Modify a 400hz clock to use 60hz

[Rugel]

I need some help on using 60hz power to operate a clock removed from an aircraft that should use 400hz input. I have opened it up to see that the 115 VAC 400z is immediately stepped down with a transformer to 8VAC 400hz. The fuse protecting the input side of the transformer is .5 A and the one on the output side of the transformer is 2 A. The label reads that input required is 115VAC, 15W, 400hz. This clock was made by Seiko in 1982 and was mounted on the bulkhead in the cabin of a DC10. Its display is LEDs that show an Airport code, date and time for Tokyo and whatever other airport code you select. My first attempt to just try 115V 60hz blew the input fuse and my second attempt to tie in downstream with 7.5 VAC @500ma 60hz just gave me some mixed flickering of the LEDs. My goal is to convert this to a nice little desktop of wall mounted gift for someone, but I don’t want to spend a lot of money. I can get to all of the board to change a capacitor or something, but I don’t know what it will take. I am sure that the first circuit after the transformer is a rectifier circuit of some sort to convert to DC. I see a couple of SCRs or FETs or something and a big capacitor there. If I knew where to tap in with a feed in of some DC from a plug-in AC adapter, I could just cut out all of the 400hz part, I think (?). I have no 400hz power to hook up to it to search for voltages on the board, so that makes experimenting a problem.

Much obliged for any ideas.

 

[Diver300]

The 400 Hz transformer will not work with 60 Hz. The core will be nearly 7 times too small, so the transformer will saturate and blow the fuse.

The capacitors that smooth the rectified AC will also be too small.

I would find a 2 A supply at about 10 V DC and connect that to where the output of the transformer went.

It is possible that the clock needs the 400 Hz for some reason, in which case your clock won't run, but it will probably light on DC. If that is the case, more investigation is needed.

 

[Rugel]

Thanks. So let me get this straight. You are saying to go in with 10 VDC (2A)where the 8 VAC(400hz) used to come out of the stepdown transformer? I'm guessing from that suggestion that the DC will just go through the rectifier circuit as is? I understand what you mean about the AC maybe being needed for the clock. I am trying to get a schematic of this thing to be able to check that out. I will give it a try and let you know.

 

[crutschow]

Thanks. So let me get this straight. You are saying to go in with 10 VDC (2A)where the 8 VAC(400hz) used to come out of the stepdown transformer? ..........................

Make sure the transformer output is not connected when you do that since the transformer will short the DC.

 

[Rugel]

Thanks, crutschow. Yes, I had done that before when I tried the 7.5 VAC. Luckily, there is a nice disconnect right there that leaves the two leads real easy to get to right off the CB.

 

[Mr RB]

Please post photos.

If the transformer has just one output and goes to one bridge rectifier, and one large filter cap, you can just inject a DC supply (of the right voltage!) into the filter cap. That should get the clock powered up.

If the transformer has multiple ouptuts it will be trickier. Photos will help a lot!

 

[Diver300]

Thanks. So let me get this straight. You are saying to go in with 10 VDC (2A)where the 8 VAC(400hz) used to come out of the stepdown transformer? I'm guessing from that suggestion that the DC will just go through the rectifier circuit as is? I understand what you mean about the AC maybe being needed for the clock. I am trying to get a schematic of this thing to be able to check that out. I will give it a try and let you know.

You have that right. The peak voltage of 8 VAC is around 11 V, so a 10 V supply is about right. The DC will go straight through the rectifier, which also means that it doesn't matter which way round you connect it. The 10 VDC supply will have the transformer and capacitors rated for 60 Hz.

As Crutschow says, you have to disconnect the 400 Hz transformer, which would otherwise short the DC supply.

I think that the flickering lights were because the capacitors to smooth the 400 Hz would have been too small, so in the much longer time that the 60 Hz waveform is small, the capacitors ran down, so the clock itself was turning on and off many times each second.

 

[Rugel]

Thank you. The transformer has just the two wire output, so hopefully that will work. If not, I'll get some photos out to all.

 

[Rugel]

Thank you again. I'll have time to try that by the weekend and will let you know.

 

[dr pepper]

You need to find out if your clock uses the 400hz to keep time, 50/60hz domestic bedside alarm clocks often use the mains as a way of keeping time as its cheep, 400hz on an aircraft however probably isnt bang on 400hz not all of the time anyway.

So as allready mentioned you might be able to power the clock with a 10v dc supply, see if it works, be carefull on polarity if you go on the transformer secondary side of the rectifier then polarity wont matter so much but you'll need to disconnect at least one sode of the secondary.

If the clock powers up but the time doesnt increment then it does use the input freq for keeping time.

 

[Diver300]

If the transformer output only goes to the rectifier and nothing else at all, it is unlikely to be used for timing. I don't think the 400 Hz is accurate like 50 / 60 Hz mains is.

 

[unclejed613]

i wouldn't think the 400hz is used for keeping time, since the 400hz isn't there when the plane engines are off. i would guess the clock has a crystal oscillator or something similar to keep time. of course this would mean there must be a source of backup power such as a battery for the clock to keep time when the plane is not in use.

 

[ericgibbs]

Interesting link, ref DC10 electrical power.

https://books.google.co.uk/books?id...&q=dc10 airplane electrical batteries&f=false

 

[JMW]

Why are you contacting a free site for assistance? It was made by Seiko, I think they are still in business, you could also try Douglas Corp, they too are still around.

 

[Rugel]

I did. That division is long ago gone from Japan and Douglas was bought by Boeing over 15 years ago and this clock was never an OEM part. It was a JAL add-on. Thanks.

 

[Rugel]

That is exactly right. there is a little 6 volt AA battery pack that keeps the time when the plane is powered down, so I guess that answers that question. I should have mentioned that before. Thanks.

 

[unclejed613]

you can build a 10V regulator that will source 2A quite easily with a LM7810, a power transistor, and a diode (and the usual 3 or so capacitors). this is a standard 3-terminal regulator driving a transistor. the diode compensates for the 0.7V drop across the transistor junction. the 6 ohm resistor is the load (in your case, the clock). the input voltage needs to be about 13V. i have 16V shown here, because i used it to test the circuit, and i have a 7812 model, but not a 7810.

 

[Mr RB]

i wouldn't think the 400hz is used for keeping time, since the 400hz isn't there when the plane engines are off. i would guess the clock has a crystal oscillator or something similar to keep time. of course this would mean there must be a source of backup power such as a battery for the clock to keep time when the plane is not in use.

I agree. Most of the early 400Hz (and large 'planes especially) used rotating 400Hz alternators, so I would not expect that 400Hz freq to ever be used for timekeeping purposes, especially in aero clocks where an accurate time is pretty important.

The clock will have a xtal timer.