Micro-trak Sport 5

[techie]

Any sources out there where I might find a manual for a Micro-Trak Sport 5 remote mixer?

I have one that is functional but has a bit of a hum when the mic inputs are potted up. Thinking that it was poor rectification in the power supply section, I replaced the caps in there but it didn't solve the problem.

The unit appears to have been modified by a previous owner. Tracing it makes it look like the headphone input selector has been bypassed and fed straight off the phone talkback. Something in this mod may be causing the problem but I'm not sure. Would like to find a schematic or something if possible.

 

[Chuck]

> Any sources out there where I might find a manual for a
> Micro-Trak Sport 5 remote mixer?
>
> I have one that is functional but has a bit of a hum when
> the mic inputs are potted up. Thinking that it was poor
> rectification in the power supply section, I replaced the
> caps in there but it didn't solve the problem.
>
> The unit appears to have been modified by a previous owner.
> Tracing it makes it look like the headphone input selector
> has been bypassed and fed straight off the phone talkback.
> Something in this mod may be causing the problem but I'm not
> sure. Would like to find a schematic or something if
> possible.
>
If you find one, please let me know. We have one which exhibits the same problem. Somebody has added a couple of larger filter capacitors on the output of the power supply, but I'm doubtful that they helped a lot.

Part of the hum seems to be inductively introduced through the Beyer input transformers, or perhaps just from awful circuit board layout and non shielded hard wiring. I've been intending to dig into this project, but a schematic would certainly help.

Although many people have suggested the dumpster for these things, I'm willing to give some modifications a try.

Mysery loves company.

Chuck
KZQX Radio

 

[techie]

> > Any sources out there where I might find a manual for a
> > Micro-Trak Sport 5 remote mixer?
> >
> > I have one that is functional but has a bit of a hum when
> > the mic inputs are potted up. Thinking that it was poor
> > rectification in the power supply section, I replaced the
> > caps in there but it didn't solve the problem.
> >
> > The unit appears to have been modified by a previous
> owner.
> > Tracing it makes it look like the headphone input selector
>
> > has been bypassed and fed straight off the phone talkback.
>
> > Something in this mod may be causing the problem but I'm
> not
> > sure. Would like to find a schematic or something if
> > possible.
> >
> If you find one, please let me know. We have one which
> exhibits the same problem. Somebody has added a couple of
> larger filter capacitors on the output of the power supply,
> but I'm doubtful that they helped a lot.
>
> Part of the hum seems to be inductively introduced through
> the Beyer input transformers, or perhaps just from awful
> circuit board layout and non shielded hard wiring. I've
> been intending to dig into this project, but a schematic
> would certainly help.
>
> Although many people have suggested the dumpster for these
> things, I'm willing to give some modifications a try.
>
> Mysery loves company.
>
> Chuck
> KZQX Radio
>

I'm thinking about just wiring a DC power supply through the battery connectors and seeing what that does. The only thing I need to figure out is whether the batteries are in series or parallel.

 

[Chuck]

> I'm thinking about just wiring a DC power supply through the
> battery connectors and seeing what that does. The only
> thing I need to figure out is whether the batteries are in
> series or parallel.
>

An external power supply won’t make it quieter, or at least, it shouldn't. Batteries should be a cleaner DC supply than any external power supply you are likely to find. This unit uses a split power supply giving plus and minus 18 volts. Two batteries are wired in series to give +18 volts; two are wired in series giving minus 18 volts. The common point is wired to chassis ground. On mine, the batteries connect to the PC board via a three pin Molex connector. If you try substituting the supply, make sure you use a split supply that has a common, as well as a plus and minus 18 volt output. Otherwise, you are likely to blow up some IC's.

The mixer we have has both a 120 V AC supply and a battery power supply. Our unit is better when running on batteries, but still is very noisy. It still picks up stray hum, especially when the inputs are un-terminated. Of course, with no mic plugged in, there is little need to have the pot turned up.

The noise is probably due to a number of factors, including poor decoupling between stages, and poor grounding techniques. When it is powered from AC, the hum greatly increases. I believe that also has multiple causes: Poor power supply regulation and/or impedance (a very low impedance power supply can mask a lot of other problems), poor or non-existent decoupling between stages, inadequate grounding, poor transformer shielding, etc. Most likely, it’s a combination of “all of the above.”

The hiss is also fairly objectionable as well. Upgrading to new IC’s might help. I’m not sure what it is supposed to use. The one we have has been “rigged” by a number of previous engineers, so I’m not positive what the manufacturer had in mind. It has a variety of op amps in the sockets. A schematic would be really handy, if for no other reason than to see what things are supposed to be.

Of course, it may have always been a POS. It was intended to feed a 3 KHz phone line, and you could always blame the crummy audio back to the studio on the Telco. That’s hard to do when the noise in your headphones is enough to drive you nuts. The best cure may be placing it firmly into the dumpster.

Chuck

 

[techie]

> > I'm thinking about just wiring a DC power supply through
> the
> > battery connectors and seeing what that does. The only
> > thing I need to figure out is whether the batteries are in
>
> > series or parallel.
> >
>
> An external power supply won’t make it quieter, or at least,
> it shouldn't. Batteries should be a cleaner DC supply than
> any external power supply you are likely to find. This unit
> uses a split power supply giving plus and minus 18 volts.
> Two batteries are wired in series to give +18 volts; two are
> wired in series giving minus 18 volts. The common point is
> wired to chassis ground. On mine, the batteries connect to
> the PC board via a three pin Molex connector. If you try
> substituting the supply, make sure you use a split supply
> that has a common, as well as a plus and minus 18 volt
> output. Otherwise, you are likely to blow up some IC's.
>
> The mixer we have has both a 120 V AC supply and a battery
> power supply. Our unit is better when running on batteries,
> but still is very noisy. It still picks up stray hum,
> especially when the inputs are un-terminated. Of course,
> with no mic plugged in, there is little need to have the pot
> turned up.
>
> The noise is probably due to a number of factors, including
> poor decoupling between stages, and poor grounding
> techniques. When it is powered from AC, the hum greatly
> increases. I believe that also has multiple causes: Poor
> power supply regulation and/or impedance (a very low
> impedance power supply can mask a lot of other problems),
> poor or non-existent decoupling between stages, inadequate
> grounding, poor transformer shielding, etc. Most likely,
> it’s a combination of “all of the above.”
>
> The hiss is also fairly objectionable as well. Upgrading to
> new IC’s might help. I’m not sure what it is supposed to
> use. The one we have has been “rigged” by a number of
> previous engineers, so I’m not positive what the
> manufacturer had in mind. It has a variety of op amps in
> the sockets. A schematic would be really handy, if for no
> other reason than to see what things are supposed to be.
>
> Of course, it may have always been a POS. It was intended
> to feed a 3 KHz phone line, and you could always blame the
> crummy audio back to the studio on the Telco. That’s hard
> to do when the noise in your headphones is enough to drive
> you nuts. The best cure may be placing it firmly into the
> dumpster.
>
> Chuck
>

This one uses the same +/- 18 volts. I looked at the traces on the circuit board.

It also appears to have been "rigged" with the phone line talkback bypassing the monitor switches. The problem I was having was that I couldn't regulate the headphone volume very well, I think it was attached at the wrong point on the pots. It also looked like the null control had been disconnected.

I think I will either turn it into dumpster food or use it as an "if all else fails" spare. My other phone mixer solution works better though it's not a 1-box solution (Remotemix 3 + Shure M267 + Rolls Headphone DA).

 

[Chuck]

> I think I will either turn it into dumpster food or use it
> as an "if all else fails" spare.

If you decide to turn it into dumpster fodder, I'll pay the shipping to throw both of them into our dumpster. I might be able to get one good working version out of two of them. We're a 501(c)(3) noncom. You can email me at kzqxlp (at) aol.com if you take out the spaces and substitute @ for "at".

Chuck