Quantcast
How to increase voltage in erase-head? (Otari MX7800) - Gearspace.com
The No.1 Website for Pro Audio
How to increase voltage in erase-head? (Otari MX7800)
Old 2 days ago
  #1
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
How to increase voltage in erase-head? (Otari MX7800)

Hi guys!

I’ll try to make it short.
I’d like to know if it’s possible to increase the voltage of the erase-head in my Otari MX7800. I know for a fact, that the later MTR models are equipped with separate potentiometer for this purpose. This is not the case with my Otari. But I figured, it must be possible to modify the circuit of the EH so it delivers more voltage to it. I attached schematics of the EH circuit (see pdf file below). Would you have a look at it and tell me if it’s possible?

Question: There is a transformer (T1) with a potentiometer on the EH board (see photo). What is the function of this potentiometer?

Thanks in advance!

Rob.

How to increase voltage in erase-head? (Otari MX7800)-eh-board.jpg
Attached Thumbnails
How to increase voltage in erase-head? (Otari MX7800)-eh-board.jpg  
Attached Files
File Type: pdf Otari MX7800 Schematics_3.pdf (226.6 KB, 2 views)
Old 2 days ago
  #2
Lives for gear
 
JohnRoberts's Avatar
 
🎧 10 years
Quote:
Originally Posted by Yoski ➡️
Hi guys!

I’ll try to make it short.
I’d like to know if it’s possible to increase the voltage of the erase-head in my Otari MX7800. I know for a fact, that the later MTR models are equipped with separate potentiometer for this purpose. This is not the case with my Otari. But I figured, it must be possible to modify the circuit of the EH so it delivers more voltage to it. I attached schematics of the EH circuit (see pdf file below). Would you have a look at it and tell me if it’s possible?

Question: There is a transformer (T1) with a potentiometer on the EH board (see photo). What is the function of this potentiometer?

Thanks in advance!

Rob.
I don't have first hand experience with that model but trim pots in bias circuitry are usually to peak out the resonant circuit. Maybe put a scope on it and see what it does.

JR
Old 2 days ago | Show parent
  #3
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
Quote:
Originally Posted by JohnRoberts ➡️
I don't have first hand experience with that model but trim pots in bias circuitry are usually to peak out the resonant circuit. Maybe put a scope on it and see what it does.

JR
I hesitate to mess with the pot, but I will give it a try. But what is that transformer doing in the circuit? Any idea?
Old 2 days ago
  #4
Lives for gear
 
SP2016's Avatar
 
🎧 10 years
It is not a trim pot, but an rf transformer with two coils and with a turnable powdered iron core.
So "pot" is not the right expression here.
The rf transformer is for buffering and adapting to the impedance of the (in this case) erase head.
You will need the right tool to turn the core, as the material is very brittle and will break easily.....
(Experience has teached me that.)
Try to get an original service manual.

Last edited by SP2016; 2 days ago at 08:09 PM..
Old 2 days ago
  #5
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
Shoot, I already turned one back and forward just a bit. Don’t think I broke something, the ‘pot’ turned very smoothly without any resistance.
What do you mean with ‘right tool’?
Can I theoretically adjust the voltage for the erase head with this ‘pot’?

The bias/erase-head board has measurements points for bias and erase head voltage. I made two screen shots from the measurements for reference, before I start to mess with the board (see below).

How to increase voltage in erase-head? (Otari MX7800)-bias-1.jpg

How to increase voltage in erase-head? (Otari MX7800)-erase-head-1.jpg
Attached Thumbnails
How to increase voltage in erase-head? (Otari MX7800)-bias-1.jpg   How to increase voltage in erase-head? (Otari MX7800)-erase-head-1.jpg  
Old 2 days ago
  #6
He means something like this (nonmagnetic, nonconductive, designed to strip out the tool tip before splitting the ferrite core):

https://www.amazon.com/Aven-13016-Pi.../dp/B07BSF97BQ

My advice would be to not tinker with these controls without a service manual telling you to do so.
Old 2 days ago | Show parent
  #7
Lives for gear
 
SP2016's Avatar
 
🎧 10 years
Quote:
Originally Posted by Philbo King ➡️
He means something like this (nonmagnetic, nonconductive, designed to strip out the tool tip before splitting the ferrite core):

https://www.amazon.com/Aven-13016-Pi.../dp/B07BSF97BQ

My advice would be to not tinker with these controls without a service manual telling you to do so.
Philbo King is right: Do use the right tool !!!
And he is also VERY right with the advice to get a service manual as a first step !
Old 2 days ago
  #8
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
For the record, I will not do anything unless I 100% know, what I’m doing. The SM says nothing about voltage adjustment of the erase-head. If it was the case, I wouldn’t bother you. The only thing I can offer you is the schematics of the bias/erase-head board (see post #1 ).

Pardon my ignorance, but I don’t know how a ferrite core looks like and have therefor no idea why I have to use the special tools you have linked for me. It looks like normal screw drivers to me but anti-static. Could you elaborate on this a bit more?
Old 2 days ago
  #9
Lives for gear
 
🎧 15 years
A plastic hex alignment tool should be used for two reasons:

The non-ferrous material won't detune the circuit while you're adjusting it, like a metal tool would.

A metallic tool could more easily damage the fragile ferrite core.
Old 2 days ago
  #10
Gear Guru
 
Thomas W. Bethe's Avatar
 
1 Review written
🎧 15 years
Stupid question but WHY do you want to increase the voltage to the erase head???
Old 2 days ago
  #11
Lives for gear
 
crosscutred's Avatar
 
🎧 10 years
I just had a quick look online for a manual, I think this is the one

https://mpe.berklee.edu/documents/st...ri%20MX-70.pdf

Page 71 is what you need

If it's the wrong machine then the relevant info for you machine should be available, this is what people are talking about when they say to consult the manual, not just the schematics.

Good luck
Old 2 days ago
  #12
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
@ crosscutred
I have consulted manuals from other Otaris before I opened this thread here. But it didn’t help me much since I have only one transformer in the circuit and other Otari models have two. So I was not sure what purpose my transformer fulfills. Also the way you handle the transformers varies from model to model. And as I already mentioned, the SM for my Otari is silent about this topic.

But user SP2016 thinks that T1 transformer belongs to the erase had, so I could try to adjust it for the peak reading (for instance) and see what happens. The problem is, that without an extender-card I’m flying blind. I have to take out the card, turn the core a bit, then put it back in and see what happens. I will be back as soon I have some news.

And thank you guys, I appreciate your help.

@ thomas . Half of the tracks don’t erase properly as they should. The tape path is OK, heads cleaned and demagnetized and freshly re-lapped. But the problem was before I sent them for re-lapping. I just want to bring my machine back to specs. Taking care of the erase problem is next on my list.
Old 1 day ago
  #13
From what I see in the schematic, those adjustable transformers act as inductors in a tuned tank circuit. When properly set, they will resonate with the capacitor at the erase/bias frequency, at which point the AC voltage across the tank will be maximum. So you might be on the right track. Or you may not be.

It is quite possible you are, in effect, (to use a metaphor) trying to compensate for a bad spark plug by adjusting the muffler.

If some component has drifted in value with age, causing the erase head AC voltage to drop, the right thing to do is to isolate and replace the faulty component.
Old 1 day ago
  #14
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
Indeed the peak voltage is to aim for when you adjust the T1 transformer. But still half of the tracks don’t erase properly (see spreadsheet). In the last column I attached the inductance measurements which was made by the professional who re-lapped the heads (Terry Summers from summertone.uk)

Conclusions? What do I know.

@ Philbo King

This would be my best uneducated guess too. But where to start? Resistors? Voltage regulators? Any suggestions? The electrolytic caps has been already swapped.

How to increase voltage in erase-head? (Otari MX7800)-screenshot_2021-06-11-erase-head-pdf-2-.png
Attached Thumbnails
How to increase voltage in erase-head? (Otari MX7800)-screenshot_2021-06-11-erase-head-pdf-2-.png  
Old 1 day ago
  #15
Lives for gear
 
Radardoug's Avatar
 
🎧 10 years
Get the correct tool, adjust the coils for peak output on the bias output. Its not difficult. The fact that half the tracks erase means you are very close.. Also check bias frequency first, the bias oscillator might have shifted, which will affect all tracks tuning.
Old 1 day ago | Show parent
  #16
Quote:
Originally Posted by Yoski ➡️
Indeed the peak voltage is to aim for when you adjust the T1 transformer. But still half of the tracks don’t erase properly (see spreadsheet). In the last column I attached the inductance measurements which was made by the professional who re-lapped the heads (Terry Summers from summertone.uk)

Conclusions? What do I know.

@ Philbo King

This would be my best uneducated guess too. But where to start? Resistors? Voltage regulators? Any suggestions? The electrolytic caps has been already swapped.

The chart shows roughly the same voltage reaching all erase heads, but some don't erase. Some with highest voktage don't erase while others with lower voltage do. Seems quite suspicious...

You might do continuity checks on the erase windings with a digital ohmmeter, just to verify all show comparable readings.
Old 1 day ago
  #17
Lives for gear
 
🎧 15 years
Maybe the erase head is mechanically out of alignment?
Old 1 day ago | Show parent
  #18
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
Quote:
Originally Posted by Radardoug ➡️
Get the correct tool, adjust the coils for peak output on the bias output. Its not difficult. The fact that half the tracks erase means you are very close.. Also check bias frequency first, the bias oscillator might have shifted, which will affect all tracks tuning.
I don’t need an anti-static screw driver, since I have to take out the board out of the chassis in order to turn the ‘pot’ in the transformer. No current flows through the transformer when the board is out. Bias frequency is 133 kH and is very stable in all channels. See the screen shot below as an example. Red color is the bias signal and blue is for the erase head.

How to increase voltage in erase-head? (Otari MX7800)-ch1-eh_2.jpg
Attached Thumbnails
How to increase voltage in erase-head? (Otari MX7800)-ch1-eh_2.jpg  
Old 1 day ago | Show parent
  #19
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
Quote:
Originally Posted by Philbo King ➡️
The chart shows roughly the same voltage reaching all erase heads, but some don't erase. Some with highest voktage don't erase while others with lower voltage do. Seems quite suspicious...

You might do continuity checks on the erase windings with a digital ohmmeter, just to verify all show comparable readings.
Yes, I agree it’s strange. But as I mentioned it above, the heads has been recently re-lapped by Terry Summers from summertone.com in the UK. He checked the windings of course. The last column of the chart I posted above are his measurements of the inductance in the erase head. He of course realigned the whole head-block after the re-lapping.I really don’t think the problems have something to do with the erase head. Let's say, it is not my main suspect at the moment.
Old 1 day ago
  #20
Lives for gear
 
Radardoug's Avatar
 
🎧 10 years
You cant do this without the card being in and going. If you need an extender then get an extender. Why is it that people on Gearslutz dont want to buy the right tool, and take the good FREE advice we give you?
DO IT RIGHT OR DONT DO IT AT ALL!
Old 1 day ago | Show parent
  #21
Lives for gear
 
🎧 15 years
Quote:
Originally Posted by Yoski ➡️
Yes, I agree it’s strange. But as I mentioned it above, the heads has been recently re-lapped by Terry Summers from summertone.com in the UK. He checked the windings of course. The last column of the chart I posted above are his measurements of the inductance in the erase head. He of course realigned the whole head-block after the re-lapping.I really don’t think the problems have something to do with the erase head. Let's say, it is not my main suspect at the moment.
Yes, relap can change head inductance which can require re-tuning of the peaking circuits.

But usually, heads need a final alignment while in place on the machine, with tape rolling. A slight mis-adjustment could cause erase depth problems. So I wouldn't be too quick to discount that.

Was the machine erasing properly before the heads were relapped?
Old 1 day ago
  #22
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
@ Radardoug

Try to find an extender card for my Otari model. Good luck with that. I was thinking about building an extender cable, as soon as I became aware that I need one. But I will not stop looking for answers only because I don’t have extender card at hand.
Old 1 day ago | Show parent
  #23
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
Quote:
Originally Posted by David Kulka ➡️
Yes, relap can change head inductance which can require re-tuning of the peaking circuits.

But usually, heads need a final alignment while in place on the machine, with tape rolling. A slight mis-adjustment could cause erase depth problems. So I wouldn't be too quick to discount that.

Was the machine erasing properly before the heads were relapped?
No, similar problems. But I don't recal which tracks were problematic.
Old 1 day ago
  #24
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
Problem solved!

Today morning I decided to interchange bias boards between working and nonworking channels. It turned out that the problem stayed with the problematic tracks. So it had to be the erase-head as David Kulka suggested. But what? I decided to go with the simplest solution and demagnetized the sucker, although I did it twice since the heads came back two weeks ago from re-lapping. I gave him two extra long demagnetizing sessions. Almost burned my demagnetizer, hahaha. But after that, the head started to perform as it should on all tracks. Zero audio residue on all tracks.

As I said I demagnetized the heads twice since they came back from re-lapping. But apparently it was not enough after the torture they have been trough. Now when I think about it, I probably payed to little attention to the erase head. I obviously concentrated on the other heads more. Well, that was a mistake. You live and learn.

I thank everybody who participated in this thread and offered his support.

Regards!

Rob.
Old 1 day ago
  #25
Gear Guru
 
Thomas W. Bethe's Avatar
 
1 Review written
🎧 15 years
It is strange that by simply demagnetizing an erase head that everything is now working. Glad you got it solved but something still does not seem right. Why would some tracks work OK and some not? Just wondering????
Old 1 day ago
  #26
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
@ thomas

Your post really unsettled me. I ran over and checked all tracks once again. It has really worked. The erase head works perfectly now. Fortunately.
Old 22 hours ago
  #27
Excellent! I'm glad you got it resolved!
Old 21 hours ago
  #28
Lives for gear
 
JohnRoberts's Avatar
 
🎧 10 years
may have been a dirty connector that was exercised into working by replugging. Take the win...

JR
Old 18 hours ago
  #29
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
Thank you Guys! I'm happy it was an easy fix too.
📝 Reply
Topic:
Post Reply

Welcome to the Gearspace Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…

Forum Jump
Forum Jump