Ampex MM1100 Tension Problems

[dawsonaudio]

I am having trouble with the stop tension on my Ampex MM1100 2 inch/16 track machine.  I've been trying to get some help for a week or two now over on the Ampex mailing, but haven't made much progress.  Basically, I can normalize the reels, adjust the play tension, rewind and forward tension, but the stop/idle tension can't be achieved.  The supply reel can be adjusted properly, but the take-up reel barely begins to add tension to the tape even after adjusting the stop trimmer pot all the way.  I've tried adjusting the master gain trimmer on the transport control board, but this doesn't help.  I've checked all the voltage/s at the power supply and they are on point.

I've been thinking of replacing all of the tantalum capacitors on the transport control board (like 5 or 6) of them.  Not sure if one of those could be the culprit. 

One other question, would electrolytic caps be able to substituted for tantalum caps in this situation?  Not sure the answer to that.

Thanks,
Nate

 

[radardoug]

The 1100 was notorious for tension drift, which is why the 1200 came out fairly quickly, and Ampex completely changed the way the MDA's worked. From memory, the MDA's were opto coupled, and the optos wore out, so you should look there. As to tantalums, they either work or generally they go short. These days you can replace with electros, electros today are far better than they were. Use 105 degree caps.

 

[dawsonaudio]

How would I go about testing the optos to see if they have failed?  I've got the tension assembly cover off and can see what looks like two mirrors/opto units? visible with wires soldered to each.  And then there is the light bulb. 

 

[dawsonaudio]

I've got another question.  With the Ampex Transport control pwa board out of the machine, I've checked all the resistors and there is only one that isn't measuring correctly in the circuit (r38).  I de-soldered the end and it measures correct.  It is supposed to be a 2k resistor and in the circuit it measures around 500ohms.  All of the other resistors in the circuit read fine/correct.​  The other thing that I noticed is that two of the many diodes have a positive diode reading on my meter in both directions when in the circuit.  If I disconnect either end, they read only one way.  Is that normal or does this point to something else on the board causing issues...

And I know that the resistor is fine as I've installed a new one that exhibits that same issue with a drastic reduction in resistance when in the circuit.  All of the other resistors measure fine on the board except for this one.  I don't have additional diodes at this point, but I'm assuming they are fine since since the read fine out of the circuit.

 

[mrclunk]

It's been a while since i fiddled with ours but if you gently move the tension arm does it have any effect?
Is the tension lamp working, take the cover off by the tension arm and check?

EDIT, ignore me. I didn't read your post properly... duh

 

[radardoug]

How would I go about testing the optos to see if they have failed?  I've got the tension assembly cover off and can see what looks like two mirrors/opto units? visible with wires soldered to each.  And then there is the light bulb.

You need to look at page 157 of the manual, the MDA board. The lamp under the tension assembly cover also needs to be good. What you call optos there are LDRs.

 

[dawsonaudio]

The lamp is good.

I will look for the MDA page in my manual...I've also got a custom MDA board, looks like a one off MM1200 MDA board in my MM1100. 

Thanks

 

[radardoug]

You have to have the optos for an 1100. So if you have a 1200 board it wont work properly, and may be hazardous. The original 1100 board ran direct off the mains, and needed the optos for isolation. On the 1200 they had separate windings on the power transformer for the supply and takeup, and so did not need to isolate.
Tension control on both these machines was less than optimal, but the 1100 was bad and drifted a lot.

 

[dawsonaudio]

When I say I think it's a one off of a MM1200 schematic, I'm not entirely sure.  It just looks like a custom board with no labeling as my other boards have on them.  Attached is the image of the board in question or there is a link.

https://photos.app.goo.gl/krEmQjrqV8LRocYX7

I did have the machine gone through when I first purchased the deck 10-15 years ago by a known ampex technician.  I had the green light when he left so I would imagine he made sure everything was compatible/functional.

 

[radardoug]

Doesn't look like an Ampex board, so may not be the same schematic as in the manual. You would need to go over it and compare it with the schematic.

 

[dawsonaudio]

So I traced the MDA board that I have on my MM1100 and it's a custom board made from the Ampex MM1200 mda schematic in the MM1200 manual.  I looked through the original manual that came with my deck and someone wrote on the mda schematic "No Longer In Service."  The company before me must have ditched the MM1100 mda that came with the deck.  Instead of the opto's on the MM1100 MDA it uses UA741pc op-amps like in the MM1200 , along with all of the other components on the MM1200 MDA schematic.

So maybe my MM1100 has been modified to work with the MM1200 MDA.

 

[radardoug]

You need to have a very good understanding of how this MDA works, and what voltages are applied. There is mains voltage straight on the MDA board on the 1100. The 1200 used separate floating windings for each side of the MDA, and so they could run the control circuitry at ground potential. The 1100 has to have that isolation provided by the optos. It is possible you have a Frankenstein machine. It might have a 1200 power transformer fitted, and be completely rewired to be a 1200. The only way to find out is check the wiring.
AND BE VERY CAREFULL!! Voltages present on the MDA can kill you.

 

[dawsonaudio]

Here is my power supply/s.  Looking at the 1100 manual, there is only one transformer for the 39/27/15volt supply/s.  My supply has one for the 39 volt and a separate for the 27/15 volt supply.  Those are the large metal boxes to the right in the attached picture link.  My supply also has a separate transformer for the Take Up motor.  This is the large black hammond transformer on the bottom/center in the picture link. These changes were added/redrawn onto the existing power supply diagram by whomever modified the supply.  I'm not sure how this compares to an original MM1100 power supply.


https://photos.google.com/photo/AF1QipPJ21-j9oyDtwBGu0cobAvnXleH7-WEFhyRrrdn

 

[radardoug]

Yes so its a Frankenstein. You need to check you have all the right circuitry for the 1200 mod.