Otari MX5050 newbie questions

[jcmusic]

More questions to add to my newbie list:

1) Is it a bad idea to use a "dry erase" marker to identify the path that the tape makes over the heads? I recall reading this was a helpful debugging technique for head alignment somewhere on one of the R2R guides. I can't seem to find anyone else using this approach in any of the forums, however.

2) Is the Otari's 2-Track/4-Track switch's output contained within the head block? I am assuming that I don't need to worry about the output of each individual repro head if/when I directly wire to an external preamp. I find it strange that the 2T/4T switch doesn't appear anywhere in the MX5050 schematics (I haven't found it yet)....


Prior to testing out my deck, I had done a thorough cleaning (with CL-100 cleaner).  I will clean the pots and switches with contact cleaner later tonight.

Chris

p.s. I did happen to find a toe nail stuck in one of the shielded repro heads. It might be Magic Johnson's but the burnt-in wear pattern looks more like the image of the Virgin Mary. Do you want me to send it to you? ;-)

Chris,
As to question #2 no you don't have to worry about the switch, when you wire via direct.

Jay

[ironbut]

Hey Chris,

The 1/2 track-1/4 track switch is just above the heads in the headblock. If you have the same Mk2 manual I was looking at, it's illustrated on page 1-4. It may be that the rest of the manual is generic for the 5050. Some didn't come stock with the 1/4 track head but I would imagine the switch is there all the same.

[squasher]

Yes, you're right, the 2T/4T switch is in the Instruction Manual.

What I meant is that the switch is not shown in the schematic that I found somewhere on the web. The "Record Reproduce Amplifier; A1028.." schematic shows a generic connection to the repro heads but not the switch itself.

No big deal as Jay already answered this question. 

Incidentally, after removing the headblock and cleaning the switch and headblock connector (with contact cleaner), the source of my problem still exists. I'm not getting any output from the left channel. I'll need to dig further to find where the signal is getting dropped.

Chris

[Ki Choi]

Hi Chris:

You are in luck to have working right channel to use as your reference in troubleshooting the problem left channel.

You may already be thinking about the next step but if I was to dig in deeper, I would record a 1 Khz sine wave from a signal generator on to a tape using a working machine and play it on your problem machine while tracing the signal with an oscilloscope (or a Multimeter) from your tape head on down comparing your reference right channel and lookiing for same wave from the left channel, etc.

Unfortunately, there's a chance your left channel head might be dead...

[squasher]

I have a friend who has an alignment tape (from years ago). I was planning on using that to debug my machine's problem. The only thing holding me back is finding the time to do the testing.

Can anyone point to what the likely causes are for these issues (leaky electrolytics, blown transistors)?

To reiterate what I had said in an earlier, I plan to hardwire the head outputs to an external preamp. With that in mind:

  1) What's the likelihood that something is wrong with one of the repro heads on my deck? I assume the probability is low.
  2) What's a good wire to use for the connection between the heads and the external preamp? Shielded twisted pair? coax? Does it matter?

Chris

[ironbut]

Hi Cris,

Having a completely dead playback head is pretty unlikely. In fact, if you're getting on sound from the one channel with the head selector switch in both the 1/4 track and 1/2 track position effectively doubles the odds that it isn't a head problem.
What's much more likely is that the wiring somewhere between the switch and the output amps has a problem.

Before you or someone else starts to troubleshoot this problem, be aware that the voltages going into the record head and out of the playback heads is very low. It's not a good idea to connect a multimeter to either of them to check continuity since this applies a voltage much higher than the heads are designed for. It will magnetize the heads and it will be tough to demag them afterwards.

 I don't know how much experience you have with this kind of stuff but after you're sure that it isn't a switch, pot or the tape, I'd check to be sure all the internal connectors and circuit card connectors are clean and well seated. Many times, the cards will slip a tiny bit in shipping or something and the connection will be sitting on a totally oxidized spot. Moving them in and out a little often times solves this problem for a while. Be careful with the cards since some can become brittle with time.

 If you're able to follow the circuit coming from the head selector switch, follow that till it gets to the playback amp section. If that connection is made with a multi-pin connector, try unplugging and plugging it back in a couple of times. If that has no effect, you should be able to make a little troubleshooting jumper system. Just take some uninsulated wire of about the same size a the male pins on the board (probably about 20AWG) and put them into the female connector. Get some jumpers with small hooks on each end (Radio Shack) and connect one end to the wires sticking out of the female plug and connect them to the correct pins on the board. Figure out which ones are the signal wires from the switch and exchange them left to right. If the signal follows the hook/clip, it's almost certainly the switch and it would be a good idea to take the switch apart and clean or replace it. If not, and you end up with no working channels when the jumpers are switched, try cleaning all the connectors leading out of the amp board.

Give that a try and get back to us.

[squasher]

Thanks for the helpful advice Steve.

I cracked open my deck and reseated all the internal connectors (after applying Deoxit 5). I don't know if that was the missing ingredient but, thankfully, I'm getting some music out of both channels. I'm getting full output when in SRL (Standard Reference Level) mode but there's a level mismatch somewhere in the signal path when using the Output Level Pots.

Given that I bought an Eros Preamp earlier today, I'm satisfied enough that the transport is working and will concentrate on getting the motor controllers tuned up (replace leaky electrolytics) and the tape path working as smoothly as possible.

Chris

[ironbut]

Hey,
Good deal Chris!
The remaining problem is almost certainly the output pots themselves, the connectors leading to them or the SRL switch. When the machine is in SRL, it bypasses the pots and the signal gets routed to a set resistor value (voltage divider).
Getting in there and seeing where stuff is and the signal paths will come in handy when you "hot wire" the heads (most folks wire directly from the head selector switch to back panel output jacks. I really helps to demystify the whole thing. Just be sure and use some nice shielded wire.

The Eros sounds way above it's price range. I really think it's the "sweet spot" in available tape preamps and the design of all of the Bottlehead kits really lends itself to modification on down the road.

Enjoy!

[squasher]

Does anyone know where I can get the (preferably free) MX5050 B2HD Service/Maintenance Manual ?

I am not looking for the Instruction Manual, the schematics, or Parts Lists (I have all of those). I am looking for the manual that specifies technical details (proper tape tension, adjustment screws on the head block, etc.). The Instruction Manual doesn't contain this information.

Thanks in advance,
Chris

[progdrums]

Hi Chris and Mr IronBut :)
I have a Otari 5050BII-2 and desperately need schematics as the transport is dead. Does this unit need the dummy remote plug installed to allow the transport to function?
Any help would be greatly greatly appreciated as I am really friggin stuck here
Cheers

[vector]

So, how is yout project going? You seem to have gotten lots of response. I consider you pretty brave taking on the project. I came he in hope of finding out how difficult my problem isl  My Otari 50-50 has been sitting for 3 or 4 years. Play back is great and the 15 ips works great and is ON SPEED.

However, 7 1/2 just sits dead as a doornail. I can hardly turn the capstain can hardly be turned by hand.

How daunting do you think that problem might be to tackle?

Victor

[sound signal]

Greetings,

My Otari 50-50 has been sitting for 3 or 4 years. Play back is great and the 15 ips works great and is ON SPEED.

However, 7 1/2 just sits dead as a doornail. I can hardly turn the capstain can hardly be turned by hand.

I don't know about Otari, but similar problems with Revox (high speed OK, low speed - capstan is stuck, especially after the machine has been sitting unused for some time) are usually due to capstan shaft lubrication issues (lubricant dried out or congealed).

The higher motor torque at the high speed setting overcomes the stiction and gets the capstan going.  Lower motor torque at the low speed setting can't overcome the stiction and the capstan remains stuck.  The fact that you can't turn the capstan by hand seems to confirm this diagnosis.

Straightforward disassembly, cleaning and relubrication of the capstan shaft and bearing should cure it.

Best regards.