Otari MX-50 - Input Amplifier Issue

[smilan]

Hi, I have an Otari MX-50 with a problem at the input amplifier.
At the junction of C301 and R301 the signal is equal to the signal at the junction of C401 and R401 (0.450V AC).
At the junction of C302 and R302 the signal is equal to the signal at the junction of C402 and R402 (0.454V AC).
On the other side of R301 the signal drops to 0.026V while at the other side of R401 the signal voltage is 0.065V.
On the other side of R302 the signal drops to 0.120V while at the other side of R402the signal voltage is 0.432V.
What else I should check before replacing IC509 (M5219p)?

 

[samgraysound]

Looks to me like the opamp is the most likely culprit. You can test the other components but I don't think it's likely they are failed. I think to get the readings your getting, multiple passive components would have to fail simultaneously.

But if you like:
Do a visual inspection of the underside of the board and look for cold or cracked solder joints and cracked or broken traces
Test the resistance of R301-R306
Test for a short across C303 or C304
No harm in replacing C305
Test the op amp power rails, look for DC on Pins 4 and 8. Presumably it will be +/-15

What AC and DC voltages do you have at Pin 7 of the opamp and how does it compare to the other channel?

Sam

 

[user 130681]

Is there output on IC509?
If none, check by fitting a new chip.
That will tell you straight away.
If you can't get one, dropping the +- 20v to +-18 and you can try a 358.

 

[smilan]

Thank you Sam and Jon

But if you like:
Do a visual inspection of the underside of the board and look for cold or cracked solder joints and cracked or broken traces

All looking good to me.

Test the resistance of R301-R306
Test for a short across C303 or C304

All seems to be right.

No harm in replacing C305

I test C305 with a LCR meter, it looking good. I don't have here a bi polar cap at this size.

Test the op amp power rails, look for DC on Pins 4 and 8. Presumably it will be +/-15

The supply rails for the op amp are +20.21V nd -20.30V

What AC and DC voltages do you have at Pin 7 of the opamp

With no input signal there's 0.004mV DC and 0.024V CA
This signal there's 0.004mV DC and 0.024V CA

and how does it compare to the other channel?

With no input signal on pin 1 there's 0.006mV DC and 0.024v AC
With signal there's 0.006mV DC and 0.136v AC

Is there output on IC509?

Yes, there's an output on both channel, but channel 2 is significantly lower than channel 1

If none, check by fitting a new chip.
That will tell you straight away.
If you can't get one, dropping the +- 20v to +-18 and you can try a 358.

I'll see if I can find a M5219p in the local electronics store. If I'll not be able to find one I'll drop the supply voltage to +/-18V and put a 358.

The only problem is that on the schematic I found online the PSU section is missing...
Maybe you have a PDF copy of it?
This is the service manual that I have

 

[smilan]

I replaced the M5219 with a 4558 and now both inputs are equal.
Thank you for your help!

 

[smilan]

I received this machine in a trade after fixing some gear for a friend.
The transport seems to work good as well as the repro erase and heads.
The record level is pretty low on both channels due to a very worn head.
Do you think it's worth to invest $250 and buy a recording head?
How long would the repro head keep working?

 

[Tubetec]

It looks like you have very uneven wear on the heads , that points to other problems with the tape path .
little point in replacing anything without at least attending to the underlying tape path issues .

 

[smilan]

It looks like you have very uneven wear on the heads , that points to other problems with the tape path .
little point in replacing anything without at least attending to the underlying tape path issues .

I don't know nothing about the history of this machine and how it was maintained.
Of course the tape path should be realigned if a new head will be installed.

 

[user 130681]

There are a few companies that can re lap the head as the gap is still correct and not worn through.
The joys of many years using metal tapes.

You can perfectly well re-lap tape heads yourself providing there is sufficient 'material' left to provide the necessary magnetic contact - and you do it with extreme caution and very carefully. I have done many myself with great success in professional life - including 24 track 2" heads used everyday in professional recording. This is how we do it:

Use increasingly fine wet/dry rubbing paper in stages to bring down the the unworn parts of the head each side of the tape path to the the remaining level of the worn tape path. To keep the grinding surface completely flat, I used a thick glass block with the abrasive paper wrapped over the top and mounted on a sturdy desk surface. This works for every material I have tried. Hold the head in a plastic jawed vice.

The required profile is visible on the unworn parts and it is important to retain this as closely as possible, making sure that the face of the head stays even both vertically as well as horizontally. With some heads it's useful to draw round it in pencil to get an image of it to work to.

It's important to always grind in a widthwise motion (the direction of the tape), looking frequently to ensure it is going evenly and the profile is being maintained. Remember that significantly changing the 'wrap' profile of the tape contact on the head will change LF response.

Never ever grind in a lengthwise manner, as this will damage the magnetic gap by clawing out the material in between the pole pieces :-(

In fact wider 2" heads are easier to lap because they are more likely to stay level while you take off the excess. 1/4" and 1/2" heads need more care. More care is also required for ceramic heads as this material is harder than the metal types, but they are still perfectly doable with ordinary wet/dry abrasive paper.


The grinding is finished when the wear marks between the the worn and unworn parts of the head just about disappear - do not do anymore as this will reduce the life of the head without giving any benefit. All you are trying to do is take out the extra edges that have been caused by tape wear and restore the original wrap and contact with the pole pieces. It is ok the leave some vestige of the upper and lower wear marks as these act as a guide to (and confirmation of) the evenness of your efforts. They also act as a useful guide when re-installing the head - especially if you have height adjustment.

Then polish the surface to a shine firstly using car rubbing compound on a wet rag - and then further finishing with something less abrasive like Brasso or other gentle domestic metal polish.

At this point the head will still perform less than perfectly because no amount of polishing will totally restore the surface within the magnetic gap. So the next stage is to re-install the head on the deck in the best possible position - and let the tape itself 'finish' the head by running a whole spool of (gash) tape over it (repeatedly if necessary) until HF response is restored. Then finally set up the angle and azimuth - and with any luck you're done.

From experience starting from significant wear, you can normally do this two or 3 times and restore original performance on professional machines. After a few times though, performance declines because of loss of head material, and/or geometry - or simply because the guides on the machines do not allow enough wrap with the smaller intrusion the head causes in the tape path.

Hope this helps.

 

[smilan]

There are a few companies that can re lap the head as the gap is still correct and not worn through.
The joys of many years using metal tapes.

You can perfectly well re-lap tape heads yourself providing there is sufficient 'material' left to provide the necessary magnetic contact - and you do it with extreme caution and very carefully. I have done many myself with great success in professional life - including 24 track 2" heads used everyday in professional recording. This is how we do it:

Use increasingly fine wet/dry rubbing paper in stages to bring down the the unworn parts of the head each side of the tape path to the the remaining level of the worn tape path. To keep the grinding surface completely flat, I used a thick glass block with the abrasive paper wrapped over the top and mounted on a sturdy desk surface. This works for every material I have tried. Hold the head in a plastic jawed vice.

The required profile is visible on the unworn parts and it is important to retain this as closely as possible, making sure that the face of the head stays even both vertically as well as horizontally. With some heads it's useful to draw round it in pencil to get an image of it to work to.

It's important to always grind in a widthwise motion (the direction of the tape), looking frequently to ensure it is going evenly and the profile is being maintained. Remember that significantly changing the 'wrap' profile of the tape contact on the head will change LF response.

Never ever grind in a lengthwise manner, as this will damage the magnetic gap by clawing out the material in between the pole pieces :-(

In fact wider 2" heads are easier to lap because they are more likely to stay level while you take off the excess. 1/4" and 1/2" heads need more care. More care is also required for ceramic heads as this material is harder than the metal types, but they are still perfectly doable with ordinary wet/dry abrasive paper.


The grinding is finished when the wear marks between the the worn and unworn parts of the head just about disappear - do not do anymore as this will reduce the life of the head without giving any benefit. All you are trying to do is take out the extra edges that have been caused by tape wear and restore the original wrap and contact with the pole pieces. It is ok the leave some vestige of the upper and lower wear marks as these act as a guide to (and confirmation of) the evenness of your efforts. They also act as a useful guide when re-installing the head - especially if you have height adjustment.

Then polish the surface to a shine firstly using car rubbing compound on a wet rag - and then further finishing with something less abrasive like Brasso or other gentle domestic metal polish.

At this point the head will still perform less than perfectly because no amount of polishing will totally restore the surface within the magnetic gap. So the next stage is to re-install the head on the deck in the best possible position - and let the tape itself 'finish' the head by running a whole spool of (gash) tape over it (repeatedly if necessary) until HF response is restored. Then finally set up the angle and azimuth - and with any luck you're done.

From experience starting from significant wear, you can normally do this two or 3 times and restore original performance on professional machines. After a few times though, performance declines because of loss of head material, and/or geometry - or simply because the guides on the machines do not allow enough wrap with the smaller intrusion the head causes in the tape path.

Hope this helps.

Nice, thanks for the information. this one seems to be an interesting project to dive into.
What levels of grit do you recommends?

 

[user 130681]

I would start with P180 and finish with P800 maybe even finer for a good polish with matal polish.
The art is to keep the shape of the head and never run up or down, always across the head, like tape does.