Noisy Otari MX5050 BII-2 Tape deck

[KilgoreCemetery]

As soon as the unit is powered on, there is a constant static noise in both channels. You can hear it through the headphone jack, as well as the output connection. It doesn't register on the VU Meters, but it also makes any playback with known good tape extremely quiet.

I've been using an audio probe to "listen" to where the noise is coming from, but get stuck at the preamp chip, IC501. There's a constant buzz in my audio probe, and when I turn up the volume to hear anything before the preamp chip, it gets drowned out by it. I did try swapping out the chip itself with one from a working unit, and even tried swapping out the socket that it goes in as a last-ditch effort. Neither change fixed the problem.

I've also tried unplugging as many wires as I can from the amp board in an attempt to isolate the problem, but it's constant unless I remove one of the plugs that supplies power. More notably, the noise is still there if I unplug all inputs and outputs, like tape heads, microphone, and line in.

Unfortunately, I don't have an oscilloscope, so the only other thing I can think to do is to start pulling components and testing them, but I'm concerned that even if I pull the bad part, it will still test good. Can something like a noisy transistor show up on a multimeter or an all-in-one component tester?

Is there another, possibly better, way of tracking it down? I do have a nearly-identical (same model) unit to compare with, but all the voltages I've checked so far are similar.

 

[kellys_eye]

You need to measure the signal-to-noise ratio (s/n ratio) and compare this with the specifications. It may just be 'what it is'.

 

[KilgoreCemetery]

I understand what you're saying, but the way it's running now, you can't hear anything but static even when the tape is playing. At max volume the static is louder than the playback signal. I have two of these Otari's and the tape plays fine on the other one. There's almost zero noise on the good Otari, even at full volume.

 

[kellys_eye]

Check the tape bias oscillator.

Check also the tape playback head (by replacement). Can you see any heavy wear on it?

 

[KilgoreCemetery]

Ah, that seems a bit over my head, but I've been doing some reading on it. Would the bias oscillator affect both channels evenly?

Would that be before the preamp typically?

I'll have to do a lot more research on it apparently

 

[kellys_eye]

Would the bias oscillator affect both channels evenly?

Yes

Would that be before the preamp typically?

Yes

I'll have to do a lot more research on it apparently

Yes

 

[KilgoreCemetery]

I've been using a schematic that I downloaded from HiFiEngine, but I don't really know what I'm looking for to identify the bias oscillation circuit. However, if it's on the board with the preamp and main amp, I'm pretty sure it's DC powered. There are only two circuits that go into the preamp IC.

The first is an array of 2SK105 JFETs and diodes that are powered by +18V and -18V that feed into the negative side of the preamp IC, and seem to be dependent on what speed settings are selected for playback/record. They also have some trim pots for adjustment. I'm assuming that this is the circuit I'm looking for, but other than part swapping, I'm not sure how to proceed. Is there a way to test for noisy transistors?

 

[kellys_eye]

I've been using a schematic

...that might be useful if it was posted here!

 

[KilgoreCemetery]

Yeah, unfortunately, the file is too large to upload here and snapping a screenshot will just make everything unreadable when you zoom in. HiFiEngine is a reputable website. They do require you to register, but everything is free and they have thousands of schematics and service manuals.

I've already pulled each of the JFETs and tried swapping them into a working unit one at a time to see if it causes the noise to appear, but it hasn't. I'm gonna spray the trim pots again and see if that makes any difference, but I still don't know how else to verify whether a component is causing the noise

 

[KilgoreCemetery]

Hopefully this will work. It only covers the board I'm working on.

The preamp, IC501, is in the upper left section. I'm guessing that the bias oscillator part is either under it, or to the left

 

[KilgoreCemetery]

I've also replaced the electrolytic capacitors with brand new Nichicon ones

 

[kellys_eye]

I find it difficult to read that schematic but can see that VR501 and VR502 set the oscillator levels.

 

[KilgoreCemetery]

Ah.. I see it. Not quite sure how that ties in to the preamp just yet. I'll have to study the schematic a bit more later.

I swapped IC506 with the one from the working unit and adjusted VR502 after marking where it was. Neither change affected the noise.

 

[KilgoreCemetery]

All the resistors in the oscillation circuit are well within tolerance and the diodes seem to be working fine. I tested them in circuit with my multimeter in diode mode. All the electrolytic caps in this section were replaced prior to discovering the noise issue, but I'm considering swapping them out in case I received a bad batch or something. I swapped out Q511, Q512, and Q503, and verified that the incoming voltage is about where it should be (+17.92 and -17.96 when SW506 is pressed).

The only other components in this area are the non-electrolytic caps, or just plain swapping out VR502. If it's not those, then I either missed something or am barking up the wrong tree entirely

 

[kellys_eye]

Have you checked the heads?

 

[KilgoreCemetery]

Visually, they're in pretty good shape. There are a few speckles, but little wear. I've cleaned them two or three times over the course of about a week and a half just to make sure they're good. Also, I can unplug where the heads connect to the amplifier board and the noise is still present. I should note that the noise is overwhelming even without anything playing or the capstan spinning

 

[kellys_eye]

Break the audio path at strategic locations to isolate the relevant sections (PA, preamp, pre-emphasis etc). Is it both channels? If so then the common denominator is the PSU.

 

[KilgoreCemetery]

It is on both channels. By breaking the audio path, do you mean jumper it to ground?

 

[kellys_eye]

No, disconnect the signal path (break it). You could short that point to ground to prevent stray pickup at the input point but for the higher level sections this isn't strictly necessary.

But since the noise is present on both channels you must approach this from a common point - either the PSU of any part of the circuit that is common to both channels (the tape bias oscillator is, as it happens). Start by applying a separate, regulated DC source to the circuit and see what, if any, changes occur (just to isolate the tape decks PSU as being the source of noise).

In the schematic you posted, one channel is shown in the top half of the page and there's a 'dotted box' illustrating the duplication of that top section as the other channel.

The LOWER half of that page schematic is the COMMON CIRCUITRY to both channels (for the most part) so whatever fault you have will lie down there......

There is a 'string' of FETs (Q106 - Q111?) used to set the channel gain level - check that the switching voltages are correct and that only one FET is on at any one time - damn, that schematic is hard to read!

 

[KilgoreCemetery]

I have not yet tried disconnecting the signal path to isolate the problem, but I had previously pulled, and tested the JFETs Q106 - Q111 using the working Otari. None of them caused the noise in the other unit. However, I did go ahead and test voltages on them today. With current settings (Low Speed, Small Reel, etc), all of them show the same voltages for the most part. If I'm reading the datasheet correctly, Drain and Source are both at 147mV, and Gate runs around -17V for all but two of them, Q110 & Q210, which have values ranging from -25mV to -30mV. It fluctuates a little.

Also, I don't have a DC power supply that I can hook up, so I actually set the good unit as close as I can to the bad one and just ran the wiring across. By doing this, I was able to use the power from the good unit and the "Rec and Repro Amp" board from the unit with the constant noise. This included six connectors (4, 6, 7, 8, 12, 30), plus a chassis ground wire that went from the power board of the good unit to the chassis of the noisy unit where the board in question is mounted. The noise was still there when I powered on the good unit.

For a second I thought that would rule out the part of the schematic that we've been looking at with the transformer, but I just realized that it does, in fact, have a cylinder-type transformer. I had previously assumed it was just another coil. I'll be checking over the rest of the power section of this board in a new light, I think.