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Author Topic: Otari mx5050 bII - constant hum - potential PSU issues?  (Read 1505 times)

Offline thraxeh

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Otari mx5050 bII - constant hum - potential PSU issues?
« on: November 29, 2014, 01:35:16 AM »
I picked up my first RTR a few days ago an Otari mx5050 bII

I grabbed it for very cheap as it was advertised as "requires repairs" as i was after a project to see if I could get this up and running again. I checked it out and it looked complete - I got it home powered it up and only the capstan was spinning - so I replaced all fuses and now it appears all transport controls work, all reels spin and the breaks/solenoids work fine. So far so good. I also deoxit all switches and connections.

The next issue is that the meters peg as soon as I turn it on and there is a constant hum on the headphone output - I have yet to plug anything into it other than power and headphones. The hum doesn't change at all when I turn any input or output knobs, and I can't hear the test tones or any kind of change when I switch the monitoring switches etc.

I measured DC across all Pins on the inputs and outputs and came up with some interesting results

Both Line Outputs
Pin 1: 10mv
Pin 2: 1v
Pin 3: 3v

Both Mic Inputs
Pin 1: 16v
Pin 2: 10mv
Pin 3: 500mv

Both Line Inputs
Pin 1: 1.2v
Pin 2: 10mv
Pin 3: 1.2v

My main concern is 16v on Pin 1 of the Mic inputs - this doesn't seem right, or maybe this is expected? At this point I don't want to plug anything into the inputs or outputs as I appears something is wrong.

As it is occurring consistently across left and right channels I guess I have to focus on the power supply - is there any common points I should check? I'm going to check the voltage rails and see that they are consistent.

Offline thraxeh

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Re: Otari mx5050 bII - constant hum - potential PSU issues?
« Reply #1 on: November 29, 2014, 02:03:16 AM »
My -18 rail is only showing ~200mv

Need to investigate further!

Edit: further testing...

While testing around the -18v rail - I accidentally shorted the vreg chip with my multimeter and it ended up fixing it?!

Anyway, time to clean the heads and test this further

*** Cleaned heads and put first reel on - recorded some acoustic and vocals through mic inputs - works well so far.
« Last Edit: November 29, 2014, 03:28:22 AM by thraxeh »

Offline thraxeh

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Re: Otari mx5050 bII - constant hum - potential PSU issues?
« Reply #2 on: December 06, 2014, 11:51:04 PM »
So my Otari is going well. I will just add more information as I go on...

The heads were worn with grooves, so I lapped them and the tape lifters. Made a HUGE difference to high freq consistency on the 2nd channel. Also revived the pinch roller rubber by sanding away the smooth layer and cleaning with soaping water - brought back some of its grip - its now usable, but I have a replacement on order which I think is necessary.

waiting on some MRL tape to arrive so I can calibrate it properly. having a blast though, should have got into tape sooner.