Otari MTR-90 MKIII Tape Path and Nonerasure !!! SOLVED !!!

[digitall2000]

hi
i am rebuilding an otari mtr-90 mkIII, the one in - Otari MTR-90 maintenance 2 (too)
so now that the transport is working fairly well (i think we have a small MDA issue)
we have 2 channels that are not erasing
as of friday last when i had to leave, the situation was that the 2 audio cards were receiving
erase signal from the main bias board (85khz sine wave) but the erase signal was not being
passed on to the erase head
this erase issue is following the 2 specific cards
there is a relay involved and also at least a half a dozen logic points to send the signal to the erase head

so i was wondering:
1.  if anyone has had the same issue with these machines?
2.  what is the usual culprit here, the relay or the logic?
3.  does anyone know the specs on the relay it is an  OKI  F51A05D? 
      it is a single line relay but i don't know the voltage i have found 3 for sale but absolutely no specs

the other thing i was hoping someone could enlighten me on is the use of "newer formula" tapes
my client wants to use G9 and the likes so again i am wondering:
1.  do i need to apply more tension to the thicker tapes?
2.  do i need to apply more erasure current (otari's term) to the thicker tapes?
any experienced users here?

thanks for any insight you can offer
sincerely,
lance

 

[Matt C]

Regarding your tape formulation question, FWIW I've used Quantegy 499, GP9, and RMGI 911 on my machine and it handled them all reasonably well without any adjustments. Granted I did not get down to measuring the extreme nit picky details with each tape, but I had no noticeable issues.

No clue about the erase issue but couldn't you just do some signal tracing to see where the erase signal is being interrupted?

 

[digitall2000]

Thanks matt

yeah signal tracing is what  i am going to do today

the other thing i forgot to mention  regarding the tape tension is that the return to zero is not good
if i play the machine for about let say 6 minutes and hit the locate zero button i loose about 3/4' on my zero mark

lance

 

[Matt C]

the other thing i forgot to mention  regarding the tape tension is that the return to zero is not good
if i play the machine for about let say 6 minutes and hit the locate zero button i loose about 3/4' on my zero mark

nine inches off doesn't seem too terrible, but the advice I've gotten from other forum members here was that this is caused by tape slippage on the capstan roller. Which might mean your rollers should be replaced if they are old.

If you look in the manual's transport calibration procedure, it mentions dealing with this type of slippage in a section called "6.1.9.5 Fine Adjustment of Position and Gain"

 

[Winetree]

Unless it's mechanical adjustment;
Are the rollers the original brownish Otari Rollers/ Are they glazed?
IF so it's the rollers.
The Athen REDS are good.

 

[digitall2000]

first of all thank you guys

matt that was suppose to be 3/4" not 3/4 foot,  so 3/4" would be very acceptable, but
that does not seem right to me.  on a system like this i don't think there should be so
much drift on just one pass.  but then again these are the otari brown rollers.  i have
"deglazed" then with 1000 grit attached to a piece of melamine and then cleaned them
anything else i might do to revive the rollers?

still again does anyone know of a logic table for the rec/erase procedure?
it is a lot more complex/convoluted that i thought it would be

thanks again
lance

 

[gyraf]

A classic problem with the high-output tapes is the  tape slippage on MTR90/100 type pinchroller-less drives: For high-output tapes it is necessary to apply an extra-slippery tape backing coat in order to ensure that the tape moves just-slightly when resting on the reels - in order to keep print-through under control. Had a lot of fights with 3M over this - and it seems the only way to fix it is upping tape tension - greatly increasing head- and tape path wear..

On topic: Suspects for not erasing on MTR90, in approximate order

Head dirt
Head Connector
Safety relay
The inductor in series with the erase voltage, after the erase buffer

 

[digitall2000]

thanks for the insight gyraf

can you clue me in on how much tension is appropriate?
should i raise the mechanical tension of the swing arm?
should i raise the gain on the reel control card?
both?

on the erasure problem
the problem is following the cards when i swap them out
so that means that it is not heads or connectors, correct?

on the inductor, the only inductors i see in series are the ones on the +- 20v supply rails
then there is one across the erase signal and ground at the input of the circuit
and one across the signal to ground after the first buffer amp  IC401
is IC401 being turned off/on by the signals at points I and J ?
because it is not just the relay controlling the flow of the erase signal

on the edge card connector "fingers"/ pins could someone clue me in on
what these mean, especially #28

#12  CUE AT
#15  LOW COMP
#27  READY
#28  P.  ON  C.
#33  ODD EVEN
#34  INHIBIT
#37  REC HOLD

thanks again
lance

 

[pucho812]

Have you swapped the cards around to see if the erase fault follows the card?
Are the trim pots working?  I used to tech at one of the local audio schools and the biggest mtr 90 problem we ran into was people over turning the trim pots and breaking them.

 

[digitall2000]

yes i swapped the cards and the issue follows the card
and the trim pots are working

the erase signal is getting to the card but not through it
relay maybe, but you can't just short the relay, i tried that with a working card and it would not pass signal
there seems to be something else  controlling the erase signal along with the relay

these three cards didn't want to bias correctly
also i fergot to mention that the cards seem to record
i record a tone and record another tone at a different frequency
i get a modulated tone --- wasn't listening just viewing on a scope

just hoping someone has had this problem and had a quick fix
haven't been able to allocate to much time to this lately

thanks for the suggestions
lance

 

[pucho812]

well the upside is you know it's on the card. The downside is you know it's on the card.
So at least we have the fault isolated.

 

[gyraf]

This section rather different from MTR90 mkI/II..!

IC401 looks like some sort of OTA, acting on control voltages from inputs I, J through IC515

I would check for signal presence at C402 (input), C405 (after OTA, before driver opamp), and C412 (output from driver)

If these are present, check signal after relay, or possibly short relay out to test (but note that your channel now is in permanent erase!!)

Last, with card in circuit (including path to/from erase head) check continuity from relay (output) to  R421-R422 (erase current measuring resistors).

Note, that if erase current return path through  R421-R422 (30 Ohms total) is not connected and going properly into gnd, no erase current will flow through your head.

/Jakob E.

 

[digitall2000]

thank you gyraf

not sure if i can get back to the mtr today
but i will let you know of any progress

the record relay is 5V, right
what should the contacts be rated at
tried google and found no info

this is very good insight
here on the west coast very few people have experience with the mkIII
and from where we are they are hundreds of miles away

thanks again
lance

 

[digitall2000]

audio card erase section

 

[digitall2000]

IC 401 trans amp  --  LM13600

 

[digitall2000]

so now let me see if i can do this justice

the LM13600 is a transconductance amp  (current controlled amp) 
an OTA as gyraf pointed out

"IC401 looks like some sort of OTA, acting on control voltages from inputs I, J through IC515"

on the cards that fail to erase the "safety relays" are operational
the "current path" through the erase head to ground is good
the common point of erasure failure is IC401
if IC401 is current controlled i do not know how to figure this out

dc voltages on the pins of IC401, at rest, only one side being used
9    -1.16
10    0
11  +14.7
12    0
13  +.23
14  +.23
15  +.88
16  -14.2
so when you go into record the only dc voltage  that changes is
the one on pin 16 which drops (rises?) to -13.4V,  the amp opens
up and passes the erase signal to the IC402 and on to the relay

so up stream a little
i'm going to use top and bottom and bottom being closer to IC401
i.e. the junction of D509 and D510 is the top of R404
the bottom of R404 is the bottom of R423 and  pin16 of IC401
if this is wrong please let me know

at rest IC515 inputs pins 3 and 5 point I and J respectfully are at -13.8V
and so are out puts pins 5 and 7

when you go into rec mode point J goes to +11.7V and hence the output
pin 7 of IC515 rises to +11.7V so the the top of R404 is at +11.7V and the
top of R423 is still at +14.2V and now the junction of R404, R423 and
pin 16 of IC401 is at -13.2V and the amp passes the signal on to IC402

could some one explain this to me

on all failures to erase the amp IC401 is not passing signal to IC402
one card where the voltage on pin 16 0f IC401 is about -1.4V and that
card passes very little signal to IC401
one card has no erase signal on pin 9 but it does have erase signal on pin 15

what would have caused that?

and on being an OAF i accidentally touched my probe to pin 15 and 16 0n IC401 at the same time
would that kill the amp?

thanks for any and all advice in advance
lance

 

[abbey road d enfer]

Pin 16 is the base of an NPN Darlington, that controls  current in the input LTP and in the single-ende output stage. The voltage there cannot be more than 2Vbe above the negative rail. If it is, that means one of the junctions is shot.
I'n certain you need to replace this LM13600 (or re-wire to the unused half ).
Touching it with a grounded probe is guaranteed to kill it.

 

[digitall2000]

thanks abbey road d enfer

i also wanted to ask if i should be wearing a ground strap while working on this circuit?

and how do you figure what voltage you need for the amount of gain you want?


lance

 

[abbey road d enfer]

thanks abbey road d enfer

i also wanted to ask if i should be wearing a ground strap while working on this circuit?

I wouldn't worry too much about it. Now if you have to work on the CMOS logic you may take the usual precautions. I have worked with CMOS logic quite often; I never wore a strap.

  and how do you figure what voltage you need for the amount of gain you want?

I don't understand your question... What voltage? Do you mean the voltage on pin 16? This pin is not voltag-drivene, it's current driven. There is a permanent pull-down of 30k going to the negative, that ensures current is nil when there is a logic low (-15) from the logic. When the logic goes high, it sends 15V into R404 (I think it's also 30k) resulting in 0.5mA into pin 16, which turns on the transconductance.

[/quote]

 

[digitall2000]

hi
finally back to this erasure problem
now i get the current driven idea
but what is the benefit of that over a regular vca ?

in this erase amp circuit there are small chokes in the psu rails
on one of the non working cards the neg. rail choke is open
would harm any thing if i jumper across the choke just to see if that card works ?
maybe using the same value resistor ?

thanks
lance