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[lucey]

Both Mr. Joel Katz and the good folks at ATR have no idea what's up here ... please help if you can ... it's been 2 months on this.  I'll pay you if needed.


Machine:  Ampex MM1200

Issue:   27V/15V card is putting out nothing with a full load on it, thus the transport box shows no lights (after the first split second at power up) and the motors spin, etc.

Notes:
1. When J2 (27/15 power supply connector) load is removed the 27/15 reg card puts out 27/15.  Similarly, when the transport box is removed the reg card reads 27/15.

2. When J1 (39V power supply connector) is removed, 27/15 reg card puts out 27/15.  Transport box lights, all Ch. REC lights are ON showing Rec.




These components (everything upstream of the PSU except cables/connectors) have been checked over and work in 4 other 1200s:

transport box
27/15V card
39V card
MDA assembly (complete)
3 transport cards + tention kit card

The 27V reg at the PSU has been tested working with a load of up to 6.75A at 4ohm


Attempts to track down a short or a high resistance part causing shut down include:  
1. Removing all audio cards + 4 transport cards.
2. Unplugging Capstan Motor.


Fixes so far:

1. 10A Fuse Holder F3 on PSU was replaced due to a fried wire on the back side that was 'working' but highly resistant.  We thought that was it, but no!  
2. IN a check over, MDA had some transistor issues, transport box had some minor regulator issues.



Next ideas:
1. Remove the audio cages one row at a time to find a short or a point of over amping the regulator card (to shut it down)
2. Going to RADAR
3. Hari Kari



Note:  The pot on the 27/15 reg card that controls shut down is at 46.0 ohms, as it should be.  All fuses are fine.

[David Kulka]

Indeed a tough problem.  From a distance, and not having seen the insides of a 1200 for a while, I can only offer a couple of general suggestions.

My hunch would be some combination of inrush current + operating current, causing the power supply to crap out, or maybe go into protection mode.

You said "the 27V reg at the PSU has been tested working with a load of up to 6.75A at 4ohm".  That's a lot of current and it sounds pretty good, but I would try to look at the power on current, using a scope, and see how it compares to that 6.75A.  Maybe borderline capacitors or semiconductors are drawing strange currents when the machine is switched on.

A connector that is flaky or tired or has been running hot for many years might cause similar problems.

You might disconnect the 27 and 15 volt outputs one at a time, to see which one is the bad guy that's not coming up with the machine connected.

Hope this helps.  Again, I don't see these machines very often these days and don't quite recall all the details and weirdnesses of their circuitry!

BTW - how did this problem begin?

[amorris]

a power short would pop fuses, right? i think that the ground return is getting lost or contaminated. plugged in, can you measure at the supply and get nothing? or are you measuring from the front of the machine. try it with out the transport remote. there's a lot to go wrong in there.

[lucey]

I dont recall how this all started David, it was 2 months ago+ and it's been a nightmare A/Bing and sending all the cards in for testing so I'm confused. I'm no tech.

amorris wrote on Fri, 30 September 2005 15:05

a power short would pop fuses, right? i think that the ground return is getting lost or contaminated. plugged in, can you measure at the supply and get nothing? or are you measuring from the front of the machine. try it with out the transport remote. there's a lot to go wrong in there.

If the PSU is plugged in with no 27/15V and no 39V load it measures fine on 27/15 and on 39 at the regulator cards for each.

If the 27/15 is the only thing plugged into the PSU it shows 27/15.

If the 39V is the only thing plugged into the PSU it shows 39.

If both are plugged in the 27V shuts down on start up after a split second flicker.

(If the remote is removed, the 27V appears at pins A and C on the 50 pin as well as at the test points on the PSU reg card.)




The 27/15 runs nominally in a 1200 at around 7.25Amps, and on the bench my PSU fuse blows at 8A, as it should.

If the transport box is removed (on the 27/15 line) then the 27/15 reads fine at the PSU test points. The transport box is not faulty and has been tested in a number of machines by both techs.  This lead me toward the idea of a resistance lack where that load puts the regulator shut-off over the line.  Yet the regulator does not shut off according to the local tech today, until the 10A fuse blows at 8.25 Amps.

The Capstan Motor was removed, no changes as a result.  

Both reel motors turn full speed when the 27/15 is not at the Transport, so they are both working at least, no dead short.




Unless you have another idea the plan is:

1. Disconnect each of the reel motors and start up, to see if that changes anything in the way of the inrush current.

2. Disconnect each of the 4ch Audio Card Cages, one it a time, to see if that matters.  If it can be isolated to one cage, there we are.

3. If not, I'll desolder/remove the 15V and 27V from J2 one a at time to isolate the issue on 15 or 27 at least.



???


thanks,

[David Kulka]

The 27v rail is most likely the bad one but I'd disable the 15v supply, just to see if that gives any clues.  It can't hurt to disconnect the reel motors, but I doubt they have anything to do with this.

Assuming it's the 27v rail, I think I'd try to determine whether the supply or the rest of the machine is at fault.  After re-reading your comments and looking at the schematic, my hunch would be the supply.

That 27/15 regulator card has a lot of zener diodes.  Ampex liked to use zeners but they are rather imprecise, and can cause all sorts of problems.  Can you get hold of another 27/15 card and try it?  If it proves to be the card, it should be a fairly simple fix.  Other thoughts:

Does R8 on that card get hot when the machine fails?

Turn the 27v adjust all the way down, then see whether the supply powers up.

R6, a trimmpot on the 27/15 card, looks like an overvoltage protection adjustment.  It might be a little too trigger-happy.  Maybe remove Q3 to disable that circuit, and see what happens.  Slight risk associated with this idea, but should be ok.

I like the idea of disconnecting the audio chassis.  Maybe disconnect all of them, see if the machine comes up, then reconnect them one at a time.

Maybe measure the 27v turn on current with a scope, to see if there's some abnormally big spike.

Maybe begin to replace ALL the semiconductors and capacitors on the 27/15 board, a few at a time, testing as you go along.  Measure the resistors, inspect for physical damage or hot spots.

Maybe disconnect the output of the 27v supply and put a 4 ohm load across the supply, see if anything interesting happens.

Maybe try an external power supply for the 27 volts (with fusing and/or current protection, of course) to see whether the machine runs in that state.  Perhaps a couple of medium size 12 volt batteries?

Of course, as with all troubleshooting exercises, the fast and simple ideas are the ones that should be tried first.  Good luck!