Janome (New Home) Memorycraft 6000 sewing machine

[steve_tuck]

Hi,
I have this item to investigate. I've had it looked at by a sewing machine mechanic but the problem is electronic, not mechanical.
I purchased the service manual, but this only gives faultfinding down to "replace the board" level.

Being 1980s era electronics, it's not too complex. I've found the board that has a fault. Unfortunately I don't have a schematic (does anyone?) the board is marked "822-B-3" and is no longer available as a spare as far as I can find.

It's an interesting board, comprising classic 40-pin DIL microcontroller and 74LS logic parts (some proprietary). It's got two large heatsinked ICs, which are stepper motor controllers, which are Janome / Sanken SI-7200J. These are quite high-power and advanced devices for the time. There are some parts listed on a well-known Chinese site for around £15, if that turns out to be the fault.

So far I've established the 5 volts PSU to the logic section is working. The high-voltage side for the stepper controllers blows the 2.5A fuse. The circuit starts with a bridge rectifier and smoothing cap, then it has the following part:

MI-10

This has two diode symbols on it, but I can't find any precise info. I wonder if it's a zener stabiliser of some kind? That kind of thing could fail short-circuit over time ..

Also it would be helpful to know what those diodes are. I think they'll be rated for at least 100v and a few amps, based on the likely supply voltage (the smoothing cap is 4700u at 50v).

[paulsherwin]

It's probably a pair of rectifier diodes. Presumably it has 3 legs? It's difficult to imagine the function in the circuit you describe though.

I think you're going to have to trace out the circuit.

[Radio Wrangler]

There may be two different power supplies. One with the four individual diodes and one with that little rectifier block. Stepper motors need a DC supply, and usually well over 5v for efficiency and speed. The logic area will want 5v in that era. Is there a transformer?

How confident are you that the stepper driver parts your search turned up are genuine? These are the sorts of things where you run into a mix of bona-fide traders and scammers. Some checking would be wise.

Before spending money on parts check the windings in the steppers are intact.

David

[factory]

There is larger bridge rectifier in the picture too.

David

[steve_tuck]

Originally Posted by Radio Wrangler

There may be two different power supplies. One with the four individual diodes and one with that little rectifier block. Stepper motors need a DC supply, and usually well over 5v for efficiency and speed. The logic area will want 5v in that era. Is there a transformer?

How confident are you that the stepper driver parts your search turned up are genuine? These are the sorts of things where you run into a mix of bona-fide traders and scammers. Some checking would be wise.

Before spending money on parts check the windings in the steppers are intact.

David

Yes, I have more detail now. The transformer provides two secondaries, connecting to the board top right via a 4-pin connector. Pins 1 and 4 of this (purple) measure 29v rms (~40v peak). This passes through a fuse F2 2.5A (in my case, blown already) and through the four discrete diodes forming a full-wave rectifier and to the large cap C9, and via the "mystery" IN10 component and from there to the large stepper driver ICs.

The other secondary enters on pins 2 and 3 measuring 16v rms. This passes through the packaged full-wave rectifier (an S4VB) to cap C10, 3300u, 25v. This side is working; it then passes through a regulator IC11 3052P which drops it down to 5v for the logic.

As it's hard to find where the short might be occuring, I'm going to try some beginner's luck and remove/examine C9.

Originally Posted by paulsherwin

It's probably a pair of rectifier diodes. Presumably it has 3 legs? It's difficult to imagine the function in the circuit you describe though.

I think you're going to have to trace out the circuit.

As there are already two separate, obvious full-wave rectifiers (one a S4VB, one made from discrete diodes) it's not obviously doing that.

Originally Posted by factory

There is larger bridge rectifier in the picture too.

David

Yes, that's for the lower voltage side (it's an S4VB) which is working.

[steve_tuck]

Update: removed the main smoothing capacitor C9, 4700u, 50v. Measured as a short. Replaced it with nearest equivalent in my parts box as a temporary measure. Tested the machine and it's now working.

Now I'll replace with correct values etc.

[steve_tuck]

Here's a better pic of the mystery component. While it's likely working OK, I would like to know what it is. The number on the side is "1402024", the designation on the top is "MI-10", and those look like two diode symbols on the side. It's in a 4-pin package.

[TonyDuell]

If the 'AC squiggle' and '+' on the silkscreen refer to that component (is there another AC symbol at the right hand end?) then I think it's what it appears to be. 2 diodes, common cathode connection.

Sort of like a silicon EZ80

Could be used as a biphase rectifier on a centre-tapped transformer winding, for example

[ex 2 Base]

Good to hear is now up and running again, better than the bin. Ted

[Silicon]

Power diodes are not just for power supply rectification.

They can be used as Steering diodes, Clamping diodes and for protecting semiconductors from the back EMF from coils and motors.

[RalphFlow]

Hi. I am a new member, came here because of this thread.

My question to Steve if listening is....
"Hi Steve. I came across your post about the Janome sewing machine. I have the same model, and an electrical problem. I was wondering with your machine, although nothing worked, did the lamp above the needle work? My lamp works, but nothing else, no mechanical, no red dots for the stich selection, and no red LED numbers. Looking at your post I am thinking about just going in and replacing the caps as you did and checking the fuse. I am not as knowledgeable by any means as much as you, with respect your post, but can replace components etc...What do you think, shall I give it a go? I live in NZ. Thanks Ralph".