Mackie ms 1202 mixer problem - Page 1
Forum section: electronics / repair

author: diskosteppas
author_rank: Contributor
timestamp: January 26, 2021, 08:58:53 pm
content: Hi all I have a non vlz mackie ms 1202 and I have a problem with the last stereo input line channel.It passes audio in the L (mono signal) jack insert but only on one side. Right jack insert passes faint audio. I have cleaned, replaced opamps, regulators and psu capacitors. I have not found a service manual and although I have some diy synth building experiece ive never worked on a mixer before. Please could anyone help me get this channel working at all?

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: January 26, 2021, 09:13:05 pm
content: Service manual (such as it is) attached.

author: diskosteppas
author_rank: Contributor
timestamp: January 26, 2021, 09:29:53 pm
content: Thanks Jonny I had a look as this is the same pdf I found online but I thought I needed a schematic which is not included. All the other channels work perfectly. Does the symptom not inspire any place to on the board to look please?

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: January 26, 2021, 09:52:44 pm
content: If it's like my unit (with fewer channels), if you plug something only into the left of a stereo input, it behaves as mono and so should come out on both channels in the mix. You can use that to help start tracking down the problem. i.e., it could be the right input socket is damaged or the output from that channel's right op-amp is faulty. If you connect just a left signal (i.e., physically nothing in the right socket), you ought to get one of two things: audio coming out on the left and right (since it is acting as a mono input) or nothing on the right. In the former case, it would point to a physical connection problem or a problem in the right channel input stage. In the latter, it would suggest an electronic problem after the input stage.Additionally, with the power off, I would make resistance readings throughout the circuits for two of the good stereo channels and compare to the faulty one. I say two since that will confirm if you ought to get similar readings for each channel.

author: diskosteppas
author_rank: Contributor
timestamp: January 26, 2021, 10:19:24 pm
content: Thanks again for the replyleft channel should be putting out in both channels but this is not the case at the mo. Right channel just so faintly hear the signal. Ive pulled the plug so will have a probe! So sorry for the questions it really does give me a much nicer stereo than just my soumdcard and love to have it working again

author: diskosteppas
author_rank: Contributor
timestamp: January 30, 2021, 07:47:41 pm
content: Hi all what if I measured a square wave from oscilloscope and trace the signal towards the ouptput till it goes wonky on the scope? Or maybe from the output until the square shows on scope?Never used a scope tho I have a 2mhz one donated

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 30, 2021, 08:11:29 pm
content: Put a signal into the mono (L) socket.Then scope the channel fader to see whether both sides are gettting to the fader and the fader isn't faulty one side.If only one side is getting to the fader, scope the gain pot and see if the stereo gain pot is getting both sides.This will narrow down your possibilities enormously in a couple of minutes.

author: diskosteppas
author_rank: Contributor
timestamp: January 31, 2021, 04:15:35 pm
content: Thanks so much for the help I have taken my first steps to using a scope! I put 1 khz test signal into the mono insert. In the good channel gives readings across the stereo channel gain pot and the stereo main output pot.Putting the same signal into the faulty channel I get the 1khz in the stereo channel gain pot but only one side in the main output fader.I have a schematic for the mixer now!I have been referring to channel8 on page 3 could I please have any guidance suggestions from here?

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: January 31, 2021, 05:47:51 pm
content: My suspicion is that U502A or U501B is dead (or both) for that particular stereo channel or polarized capacitor C516 could have died.I would systematically probe the left- and right- circuits with the test signal going to both until you find a significant difference.Hopefully, the amp chips are DIP-8 package and not a surface mount variety:https://www.mouser.com/ProductDetail/NJR/NJM4560D/?qs=cYKsvIpO1PiW/gHTlBnKAg==

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 31, 2021, 06:40:50 pm
content: If an op-amp goes faulty, it invariably has full DC on its outputs, either +15v or -15v.The circuit diagram you posted appears to have incorrect pin designations on the op-amps, however.It shows the outputs to be pins 2 and 8 which is not correct, they should be pins 1 and 7, like most every other 8-pin dual op amps in existence.https://www.mouser.com/datasheet/2/294/NJM4560_E-365030.pdfI actually think, from the diagram, that the op-amps are probably OK, since a blown dual op-amp usually has both outputs on DC.This design (stupidly IMHO) uses 2 dual op-amps where each op-amp has one half powering the left side, and the other powering the right, so should this op-amp fail both left and right are likely to dissapear.Anyway, forget the pin numbering issue for now, the safest thing to do is to find the 2 capacitors on the op-amps outputs, C715 and C716, and probe there to see if the opamp has a DC output, and/or this is where the signal disappears.I suspect C716 is faulty.

author: diskosteppas
author_rank: Contributor
timestamp: January 31, 2021, 07:19:58 pm
content: This was absolutely on the money, I did wonder why I could see the test tone at the minus point of the capacitor around this point!! Thank you so much It's a FIX!!As for the opamps yes I was so confused. The mixer uses a sip9 package with some pins connected- which I do not know which ones and data sheet was not available.I cannot believe the support I have been given. My sound has just got that much bigger thanks to all!!Thank you so much now to bolt it back together and enjoy my much needed mixer summingBUZZING!!!