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1980's electronic typewriter transformer bad?


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This is a Brother EM711 typewriter. It would randomly reboot as if you had turned the power off then on. Now it's totally dead. Took it apart and checked the transformer.


there is 120V on the black and white wires going up to the transformer, but only like 30 mV coming out on the two red wires on the left. Bad transformer? And where do I find a replacement? Found nothing by searching those numbers and have no idea what the output should be.
Agree. There is something "open" on the primary side, which is uncommon. I would carefully unwrap some of the cover layers (paper?) to look for the possibility of a thermal fuse or some other element that acted as a fuse on the primary, buried under the wrapping.

Hardly 'uncommon', it's by far the most common failure in a mains transformer, particularly ones where there's a thermal fuse inside the transformer.

It's really a very common failure.

From a service engineers point of view, when something uses a conventional transformer and it's dead, the first thing you do is put a meter across the mains plug to see if the transformer (or a potential mains fuse) is O/C - you don't initially bother taking it apart.

Once you've confirmed it's O/C you then take it apart and look for a mains fuse - just in case, although it's most likely going to be the transformer anyway, even if there is a mains fuse.

I've replaced hundreds of failed mains transformers over the decades :D
I agree with Nigel. I believe the primary coil fusing is more common than secondary due to the thermal resistance of the finer magnet wire is higher than the coarse secondary.

He was given a good estimate of voltage by ZZ and VAR or apparent power rating can be estimated by size and weight of the CRGOS laminated steel. So pick any at least as big for your primary AC V-Hz which I read as 117V.

You can also simply use a DC adjustable supply across the worn bulged Cap and increase voltage until it starts working.
Then you have more cheap choices for a solution for this relic, err, ehm, antique.

The reasons can be extrapolated from the plain Cu wire fusing power thresholds using Lewis Onderdonk's equation, and the winding engineers over a century later, have probably, like me, never hear of him before. Fun Fact. It was used in mid 1800's to compute the trajectory of cannonballs with fuses that ignite explosive material inside for military purposes and wealth. He also built railroads.
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I don't have but found this
I recently got a WZ3605E DC DC Buck Boost module from AliExpress for about $10. It is be far the simplest buck/boost converter module I've seen. Supplied with 12 volts from an Xbox power supply, it can deliver from 0.6 volts to more than 30 volts at up to 5 amps/80 watts.

It features constant voltage/constant current modes, preset settings, a plot of voltage and current vs time and a whole lot more.

Coupled with an Xbox power supply as I described in the link above, you get a fixed 5 volt, 1 amp supply, a fixed 12 volt, 16 amp supply and the variable supply. Not bad for a total cost of less than 25 bucks.


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The AC feeds a diode bridge then to the big e-Cap which might be getting about 75% of its rated voltage (TBD?) So If you had a variable ACDC supply or a computer DC supply and the above $10 variable DCDC you can determine a working DC voltage. The Vdc will be slightly higher than Vac. (40% with no load)

I could then choose a transformer to replace it, such as one of these. ~ $16 usd 45VA (~watts)
I would just figure out what voltage it needs to run and replace the transformer with a very easily accessed, and safety feature packed DC power adapter. Add a barrel connector and just plug it in. Modern DC adapters have nice overcurrent protection, and give a very stable dc supply so your old, big and expensive cap won't have to be replaced any time soon. You may even have a 19v laptop supply or a 12v supply (depending on the result of your power supply test). If that works and you can fix this at no cost (other than your variable power supply, a barrel connector and some wire.
Got the $50 power supply off Ebay delivered today. Hooked it up starting with 5 volts, then 10, 12, 15.... and around 20v it started to work but not very well. Wasn't until I hit 24 volts that it seems fully functional. Will put all the rollers back in for another test to confirm everything works and it takes a 24v transformer before searching for one.

Works great with 24VDC. Someone in another forum said I should use a 18V transformer because it's AC. Really?
Yes, 18VAC will produce about 25VDC at peak. Under load, the voltage will normally drop to the 22 to 24VDC while operating under a light load. Under heavy load, it might drop to 20V. A 24VAC transformer will give almost 34VDC peak. The question to that is, would 34V damage anything? Under heavy load, that 34V will drop to 26 to 30VDC, depending on transformer VA rating.
Recheck the component voltage ratings around the rectifier diodes, if they are ALL rated 50VDC or better, then a 24VAC may work ok. Test your printer at 30VDC first, see if it works ok. Measure the maximum current drawn. 30V x current will give approximate VA rating required for transformer
There's a big black reservoir capacitor on the left of the first picture - what voltage rating does it have? - the required DC voltage is probably going to be between 24V DC (which just works) and the voltage rating of the capacitor, probably 35V?.
18 volt transformers in this footprint are around $50 with shipping. Any reason I couldn't use this 20V model I could get for $15? Don't think I have this typewriter on for more than 15 minutes at a time.

if 24 V is minimum reliable DC and the grid is +/-10% Vac then you want 24Vdc minimum when the grid -10%

I agree with Diver300 that 20 to 40 is safe. A 24Vac transformer is my answer as others also predicted. This will produce 40% higher Vdc with no load and 24 Vdc at max load with min grid Vac. -10%

The one I found earlier for 115V or 120V should do. $16.25 + ship + tax
If you test it by raising the voltage towards 35V, the symptom will be that the waste heat will show up on the regulator being too hot to touch.>60'C This may fine tune our design by committee answer. +85'C is safe for a case temp. The Xfmr VA rating must be 30% higher than the Vdc VA product because RMS current only flows <=25% of the time near Vpk charging the 10 mF cap and poor power factor affects losses. <= my sim

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