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Otari MX-7800 very low output on 2 channels
Old 3rd August 2020
  #1
Otari MX-7800 very low output on 2 channels

I have a vintage Otari MX-7800 8 track 1 inch tape machine that I bought 10 years ago, used very little, then moved and just started working with it again after about 7 years.

Went through the manual to do alignment and VU meter calibration. Did some test recordings, all but 2 channels work OK.

Problem is very low, almost inaudible output on channels 3 and 7. During recording to channel 3, the VU meter shows plenty of signal hitting the input.

I swapped both the input and output amplifier boards between channel 1 (which works) and channel 3 to see if it was on one of these boards, but the problem remains.

Tried direct input of 4 dBu 1000khz signal of my tone generator to the line input on channel 3 to eliminate any issues with the console, and problem remains.

Heads cleaned, cables checked. Any more suggestions on what to look for?
Old 15th October 2020
  #2
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
Quote:
Originally Posted by diamondjim ➡️
Problem is very low, almost inaudible output on channels 3 and 7. During recording to channel 3, the VU meter shows plenty of signal hitting the input.

Any more suggestions on what to look for?
Hi!

I just came across your thread. Did you managed to pin down the problem?

My guess is, it is ether an oxidized/dirty relay or dirty contacts at the rotary switch knob (input/tape/save etc.). Cleaning the switch is an easy thing. But to get access to the relays you’d have to take the ‘main-board’ out of the audio unit. If you want to do it and need assistance, let me know. I could also provide you with the info on how to clean a relay the right way if you want.

Cheers!
Old 15th October 2020
  #3
Thanks Yoski,
Yes any help will be appreciated. I think either the Input/Safe etc rotary switch or the SRL Playback rotary pot may be the culprit. So any info on how to best clean them will be welcome.

Jim
Old 15th October 2020 | Show parent
  #4
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
Quote:
Originally Posted by diamondjim ➡️
Thanks Yoski,
Yes any help will be appreciated. I think either the Input/Safe etc rotary switch or the SRL Playback rotary pot may be the culprit. So any info on how to best clean them will be welcome.

Jim
That was quick

I would try to clean the switches first. Any contact/deoxy spry will do. Just take the audio unit out of the machine and .... I'm sure you have cleaned a pot before so I don't need to elaborate on this. If it won't help, let me know and I'll take you through the relay-cleaning process.

Cheers!
Old 15th October 2020
  #5
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
I need to correct myself. If you want to clean the SRL pot too, then any deoxy-spry won’t do. You need one that won’t destroy the coal layer. And here is the problem. Everyone recommends something else. Till this day I’m not sure if I’m using the right thing. But I made good experiences with it, so it can’t be wrong. I use this here. But it’s only available in Germany:
https://www.ebay.com/itm/Teslanol-T6...u/362763505987

I'm sure you've got planty of good stuff over in the US.
Old 15th October 2020
  #6
Yes I see that won't ship to U.S.
I am sure I can find something here.
Is Teslanol product just a pure compressed air spray that won't leave a residue?

Something like this maybe?
Chemtronics ES1020 Ultrajet Duster
Old 15th October 2020
  #7
Lives for gear
 
nosebleedaudio's Avatar
 
🎧 15 years
Will add that if the D5 deoxit does NOT help, DONT use some other chemical because you could have a chemical reaction that is NOT good..Seen switches that melted..
Old 15th October 2020 | Show parent
  #8
Quote:
Originally Posted by Yoski ➡️
You need one that won’t destroy the coal layer.
I'm sure you've got planty of good stuff over in the US.
Not sure what you mean by coal layer, but here are the compatibility specs for the Chemtronics duster:

Material Compatibility

Buna-N Excellent
Graphite Excellent
HDPE Excellent
LDPE Excellent
Lexan Excellent
Neoprene Excellent
Cross-Linked PE Excellent
Polyacrylate Excellent
Polystyrene Excellent
PVC Excellent
Silicone Rubber Excellent
Teflon Excellent
Viton Excellent
Old 15th October 2020 | Show parent
  #9
Lives for gear
 
nosebleedaudio's Avatar
 
🎧 15 years
Quote:
Originally Posted by diamondjim ➡️
Not sure what you mean by coal layer, but here are the compatibility specs for the Chemtronics duster:

Material Compatibility

Buna-N Excellent
Graphite Excellent
HDPE Excellent
LDPE Excellent
Lexan Excellent
Neoprene Excellent
Cross-Linked PE Excellent
Polyacrylate Excellent
Polystyrene Excellent
PVC Excellent
Silicone Rubber Excellent
Teflon Excellent
Viton Excellent
I guess he means carbon..
Old 15th October 2020 | Show parent
  #10
Gear Maniac
 
Yoski's Avatar
 
🎧 5 years
Quote:
Originally Posted by diamondjim
Not sure what you mean by coal layer, but here are the compatibility specs for the Chemtronics duster:
Sorry, bad translation. I’m not familiar with the correct English term, but a potentiometer is in essence a round variable resistor, and like a resistor it has a carbon-composit component to it that serves as resistance layer. See attached picture. You don’t want to destroy this layer with aggressive deoxidizing chemicals. The specs for Ultrajet says that it is excellent with graphite so it will handle potentiometers very well, but you don’t need high pressure to clean pots. And it won’t serve very good for cleaning the fragile contacts in relays, so maybe you should look for something else. On the other hand, for the method I use to clean relays high pressure is irrelevant. Maybe they have the same product but as a normal spray? An ordinary spray will serve you better in most applications.

I hope that someone else will chime in and help you with the right choice.

Quote:
Originally Posted by diamondjim
Is Teslanol product just a pure compressed air spray that won't leave a residue?
No, it adds a film that enhances conductivity of the contact and protects it from oxidation.
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