Hmmm. Ok. I certainly trust your expertise here. If/when it acts up again, I'll try freeze spray drops. Should I focus on the Left Output Assy first or the Voltage Amplifier Assy?
Can anyone recommend good replacements for the 100 Ohm Trimmer Pots (VR1 & VR2) on the EQ Board. The original ones are designed so that they can be adjusted via 2 holes in the circuit board. From what I have found, these would be referred to as Bottom Adjust type. But I can't find a source. Couldn't find them on mouser or digikey... I thought about just getting normal top adjust type and mounting them on the back of circuit board, but the EQ board is right up against the side of the case so I doubt they would fit that way. And the component side is not accessible without unscrewing and removing board, so making adjustments from that side is not an option.
My originals are very sensitive to the slightest adjustment so I'd like to try a multi-turn type. Seems like every time I check the voltage, it's off by as much as 400mV. It should be set to less than 100mV. But no only are my pots very sensitive, voltage constantly fluctuates, so I'm having difficulty setting it and getting it to stay.
My originals are very sensitive to the slightest adjustment so I'd like to try a multi-turn type. Seems like every time I check the voltage, it's off by as much as 400mV. It should be set to less than 100mV. But no only are my pots very sensitive, voltage constantly fluctuates, so I'm having difficulty setting it and getting it to stay.
Can't help you with that one I'm afraid. It would be a case of having to see what was possible and do-able.
you can set if for 0 mv output, then measure the resistance and put in a fixed resistor of that value. if it's odd you can parallel 2 rsistors for the correct output. Use 1/6 w for size if space is tight.
Just put 9240, 240 mosfets into SX-780 huge difference from STK0050II module
offset stays at 5mv
Just put 9240, 240 mosfets into SX-780 huge difference from STK0050II module
offset stays at 5mv
Latest news...I have the amp hooked back up and am using it again. But last night I hooked up a different turntable and left channel immediately went out. Could smell overheating on left output board again.
Turned things off, then tried CD player and got music out of both channels. Adjusted phono input connections (even though I paid close attention when I hooked them up initially, to not push them on too far) and tried phono again and it started working on both channels.
Indeed sounds like something physical, so this weekend I am going to take a close look at the RCA inputs on the back and see where there might be a short.
Turned things off, then tried CD player and got music out of both channels. Adjusted phono input connections (even though I paid close attention when I hooked them up initially, to not push them on too far) and tried phono again and it started working on both channels.
Indeed sounds like something physical, so this weekend I am going to take a close look at the RCA inputs on the back and see where there might be a short.
That really does seem to further confirm the 'something physical' theory. As well looking for anything that might be intermittently open, also look and see if any movement could cause anything to short as well. I would have thought anything going open on a phono input would cause horrendous noise/hum etc. Something shorting could turn a stage into an oscillator.
Got home and fired her up and got no output on left speaker playing phono. Turned it off, unplugged speakers and w meter connected to left speaker outs it went straight to 800 mV. Jiggled phono jacks, unplugged CD cables and jiggled phono some more and it dropped back down to 7.4 mV. Jiggling phono more I could not get it to go back up. Even after reconnecting CD it's still good. Hmmm.
I might add that right speaker is bit louder, sounds like it has more bass than left. I noticed this early on as well. Could that be an indication of caps failing on left. To be truthful, as this amp does not have a loudness button, the bass on both is a bit weak compared to my other amps. And switching tone on and turning up bass doesn't add much to overall sound.
Ok. Just tried turning tone on and I get barely any sound out of left. Right speaker is ok. Left is low and distorted trying to adjust treble and bass. And with tone on, jiggling phono jacks gets a response, but doesn't clear up completely. I may just be jiggling board inside. Seems to me something amiss on EQ/Input board. And/or dirty contacts on tone switch, bass and treble pots. More Hmmmm.
Final thoughts for the evening. I pulled the Pioneer and hooked up my Maranta 2225 and both channels sound much richer. More bass. (It has had some caps replaced by previous owner)
The 800mv you measured is sounding more and more like oscillation caused by a physical issue somewhere. That kind of reading would be how a typical DVM interpreted a high frequency asymmetric waveform.
Issues such as feeling the bass is lacking or an imbalance between channels always need proving by measurement (scope 🙂) before just diving in and replacing caps. Not least because it pinpoints the problem and its location. A dried out cap wouldn't normally respond to physical input so it sounds like you may have more than one problem at work here.
Switches, contacts, plug/sockets and crimping of wires are all likely problem areas but of course not the only possibilities.
Issues such as feeling the bass is lacking or an imbalance between channels always need proving by measurement (scope 🙂) before just diving in and replacing caps. Not least because it pinpoints the problem and its location. A dried out cap wouldn't normally respond to physical input so it sounds like you may have more than one problem at work here.
Switches, contacts, plug/sockets and crimping of wires are all likely problem areas but of course not the only possibilities.
Mooly, I have come across a Tektronix 475M scope that I can get pretty cheap (compared to prices I see on eBay). Is this an adequate scope to work on this amp, other amps? Seller says it is in working condition, doesn't know when it was last calibrated, only used it a couple of times for a project and its been sitting in his closet. How would I know that it's working, or needs calibration?
That's way more than adequate.
A basic check on functionality is to display the 'cal' or calibrate signal that will available of the front of the scope. This is typically a 1kHz squarewave of known amplitude, usually 1volt. If both channels of the scope can display that correctly then its a good start.
Make sure the trace is clear and bright and that it can be focused sharply.
A basic check on functionality is to display the 'cal' or calibrate signal that will available of the front of the scope. This is typically a 1kHz squarewave of known amplitude, usually 1volt. If both channels of the scope can display that correctly then its a good start.
Make sure the trace is clear and bright and that it can be focused sharply.
Ok. Thanks Mooly. I'll keep you posted. Also if I get it do you have particular areas I should check. I've been watching a few YouTube videos on how to use a scope. But haven't found any that show troubleshooting solid state amps yet. This scope has a single probe.
Google tells me its a 200 MHz dual trace scope. Very nice 🙂 You may or may not get probes with it as they are classed as consumables. For basic initial work a couple of 'BNC to croc clip' type of leads will get you a long way.
Always remember the scope is really a voltmeter displaying voltage on the Y axis against time on the Z axis.
This shows typical instability in small signal circuitry:
http://www.diyaudio.com/forums/anal...u-have-checked-see-its-stable-havent-you.html
And more:
http://www.diyaudio.com/forums/anal...rent-feedback-opamps-simulation-practice.html
The 5th picture in post #1 above shows how a 1kHz 'cal' signal would look if it were a 1 volt peak to peak voltage and the scope were set to 500mv per vertical division. The time or Y axis would be set to 0.5 milliseconds per division. 1000Hz has a period of 1/1000 which is 0.001 seconds (1 millisecond). So the oscilloscope would be set to 0.5ms to get the view in that picture assuming that the signal were 1kHz in frequency.
You can make your own test discs of audio signals (sine wave etc) using Audacity:
http://www.diyaudio.com/forums/soft...dacity-get-you-started-guide.html#post3696956
Always remember the scope is really a voltmeter displaying voltage on the Y axis against time on the Z axis.
This shows typical instability in small signal circuitry:
http://www.diyaudio.com/forums/anal...u-have-checked-see-its-stable-havent-you.html
And more:
http://www.diyaudio.com/forums/anal...rent-feedback-opamps-simulation-practice.html
The 5th picture in post #1 above shows how a 1kHz 'cal' signal would look if it were a 1 volt peak to peak voltage and the scope were set to 500mv per vertical division. The time or Y axis would be set to 0.5 milliseconds per division. 1000Hz has a period of 1/1000 which is 0.001 seconds (1 millisecond). So the oscilloscope would be set to 0.5ms to get the view in that picture assuming that the signal were 1kHz in frequency.
You can make your own test discs of audio signals (sine wave etc) using Audacity:
http://www.diyaudio.com/forums/soft...dacity-get-you-started-guide.html#post3696956
I have a Signal Generator app (20 Hz - 20k Hz) on my phone that I can connect via aux cable, but it sounds like I may need a dummy load setup as well, or can I just use cheap bench speakers here.
A dummy load is very useful although a fair proportion of testing is done with no load. Speakers... you would never stand the noise once you got over a few tens of milliwatts (honest).
Square waves are also useful when testing amplifiers, speakers definitely don't care for those. Get some dummy loads, Parts Express has some fairly inexpensive resistors made for that purpose. The big Dale Non-Inductive resistors used to be reasonable but now they're several hundred $ each for the 250 Watt versions.
Craig
Craig
How much 😀
Well call me a cheapskate but something like these bolted to a bit of metal to act as a heatsink will take a massive amount of abuse and would cover 99% of requirements.
WELWYN - WH25 4R7 JI - RESISTOR, 25W 5% 4R7 | CPC UK
WELWYN - WH25 8R2 JI - RESISTOR, WW 25W 5% 8R2 | CPC UK
WELWYN - WH50 4R7 JI - RESISTOR, WW 50W 5% 4R7 | CPC UK
Well call me a cheapskate but something like these bolted to a bit of metal to act as a heatsink will take a massive amount of abuse and would cover 99% of requirements.
WELWYN - WH25 4R7 JI - RESISTOR, 25W 5% 4R7 | CPC UK
WELWYN - WH25 8R2 JI - RESISTOR, WW 25W 5% 8R2 | CPC UK
WELWYN - WH50 4R7 JI - RESISTOR, WW 50W 5% 4R7 | CPC UK
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