The plate amp's power LED is still on, but the sub will not produce any sound from either RCA input or speaker input.
I can't find anything obviously fried inside, and this plate amp is discontinued and difficult to find replacement. Box opening is about 9.25" x9.25".
Any known quick fix for this amp or what might replace it?
I can't find anything obviously fried inside, and this plate amp is discontinued and difficult to find replacement. Box opening is about 9.25" x9.25".
Any known quick fix for this amp or what might replace it?
I have a m&k volkswoofer that had a blown amp in it when I got it. I didn’t troubleshoot much but the ps caps looked suspect and the online wisdom was these were not worth fixing. A parts express plate amp mounted on plywood then mounted in the original plate amp opening should fix this. For my setup I just removed the original amp, covered and sealed the hole and run an external triad subwoofer amp.
Neither of those large laid-down-mounted electrolytics on the low level board look perfectly healthy. Have you checked for proper supply voltages? The 2-pin connector with black and red wires to the low-level board would be a good place to start.
One of those electrolytics is rated 100V -- a little peculiar, unless they've put the Speaker-Level processing on that board (Uu-u-gh 🤢 ). If 'Open', that would only affect the speaker-level inputs, of course; line-in RCA's would still work, and you said they don't. On the other hand, if it/they failed 'Shorted', all subsequent parts would probably be toast.
We can't see the health of the 4 output devices, or their drivers, that are under the power board. This might actually be a not-too-difficult repair for the 'right people'. Is there no capable repair facility in your part of the world?
This doesn't sound/look to me like a 'Known Quick Fix' sort of problem. But to the good, you have a linear power supply, and the power transformer looks adequate. Thus, many of the 'likely failures' -- switching supplies have many -- are not an issue.
And I wouldn't underplay Indigent Audio's advice, which is also excellent. 😉
Cheers
One of those electrolytics is rated 100V -- a little peculiar, unless they've put the Speaker-Level processing on that board (Uu-u-gh 🤢 ). If 'Open', that would only affect the speaker-level inputs, of course; line-in RCA's would still work, and you said they don't. On the other hand, if it/they failed 'Shorted', all subsequent parts would probably be toast.
We can't see the health of the 4 output devices, or their drivers, that are under the power board. This might actually be a not-too-difficult repair for the 'right people'. Is there no capable repair facility in your part of the world?
This doesn't sound/look to me like a 'Known Quick Fix' sort of problem. But to the good, you have a linear power supply, and the power transformer looks adequate. Thus, many of the 'likely failures' -- switching supplies have many -- are not an issue.
And I wouldn't underplay Indigent Audio's advice, which is also excellent. 😉
Cheers
As the board looks quite good, no obvious burn marks etc. a simple question: Did you check the fuse? It is hidden where the mains line get's plugged in.
The MegaBass amp is supposed to be a drop in replacement for the M&K 250 amp.
The MegaBass II amp appears to be a dead ringer for your amp.
https://www.apexjr.com/amps.html
Apexjr has a forum on this site, but it doesn't appear to be very active.
The MegaBass II amp appears to be a dead ringer for your amp.
https://www.apexjr.com/amps.html
Apexjr has a forum on this site, but it doesn't appear to be very active.
"The MegaBass amp is supposed to be a drop in replacement for the M&K 250 amp.
The MegaBass II amp appears to be a dead ringer for your amp."
Thank you bringing this up. I have actually been in contact with Apex Jr, but unfortunately, their amp has different plate size(10x10") from M&K (10-3/8" x 10-7/16"). My box's cutout is 9.5" x9.25" and will just miss the screws on Apex Jr amp 🙁
I've been thinking about ways to somehow screw/secure the Apex Jr amp but can't come up with anything reliable so far... Perhaps some way to use external screw-down clamps or something?
The MegaBass II amp appears to be a dead ringer for your amp."
Thank you bringing this up. I have actually been in contact with Apex Jr, but unfortunately, their amp has different plate size(10x10") from M&K (10-3/8" x 10-7/16"). My box's cutout is 9.5" x9.25" and will just miss the screws on Apex Jr amp 🙁
I've been thinking about ways to somehow screw/secure the Apex Jr amp but can't come up with anything reliable so far... Perhaps some way to use external screw-down clamps or something?
Not sure of your comfort level with electronic troubleshooting/repair, but here are a couple quick tests that will help isolate the problem.
1) The LED should be red when the switch is in the OFF position and no sound will be produced.
Move the switch to the ON position, does the LED color change to green? any thumps or sound from the woofer?
2) Check the preamp power supply voltages on the orange and brown wires at the connector on the amp board. They should be +/-12VDC with little or no AC ripple.
If the voltages are low or lots of ripple, usually this means one or more of the capacitors circled in red need to be replaced.
3) Check to see if the signal coming from the preamp board to turn the amp ON/OFF(black wire) is working properly.
It should toggle between +9VDC to -9VDC as you change the position of the ON/OFF switch.
4) Check to see if the current source supply for the amp input stage is getting activated by the signal measured in 3).
Measurement location is shown with blue arrow pointing to one end of 100K resistor; should switch between +67VDV and -67VDC as you change the position of the ON/OFF switch.
Report results and we can go from there.
In the mean time, I'll see if I can locate the schematic...the above is from memory troubleshooting those amps years ago.
Attachments
Hello Bolserst,
I have the almost the same subwoofer amp on the M&K VX850. My subwoofer plays audio, just not quite right. In the very low range, 20-40hz, it’s a bit intermittent.
Looking at the picture you posted with the voltages and arrows, I have a problem with the orange arrow. I have 12v DC but also a 15v AC ripple. I have already changed the main capacitors 80v 8600micro farads and the 4 you have circled in red. But the issue remains unchanged. Could you offer any more advice and maybe even a schematic?
I have the almost the same subwoofer amp on the M&K VX850. My subwoofer plays audio, just not quite right. In the very low range, 20-40hz, it’s a bit intermittent.
Looking at the picture you posted with the voltages and arrows, I have a problem with the orange arrow. I have 12v DC but also a 15v AC ripple. I have already changed the main capacitors 80v 8600micro farads and the 4 you have circled in red. But the issue remains unchanged. Could you offer any more advice and maybe even a schematic?
Hello Bolserst,
I have the almost the same subwoofer amp on the M&K VX850. My subwoofer plays audio, just not quite right. In the very low range, 20-40hz, it’s a bit intermittent.
Looking at the picture you posted with the voltages and arrows, I have a problem with the orange arrow. I have 12v DC but also a 15v AC ripple. I have already changed the main capacitors 80v 8600micro farads and the 4 you have circled in red. But the issue remains unchanged. Could you offer any more advice and maybe even a schematic?
Hmmm...is it possible that it is 15mV AC ripple instead of 15V? If it was 15V you would have a very loud hum coming from the sub even with no input signal applied.
Can you describe your 20-40Hz issue in more detail? Do you have a signal generator where you can input a constant 30Hz tone and slowly turn up the volume to see what is happening? I'm wondering if it is an issue with the M&K Limiter/compressor circuit on the daughter board attached to the pre-amp PCB.
I don't have a schematic for the M&K amp, only the Parts Express Amp which has essentially the same power amp circuit. It is the preamp that is different/improved on the M&K.
Can you describe your 20-40Hz issue in more detail? Do you have a signal generator where you can input a constant 30Hz tone and slowly turn up the volume to see what is happening? I'm wondering if it is an issue with the M&K Limiter/compressor circuit on the daughter board attached to the pre-amp PCB.
I don't have a schematic for the M&K amp, only the Parts Express Amp which has essentially the same power amp circuit. It is the preamp that is different/improved on the M&K.
Attachments
I have a Parts Express #300-793 subwoofer plate amp with remote control that is the size you need (10-3/8" x 10-7/16") which I just got at the BAF raffle. Looks completely fresh and untouched NIB, and I haven't even plugged it in yet to check... Anyone need it?
Hello again. The route cause of the AC ripple is a poor quality multimeter, not the sub amp. After re-testing a fluke meter the AC ripple is not there. My bad for using cheap tools.
In the mean time I purchased an ESR meter and tested every capacitor on the power amp circuit. 2 of them were defective and another 2 were about twice the maximum recommended Ohms. I then replaced every electrolytic capacitors and put new thermal paste on the transistors. This has improved the sound level.
The 20 -40Hz issue is still present. It is inconsistent bass at low levels. For example when listening to Elvis - All shook up, these is a meant to be a bouncy base line. But it sounds chopped and interrupted rather than melodic. When I am playing music loudly this is less of a problem, if my amp plays more quietly sometimes the sub plays nothing or only the loudest notes. I use an app on the phone through my main hofi amp as a “signal generator“. what I have noticed is that the first half of A second the sound is louder than after this time. I have checked this with a decibel meter.
im thinking it is to do with the pre amp side. Using Hi level input or low level input makes no difference.
recently my HT sub stopped working with same simptoms. led on, auto off working perfect, PSU ok, output devices fine, etc. but no sound after applying signal.
after a lot of trouble shooting , finally i found that the input coupling cap (electrolytic) was damaged, may be a spike or something.
after changing that, its working perfectly again.
after a lot of trouble shooting , finally i found that the input coupling cap (electrolytic) was damaged, may be a spike or something.
after changing that, its working perfectly again.
I have checked with the switch in both ”Auto ON” vs “ON” and there are was no difference. I will look into the input coupling capacitor to see what I can find.
To show what I have been trying to explain, as a picture is better than words. I created a frequency generator that started at 20hz, going up by 1hz per second, up to 60hz. you can see between 20hz and 30hz there is almost no output, a peak output at 35hz, and pretty level after that.
