Help With OTARI MX 5050 BIII - 2. How do you Switch to 15ips?

[J_West]

Have an Otari MX 5050 BIII-2 Reel to Reel that runs at 3 3/4 & 7 1/2
ips. We need it to run at 15 ips. Does anyone know how, or if, you
can switch speeds?

The BII units have an internal switch on the PC board under the back
lid, but not on this one. The Otari Website lists a low and high speed
version. But it's not clear on how this is indicated on the unit.

Thanks for any help.

[William Sommerwerck]

That's surprising, as the speed change components add little to the unit's
cost.

I'd suggest opening the unit, and looking carefully. I can't believe it's
not there -- it was probably moved from II to III.

[Henry Howard]

Is it a Mark IV ?
Manual can be found at http://www.alta-publishing.com/forums/otari-manual-MX5050BIII.pdf
See page 12 (pdf page 14)

Normally, the MX-5050BIII-2 is shipped from the factory with set at
High Speed (15/7.5 ips).
If you want to change this to Low Speed (7.5/3.75 ips), follow the
steps below.

1. Remove the bottom panel to access the CONTROL PCB. Rotate the
CONTROL PCB.

2. The Speed Version Select SW1-1 is located on the CONTROL PCB.
Change
the switch position to the LOW position. (☞§2.3.)

3. Replace the bottom panel on the machine.

4. Make all necessary adjustments (Reproduce EQ, SRL, Bias Record EQ,
Record Level) referring to the corresponding explanation in Section 7.

NOTE: The MX-5050MKIV-2 cannot be changed to Low Speed.

[Scott Dorsey]

I think there is a switch on the control board which changes the speed
between high and low speed variants. You cannot have more than two
speeds on the machine.

When you do this, I think there is also some re-strapping you have to
do on the individual audio cards to make the emphasis and de-emphasis
frequencies correct. I don't recall the details but if you pull the
cards you should see it if it's there.

You really need to get the service manual for the exact variant of
the machine you've got. Otari may still have one, and it won't be
cheap, but it will be worth it.

Also, of course, once you have changed the speeds you need to do a full
alignment at the new speeds and set everything right. Be sure to run
a full tone ladder so that you can assure yourself that all the EQ
settings are right. If you have the EQ constants wrong, you may be
able to line the thing up so that the response at 10KC, 1KC, and 100C
are all correct but the response nowhere else on the chart is accurate.
The tone ladder will let you know where you really are. Do it both from
the alignment tape and from an oscillator in record, so you know both
record and play are individually correct.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[J_West]

Thanks a Million Howard! Got 'er done....... and thanks Scott for the
other setting and alignment considerations. Will check those.

That manual is not easy to find (nor are those dip switches on ther
BACK of the pc board. tiny suckers) ........ Had the seller of this
unit included the manual as promised, I wouldn't have had to come
crying to the group. :)

J West

[William Sommerwerck]

> When you do this, I think there is also some re-strapping you
> have to do on the individual audio cards to make the emphasis
> and de-emphasis frequencies correct. I don't recall the details
> but if you pull the cards you should see it if it's there.

No. There are FETs that switch the resistors. Moving the speed switch turns
the FETs on and off as appropriate.

The playback EQ might be wrong. On mine, the playback response of an MRL
tape was incorrect. When I looked at the circuit, it was "obvious" the
resistor value for the higher-frequency time constant was wrong. Replacing
it with the theoretically correct value flattened the response. Otari, of
course, said I didn't know what I was talking about.

This change isn't needed if you don't play back commercial recordings, or
recordings made on other machines. But it will give you about 1dB (!!!) more
HF recording headroom.

[Scott Dorsey]

J_West <johnst...@hotmail.com> wrote:
>Thanks a Million Howard! Got 'er done....... and thanks Scott for the
>other setting and alignment considerations. Will check those.
>
>That manual is not easy to find (nor are those dip switches on ther
>BACK of the pc board. tiny suckers) ........ Had the seller of this
>unit included the manual as promised, I wouldn't have had to come
>crying to the group. :)

Otari still has the manuals in stock. They are not cheap. While you
are ordering it, get a new pinch roller too because odds are you will
need it....