Question about PSU transformer substitution - Tandy 5.25in disk drives

[TRS-Ian]

I have a small but growing pile of 5.25in disk drives from the USA which are piling up - they are 120V and here in Australia we use 240V.

(By disk drive I mean the external unit comprising of the disk drive itself, case, and power supply.)

The power supply comprises of a transformer with two separately wound secondary windings. One supplies 8.5V the other supplies 14.5V (both unloaded), so I am looking for a replacement 240V transformer with similar outputs. These then feed into two full wave rectifiers, a large filter e-cap, then a voltage regulator and out to the disk drive.

The unit itself it marked as 120V 60Hz and 0.25A on the outside, so will a 1A (15VA) multi-tap transformer be adequate for the job? I was hoping to tap off 7.5V and 12V

Alternately 2A (30VA) multi-tap transformers are available for considerably more money and I'm happy to use one if that is what's best for the job.

If I'm off the mark can someone advise what I should be using? Like two small transformers etc.?

Another questions is that in general, on 5.25in drives which require a 5V and 12V supply, what typically is the required amperage for each supply? The drives I'm trying to get working are TEC FB-501 which are 1/2 height and more modern than the old full height belt-drive Tandons... though I do set up the odd Tandon from time to time.

Thanks in advance,

Ian.

 

[Chuck(G)]

I suspect that you're best off in the long run by employing a step-down 220VAC to 120VAC transformer and leaving the drive boxes alone. If you were in the UK, I'd advise the employment of a portable tool transformer (output is 120V, with a grounded center tape, IIRC). Still, any good-sized transformer with a "split" primary should work as an autotransformer. The secondaries are unimportant.

I've used a similar solution for 220/240VAC gear from overseas; it's just that the wiring is a bit different.

 

[Hugo Holden]

Hi,

I agree with Chuck(G), it is best to use an external 240V/120V step-down transformer.

Jaycar Electronics in Australia have some very nice small ones fitted with American mains connectors, which also bring the earth connection across and they have an isolated secondary winding for safety and act as an isolating transformer. Though, be aware that an isolating transformer defeats the action of the RCD (earth leakage detector) on your fuse-box so its important to only run one appliance from an isolating transformer at any time, otherwise a fault in the other appliance could ground one of the voltage feed wires taking the other wire high above ground and create a shock hazard that the RCD cannot detect (this is not a widely known or thought about issue).

Many step-down transformers are Auto-transformers, as this keeps their size smaller and they do not defeat the RCD like an isolation transformer does, but probably, overall, the isolation transformer is actually safer than even the RCD, if it is just the one appliance connected to it that is, like when you are outside using one item like an electric drill. There is always a chance that a RCD might not actually trip off with an unbalanced current, if faulty, but usually they will, but I once saw an RCD where the plastic casing had melted a little, due to heating, and it jammed the trip mechanism !

So what I'm saying is don't use an isolated type mains step-down transformer to run a power board (strip of power sockets) with multiple 120v items powered. If you want to have a 120V power board running a few 120V appliances make sure it is an auto-transformer step down design, that way your fuse-box RCD's will still work.

Jaycar also have dual voltage switch-mode PSU's with a number of voltage & power output options.

 

[TRS-Ian]

Thanks for your thoughts, both of you. Just plugging the drive into a step-down isn't going to be a viable solution as I those buying these off me don't want an additional box. Also the least expensive step-down that would be suitable and safe blows out the potential sales price to that where no one will buy them. Sorry to sound like it's all about the mighty $ but I do buy and sell a fair amount of hardware in order to perpetuate my hobby.

Fortunately a PSU specialist dumbed down the required calculations to allow me to replace the 120V transformer with two small 240V transformers from Jaycar. A bit of work to mount the transformers inside the box, a new Australian power cord and the unit works. It's been running non-stop for 3 days now.

Cheers,

Ian.