Need help with Ampex 440-B 1" 8 track PSU connections

Greetings.  I was given an Ampex 1" 8-track.  It's extremely clean, and I was told it was in working condition except for a bad bias oscillator. Heads look to be in surprisingly good shape.

These machines, of course, have two power supplies-- one with a master bias oscillator (powering channels 1-4) and one without (powering channels 5-8). 

There are lots of multipin connectors on both PSU enclosures.  Some were disconnected when I acquired the machine.  I identified the one with the master bias oscillator, labeled where each multipin cable went, and pulled it.  Identified an exploded electrolytic cap, replaced it (and a few others in the enclosure, for good measure), thinking I had probably fixed the machine.  I did not think to attempt power-up before pulling the supply and performing the repair.

I attempted power-up, and it tripped the breaker in the studio control room.  There are 3 fuses on the front of the machine, under the transport controls-- all three fuses were intact and not blown.  There is a 1.5A slo-blo fuse on each PSU enclosure.  I checked these 2 fuses and they were... missing?

So my question is, what could cause a fault that would make the breaker in the studio trip, even without a fuse in either power supply?  If anyone knows these machines, I'd appreciate any leads.  I feel there must be something connected incorrectly with the two PSU (I admit to not having researched how it all connects, I just labeled the connectors and tried to put it back how it was).  I do have a 440-B service manual, but not for the 8-track. 

I feel like it has to be something simple.  Any leads?  I know Brian Roth knows these machines-- does he still post here?

Thanks!

1st thing I would check is the bridge rectifier.  You may have a shorted diode in the bridge thats allowing raw AC (unrectified)  into that filter cap.  It will blow breakers or burn fuses.

If the 440 is blowing wall breakers with no fuses inserted, check for a shorted conntector for the power cord. Or, for that matter, a shorted power cord.

Peace,
Paul

Thanks to both of you.

If it was a shorted power cord, wouldn't it trip breakers immediately upon being connected to the wall?  This requires the power switch to be flipped "on" before it trips the breaker. This leads me to believe the fault to be downstream of the power cable, at least.

However, the bridge rectifier idea is a good thought-- but it seems strange that this would happen whether or not there was a fuse in place in the PSU or not.

A helpful member has offered me the 8-track addendum to the service manual.  Once I see that, I can have a better idea of how everything connects between the two PSU units.

soapfoot said:

Thanks to both of you.

If it was a shorted power cord, wouldn't it trip breakers immediately upon being connected to the wall?  This requires the power switch to be flipped "on" before it trips the breaker. This leads me to believe the fault to be downstream of the power cable, at least.

However, the bridge rectifier idea is a good thought-- but it seems strange that this would happen whether or not there was a fuse in place in the PSU or not.

A helpful member has offered me the 8-track addendum to the service manual.  Once I see that, I can have a better idea of how everything connects between the two PSU units.

Click to expand...

fact we know it pops a breaker when turned on.
fact we know that it does this with out fuses.

If the fuse is not in we can assume that power is not getting into the power transformer and the rest of the PSU circuit. What's left?

Maybe the power switch is wired incorrectly?  ???

This is very odd. I've looked at the schematics for the AG440B four-track model, and the AG-440V eight-track model. Both have two fuses, one in each leg of the power line, upstream from the double-pole power switch. That means that, in theory, with no fuses installed, there should be no AC power on any of the switch terminals, and what you're describing can't happen.

So...when the machine is plugged in and no fuses are installed, is there AC voltage (relative to ground) present on any of the switch terminals? (Be really careful when measuring this.) Is there AC voltage present between any of the switch terminals? If so, the only explanation I can offer is that some former owner hot-wired around the fuses, a very dangerous (and maybe illegal) thing to do. Or plugged the fuse-holders with aluminum foil, aldo dangerous and maybe illegal.

Peace,
Paul

Very interesting.  I'll check into all of this and post an update.  I have a busy couple of weeks outside the studio (and the studio can't exactly have me blowing fuses in the middle of sessions, either) but hopefully I can get to it soon.  I want to get this machine up and running for a record I'm doing in August.

pstamler said:

This is very odd. I've looked at the schematics for the AG440B four-track model, and the AG-440V eight-track model. Both have two fuses, one in each leg of the power line, upstream from the double-pole power switch. That means that, in theory, with no fuses installed, there should be no AC power on any of the switch terminals, and what you're describing can't happen.

So...when the machine is plugged in and no fuses are installed, is there AC voltage (relative to ground) present on any of the switch terminals? (Be really careful when measuring this.) Is there AC voltage present between any of the switch terminals? If so, the only explanation I can offer is that some former owner hot-wired around the fuses, a very dangerous (and maybe illegal) thing to do. Or plugged the fuse-holders with aluminum foil, aldo dangerous and maybe illegal.

Peace,
Paul

Click to expand...

Interesting point paul, Maybe they bypassed the fuses and there is a short in the power transformer?

Check the fuseholders aren't broken and shorting to ground. I have had that problem with those particular fuseholders.

radardoug said:

Check the fuseholders aren't broken and shorting to ground. I have had that problem with those particular fuseholders.

Click to expand...

Another excellent lead.  I have a busy 2 weeks of gigs, sessions, etc., in and out of town-- but hopefully I'll be able to check on these things soon.