Ampex ATR-800 Reel to Reel Appears Dead - What to do?

Picked up a cheap Ampex ATR-800 1/2" 4 track like this today:


Does not seem to want to power up, whipped the back off and checked all the fuses which are all ok. The mains transformer is hidden in a big ass metal box right at the heart of the machine and is totally inaccessible without a fair bit of disassembly.

What steps should I take to trouble shoot this thing?

Cheers,

John. 

Disconnected the motor drivers and had a look in the giant transformer box, nothing to see here but a huge potted transformer.... popped the fuse in the mains lead. Hummmm.

One or more shorted diodes (along with faulty filter caps) in the PSU rectifier circuits are a common cause of a blown mains fuse.  I hope you have a service manual or schematics.

Bri

Thanks Bri,

I have some schematics and user manuals but noting for the PSU which is a bit frustrating.

There is a central fuse board next to two large 50v 10000uf caps which visually look good,  I tested and they seem to charge up and discharge as expected. They could be 30 years old so I'm not 100% sure about them.

There is also a bridge rectifier which is shorted between the two AC legs - This is totally wrong I think(???) so I'm going take it out of circuit and see if it's an internal short or somewhere else.

UPDATE: So I took the bridge rectifier out of the circuit at it is ok. There seems to be a short across two of the wires coming out of the sealed mains transformer, these are thick beefy wires and go through a slo-blow 10A 32V fuse each and then to the AC inputs of the bridge rectifier.

Does this mean the transformer is toast? Are there any more checks I can do?

ANOTHER UPDATE: The wires that I thought were shorted give 36.8v unloaded when the unit it powered up so I guess transformer is not totally broken.

Anyone have any ideas what to try next?

Cheers,

John.

> There seems to be a short across two of the wires coming out of the sealed mains transformer

The naked transformer winding always has low resistance. Otherwise the electrons could not come out to do your work.

Say it is supposed to deliver 35V at 5A. 35V/5A is 7 ohms. If the winding were also 7 ohms, *half* the power put in would just heat the transformer. Ideally the resistance would be zero. But that means infinite size copper. Practical transformers have 5%-10% loss, which suggests 0.3ohm-0.7ohm winding resistance. On most ohm meters this is "dead short". There is usually no point in getting an exact number. Infinite ohms is bad, low-low-ohms is fine *or* there is a shorted-turn (and you can't readily identify this by measurement).

I'm sorry I don't have a clue about this specific beast. But it's just like plumbing. No water at the sink? Try the hose. If both are bad, then the problem isn't under the sink. Go where the pump/meter comes into the house and see if there's water there.

You had a blown main fuse, a shorted rectyfier, and now 35V without the rect. It is possible you just need a new rectifier. Experience (mine and now yours) says to OVER-spec that part. Turn-on current is large (look at the caps). A rect can take such abuse maybe 1,000 times, micro-cracking each time, before it quits. Over-over-size is cheaper than another failure. 25A 400V is not too large. I'd check prices/sizes for 50A.

Thanks for pointing out the winding thing, I'm glad the transformer is not dead it weighs about half a ton. I measured the rectifier with my meter set to diode and it seems fine. It must have been the transformer winding giving the shorted reading because out of circuit it's perfect. It can't hurt to replace and upgrade though.

Think I'll just have to trace where the power goes and more importantly where it stops!

So, with the bridge rectifier disconnected, the mains fuse didn't pop?  Sounds like the xfmr is probably OK.  And, the bridge might be OK, since they usually fail with a short on one or more of the four diodes....which will blow the mains fuse.

That makes the filter caps a next suspect.

Bri

Well I don't think I mentioned it in the last post but I used a 13A fuse which didn't pop. I just tested it back down at the specified 5A it gets blown immediately the main power switch is pressed.

Ooh good, lets put a bigger fuse in it!

You need to buy a variac or make a series lamp circuit. Wire a 300 or so watt lamp in series with the phase lead. This way, if the power supply has a short, the lamp will light up. If the power supply is good, the lamp will come on momentarily and then go dull. Its the perfect safety device for testing power supplies and amps. You can make it up using a plug and socket suitable for your wiring, no need to disturb the unit wiring.
Once you do this, you can meter across the filter caps and see if that is where your problem is. You would be advised to disconnect the power supply from all its loads, and get that going properly first.
The fault may be in the load.

I never had my nose in a ATR800 but if they are anything like an ATR100 look for
shorted Tant. caps across power rails.  A common problem with ATR100s.

Best

GARY

leafcutter said:

Well I don't think I mentioned it in the last post but I used a 13A fuse which didn't pop. I just tested it back down at the specified 5A it gets blown immediately the main power switch is pressed.

Click to expand...

That was really stupid. There's no nice way to put it. It's not only incredibly dangerous to you, but you might have broken things much further by now, the original fault a simple one compared to a host of melted lumps scattered around.