SOLVED - Otari MTR-90 mk2 - a few things, mainly head shields - need schematics

[Emilblanca]

Hi folks,

So I bought an otari MTR90 mk2 recently from some hifi warehouse. They told me it was all working and they recorded a record on it - i thought , great, i wont have to do any work on it, good to go...yeah I was wrong, it has a few issues.
Waiting on new rubber rollers restored from Terry, as the orange-ish rubber had pretty much turned to marmalade. also the head shields are completely non op... Someone obviously tried to revive them by the looks of things, to no avail.
Has anyone got a full set of digitised schematics for these things? I have the service manual but it lacks anything useful finding schems seems to be difficult?

Particularly I need the head shield drive circuitry. I spent a while servicing the mechanism and it can lift and drop fine on a DC bench supply although it needs around 100mA...
I measured what the Otari is trying to pull to drive the little Canon motor and its only 25mA, doesnt seem to have momentum/startup torque to push. I spent all night sorting the mechanism out.

its 24volts so im guessing from the 24v PSU rail..so I could check caps etc (every cap ive tested on other areas has been bang on, although havent tested PSU caps, also havent tested any for leakage either)

I followed the wires from the small motor PCB (white with black dots) and found them to be leading to Pin 25 (one side at least) of Card 3 and 4 (transport control and master CPU). Also I found this, in the same wire grouping just behind the main machine cards, bias etc, behind the side panel. Not sure if its from factory or something weird? Cut wires? Thought it to be odd. can anyone confirm ?

Anyway yeah sorry long winded but I guess I would like schematics for cards 3 and 4, and anything to do with head shields circuit. the SM just talks about lubing the worm gear

 

[Emilblanca]

update - PB-7DDA board ( motherboard for main cards) has R1 absolutely toasted, right next to the relays that controls the Pos/neg switching for the shield motor up/down. toasted R1 measures 60R in cct. Relays tested fine. Really need to inspect downstream .. these relays have continuity with pin 25 on connectors on CN3 - transport control. Followed PCB traces to outputs on a uPA2004c chip - 16 pin darlington transistor array ic. only 2 on the board, i wonder if the other one is for the tape lifters next to Q7.

Thats as far as ive got anyway.

the whole process would be much easier with schematics

 

[cavedata]

Transport control card in Tech docs section - check upper right side of schemo for shield control

 

[Emilblanca]

this is the only manual i have. where are the schematics? which page? i dont see anything

 

[Mihobro]

Hi folks,

So I bought an otari MTR90 mk2 recently from some hifi warehouse. They told me it was all working and they recorded a record on it - i thought , great, i wont have to do any work on it, good to go...yeah I was wrong, it has a few issues.
Waiting on new rubber rollers restored from Terry, as the orange-ish rubber had pretty much turned to marmalade. also the head shields are completely non op... Someone obviously tried to revive them by the looks of things, to no avail.
Has anyone got a full set of digitised schematics for these things? I have the service manual but it lacks anything useful finding schems seems to be difficult?

Particularly I need the head shield drive circuitry. I spent a while servicing the mechanism and it can lift and drop fine on a DC bench supply although it needs around 100mA...
I measured what the Otari is trying to pull to drive the little Canon motor and its only 25mA, doesnt seem to have momentum/startup torque to push. I spent all night sorting the mechanism out.

its 24volts so im guessing from the 24v PSU rail..so I could check caps etc (every cap ive tested on other areas has been bang on, although havent tested PSU caps, also havent tested any for leakage either)

I followed the wires from the small motor PCB (white with black dots) and found them to be leading to Pin 25 (one side at least) of Card 3 and 4 (transport control and master CPU). Also I found this, in the same wire grouping just behind the main machine cards, bias etc, behind the side panel. Not sure if its from factory or something weird? Cut wires? Thought it to be odd. can anyone confirm ?

Anyway yeah sorry long winded but I guess I would like schematics for cards 3 and 4, and anything to do with head shields circuit. the SM just talks about lubing the worm gear

It may seem obvious, but I have had several decks with head shield problems caused by damage to the mechanism caused by having the shields down, and front doors closed and lifting or lowering the top. It is a good practice to always have the shields up when lifting the top.
Second, the screw lift is often bound by the 40 year old grease. Removing all the old grease and regreasing with a synthetic grease like Lobell sells, as well as lubricating the motor will reduce the strain on the drive circuit. Finally, the drive circuit, being burned out, would need to be repaired. Neglecting the load exerted on the circuit by the sticky mechanical parts could end up revising burned out drive circuits. Also check the top and bottom travel cut off switches as these can also cause the drive circuit to fail.
Usually the drive circuit resistors or diodes will burn first so you probably not have too bad a repair.

 

[Emilblanca]

i should have given an update on this - this is now solved. The problem was a burned out resistor next to the head shield relays on the back of the unit, next to the fan. diodes were all fine. i also replaced the motor with a modern dc motor i bought on amazon. I replaced 20 ohm power resistor with slightly higher value and wattage thanks to the motor replacement being a bit bulkier. oh , also to note was that the head shield sensor guides were not aligned, this is what was causing there to be the 24v left on the resistor and it burning up, so after i adjusted them, no problems.
that combined with ancient dry grease on the motor mechanism, i took the whole mechanism apart, cleaned and regreased.
head shields are now very strong and fast.

 

[JeffKall]

i should have given an update on this - this is now solved. The problem was a burned out resistor next to the head shield relays on the back of the unit, next to the fan. diodes were all fine. i also replaced the motor with a modern dc motor i bought on amazon. I replaced 20 ohm power resistor with slightly higher value and wattage thanks to the motor replacement being a bit bulkier. oh , also to note was that the head shield sensor guides were not aligned, this is what was causing there to be the 24v left on the resistor and it burning up, so after i adjusted them, no problems.
that combined with ancient dry grease on the motor mechanism, i took the whole mechanism apart, cleaned and regreased.
head shields are now very strong and fast.

Hi @Emilblanca …
You wouldn’t mind sharing the resistor upgrade specs and the manufacturer’s name and part number for your Amazon motor by chance?

The MTR90 III that I acquired came with the circuit board and motor base laying in the bottom of the machine cabinet from the top deck being opened with the meter bridge closed …

I was able to reassemble the motor base … and clean the shield assembly and apply fresh grease to the drive screw… but it still operates very anemically …

It sounds like your Amazon motor upgrade would solve it !

Thank you in advance !!!

Best regards !

 

[Emilblanca]

Hi @Emilblanca …
You wouldn’t mind sharing the resistor upgrade specs and the manufacturer’s name and part number for your Amazon motor by chance?

The MTR90 III that I acquired came with the circuit board and motor base laying in the bottom of the machine cabinet from the top deck being opened with the meter bridge closed …

I was able to reassemble the motor base … and clean the shield assembly and apply fresh grease to the drive screw… but it still operates very anemically …

It sounds like your Amazon motor upgrade would solve it !

Thank you in advance !!!

Best regards !

no problem. it was this one. 24VDC

https://www.amazon.co.uk/gp/product/B07WBZTFXW/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Make damn sure those guide sensors are doing their job or the relay wont open back up, leaving all that current on the green 2W resistor, which will burn up, regardless of what value you choose.

Im pretty sure I used a 3 or 5 watt, i remember experimenting with different values until i got the right amount of torque on the head shield snapping up and down. Too low resistance and theyre gonna snap like a gun, too high and theyll be too weak to lift against gravity. my gut tells me i used 40 ohm but maybe try 60 or 80. I think this could depend on the wear and tear of the mechanism so youre best alligator clipping different values in until something works (and doesnt overheat)

 

[Emilblanca]

also , something that came up recently on the otari fb group - if you take apart the mechanism dont forget to put the black tube stopper on the screw gear or the slider will always drop down too far and lock up, wont be able to lift itself back up. that black stopper piece is crucial

 

[JeffKall]

also , something that came up recently on the otari fb group - if you take apart the mechanism dont forget to put the black tube stopper on the screw gear or the slider will always drop down too far and lock up, wont be able to lift itself back up. that black stopper piece is crucial

Thank you, @Emilblanca !
I absolutely appreciate your willingness to share your personal insights and experience !

The search for a motor was definitely a frustration … and aside from transport lamps … this was the last hurdle to clear to get my MTR90 back to health !

Best regards,

Jeff

 

[Emilblanca]

Transport lamps - midget flange 6v I believe, don't quote me on that.

Vu meter bulbs I found pretty much the exact axial rice bulb type for peanuts on AliExpress, like 5 bucks for 20 or something crazy. can dig it out if you need and no worries, I understand the importance of recording this information online.

Plus I've been on this whole journey all in the last few months myself as I acquired mine recently and wanted to get it ship-shape

 

[JeffKall]

Transport lamps - midget flange 6v I believe, don't quote me on that.

Vu meter bulbs I found pretty much the exact axial rice bulb type for peanuts on AliExpress, like 5 bucks for 20 or something crazy. can dig it out if you need and no worries, I understand the importance of recording this information online.

Plus I've been on this whole journey all in the last few months myself as I acquired mine recently and wanted to get it ship-shape

Thank you, @Emilblanca !
It has been an absolutely gratifying journey for me … and the head shields and the transport button lamps are the icing on the cake !

The meter bridge is perfect (probably due to them all having been replaced just before I acquired it) … but I have really been scratching my head on the button lamps because of the mounting ….

It sounds like the midget 24V are plug & play (light) !!!

Were they Aliexpress, also ?

Thank you, thank you, thank you, sir !!!

Best regards and happy holidays !!!

 

[Emilblanca]

i think i just got mine off ebay. you just clip out the button with a screwdriver lightly, dont even have to unscrew anything. but yes definetely check the voltage

 

[JeffKall]

i think i just got mine off ebay. you just clip out the button with a screwdriver lightly, dont even have to unscrew anything. but yes definetely check the voltage

Thank you … again, sir !!!

Best Regards,

Jeff