like a resistor green or yellow marked with XA /110V.You are wrong to say red wire feeds off the emitters of pnp when conventional complementary circuit ,the collecters are the oitputs unless it's a different topology.
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
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?
edit- I presume it was assembled and tested when you received it? 2 emitters per side total 4?
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.
Also measure the output again without speaker load. Jeez I am sorry.
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.
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.
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.
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.
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.
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.- Status
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