1980's electronic typewriter transformer bad?

[atikovi]

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.

 

[Nigel Goodwin]

You would need to source a replacement from Brother, unless you can find a service manual that tells you what voltage it is.

The original transformer will almost certainly be specially made for Brother.

 

[atikovi]

Brother no longer has parts for this.

 

[Nigel Goodwin]

Brother no longer has parts for this.

Do they have service manuals for it?, it looks to be a simple single winding transformer so would be easy to replace if you knew it's voltage (electrically if not physically). Presumably any long established Brother service agent would have service manuals for all old models, as would Brother themselves.

 

[aliarifat794]

I can see some documents here. These may help. https://www.brother-usa.com/support/em711

 

[atikovi]

I can see some documents here. These may help. https://www.brother-usa.com/support/em711

Been there. When you click on, Manuals, and search on EM711 it says An unexpected error occurred.

 

[atikovi]

Do these numbers mean anything? besides 117 volts input. Is that a logo for the manufacturer? C Power?

 

[sagor1]

The numbers are probably house numbers, don't mean anything except for manufacturer parts list. The only number of significance is the AC117V. Is there any numbers on the other side?

 

[sagor1]

Some transformers have a thermal fuse inside the wrapping tape, near the outside of the windings. However, that transformer may be too "old" to have incorporated one. That may or may not explain the intermittent operation at one point, until it failed completely.
Measure the resistance of the 117V winding. If it is open, it may be such a fuse. If the winding shows continuity, then not likely it has a thermal fuse.

 

[atikovi]

No other numbers. 575K ohms on the input.

 

[ZipZapOuch]

make sure your meter is set to AC volts when you measure. Transformers don't output DC.

Second, it is rare for a transformer to crap out unless something happened to it (like someone damaged the insulation or a lightning strike - but that could be possible that 40-year old insulation just crumbled.

Measure the resistance across the
- black/white 120v input pair
- the output pair
- the white input and each output
- the black input and each output.
- each of the four wires and the metal frame
and report back.

 

[ZipZapOuch]

No other numbers. 575K ohms on the input.

Too high for input pair - way too high.

 

[atikovi]

- black/white 120v input pair THIS MORNING 575K, NOW 2.1M
- the output pair 0.8
- the white input and each output INFINITE
- the black input and each output. INFINITE
- each of the four wires and the metal frame INFINITE

 

[ZipZapOuch]

- black/white 120v input pair THIS MORNING 575K, NOW 2.1M
- the output pair 0.8
- the white input and each output INFINITE
- the black input and each output. INFINITE
- each of the four wires and the metal frame INFINITE

Yep, the input is out of commission. Unwrap the plastic tape to see if there a burned section. No reason not to do an autopsy.

 

[For The Popcorn]

If you take some clear pictures in the area where the red wires attach to the board, it may be possible to make an educated guess about the output voltage.

There will be one or more large electrolytic filter caps near the point. Try to see the rated voltage. There probably are one or several voltage regulators nearby, possibly bolted to a heat sink. They may have numbers like LM7805 or LM7812. These are some clues to the needed voltage.

 

[atikovi]

 

[ZipZapOuch]

Likely a 24vac transformer that will make about 34v rectified. Could be lower but I doubt it.

 

[ZipZapOuch]

Is the top of that cap a little domed? The photo is so close up it could be a lens issue but it looks like a bit of a dome (as if the cap was overheated and possibly shorted internally). Something made that transformer pop so, you might want to do some inspections before connecting a new one.

 

[ZipZapOuch]

You could just look for some misc transformers snd try them 6v, 12v, 18v, 24v until you get to the minimum size that works. Look for parts off old clock-radios, printers, etc.

Or, find a friend with a DC power supply and connect through the bridge rectifier (it doesn't care if you supply AC or DC and doesn't care about polarity - the bridge will make it right for you. Look at the heat sink (out +/- are labelled, just put the DC into the opposite two terminals - typically labelled ~ but not in your case). Start at 5v and work your way up 1V at a time until the thing comes on.

 

[sagor1]

Yep, the input is out of commission. Unwrap the plastic tape to see if there a burned section. No reason not to do an autopsy.

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.